Table of contents

Pg # Topic
Chapter 1: Energy and Respiration

Energy and why we need it

 Energy is defined as the capacity to do work.
 All living organisms require a continuous supply of energy for various,
vital biological processes.
 Examples include:
- Anabolism, where small units are used to synthesize large molecules, e.g
proteins synthesis.
- Maintaining homeostasis in body, e.g. body temperature
- Active transport of molecules in and out of cells.
- Muscle contraction and movement in animals, plus beating of cilia and
flagella.
- For growth and cell division.
- In reproduction, energy is required for gamete production, fertilization
and embryonic development.
- Electrical transmission of nerve impulses.

Source of energy
 According to the laws of thermodynamics, energy cannot be created or
destroyed; it is transformed from one form into another.
 Only two types of energy are suitable for living organisms, i.e. light and
chemical energy.
 During photosynthesis, light energy from the sun is captured by pigments
called chlorophyll within specialized organelles called chloroplasts, found
in plant cells.
 This energy is then converted into chemical potential energy in the form
of nutrient molecules, e.g, carbs, fats and proteins.
 These molecules are then used for cellular respiration, which releases
energy in form of ATP.
 ATP (adenosine triphosphate) is a phosphorylated nucleotide and is the
universal energy currency for cells.
 The energy in ATP molecules can then be used to fuel various cell
activities.

Structure and hydrolysis of ATP

 ATP = Adenosine tri-phosphate
ADP = Adenosine di-phosphate
AMP = Adenosine mono-phosphate
 ATP is a phosphorylated nucleotide, made up of: ribose sugar, adenine
base and three phosphate groups.

 ATP can undergo hydrolysis, a reaction which requires addition of water.

 When one phosphate group from an ATP molecule is removed, it releases
30.5kJ mol of energy and forms ADP (plus one inorganic phosphate
molecule- Pi).
 Pi = Inorganic phospate molecule = H3PO4
 When another phosphate group is removed, AMP is formed and another
30.5kJ mol of energy is released (plus one inorganic phosphate molecule-
Pi).
 However, removal of the last phosphate group only releases 14.2kJ mol of
enrgy, leaving behind only the nucleoside- Adenosine

 The enzyme which catalyses the hydrolysis of ATP is called ATPase.

 The breakdown of ATP is a reversible reaction.

 ATP can be reformed from ADP and Pi, and ADP from AMP and Pi.
 It is a phosphorylation reaction.
 Since it requires water, it is also a condensation reaction.

 Equations:
 ADP + Pi -------> ATP + H2O
 ATP + H2O -------> ADP + Pi
 Features of ATP
 ATP is known as the universal energy currency or carrier because it is
used in all organisms, as a source of energy.
 It is perfect for its role for many reasons:

 Hydrolysis of ATP can be carried out quickly and easily, whenever energy
is required, by just one enzyme- ATPase.
 The rate at which ATP is synthesized can adjust very quickly to demand.
 A useful amount of energy (small but sufficient) is released, enough to
drive important metabolic reactions while keeping energy wastage low.
 It exists as a relatively stable molecule, meaning it won’t break down
unless ATPase is present, so energy would not be wasted.

Synthesis of ATP
 On average humans use more than 50 kg of ATP in a day but only have a
maximum of ~ 200g of ATP in their body at any given time.
 This is because ATP is an energy carrier, not an energy store.
 Molecules like fat and glycogen are made for long term storage of energy.
 ATP is made from ADP and inorganic phosphate, in a phosphorylation
reaction.
 This can be done in two ways- Chemiosmosis and Substrate-linked
reaction.

Substrate-linked phosphorylation:
 Substrate-linked phosphorylation is the process by which ATP is formed,
through the direct transfer of a phosphate group from a high-energy
molecule to ADP.
 Energy for the reaction is supplied by other metabolic pathways, such as
glycolysis and the Krebs cycle, where energy-rich molecules are broken
down.
 An example of a high-energy molecules is succinyl-CoA, which donates a
phosphate group to ADP, in the Krebs cycle, generating ATP.
 This type of ATP synthesis occurs in the cell cytoplasm and in the matrix
of the mitochondria.
 It only accounts for a small amount of the ATP synthesised during aerobic
respiration, 4-6 ATP per glucose molecule.

Chemiosmosis:
 Chemiosmosis is a process which involves proton movement across a
membrane, resulting in a proton gradient, that can then be used to drive
ATP synthesis.
 It takes place in the inner mitochondrial membrane in animal cells during
cellular respiration.

 In plant cells, chemiosmosis occurs in the thylakoid membrane of

chloroplasts during photosynthesis.
 It is part of oxidation phosphorylation and accounts for 32-34 ATP per
glucose molecule.

 Chemiosmosis in mitochondria generates ATP during cellular respiration.

 The electron transport chain in cellular respiration creates a proton
concentration gradient by pumping protons from the mitochondrial matrix
to the inter membrane space.
 ATP synthase uses the energy from proton movement back into the matrix
to convert ADP and inorganic phosphate (Pi) into ATP.

 Chemiosmosis also occurs in chloroplasts during photosynthesis.

 Chlorophyll captures light energy, which excites electrons and causes
them to flow through an electron transport chain in the thylakoid
membrane.
 The electron movement through the chain pumps protons (H+) from the
stroma to the thylakoid space, creating a proton gradient.
 Protons returning to the matrix through ATP synthase generate energy
used to convert ADP and inorganic phosphate (Pi) into ATP.
 Aerobic Respiration
 Aerobic respiration refers to the metabolic process by which cells use
oxygen to convert nutrients, such as glucose, into energy in the form of
ATP.
 It occurs in the presence of oxygen and involves a series of chemical
reactions.

 There are four stages in aerobic respiration which occur in eukaryotic

cells:
- Glycolysis in cytoplasm
- link reaction in the mitochondrial matrix.
- Krebs cycle in the mitochondrial matrix.
- oxidative phosphorylation on inner membranes of mitochondria.

Glycolysis:
 It is the first stage of aerobic respiration and occurs in the cytoplasm of
cells.
 Glucose molecules (6C) are broken down into two pyruvate (3C) each.

 Phosphorylation:
 Glycolysis does not require the presence of oxygen, but does require an
initial energy investment to activate glucose. Two ATP molecules are
needed to activate one molecule of glucose.
 Addition of two phosphate groups to a glucose forms a more reactive
molecule called fructose 1,6 biphosphate (6C).

 Lysis:
 The phosphorylated 6C sugar is split into two 3C triose phosphates called
G3P (glyceraldehyde 3-phosphate).

 Oxidation:
 Next, each triose phosphate is oxidized, by undergoing dehydrogenation.
 Hydrogen is removed from each molecule of triose phosphate and
transferred to coenzyme NAD to form two NADH (reduced NAD).

 Substrate-linked phosphorylation:
 Intermediate molecules generated in glycolysis undergo a series of
enzymatic reactions. E.g glyceraldehyde-3-phosphate, is converted to 1,3-
bisphosphoglycerate.
 Phosphates are transferred from these high-energy intermediate
molecules, to ADP, forming four ATP molecules.

 End product:
 Each triose sugar is converted to a pyruvate.
 If oxygen is available, a pyruvate will move into a mitochondrion and
complete aerobic respiration, otherwise it will be converted to ethanol or
lactate, for anaerobic respiration.
 In total, glycolysis yields a net gain of two molecules of ATP and two
molecules of NADH for each molecule of glucose that enters the process.
 The ATP produced during glycolysis serves as an immediate source of
energy for the cell, while NADH carries high-energy electrons that can be
used in later stages of aerobic respiration.

 Equations:
 Glucose + 2ATP → Fructose bisphosphate.
 Fructose bisphosphate → 2 Triose phosphate
 4H + 2NAD → 2NADH + 2H+
 4Pi + 4ADP → 4ATP
 2 Triose phosphate → 2 Pyruvate

Link reaction:
 Also called pyruvate decarboxylation, it is the second stage of aerobic
respiration and takes place in the mitochondrial matrix.
 It is the transition stage between glycolysis and kerbs cycle and links the
two processes.
 After a pyruvate enters the mitochondrion, it is undergoes a series of
 transformations.
 Decarboxylation and dehydrogenation:
 Removal of carbon dioxide and hydrogen converts the pyruvate into an
acetyl group (CH3COO-).

 Combination with coenzyme A:

 The acetyl groups combine with coenzyme A (CoA), to form acetyl-CoA.
 This compound is a carrier of the acetyl group and is involved in
transferring it to the next stage, the Krebs cycle.
 Acetyl-CoA is a 2C molecule, as a result of decarboxylation; CO 2 is a waste
product.
 The dehydrogenation reaction reduces NAD to NADH.

 Equation:
 Pyruvate + NAD + CoA → acetyl CoA + CO2 + NADH

 Role of coenzyme A:
 A coenzyme is a molecule required to catalyze a reaction, but does not
take part in it.
 Coenzyme A acts binds to the acetyl group, resulting in the formation of
acetyl-CoA.
 This reaction is catalyzed by an enzyme called pyruvate dehydrogenase.
 The formation of acetyl-CoA is significant because it serves as a key entry
point into the Krebs cycle.
 Acetyl-CoA acts as the carrier molecule that carries the 2C acetyl group
derived from pyruvate into the Kerbs cycle.

Krebs Cycle:
 The third stage of aerobic respiration, also called citric acid cycle or
tricarboxylic acid (TCA) cycle.
 It takes place in the mitochondria of cells and is a series of chemical
reactions that further break down the products of glycolysis to generate
energy-rich molecules.
 Acetyl CoA (2C) enters the circular pathway and combines with
oxaloacetate (4C) to form a 6C compound, citrate.
 Through a series of enzymatic reactions, citrate is gradually broken down,
releasing carbon dioxide as a waste product. As the cycle progresses,
energy-rich molecules such as ATP, NADH, and FADH2 are produced.
 Oxaloacetate is regenerated at the end of the cycle, ready to accept
another acetyl group.

