CELLS

A cell consists of living matter called a protoplasm

And the protoplasm consists of the nucleus cytoplasm and cell surface membrane

The nucleus consists of:

- Chromatin threads that contain hereditary material dna which condense into chromosomes
during cell division. Specifically they have instructions for creating proteins.
- Nucleolus, a spherical piece of shit where components of ribosomes are synthesised and
assembled
- Nucleoplasm which is a denser protoplasm for the nucleus like what the hell why do you
need another plasm for the nucleus
- there are two membranes for the damn nucleus, the outer one and the inner one and both
are covered in nuclear pores
The nucleus controls protein synthesis in the cytoplasm as well as cell activities such as
biochemical reactions within the cell. Nuclear division is the basis of cell division and
reproduction and the continued survival of the organism

Cytoplasm, which consists of a semi-fluid medium called cytosol which surrounds the nucleus

It is the place where all the shit in the cell happens and process, such as chemical reactions
Inside the cytoplasm are organelles or small structures that carry out a certain function each such
as mitochondria the site of release of energy from food during cellular respiration in the form of
molecules called adenosine triphosphate; it consists of an outer membrane, inner membrane,
ribosomes, matrix, and dna.

The outer membrane of mitochondria is a smooth continuous boundary whereas the inner
membrane is extensively folded to form cristae which projects into the semifluid matrix
And they contain circular dna dont ask me how that works

Chloroplastas are organelles where photosynthesis takes place (no shit), and they’re usually a few
um and 5-10 um in length

Chloroplasts are made up of the following:

- Ribosmoes
- Starch grains
- Stroma
- Lamellae
- Genetic material (dna)
- Granae
 - Thylakoids
- A double membrane envelope

Thylakoids stack to form grana

And the interior of a chloroplast consists of stroma which contains soluble enzymes and circular
dna

Vacuoles are essentially things that store shit and food (literally)

In plant cells they have a large central vacuole enclosed by a tonoplast or a membrane which
pushes the cytoplasm to the edge of the cell.

The cell sap within the vacuole consists of dissolved sugars, salts, and amino acids.

In contrast animal cells contain many small, non-permanent vacuoles

Ribosomes are sites where the cells make proteins

Free ribosomes are suspended within the cytoplasm.
While bound ribosomes are attached to the endoplasmic reticulum and make proteins for
secretion from the cell.

The endoplasmic reticulum can either be rough or smooth. It consists of a network of flattened
membrane bound sacs known as cisternae.
ER usually originates from the outer membrane of the nucleus and attaches to it. Rough er
appears rough because ribosomes are attached to the outer surface. This is where proteins are
synthesised and are packaged in membranous vesicles and secreted across the cell membrane.
Smooth er lacks ribosomes, and synthesises lipids and detoxifies drugs and poisons.

The golgi apparatus or golgi body consists of the SAME DAMN STACK OF FRICKING
cisternae and a system of vesicles known as golgi vesicles.

It receives proteins and lipids from the ER and modifies them chemically before sorting and
repackaging them into secretory vesicles. These then fuse with the cell membrane and release
the secretory proteins e.g. enzymes to the exterior of the cell.

Process:
Vesicles are fused from the er with the golgi apparatus
The golgi apparatus cuts off vesicles which move out into the cell
Some vesicles are retained in the cytoplasm e.g. lysosomes and peroxisomes
Vesicles with contents discharged from the cytoplasm such as digestive enzymes, suberin in cell
walls, antibodies, the waxy circle of the epidermis and so on are removed from the cell.

Some cells secrete proteins produced by ribosomes into the er, such as pancreatic cells which
secrete insulin into the bloodstream.
As the polypeptide chain grows from the bound ribosome, it enters the cisternal space of the er
and folds into its native conformation. Secretory proteins, as they are called, are kept separate
from proteins produced by free ribosomes which remain in the cytosol.

Transport vesicles travel to the golgi apparatus from the er. At the golgi apparatus proteins are
chemically modified, sorted and sent to other destinations. The cis face of the golgi apparatus is
located nearer to the er and receives proteins from the er. The trans face of the golgi apparatus
gives rise to secretory vesicles which eventually fuse with the plasma membrane.

Most secretory proteins are glycoproteins, i.e. proteins covalently bonded to carbohydrates.