 Decarboxylation and dehydrogenation:

 Decarboxylation of citrate releases 2CO2 as waste product, while
dehydrogenation releases hydrogen atoms that reduce coenzymes NAD
and FAD.

 Substrate level phosphorylation:

 A phosphate is transferred from one of the intermediates to ADP, forming
1 ATP.
 For each turn of the cycle, 2 CO2 and 1 ATP molecule is produced while 1
FAD and 3 NAD molecules are reduced.
 Although the Krebs cycle itself generates a small amount of ATP, it
primarily produces electron carriers (NADH and FADH2) which will be
further utilized in the electron transport chain, where the majority of ATP
is produced through oxidative phosphorylation.

 Equations:
 Acetyl-CoA + Oxaloacetate + H2O → Citrate + CoA
 Succinyl-CoA + ADP + Pi → Succinate + ATP + CoA
 Malate + NAD+ → Oxaloacetate + NADH + H+
 8H + 3NAD + FAD → 3NADH + 3H+ + FADH2

Role of NAD and FAD

 NAD (nicotin-amide adenine dinucleotide) and FAD (flavin adenine
dinucleotide) are coenzymes that act as electron carriers, shuttling high-
energy electrons during cellular respiration.
 Coenzymes are organic, non-protein molecules that work in conjunction
with enzymes to facilitate various biochemical reactions in the body.

 They are initially in their oxidized forms, NAD+ and FAD, respectively.
 When they accept hydrogen atoms (H+), they become reduced forms,
NADH and FADH2.
 NAD+ accepts two electrons and a hydrogen ion (H+) to form NADH,
while FAD accepts two electrons and two hydrogen ions to become FADH 2.

 In the inner mitochondrial membrane, NADH and FADH2 donate their

high-energy electrons to the electron transport chain, which is composed
of a series of protein complexes, embedded in the inner membrane.
 As the electrons move through the electron transport chain, they transfer
their energy to pump protons (H+) across the inner mitochondrial
membrane, creating a proton gradient.
 Movement of hydrogen ions down the proton gradient, back into the
mitochondrial matrix, gives the energy required for ATP synthesis.

Oxidative phosphorylation:
 Oxidative phosphorylation is the final stage of aerobic respiration,
occurring in the inner mitochondrial membrane.
 The process harnesses the reduction of oxygen to generate high-energy
phosphate bonds in the form of adenosine triphosphate (ATP), through
chemiosmosis.
 It yields the majority of ATP produced in the body.
 Inner membrane of mitochondria has folds called cristae which increases
the surface area for embedding enzymes and proteins e.g. electron
carriers and stalked particles containing ATP synthase.

 Electron transport chain:

 The electron transport chain (ETC) is a series of proteins and electron
carriers embedded in the inner mitochondrial membrane.
 It receives high-energy electrons from molecules like NADH and FADH 2,
which move through the ETC, transfering their energy to pump protons
 (H+) across the inner mitochondrial membrane, creating a proton
gradient.

 Proton gradient:
 It is formed by the buildup of protons on one side of the inner membrane
creating a difference in charge and pH.
 This gradient acts like a reservoir of potential energy.

 Chemiosmosis:
 To balance the charge and pH, the protons flow back across the inner
mitochondrial membrane, by facilitated diffusion, through an enzyme
called ATP synthase.
 ATP synthase catalyses the phosphorylation of ATD, to form ATP
 ATP synthase acts like a turbine, and uses the energy released from the
flow of protons to attach a phosphate group to ADP, creating ATP.

 Role of Oxygen
 The final electron acceptor in the electron transport chain is oxygen (O 2).
 Oxygen plays a critical role in the process by accepting electrons and
ultimately combining with hydrogen ions (H+) to form water (H2O).
 4e- + 4H+ + O2 → 2H2O
 Without oxygen as the final oxygen receptor, reactions inside mitochondria
could not take place.
 Equations:
 ADP + Pi + H+ → ATP

Relation between structure and function of mitochondria

 Mitochondria are sausage-shaped organelles bound by a double
membrane, where the inner membrane is extensively folded to form
cristae.
 The organelle is referred to as the powerhouse of the entire cell and is the
site of oxidative phosphorylation.
 Each mitochondrion has two membranes; the outer membrane is smooth,
relatively permeable and made of porin, while the inner membrane is
extensively folded to form cristae, and is less permeable.
 The folding increases the surface area allowing for many proteins and
enzymes to be embedded e.g. electron carriers and stalked particles
containing ATP synthase.

 The space between the outer and inner membrane is intermembranal

space.
 It has a lower pH due to high conc. of protons.

 The semi-fluid, mitochondrial matrix is the site for Krebs Cycle.

 It contains circular DNA, 70S ribosomes, glycogen granules, kerbs cycle
enzyme etc.

 This multi-compartmental design allows for reactions to be kept separate,

and for different concentrations of molecules to be maintained in different
''rooms''.
 More mitochondria are present in cells that have a high energy demand,
e.g. muscle cells, liver cells, and RHC cells.
 More active cell types may also have larger mitochondria with longer and
more tightly packed cristae to enable the synthesis of more ATP because
they have a larger surface area

Anaerobic respiration
 Anaerobic respiration is a type of cellular respiration that occurs in the
absence of oxygen.
 This is because, in the absence of oxygen, there is no final electron
acceptor available in the electron transport chain, meaning that the flow
of electrons halts, and aerobic respiration cannot proceed.

Difference between energy yields of aerobic and anaerobic

respiration
 The energy yield from aerobic respiration is greater than anaerobic
respiration due to the presence of oxygen.
 During aerobic respiration, glucose is completely broken down in a series
of reactions that occur in the presence of oxygen.
 Oxygen serves as the final electron acceptor in the electron transport
chain, allowing for the efficient production of ATP.
 This process yields a maximum of 36-38 molecules of ATP per molecule of
glucose.

 In contrast, anaerobic respiration occurs in the absence of oxygen and is

less efficient.
 It involves the incomplete breakdown of glucose, producing fewer (2-3)
ATP molecules.
 Alternative electron acceptors, such as nitrate or sulfate which have lower
energy potentials than oxygen, reduce the efficiency of electron transport
chain, resulting in a smaller ATP yield.

Lactate fermentation in mammals:

 In mammals, when there is insufficient oxygen available, such as during
intense exercise, anaerobic respiration takes place in muscle cells.
 The pyruvate are reduced to lactate, instead of entering Krebs cycle, like
in aerobic respiration.

 Glycolysis:
 Glucose is partially broken down through glycolysis, just like in aerobic
respiration.
 Glycolysis converts glucose into two molecules of pyruvate, which then
undergo a chemical reaction called lactate fermentation.

 Lactate fermentation:
 It involves the reduction of pyruvate to lactate by the enzyme lactate
dehydrogenase.
 This reaction helps regenerate the supply of NAD+, which is necessary for
glycolysis to continue.
 Further metabolism:
 The buildup of lactate in muscles during anaerobic respiration can lead to
muscle fatigue and a burning sensation.
 Once oxygen becomes available, lactate can be transported to the liver
and converted back into pyruvate, through a process known as the Cori
cycle.
 It can be also be converted into glycogen for storage in the liver.

 Oxygen debt:
 The oxygen debt represents the additional oxygen that the body needs to
consume to recover fully from the intense exercise and return to a
balanced state.
 This is because the accumulated lactate needs to be cleared and
converted back into pyruvate.

 Equations:
 Glucose -> 2 Pyruvate + 2 ATP + 2 NADH
 Pyruvate + NADH + H+ -> Lactate + NAD+

Ethanol fermentation is yeast:

 In yeast, anaerobic respiration takes the form of ethanol fermentation.
 The pyruvate are converted to ethanol, instead of entering Krebs cycle,
like in aerobic respiration.

 Ethanol fermentation is used by yeast cells during the production of

alcoholic beverages and in industrial processes, like bread-making.

 Glycolysis:
 When oxygen is absent, yeast cells metabolize glucose through glycolysis,
producing two molecules of pyruvate.

 Pyruvate decarboxylation:
 The pyruvate molecules are converted into ethanol and carbon dioxide.
 First, pyruvate is converted into ethanal (acetaldehyde) by an enzyme
called pyruvate decarboxylase, releasing carbon dioxide as a byproduct.

 Reduction:
 Then, ethanal is further reduced to ethanol by the enzyme alcohol
dehydrogenase.
 This process regenerates NAD+ for glycolysis to continue producing ATP.
 Ethanol cannot be further metabolised; it is a waste product.

 Equations:
 Glucose -> 2 Pyruvate + 2 ATP + 2 NADH
 Pyruvate + Coenzyme A -> Ethanal + Carbon Dioxide
 Ethanal + NADH + H+ -> Ethanol + NAD+.

Adaptions of rice
 Rice is a crop that has unique adaptations that allow it to grow in
waterlogged or flooded conditions.

Increased stem growth:

 When rice plants are submerged in water, they experience a shortage of
oxygen because water restricts the availability of oxygen to the roots.
 To cope with this lack of oxygen, rice plants have developed a unique
adaptation known as the “internodal elongation response’’.
 Internodes are the segments of the stem between two nodes where leaves
and branches emerge.
 Regulated by ethylene, the process helps the rice plant by allowing it to
elongate above the water surface and continue to access oxygen for
respiration.

Aerenchyma:
 Aerenchyma is a specialized tissue, consisting of air-filled spaces or air
channels within the plant's tissues, especially in the roots and stems.
 It creates pathways for the exchange of gases, allowing oxygen to be
 transported from the above-ground parts of the plant, such as the leaves,
to the submerged roots.

 It also provides buoyancy to the plant, preventing submergence and

maintaining upright position.

Ethanol fermentation:
 Ethanol fermentation is a metabolic pathway that allows rice plants to
generate energy in the absence of oxygen.
 During this process, rice plants convert the accumulated sugars in their
cells into ethanol (alcohol) and carbon dioxide.

 Unlike most plants, rice can tolerate relatively high concentrations of

ethanol without experiencing significant damage.
 This is due to various factors, including higher production of ethanol
dehydrogenase.
 The enzyme catalyzes the conversion of ethanol into acetaldehyde,
lowering levels of ethanol accumulation in their cells, minimizing its toxic
effects.

Respiratory substrates
 Respiratory substrates are molecules that can be oxidized during cellular
respiration to produce energy.
 These substrates serve as fuel sources for the metabolic processes that
occur within cells.

 The main respiratory substrate is glucose, but other RS include fatty

acids, amino acids, ketones.
 Aminoa acids are usually used as a last resort.

Energy values:
 When these different substrates are broken down in respiration,
 they release different amounts of energy
 Glucose (carbs) are around 17 kJ g.
 Fatty acids (lipid) are 38 kJ g, on average.
 Amino acids (protein) are 17 kJ, on average.

 The differences in the energy values of substrates can be explained by

how many hydrogen atoms become available when the substrate
molecules are broken down.
 This means that a molecule with a higher hydrogen content will result in
a greater proton gradient across the mitochondrial membrane which
allows for the formation of more ATP, resulting in higher RS value.
 This is why lipids, which have long, hydrocarbon fatty acid chain, are
highest on the list.

Respiratory quotients:
 RQ is the ratio of the number of molecules of carbon dioxide produced to
the number of oxygen molecules taken in, as a result of respiration.
 RQ values can range from 0.7 to 1.0, representing different types of
substrates being used.

 RQ= CO2 / O2
 Carb = 1.0 RQ Lipid = 0.7 RQ Protein = 0.9 RQ

 An RQ of 1.0 indicates for every molecule of oxygen consumed, an equal

amount of carbondioxide is produced. An RQ lower than 1.0 means that
more carbon dioxide is being produced, compared to the oxygen being
used.
 Chapter 2: Photosynthesis

Importance
 Photosynthesis is a vital energy transfer process in which plants, algae,
and some bacteria convert sunlight into chemical energy.
 It occurs in specialized structures called chloroplasts and makes energy
and carbon available to living organisms and produces oxygen in the
atmosphere, which is vital for all aerobic forms of life.

 Directly or indirectly, almost all life on Earth depends on photosynthesis.

 6 CO2 + 6 H 2O + light energy → C6H12O6 (glucose) + 6 O2

Structure and function of chloroplast

 They are large, disc-shaped plastids, found in the cytosol of a cell,
containing photosynthetic pigments, such as chlorophyll.
 They are double membranous and have an intermembranal space.

 Stroma:
 A dense, aqueous fluid within the inner membrane that surrounds
thylakoids and contains the chloroplast's DNA, ribosomes, proteins,
starches, and, Calvin cycle enzymes.
 Site of light-independent reactions
 Lamelle and Thylakoids:
 Lamelle is a membrane system running through stroma, which produces
flattened, fluid-filled sacs called thylakoids.
 Site of light-dependent reactions.

 Thylakoids contain lumen (an aqueous fluid) and chlorophyll.

 The membrane covering the sacs is known as thylakoid membranes while
the spaces between the sacs are called thylakoids spaces.

 Photosynthetic pigments and carrier molecules are embedded in lamella

and thylakoid membranes.
 Most abundant pigment is chlorophyll, which comes in two forms,
chlorophyll a and chlorophyll b.
 Grana:
 Flattened stacks of thylakoids.
 The extensive folding of thylakoids provides increased surface area for the
embedding of electron carriers and ATP synthase.
 Thus chloroplast can synthesize ATP.

Photosynthetic pigments
 These are coloured substances that adsorb light energy and convert it into
chemical energy.
 They do this by absorbing specific wavelengths of light, and tranfering the
energy to electron carriers, ultimately leading to the production of ATP
and NADPH, which are utilized in the synthesis of glucose and other
energy-rich molecules.
 The energy is then used to drive light-dependent reactions in
photosynthesis.

Chlorophylls:
 Chlorophylls are the primary pigments responsible for capturing light
energy during photosynthesis.
 The two most important types of chlorophylls are chlorophyll-a and
 chlorophyll-b.

 Chlorophyll-a:
 The main pigment for photosynthesis.
 It absorbs light most efficiently in the blue-violet and red regions of the
electromagnetic spectrum while reflecting green light, giving plants their
green color.

 Chlorophyll-b:


 An accessory pigment that assists chlorophyll a.

 It has a slightly different structure compared to chlorophyll a, allowing it
to absorb light in regions of the spectrum that are less absorbed by
chlorophyll a.
 Chlorophyll b primarily absorbs blue and red-orange light and appears
greenish-yellow.

 Importance:
 The presence of both chlorophyll-a and chlorophyll-b allows plants to
capture a broader range of light energy for photosynthesis.
 These pigments are located in the thylakoid membranes of the
chloroplasts, where they are organized into light-harvesting complexes in
phtosystems.

Carotenoids:
 Carotenoids are a group of pigments responsible for the vibrant red,
orange, and yellow colors seen in fruits, vegetables, and flowers.
 They absorb wavelengths of light stronglyin the blue-violet region of the
spectrum
 They can be divided into two main types: carotenes and xanthophylls.

 Carotenes:
 Carotenes are hydrocarbon pigments that are typically orange or red in
 color and are responsible for the bright color of carrots and tomatoes.

 Xanthophylls:
 Xanthophylls also contain oxygen in their chemical structure.
 They often appear yellow or yellow-orange in color.

 Importance:
 Carotenoids expands the range of light wavelengths that can be utilized
for photosynthesis, enhancing the plant's ability to convert light energy
into chemical energy.
 They also have antioxidant properties and are classified as provitamin A
compounds.

Absorption and action spectra

Absorption spectra of chlorophyll:

 It is a graph that shows the absorbance of different wavelengths of light
by a particular pigment.

 Thus, the absorption spectra of chlorophyll refers to the specific

wavelengths of light that chlorophyll molecules can absorb.
 The absorption spectra of chlorophyll-a and chlorophyll-b show peaks in
the blue and red regions of the electromagnetic spectrum.

 Chlorophyll-a primarily absorbs light in the blue (around 430-450 nm) and
red (around 640-680 nm) wavelengths.
 Chlorophyll-b has slightly different absorption peaks, with a higher
absorption in the blue region (around 450-470 nm) and a lower absorption
in the red region (around 620-640 nm).

 Chlorophyll has limited absorption in the green region of the spectrum

(around 500-600 nm). which is why plants appear green to our eyes.
Action spectra for photosynthesis:
 It is a is a graphical representation that shows the relative effectiveness of
different wavelengths of light in driving the process of photosynthesis.

 The action spectrum of photosynthesis shows peaks of high effectiveness

in the red and blue-violet regions of the spectrum, indicating that these
wavelengths are most efficiently absorbed by photosynthetic pigments,
especially chlorophyll.
 The correlation between the action spectrum of photosynthesis and the
absorption spectrum of chlorophyll is that they are closely related, since
chlorophyll is the primary pigment involved in photosynthesis.
 Both show peaks in blue-violet and red regions, and troughs in green
regions.

Photosystems
 They are protein complexes found in the thylakoid membrane of
chloroplasts, each containing a light-harvesting complex of pigment
molecules.

 Photosystem II (PSII) and Photosystem I (PSI) are involved in the light-

dependent reactions, which convert light energy into chemical energy in
the form of ATP and NADPH.

 The light harvesting complex can contain up to 200-300 pigments.

 Each photosystem also contains a reaction centre, which consists of a pair
of specialized chlorophyll-a molecules, serving as the primary pigment for
that photosystem.

Photosystem II:
 Photosystem II is at the beginning of the electron transport chain and is
where the photolysis of water takes place.
 It has a reaction centre of two P680 chlorophyll-a molecules, since it
absorbs light at a wavelength of 680nm.
 Thus P680 is also the primary pigment of photosystem II.

 The photosystem absorbs photons of light energy and uses them to excite
electrons within its pigment molecules.
 These energized electrons are then passed through a series of electron
carriers in a process called the electron transport chain.
 As the electrons move through the carriers, they release energy, which is
used to pump protons (H+) across the thylakoid membrane, creating a
proton gradient.
 PSII also plays a crucial role in splitting water molecules (photolysis) to
release electrons, protons, and oxygen as byproducts.

Photosystem I:
 It is in the middle of the electron transport chain
 It has a reaction centre of two P700 chlorophyll-a molecules, since it
absorbs light at a wavelength of 700nm.
 Thus, P700 is the primary pigment of photosystem I

 The system absorbs additional photons of light energy and further

energizes the electrons received from PSII.
 The energized electrons from PSI are then used to reduce NADP+
(nicotinamide adenine dinucleotide phosphate) to NADPH.
 NADPH is an energy-rich molecule that carries the high-energy electrons
and hydrogen ions (protons) to the Calvin cycle.

Photosynthesis
 There are two stages in photosynthesis:
- Light dependent reactions in lamella.
- Light independent reactions in stroma.

Light-dependent reactions:
 The light-dependent reactions use light energy to make two molecules
needed for the next stage of photosynthesis: the energy storage molecule
 ATP and the reduced electron carrier NADPH.
 It occurs in lamella membranes of chloroplast.
 There are two processes, cyclic photo-phosphorylation and non-cyclic
photo-phosphorylation.

Non-cyclic photo-phosphorylation:
 Involves photosyestem I and II.
 It generates ATP and NADPH

 Photoactivation:
 Light is absorbed by pigments in photosystem II and passed to the
reaction centre.
 The energy is transferred to the primary pigment, P680 (specialized
chlorophyll-a).

 An electron in the P680 is excited to a higher energy level and is emitted

from the chlorophyll molecule in a process known as photoactivation

 Photolysis:
 The energized electron is captured by a primary electron acceptor and is
replaced by an electron from water.
 This is done through photolysis- by splitting water molecules into
hydrogen ions (protons), electrons and oxygen.

 The reaction is catalyzed by an enzyme called the oxygen-evolving

complex.
 This releases molecular oxygen as a byproduct and replenishes the
electrons needed for the continuation of the process.
 Hydrogen ions are later used to reduce NADP.
=
 ATP synthesis:
 The excited electrons are then transferred through an electron transport
chain, consisting of proteins embedded in the thylakoid membrane.
 As the electrons move along the chain, their energy is gradually released
and used to pump protons (H+) across the thylakoid membrane from the
stroma to the thylakoid lumen, creating a proton gradient.
 Protons flow back into the stroma through an enzyme called ATP synthase,
which harnesses this flow to produce ATP from ADP and inorganic
phosphate (Pi).
 This process is known as chemiosmosis.

 Light absorption in photosystem I:

 The excited electron from P680 joins the P700 pair in photosystem I, via
electron transport chain.
 When an electron in PSI undergo photoactivation, the electron from PSII
replaces it.
 Since the electrons lost by P680 are not reverted back to P680, the
process is called non-cyclic phosphorylation.

 NADPH formation:
 The energized electron from PSI is then passed through a short electron
transport chain, where it gains additional energy.
 At the end of the chain, the electron is passed to NADP +, along with a
second electron from the same pathway.
 Combined with two H+ ions, gained through photolysis, NADP+ is reduced
to NADPH.

 2H+ + 2e- + NADP ------> NADPH

 These energy-rich molecules (ATP and NADPH) are subsequently utilized

in the light-independent reactions to convert CO2 to organic molecules,
ultimately leading to the production of glucose and other important
compounds in the process of photosynthesis.

Cyclic phosphorylation:
 Involves photosystem I only.
 Produces only ATP.

 Photoactivation:
 Light is absorbed by pigments in photosystem I and passed to the reaction
centre.
 The energy is transferred to the primary pigment, P700 (specialized
chlorophyll-a)
 An electron in the P700 is excited to a higher energy level and is emitted
from the chlorophyll molecule in a process known as photoactivation.

 ATP synthesis:
 The excited electrons from PSI are transferred to an electron acceptor
molecule, via an electron transport chain within PSI.
 As the electrons move along the chain, their energy is gradually released
and used to pump protons (H+) across the thylakoid membrane, from the
stroma to the thylakoid lumen, creating a proton gradient.
 Protons flow back into the stroma through an enzyme called ATP synthase,
which harnesses this flow to produce ATP from ADP and inorganic
phosphate (Pi).
 This process is known as chemiosmosis.

 Instead of being transferred to a separate electron transport chain and

used to generate NADPH, as in non-cyclic photophosphorylation, the
energized electrons are returned to the original chlorophyll molecule in
PSI.
 Since it does not involve the reduction of NADP+ to NADPH, it does not
produce any reducing power for the Calvin cycle, which is responsible for
the synthesis of glucose.

Light-independent reactions
 Also known as Calvin cycle or dark reactions.
 They take place in the stroma and do not require light.
 They use the ATP and reduced NADP from light-dependent reactions to
convert carbondioxide into carbohydrates..
 This occurs in three main steps.

 Carbon dioxide fixation:

 Carbon dioxide molecules enter the Calvin cycle and combine with a
pentose sugar (5C) called ribulose-1,5-bisphosphate (RuBP).
 This reaction is called carboxylation and is catalyzed by the enzyme
ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as
Rubisco.

 Rubisco is also the world’s most abundant enzyme.

 The resulting 6C compound is unstable and quickly breaks down into two
molecules of a 3C acid compound called glycerate-3-phosphate (GP).

 Reduction of GP:
 The second step involves the reduction of glycerate phosphate to a 3C
sugar phosphate called, triose phosphate (TP). It is also called glycer-
aldehyde-3-phosphate (G3P)
 This is the first carbohydrate produced in photosynthesis.

 This step requires ATP and reduced NADP, which are products of the light-
dependent reactions.
 ATP provides energy, and NADPH supplies electrons and hydrogen ions
(H+) for the reduction process.

 Regeneration of RuBP
 For every three molecules of CO2 fixed, six molecules of TP are produced.
 One of the TP molecules is used for glucose synthesis, while the remaining
five TPmolecules are used to regenerate RuBP.

 The regeneration process involves a series of enzymatic reactions that

rearrange and convert the molecules of TP. These reactions require ATP
 By regenerating RuBP, the Calvin cycle prepares the cycle to accept more
carbon dioxide molecules and continue the process of fixing carbon and
producing carbohydrates.

Intermidiates of Calvin cycle:

 Intermediate molecules of the Calvin cycle (such as glycerate 3-phosphate
and triose phosphate) are used to produce other molecules.

 Glycerate 3-phosphate (GP) is used to produce some amino acids.

 Triose phosphate (TP) is used to produce:

- Hexose phosphates (6C), which can be used to produce starch, sucrose
or cellulose.
- Lipids for cell membranes.
- Amino acids for protein synthesis.

Investigation of limiting factors

 Plants need several factors for photosynthesis to occur:
- light intensity
- CO2 concentration
- water
- temperature
- photosynthetic pigments
 Shortage of any of these factors will lead to lower rates of photosynthesis.

 Limiting factor refers to any factor or condition that restricts or constrains

the rate of photosynthetic activity. It is generally in the shortest supply.
 The three main limiting factors of photosynthesis are light intensity,
carbon dioxide concentration, and temperature.

Light intensity:
 Light is essential for photosynthesis as it provides the energy needed to
 drive the process.
 When temperature and carbon dioxide concentration remain constant,
changes in light intensity affect the rate of photosynthesis.

 When light intensity is low, the production of ATP and NADPH in the light-
dependent reactions is reduced, leading to a decrease in the overall rate
of photosynthesis.
 However, there is a saturation point where further increases in light
intensity do not significantly increase the rate of photosynthesis because
other factors become limiting.

Carbon dioxide concentration:

 Carbon dioxide is essential for the synthesis of glucose during
photosynthesis.
 As CO2 diffuses into the leaf through tiny openings called stomata, it
enters the chloroplasts and is used in the carbon fixation process.
 Insufficient CO2 reduces the efficiency of the Calvin cycle, limiting the
rate of glucose production.

 Thus, the rate of photosynthesis increases as carbon dioxide concentration

increases.
 However it reaches a plateau when another limiting factor prevents the
rate from increasing.

Temperature:
 Photosynthesis (especially Calvin cycle) is a temperature-sensitive
process.
 At low temperatures, enzyme activity is reduced, limiting the rate of
photosynthesis.
 Conversely, at high temperatures, enzymes can become denatured,
impairing their function.
 The optimal temperature for photosynthesis varies among plants, but it
generally falls within the range of 25-35 degrees Celsius.
 Chapter 3.1: Homeostasis in
mammals

 Homeostasis is the regulation of the internal conditions of a cell or

organism to maintain optimum conditions for function, in response to
internal and external stimuli.
 It involves the regulation and control of various physiological variables
within narrow limits, such as body temperature, pH, blood glucose levels,
 and fluid balance.

Importance in mammals:
 Mammals are homeothermic organisms, meaning they maintain a
relatively constant internal body temperature, due to thermoregulation, a
homeostatic mechanism.
 Homeostasis is crucial for mammals because their physiological
processes, e.g. enzymatic reactions, cell metabolism, and protein function,
are highly dependent on specific temperature ranges.
 Deviations from this optimal temperature can lead to biochemical
imbalances, denaturation of proteins, and impaired cellular function.

 Homeostasis also plays a vital role in maintaining the pH balance of bodily

fluids.
 Deviations from this optimal pH range can disrupt enzyme activity, alter
protein structure, and affect overall cellular function.
 Homeostatic mechanisms, such as the buffering systems in the blood, help
regulate pH levels and prevent potentially harmful acid-base imbalances.

 Similarly, maintaining appropriate blood glucose levels is crucial for

providing a constant energy source to the cells.
 Fluctuations can result in various disorders, such as diabetes, which can
disrupt normal cellular metabolism and lead to severe health
consequences.

 Maintaining fluid balance is also essential for the proper functioning of

mammalian systems.
 Homeostatic mechanisms regulate water intake and excretion, electrolyte
balance, and blood volume.

 Overall, homeostasis is crucial for the survival and well-being of mammals.

 It allows for the stable and optimal functioning of cells, tissues, and organ
systems by maintaining vital physiological variables within narrow ranges.
 Principles of homeostasis

Negative feedback:
 Negative feedback is a key principle of homeostasis.
 It is a regulatory mechanism in which the body detects stimuli and
responds by counteracting the changes, thereby maintaining a stable
internal environment.

 This feedback loop typically consists of three components: a receptor, a

control center, and an effector.
 Receptors are specialized structures that detect stimuli and transmits the
information to the control centers of the body.

 Control centers refer to specific structures within the body that regulate
and maintain a stable internal environment.
 They compare the input received from receptors to the set point or
desired value.
 If the variable deviates from the set point, the control center initiates a
response to bring it back to the optimal range, by signaling to the effector.

 The control centre is usually in the brain or spinal cord. Examples include:
Hypothalamus is regarded as the master control center.
Pancreas are a control centre in blood glucose homeostasis.
The piturary gland regulates homeostasis in relation to endocrin system.
The spinal cord is a control centre for coordination.

 Effectors are tissues or organs that carry out the corrective actions
directed by the control center.
 Effectors include muscles or glands.
 Muscles can contract or relax, while glands secrete hormones or other
substances.

 Role of coordination systems

 In mammals, homeostasis depends on nervous and endocrine systems.
 The nervous system consists of neurons that transmit signals rapidly
through electrical impulses, enabling rapid and precise responses to
stimuli.
 The endocrine system consists of glands that secrete hormones ( chemical
messengers) into the bloodstream, influencing the activities of target cells
or organs, over a longer time frame.

Positive feedback:
 Positive feedback is a regulatory mechanism in which a deviation from the
normal range or set point of a physiological variable amplifies the
response, leading to further deviation in the same direction.

 For example, during blood clotting, platelets in the blood recognize the
injury and aggregate at the site.
 This aggregation releases chemicals that attract more platelets to the
injury site, which further promotes platelet aggregation.

 When a woman enters the labor process, the stretching of the cervix
stimulates the release of oxytocin from the posterior pituitary gland.
 Oxytocin promotes uterine contractions, which in turn stimulate further
release of oxytocin, leading to the eventual birth of the baby.

 Positive feed back loops are tightly regulated to prevent excessive or

prolonged amplification that could be detrimental to the body.
 The control and termination of positive feedback mechanisms often
involve the activation of negative feedback loops to restore balance and
maintain homeostasis once the desired response or outcome is achieved.

Urea formation
 Excretion is the removal of unwanted metabolic byproducts and toxic
waste from the body.
 This includes nitrogenous compounds such as urea, ammonia and uric
 acid, carbon dioxide from cell respiration and bile pigments.

 Urea is a nitrogenous waste produced in liver from the deamination of

excess amino acid.
 When our body digests protein, most of the amino acids are used up in
various metabolic processes e.g. protein synthesis.
 However, our body cannot store excess amino acids, so they undergo
deamination.

 Deamination:
 Deamination involves the removal of the amino group (-NH2) from the
amino acid molecule, resulting in the formation of ammonia (NH3) and a
keto acid.
 It is carried out by the liver.
 The keto acid may enter the Krebs cycle to be respired, be converted to
glucose, or converted to glycogen / fat for storage.

 But ammonia is a soluble, toxic substance and can damage physiological

processes like, increasing alkalinity of bodily fluids, disrupt cellular
metabolism and can interfere with cell signalling process.
 Thus, to prevent accumulation, it has to be readily converted into a less
toxic form.

 Detoxification:
 Hepatocytes cells in the liver are responsible for converting ammonia into
urea through a series of enzymatic reactions, including, combining
ammonia with carbon dioxide.
 Urea is non-toxic and water soluble, so it can be easily transported in the
blood, e.g. from liver to kidney, where it is excreted.

 Equations:
 Ammonia + Carbon Dioxide → Urea + Water

Structure of Kidney
 It is a major excretory and osmoregulatory organ in mammals.
 It is responsible for removal of waste and toxic products, reabsorption of
essential substances, after filtration e.g. water and glucose, regulation pf
pH in bodily fluids, etc.

 Fibrous capsule:
 The kidney is surrounded by a tough, fibrous capsule, which provides
protection and maintains the shape of the organ. It acts as a barrier
against potential infections and external damage.

 Cortex:
 Beneath the fibrous capsule lies the renal cortex.
 It is highly vascular and and appears granulated due to the presence of
numerous nephrons, which are the functional units of the kidney.
 It also houses the glomeruli, which are specialized capillaries involved in
the initial filtration of blood.

 Medulla:
 Beneath the cortex lies the medulla, which consists of cone-shaped
structures called renal pyramids.
 They contains renal tubules and collecting ducts that transport and modify
the filtrate formed in the nephrons.

 Renal pelvis:
 At the innermost region of the kidney, the renal pyramids converge to
form the renal pelvis.
 It collects the urine produced in the kidney and funnels it into the ureter
for transport to the bladder.

 Ureter:
 The ureter is a muscular tube that connects the renal pelvis to the urinary
bladder.
 It transports urine from the kidney to the bladder through peristaltic
contractions, helping to propel urine forward.
 Blood Supply:
 The renal artery supplies oxygenated blood to the kidneys, while the renal
vein drains deoxygenated blood from the kidneys.
 The renal artery branches into smaller arterioles, eventually forming a
network of capillaries within the nephrons, known as the glomerulus.
 After filtration, the blood is collected by venules and then merges to form
the renal vein, which exits the kidney.

Structure of nephron
 The nephron is the functional unit of the kidney responsible for filtering
blood and producing urine.
 It is composed of various structures, each with a specific function.

 Glomerulus:
 It is a network of specialized capillaries located within the Bowman's
capsule and functions as the initial filtration site.
 Blood enters the glomerulus through the afferent arteriole and leaves
through the efferent arteriole.
 The high pressure promotes the filtration of water, ions, and small
molecules from the blood into the capsule. Larger molecules like protein
are not filtered

 Bowman’s capsule:
 The Bowman's capsule is a cup-shaped structure that surrounds the
glomerulus.
 It serves as the first segment of the renal tubule and collects the filtrate
from the glomerulus.

 Proximal convoluted tube:

 The PCT is a highly coiled and convoluted tubular structure connected to
the Bowman's capsule.
 It is responsible for reabsorbing the majority of filtered substances back
into the bloodstream e.g. water, glucose, amino acids and ions.
 It also participates in the secretion of certain waste products.

 Loop of Henle:
 The loop of Henle is a U-shaped structure extending from the PCT and
consists of a descending and an ascending limb.
 The descending limb is only permeable to water, allowing it to be
reabsorbed, making the filtrate more concentrated.
 The ascending limb is impermeable to water but transports ions out of the
tubule, further contributing to the concentration gradient.

 Distal convoluted tube:

 The DCT follows the loop of Henle and connects to the collecting duct.
 It is involved in fine-tuning the reabsorption and secretion processes.

 Collecting duct:
 Multiple DCTs merge to form a single collecting duct, which passes
through the medulla and eventually empties urine into the renal pelvis.
 The collecting duct is responsible for the final adjustments in urine
concentration and volume.

 It is under the influence of antidiuretic hormone (ADH), which regulates

the reabsorption of water.
 When ADH is present, the collecting duct becomes more permeable to
water, allowing it to be reabsorbed and concentrated urine to be
produced.
 In the absence of ADH, the collecting duct is less permeable to water,
resulting in the excretion of dilute urine.

Ultrafiltration
 Ultrafiltration is the first step of urine production.
 It occurs in the renal capsule and glomerulus.

Blood supply:
 Blood enters the glomerulus through the afferent arteriole and flows into
the glomerular capillaries, at high pressure.
 The diameter of capillaries is much narrower than that of the afferent
arteriole, causing the hydrostatic pressure to rise.

 Small molecules, like water, ions, glucose, amino acid and waste products
(e.g. urea) are forced out of the capillaries, and into the Bowman’s
capsule.
 Larger molecules, like protein and blood cells are left behind in the blood.

Filtration barriers:
 There are three layers of filtration:
- Endothelium of capillaries.
- Basement membrane
- Podocytes

 Endothelium of capillaries:
 The glomerular capillaries are highly specialized and possess
fenestrations or small pores which allow the passage of water and small
solutes excluding larger molecules.
 The endothelium also possesses a negative charge, which repels
negatively charged particles like proteins.

 Basement membrane:
 The basement membrane lies beneath the endothelium and is composed of
a meshwork of proteins, including collagen, glycoproteins and
proteoglycans.
 It is negatively- charged and acts as a barrier to further restrict passage
for large molecules.
 Podocytes:
 Podocytes are specialized cells with finger-like extensions called foot
processes, which interdigitate to create filtration slits.
 This creates passage for smaller solute molecules while effectively
blocking larger ones.
-
Factors affecting glomerular filtration rate:
 The filtered fluid is called glomerular filtrate.
 It contains glucose, AA, vitamins, ions, nitrogenous waste like urea, some
hormones and water.
 There are two factors affecting the glomerular filtration rate (GFR) :
pressure and water potential.

 Pressure:
 Hydrostatic pressure in the glomerular capillaries is higher compared to
other capillaries in the body due to the afferent arteriole being larger in
diameter than the efferent arteriole.
 An increase in this hydrostatic pressure will result in increased filtration,
and vice versa.

 Water potential:
 Plasma proteins are too big to be filter, so they remain in glomerular
capillaries.
 Thus, solute concentration in glomerular capillaries is greater than that of
glomerular filtrate.
 This means that blood plasma has a lower water potential than the filtrate.
 This would decrease the rate of filtration.

 Overall:
 However, overall, the effect of the pressure gradient outweighs the effect
of solute gradient.
 Therefore, the water potential of the blood plasma in the glomerulus is
higher than the water potential of the filtrate in the Bowman’s capsule
 This means that as blood flows through the glomerulus, there is an overall
 movement of water down the water potential gradient from the blood into
the Bowman’s capsule
 Ultrafiltration produces about 125cm3 of glomerular filtrate per minute,
or, 180dm3/day

Selective reabsorption
 It is the second step in urine formation and occurs mostly in the proximal
convoluted tubule.
 During ultrafiltration, waste products, like urea, are filtered, but useful
substances, like water, glucose and AA, are lost as well.
 The nephron selective reabsorbs these substances for further use as they
are required to maintain the composition of body fluids.

 The PCT is the longest and widest part of the nephron and is highly coiled.
 It is the first segment of the renal tubule and leads the filtrate from
Bowman’s capsule to the Loop of Henle.

Structural adaptions:
 PCT is well-adapted for its function.

 Microvilli:
 The luminal surface of the PCT cells is lined with numerous microvilli,
forming a brush border.
 This increases the surface area available for reabsorption, enhancing the
efficiency of nutrient and ion uptake.

 Tight junctions:
 The PCT cells are tightly packed and connected by tight junctions, which
prevent leakage inbetween cells, allowing for greater control of the
reabsoption process.

 Mitochondria:
 PCT cells contain abundant mitochondria, providing the necessary energy
 (ATP) for active transport processes involved in reabsorption.
 These processes include the functioning of sodium-potassium pump
protein.

 Transport proteins:
 PCT cells possess a variety of transport and co-transporter+ proteins on
their luminal and basolateral membranes, for transport of specific
molecules.

Procedure:
 Over 80% of the glomerular filtrate is reabsorbed.
 Whe going through renal tubules, it is called tubular filtrate

 Sodium reabsorption:
 The primary driving force for selective reabsorption in the PCT is the
active transport of Na+.
 Sodium ions are actively transported out of the PCT cells into the
interstitial fluid by sodium-potassium ATPase pumps, after which it
diffuses into the capillaries.

 Water reabsorption:
 The active uptake of sodium, and other ions, increases the water potential
of tubular filtrate.
 This creates an osmotic gradient that facilitates the reabsorption of water
and other solutes, such as glucose, amino acids.

 Ion reabsorption:
 Various ions, such as chloride (Cl-), potassium (K+), calcium (Ca2+), and
bicarbonate (HCO3-), are reabsorbed via specific transporters in the PCT
cells, maintaining electrolyte balance in the body.

 All this concentrates the filtrate, reducing its volume.

 All glucose, amino acids, vitamin and hormones in the glomerular filtrate
are reabsorbed into the blood.
 Around 80% of water and ions like sodium and chloride are reabsorbed, as
well as 40-50% of urea.

Osmoregulation
 Osmoregulation is the process by which by which organisms maintain the
balance of water and solutes within their bodies to regulate osmotic
pressure and ensure internal stability.
 It involves hypothalamus, posterior pituitary gland and the kidneys.

 Osmoreceptors in the hypothalamus:

 The hypothalamus is a region in the brain that plays a key role in
maintaining homeostasis, including osmoregulation.
 It contains specialized cells called osmoreceptors, which monitor the
osmotic pressure or concentration of solutes in the blood.
 When osmoreceptors detect an increase in blood osmotic pressure (higher
solute concentration), they send signals to the hypothalamus.

 Posterior pituitary gland:

 The hypothalamus communicates with the posterior pituitary gland, which
is responsible for the storage and release of antidiuretic hormone (ADH).
 ADH is a hormone produced by the hypothalamus and released from the
posterior pituitary gland in response to high osmotic pressure detected by
the osmoreceptors.
 ADH is a hormone that acts on the kidneys to regulate water reabsorption,
particularly the collecting ducts.

 Collecting ducts:
 The collecting ducts are tubules within the kidneys that are responsible
for the final concentration of urine.
 They contain specialized water channels called aquaporins.

Affect of ADH:
 Binding with recpetors:
 ADH binds to specific receptors on the cells, triggering a signaling
cascade that results in an increased number of aquaporins into the
luminal membranes of the collecting ducts.
 ADH binds to specific V2 receptors located on the surface of cells in the
collecting ducts.
 This triggers the production cyclic AMP (cAMP), a second messenger,
which activates signalling pathways, ultimately leading to phosphorylation
of aquaporin molecules.

 Phosphorylation:
 Phosphorylation facilitates the movement of intracellular vesicles
containing aquaporins to the cell membrane, where they fuse and become
incorporated into the membrane.
 This process increases the number of aquaporins available for water
transport and enhances the water permeability of the collecting ducts.

 As the tubular filtrate travels along the collecting duct, water molecules
move from the collecting duct (high water potential), through the
aquaporins, and into the tissue fluid and blood plasma in the medulla (low
water potential).
 The filtrate becomes more and more concentrated.
 Compared to the 180dm3/day of glomerular filtrate produced at the end of
ultrafiltration, only 1.8dm3/day of urine is produced on average.

 Negative feedback:
 When there is high water intake, blood osmolarity decreases, and
osmoreceptors signal the posterior pituitary gland to decrease the release
of ADH.
 This reduces the number of aquaporins in the collecting ducts, making
them less permeable to water.
 Consequently, more water remains in the urine, resulting in increased
urine volume and the elimination of excess water from the body.
 Control of blood glucose levels
 Glucose is the primary source of energy for cells and is essential for
various cellular functions.
 Abnormal blood glucose levels can have detrimental effects on various
organs and tissues.

 Two key hormones involved in the regulation of blood glucose

concentration are insulin and glucagon, both produced by the islet of
Langerhans, a group of cells in the pancreas.
 Insulin is released by the beta cells of the pancreas in response to high
blood glucose levels, while glucagon is released by the alpha cells in
response to low blood glucose levels.

 Insulin stimulates the liver to convert excess glucose into glycogen, a

storage form of glucose, and inhibits the breakdown of glycogen back into
glucose.
 Glucagon acts primarily on the liver to promote the breakdown of stored
glycogen into glucose, a process known as glycogenolysis.

Response to low-blood sugar:

 The normal level of blood glucose is about 90mg per 100cm3.
 When blood sugar levels drop below 70mg/dL, a condition known as
hypoglycemia occurs, initiating homeostatic mechanisms.

 Specialized cells, called α-cells and β-cells, located in the pancreatic islet
of Langerhans detect the decrease in blood sugar concentration.
 β-cells stop insulin secretion, which reduces the use of glucose by liver
and muscle cells.
 α-cells secrete glucagon, a peptide hormone, which targets the liver cells.

 Binding to receptor and activation of G-protein and adenylyl

cyclase:
 Glucagon binds to specific receptors proteins located on the cell
membrane of liver cells triggering a series of events within the cell.
 The receptor undergoes conformational change, leading to activation of G-
protein, which activates an effector protein, also embedded in the cell
membrane, called adenylyl cyclase.

 Formation of cAMP :
 Adenylyl cyclase catalyzes the conversion of ATP (adenosine triphosphate)
into cAMP (cyclic adenosine monophosphate).
 cAMP serves as a second messenger, carrying the signal inside the cell,
from the surface.
 It diffuses throughout the cell, relaying the signal to specific targets.
 Activation of protein kinase A and enzyme cascade:
 cAMP binds to regulatory subunits of an enzyme called protein kinase A,
causing their release and activating the enzyme.
 PKA is a protein kinase, meaning it adds phosphate groups to specific
target proteins, activating them.
 In this case, PKA activates phosphorylase kinase, resulting in a
phosphorylation enzyme cascade that serves to amplify the initial signal.

 Cellular response:
 Phosphorylase kinase activates glycogen phosphorylase, which then
catalyzes the breakdown of glycogen into glucose-1-phosphate.
 This can be further metabolized to form glucose which is released into the
bloodstream, increasing blood glucose levels.
 The process of converting glycogen to glucose is known as glycogenolysis.

 Gluconeogenesis:
 Glucagon also stimulates gluconeogenesis, a process in which non-
carbohydrate precursors, such as amino acids and glycerol, are converted
into glucose.
 This occurs primarily in the liver, where various enzymes involved in
gluconeogenesis are upregulated in response to glucagon signaling.
High blood sugar:
 Insulin acts on muscle and liver cells.
 It is secreted by beta cells in pancreatic islet of Langerhans, in response
to high blood sugar.

 Muscle cells:
 In muscle cells, insulin promotes glucose uptake by increasing the
translocation of glucose transporter proteins, such as GLUT4, to the cell
surface allowing glucose to enter the muscle cells more efficiently.
 Once inside, glucose is either stored as glycogen for future energy needs
or utilized for immediate energy production, eitherway helping to
decrease blood glucose concentration.

 Liver cells:
 In liver cells, it stimulates the uptake of glucose by hepatocytes through a
similar mechanism as in muscle cells, involving GLUT4 translocation to
the cell membrane.
 Once inside the liver cells, glucose is either metabolized for energy or
converted into glycogen for storage.
 Insulin inhibits the breakdown of glycogen (glycogenolysis) and the
production of new glucose (gluconeogenesis) in the liver, further reducing
blood glucose levels.

Measuring concentration of glucose

In blood:
 Glucose concentration in blood can be measured using biosensors, which
involves utilizing specific biological recognition elements and transducers
to convert the biochemical reaction into a measurable signal.
 It is used by people with diabetes to show their current blood glucose
concentration.

 Glucose oxidase is commonly used as the biological recognition element in

glucose biosensors.
 It is immobilized on a recognition layer and covered with a partially
permeable membrane that only allows small molecules from the blood to
reach the immobilized enzymes

 Glucose oxidase catalyzes the oxidation of glucose to produce gluconic

acid and hydrogen peroxide (H2O2).
 The hydrogen peroxide produced undergoes an electron transfer reaction
on the surface of the working electrode.
 This generates an electrical current proportional to the concentration of
glucose in the blood.
 The higher the glucose concentration in the blood, the higher the
electrical current produced.

 The current generated is measured by the biosensor’s circuitry and the

value is displayed on the digital screen.
 This process is complete within a matter of seconds.

 Reactions:
 Glucose + O2 → Gluconic acid + H2O2

In urine:
 If blood glucose concentration increases above a value known as the renal
threshold, not all of the glucose from the filtrate in the proximal
convoluted tubule is reabsorbed and some will be left in the urine.
 Tests on urine can give indication on whether the person has diabetes.
 The test is done by using glucose test strips, which are small, disposable
strips that contain specific chemicals and enzymes, including glucose
oxidase and peroxidase.
 These chemicals react with glucose in the urine, producing a measurable
response.

 First, a urine sample is collected in a clean container.

 The test strip contains an absorbent pad that draws the urine into the
strip, allowing it to come into contact with the immobilized enzymes
 present.

 Once the urine reaches the glucose oxidase on the test strip, the enzyme
oxidizes glucose, producing gluconic acid and hydrogen peroxide.
 The hydrogen peroxide generated then comes into contact with
peroxidase present on the test strip.
 The enzyme catalyzes breakdown of H2O2 into water and oxygen gas.

 Test stripes also contain color indicators, which react with the generated
oxygen, casuing a visible color change.
 The color change on the test strip is compared to a provided color chart to
estimate the glucose concentration in urine.

 Reactions:
 Glucose + O2 → Gluconic acid + H2O2
 2H2O2 → 2H2O + O2

Chapter 3.2: Homeostasis in plants

 Homeostasis in plants refers to the ability of plants to maintain a stable
internal environment despite fluctuations in external conditions.
 Just like animals, plants need to regulate various physiological processes
to ensure their survival and optimize their growth.
 One of these processes is the regulation of water and carbon dioxide,
influenced by the opening and closing of stomata.

 Stomata are small openings found primarily on the surfaces of leaves and
consist of two specialized guard cells that surround a pore-like opening
called a stomatal aperture.
 The stomata play a vital role in the exchange of gases, primarily carbon
 dioxide (CO2) and oxygen (O2), as well as the regulation of water vapor
loss through transpiration.

Opening and closing of stomata

Response to stimuli:
 Regulation of the stomatal aperture is needed to balance intake of carbon
dioxide and minimising water loss by transpiration.
 Stomatal opening and closing are primarily regulated by two factors:
environmental conditions and internal signals.
 Environmental conditions, such as light intensity, humidity, temperature,
and atmospheric CO2 levels.
 Internal signals, including hormonal and chemical cues.

 Environmental stimuli which cause stomata to open include increasing

light intensity and low CO2 concentration in air spaces, within the leaf.
 When light intensity increases, plants need to maximize carbon dioxide
intake.
 Guard cells take up water through osmosis and become turgid.
 This turgor pressure causes the guard cells to swell and the stomatal pore
to open, allowing CO2 to diffuse into the leaf for photosynthesis.

 Conversely, when environmental conditions are unfavorable, such as

during periods of darkness, high temperature, low humidity, or water
scarcity, plants need to minimize water loss through transpiration.
 In response to these conditions, the guard cells lose water through
osmosis, causing them to become flaccid and close the stomatal pore.
 This closure reduces the diffusion of CO 2 into the leaf, conserving water
and preventing excessive transpiration.

Daily rhythms:
 Stomata open and close in a daily rhythm, even if the plant is exposed to a
continued supply of light and carbon dioxide.
 Stomata open during the day to main a constant supply of CO 2 for
 photosynthesis, which also leads to transpiration of water and diffusion of
oxygen out of the plant.
 Since photosynthesis con not occur during night, stomata close to reduce
transpiration and conserve water.

Guard cells
 Guard cells are specialized cells found in the epidermis of plant leaves and
stems, particularly the lower epidermis of leaves.
 Guard cells have a unique bean-shaped structure, with a thickened inner
wall (facing the stomatal pore) and a thinner outer wall (facing epidermal
cells).

 The outer walls of guard cells have a thick cuticle layer preventing
excessive water loss from the guard cells.
 Guard cells have specialized cellulose microfibrils that are arranged
radially, which help them to expand and contract, enabling the opening
and closing of stomata.

 The cell membrane is folded to accommodate various channel and carrier

proteins.
 It does not have plasmodesmata
 Guard cells have high density of chloroplast and mitochondria.
 They also have multiple small vacuoles instead of one large one.

 Chloroplast enable photosynthesis, but have less grana.

 They store starch, in starch grains, which expand during the day when
more storage is needed.
 High density of mitochondria (with more cristae) helps to actively
transport ions and regulate the movement of water in and out of the cells,
thus enabling the opening and closing of stomata.
Mechanism for opening of stomata:
 Guard cells open when they gain water (by endosmosis) and become
turgid.
 Thus, the opening of stomata is driven by a decrease in water potential.

 Photoreceptor detect blue light, causing ATP-driven proton pumps to

transport hydrogen (H+) ions out of the guard cell.
 This leaves the inside of the guard cells negatively charged compared to
the outside.
 This causes potassium (K+) ions to move down the electrochemical
gradient and diffuse into the cell, through channel proteins embedded in
the cell membrane.

 The influx of potassium (K+) ions increases the solute concentration inside
the guard cells, lowering the water potential inside the cells.
 Anions like chloride, nitrate and malate also enter
 Thus, water enters the guard cells through endosmosis, causing it to
become turgid.
 The thick inner walls curve to accommodate the expanding of the outer
walls, opening the stomatal aperture.
 The radial cell thickening allows the guard cells to only increase in length.

Mechanism for closing of stomata:

 When water is low, roots synthesize abscisic acid (ABA), which is
transported through the xylem to the leaves.
 ABA binds with receptors on the surface of guard cells, inhibiting the
proton pumps from transporting H+ ions out of the guard cells.

 They also causes calcium (Ca2+) ions to move into the cytoplasm of the
guard cells through the cell surface membranes.
 Ca2+ act as second messengers and open channel proteins which allow
anions to exit the cell.
 This causes an efflux of K+ ions, further increasing water potential inside
 the cell.

 Water leaves the guard cells by exosmosis.

 The guard cells become flaccid and the stomata closes.
 Chapter 4.1: Control and
coordination
in mammals

The Endocrine System

 The endocrine system is a complex network of glands and organs
responsible for producing and regulating hormones in the body.
 It plays a crucial role in maintaining homeostasis and coordinating various
physiological processes.

Glands:
 A gland is a specialized group of cells or an organ that synthesizes and
secretes substances like enzymes, hormones or fluids.
 There are two types of glands, exocrine and endocrine.
 Exocrine glands secrete product into a duct, e.g. sweat glands, salivary
glands and mammary glands.

 Endocrine glands are specialized tissues that secrete hormones directly

into the bloodstream.
 For this purpose, they have a rich supply of blood with a relatively large
number of blood vessels.
 Examples include the pituitary gland, thyroid gland, adrenal glands, and
pancreas.

 Some glands have both endocrine and exocrine functions.

 Pancreas can secrete the hormones insulin and glucagon in to the
bloodstream and also secrete pancreatic juice into the pancreatic duct.

Hormones:
 Hormones are chemical messengers that travel through the blood to
target cells, where they bind to specific, complementary receptors,
initiating a cascade of physiological responses inside the cell.
 They are small organic molecules and effective in low concentrations.
 The receptors may be on the cell surface membrane or inside cells.

 There are many different types of hormones.

 Hormones such as insulin, glucagon (pancreas) and ADH (posterior
pituitary gland) are peptides or small proteins.
 They are water soluble and cannot cross the phospholipids layer
 They bind with receptors on the cell surface membrane triggering
signaling cascades that lead to intracellular responses.

 Amine hormones are derived from amino acids such as tyrosine.

 They can be water or lipid soluble and can have receptors on sell surface
or inside cells.
 Examples include adrenaline (adrenal medulla) and follicle stimulating
hormone (posterior pituitary gland)

 Steriod hormones are derived from cholesterol and are lipid soluble.
 Thus they diffuse across cell membranes and bind to intracellular
receptors in the cytoplasm or nucleus.
 Examples include testosterone (testes) and oestrogen, progesterone
(ovary and placenta)

 Fatty acid derived hormones are also lipid soluble and can bind to
intracellular or extracellular receptors.
 Include prostaglandins and leukotrienes.

 The effects of hormones on target cells are often slow but prolonged.

Nervous system
 The nervous system is a complex network of specialized cells, tissues, and
organs that plays a vital role in controlling and coordinating various
 functions of the body.
 It can be divided into two main components:
- Central nervous system (CNS)
- Peripheral nervous system (PNS)

 The CNS consists of the brain and spinal cord.

 It serves as the command center of the body, integrating and processing
incoming sensory information, initiating responses, and coordinating body
activities.

 The brain is responsible for higher-level functions such as cognition,

memory, emotion, and voluntary movement.
 It is divided into several regions, each with specific functions.
 The spinal cord is a long, cylindrical bundle of nerve fibers that extends
from the brainstem to the lower back.
 It acts as a pathway for transmitting signals between the brain and the
peripheral nervous system.

 The PNS consists of nerves and ganglia (clusters of nerve cell bodies) that
extend outside the CNS.
 It connects the CNS to various organs, muscles, glands, and sensory
receptors throughout the body.

Nervous system vs endocrine system:

 The endocrine system and the nervous system are two essential regulatory
systems in the human body that work together to maintain homeostasis
and coordinate various physiological processes.

Structure and composition:

 The endocrine system consists of a collection of glands scattered
throughout the body, which secrete hormones into the bloodstream.
 The nervous system employs electrical impulses and neurotransmitters to
transmit signals.
 Nerve cells, or neurons, communicate through synapses, which involve the
release and reception of neurotransmitters across tiny gaps between
neurons.

Speed and duration of response:

 Hormonal response are slow but long-lasting, until the hormone is broken
down.
 Nervous response are rapid but short-lived, only until the electrical
impulse stops.

Effector:
 Hormones affect target cells or tissues that possess specific receptors for
those hormones.
 Nervous target muscles or glands.

Neurons
 Information is sent through the nervous system as nerve impulses that
pass along nerve cells known as neurons.
 Neurons are specialized cells that are the fundamental building blocks of
the nervous system.
 They receive information from other neurons or sensory receptors,
integrate and process this information, and transmit signals to other
neurons or effector.

Basic structure:
 The cell body (soma) contains the nucleus and most of the cell's
organelles.

 A very long, slender extension also projects away from the cell body,
called axon.
 It arises from a specialized region called the axon hillock, located near the
cell body.
 The axon is surrounded by an insulating fatty layer called the myelin
sheath, which is formed by support cells called Schwann cells.
 It is not uniformly covered by myelin; instead it is interrupted by regularly
spaced gaps called nodes of Ranvier, which play a crucial role in the rapid
conduction of nerve impulses.

 The end of the axon terminates in specialized structures called axon

terminals.
 These terminals form synapses, which are specialized junctions that allow
communication between neurons or between neurons and target cells,
through neurotransmitters

 Dendrites are branching extensions that receive incoming signals from

other neurons.
 These structures receive incoming signals from other neurons in the form
of chemical messages called neurotransmitters
 They often have multiple branches, increasing their surface area and
allowing for connections with a larger number of synapses, enabling
dendrites to receive inputs from multiple sources simultaneously.
 Dendrites possess specific receptor sites on their surface that are
designed to bind with specific neurotransmitters.

Types of neurons:
 There are three main types of neurons: sensory, motor and intermediate.
 Each type has distinct characteristics and functions within the nervous
system.

 Sensory neurons:
 Sensory neurons, also known as afferent neurons, are responsible for
transmitting sensory information from the sensory organs to the central
nervous system (CNS).
 They are part of the PNS.
 They have specialized receptor at their peripheral endings, which allow
them to respond to specific stimuli, such as light, sound, pressure,
 temperature, or chemical signals.

 Sensory neurons are typically pseudounipolar.

 Sensory neurons have a single axon emerging from the cell body, which
then divides into two distinct branches.
 At one end, the axon forms the peripheral process with receptor endings
to detect stimuli, while the other end extends towards the CNS, forming
central process, with synaptic endings..
 They do not have typical dendrites, so DO NOT mention that.

 The cell body or soma of sensory neurons are often clustered into a
ganglia (dense groups of nerve cell bodies) outside the CNS.

 Motor neurons:
 Motor neurons, also known as efferent neurons, are responsible for
transmitting signals from the CNS to effector such as muscles and glands.
 They enable voluntary and involuntary movements.
 The cell body lies within the CNS (spinal cord or brain).

 The large cell body is located at one end of the neuron, with highly-
dendrites extending off it.
 This provides a large surface area for the axon terminals of other neurons.

 A very long, slender extension also projects away from the cell body,
called axon.
 At the end of the axon, it branches out into numerous fine extensions
called axon terminals, forming synapses with effector or other neurons.

 Relay neurons:
 Relay neuron, also known as interneuron or intermediate neurons, are
entirely found in the CNS.
 The cell body of a relay neuron is the central region that contains the
nucleus and other organelles necessary for the neuron's metabolic
 functions.

 Dendrites extend from one end of the soma receiving incoming signals
from other sensory neurons or other relay neurons, while an axon projects
from the other end and transmits electrical signals to other relay neurons
or motor neurons, through axon terminals.

Reflex arc
 A reflex arc is a neural pathway that mediates a rapid and involuntary
response to a specific stimulus.
 It involves the coordinated activity of sensory relay and motor neurons to
detect the stimulus, transmit the sensory information to the CNS, and
generate an appropriate motor response without conscious control or
involvement of higher brain centers.

 Sensory neurons detect sensory stimuli and transmit signals from the
periphery to the central nervous system.
 Relay neurons integrate and process sensory information within the CNS,
facilitating communication between sensory and motor neurons.
 Motor neurons transmit signals from the CNS to effector organs, such as
muscles or glands, to produce a response.

Sensory receptor cells

 Sensory receptor cells are specialized cells within the sensory organs of
the body that detect and respond to various stimuli from the external
environment or within the body itself.

 Stimulus detection and transduction:

 They possess specialized structures, such as proteins or channels, which
allow them to detect specific types of stimuli.
 When it is exposed to a stimulus, it converts the stimulus signal into an
electrical signal through transduction.

 Action potential:
 The transduced electrical signals depolarize the sensory receptor cell,
creating an electrical potential difference across its membrane.
 If the depolarization reaches a certain threshold, it triggers the generation
of an action potential, which is a rapid and transient change in the
electrical potential of the cell membrane.
 This action potential serves as a means of transmitting the sensory
information.

 Transmission of impulse to sensory neurons:

 The generated action potentials are then transmitted to the sensory
neurons through a process called synaptic transmission.
 Synaptic vesicles within the sensory receptor cell release
neurotransmitters into the synapse, which is the small gap between the
receptor cell and the sensory neuron.
 These neurotransmitters bind to specific receptors on the sensory neuron,
initiating a new electrical signal in the neuron.

 Transmission to CNS:
 The sensory neurons then transmit the information to the appropriate
regions of the CNS.
 Here, the signals are integrated and interpreted, leading to understanding
of the original stimulus and initiating an appropriate response.

What starts an action potential

 Taste buds contain specialized sensory receptor cells responsible for
detecting and responding to chemical stimuli related to taste.
 Taste buds are found on the surface of papillae, which are small bumps on
the surface of our tongue.
 Each chemoreceptor is covered with receptor proteins and different
 receptor proteins detect different chemicals.
 Chemoreceptors in taste buds are categorized into five primary taste
modalities: sweet, sour, salty, bitter, and umami (savory).
 To study the sequence of events which initiate an action potential, we will
take example of chemoreceptors in taste buds that detect salt.

 Salt detection and stimulation by salt ions:

 When food containing salt is consumed, the salt molecules dissolve in
saliva and sodium ions (Na+) come into contact with the taste buds on
your tongue.
 The sodium ions diffuse through the saliva and reach the chemoreceptor
taste buds.

 Activation of ion channels:

 Sodium ions diffuse through highly selective channel proteins in the cell
surface membranes of the microvilli of the chemoreceptor cells.
 These channels are known as epithelial sodium channels (ENaC).
 The binding of sodium ions to these channel proteins leads to a
conformational change in the channels, causing them to open.
 This allows an influx of sodium ions into the chemoreceptor cell.

 Depolarization and graded potential:

 The influx of sodium ions leads to depolarization of the cell membrane and
increases the positive charge inside the cell, known as receptor

 Depolarization refers to a change in the electrical potential across a cell

membrane, specifically a decrease in the difference in charge between the
inside and outside of the cell.
 Normally, cells have a negative charge on the inside relative to the
outside, which is referred to as the resting membrane potential.

 The change in electrical charge creates a signal called graded potential.

 The higher the concentration of salt in food or drink, the more Na +ions
stimulate the taste buds and the higher the magnitude of graded potential.
 Voltage-gated channels:
 If the depolarization reaches a certain threshold, around -55 millivolts
(mV), it triggers the opening of voltage-gated calcium channels located
along the sensory neuron connected to the taste bud.
 The influx of calcium ions into the taste bud cell causes further
depolarization
 This stimulates the vesicles in chemoreceptor to release
neurotransmitters e.g. serotonin or ATP, by exocytosis.

 Sensory neuron activation:

 Neurotransmitters bind to receptors on the sensory neurons connected to
the taste bud, activating the sensory neurons and initiating action
potential generation.
 The sensory neuron then transmits an impulse to the brain

 All-or-nothing principle:
 An impulse is only transmitted if the initial stimulus is sufficient to
increase the membrane potential above a threshold potential.
 If the stimulus is very weak or below a certain threshold, the receptor
cells won’t be sufficiently depolarised and the sensory neuron will not be
activated to send impulses.
 Threshold potential is not a static value and varies depending upon the
type of neuron and given operational conditions.
 Continued stimulation may require a greater stimulus before impulses are
sent along the sensory neurons.
Changes to membrane potential

How resting potential is maintained:

 A charge refers to the potential difference existing across the cell surface
membrane of all cells.
 In nerve cells, the charge inside the cell is mostly negative.
 The membrane is said to be polarized.
 The resting membrane potential of a neuron is the stable electrical charge
difference across its cell membrane when it is not actively transmitting
signals.
 It is typically around -70 millivolts (mV) inside the cell relative to the
outside, creating a negative interior.

 The resting potential is primarily maintained by active transport and

passive diffusion of ions.

 Sodium/Potassium pumps:
 The sodium-potassium pump is an active transport mechanism found in
the neuronal membrane. It actively pumps three sodium ions (Na+) out of
the neuron for every two potassium ions (K+) it pumps in.
 This process consumes energy in the form of adenosine triphosphate
(ATP).
 By expelling more positive sodium ions than it brings in positive potassium
ions, the pump contributes to the negative charge inside the neuron,
helping to maintain the RMP.

 Potassium gradient and leak channel:

 Neuronal membranes have "leak channels" that are always open.
 These channels allow a small, continuous flow of ions across the
membrane.
 These channels are permeable to both potassium (K+) and sodium (Na+)
ions, but they are more permeable to potassium. (There are also more leak
channels for potassium)
 As potassium ions move out of the cell through these channels, they
contribute to the negative charge inside the neuron, helping to maintain
the resting potential.

 However, as potassium ions move out, they create an electrical gradient

that opposes the chemical concentration gradient, leading to an
equilibrium state where the net movement of K+ ions is minimal.
 This equilibrium contributes significantly to maintaining the RMP.
 Gated channels:
 These channels can open or close in response to specific stimuli. Gated
channels include voltage-gated channels and ligand-gated channels.
 Voltage-gated channels open or close in response to changes in the
membrane potential, while ligand-gated channels open or close in
response to the binding of specific molecules (ligands) to the channel.
 During the resting state, these channels remain mostly closed,
contributing to the maintenance of the resting potential

 Sodium ion gradient:

 The axon membrane is relatively impermeable to sodium ions, but factors
the influence the influx of these ions are:
- The steep concentration gradient, more sodium ions on the outside.
- The inside of the neuron is negatively charged, which attracts positively
charged ions.
 Remember, sodium ions cannot diffuse through axon membrane when
neuron is at rest, due to hydrophobic core pf phospholipid bilayer.

 Chloride ion concentration:

 Chloride ions are negatively charged and also contribute to the RMP.
 They accumulate outside of the cell because they are repelled by
negatively-charged proteins within the cytoplasm. helping to maintain the
negative charge inside the cell.

 Anionic proteins:
 Inside the cell, there are large, negatively charged proteins that cannot
move through the membrane.
 These proteins contribute to the negative charge within the neuron,
further maintaining the resting potential.
Taxonomy