Lysosomes are spherical vesicles bounded by a single membrane. They contain digestive enzymes
that can break down macromolecules such as proteins, polysaccharides, fats and nucleic acids.
Macromolecules found in bacteria that have been engulfed by white blood cells are broken down
by lysosomal enzymes.

Enzymes are made by the rough er and transferred to the golgi apparatus for processing. Some
lysosomes arise from budding from the trans face of the golgi apparatus.

The cytoskeleton exists in the cytoplasm of all eukaryotic cells and it consists of a network of
microtubules, intermediate filaments and microfilaments. The cytoskeleton also provides a
structural framework giving the cell its shape and stabilising membranous systems.

Centrioles are found in both animal cells and lower plant cells but are absent in higher plant cells.
Centrioles exist as a pair of rod-like structures positioned at right angles to each other and
situated near the nucleus. The transverse section of centrioles shows nine triplets of microtubules
arranged in a ring. They also play a role in nuclear division in animal cells.

The plasma membrane surrounds the cytoplasm externally. It is a selectively permeable

membrane that controls the type of substance that enters or leaves the cell. It separates chemical
compositions and conditions in the internal compartment of a cell from those external to the cell.

The plasma membrane is mainly composed of lipids and proteins and is known as the
phospholipid bilayer.
SINGER-NICHOLSON FLUID MODEL STRUCTURE

Phospholipid molecules form a bilayer, and have a hydrophilic head and a hydrophobic end. The
charged phosphate groups face outwards and interact with the aqueous environment on either
side of the cell membrane, forming the hydrophilic part.

The hydrocarbon chains of fatty acids of the phospholipid molecules face inwards and form the
hydrophobic interior of the cell membrane. Proteins associated with the cell membrane are either
peripheral proteins on the surface of the phospholipid bilayer, or integral proteins which
penetrate part of or all the way through the phospholipid bilayer.

Cell membranes are asymmetric, meaning that two halves of the membrane have different lipid
and protein composition.

Cell membranes are fluid structures as the phospholipid molecules and some membrane proteins
can move about in the plane of the membrane. The proteins are scattered throughout the
phospholipid bilayer in a mosaic arrangement. The steroid cholesterol wedged between the
phospholipid molecules reduces membrane fluidity by reducing phospholipid movement at
moderate temperatures but it also hinders solidification at low temperatures as it hinders the
close packaging of phospholipids.

Functions of other molecules in the phospholipid bilayer also include:

Transporting polar molecules and charged ions across the membrane with channel and carrier
proteins
Membrane-bound enzymes such as microvilli on epithelial cells lining the small intestine contain
digestive enzymes in their cell membranes
Receptor molecules in the membrane are proteins that can bind to hormones or neurotransmitter
molecules
Membrane carbohydrates are covalently bonded to glycoproteins or glycolipids and are
important in cell to cell recognition.

DIVISION OF LABOUR AMONG CELLS

Most cells after dividing and growing become specialised. This means that they do one particular
job, develop a distinct shape and undergo chemical changes in their cytoplasm that enables them
to carry out their particular functions.

Through the process of differentiation a cell becomes specialised for a specific function and the
structure of a specialised cell is adapted to perform its specific function.
E.g. root hair cells because bio teachers absolutely LOVE testing people on these, idk why but its
getting kinda old after all these years

Root hair cell has a long and narrow extension/protrusion

Adaptation - the long and narrow protrusion of the root hair cell increases surface area to volume
ratio for efficient absorption of water and dissolved mineral salts from the soil.
Root hair cells contain several mitochondria.
Adaptation - more energy in the form of adenosine triphosphate molecules can be released for
active transport for the absorption of dissolved mineral salts.
Root hair cells have large vacuoles containing cell sap made up of a relatively concentrated
solution of sugars and salts.
Adaptation - the root hair cell has a more negative water potential than that of the soil solution.
Water enters the root hair cell by osmosis across the selectively permeable cell membrane.

E.g. xylem vessels which transport water and dissolved mineral salts up the plant from the roots
Xylem vessels are made up of long cells joined end to end. End walls between adjacent cells are
broken down.
Adaptation - the joined ends of adjacent cells allow for a continual flow of water and dissolved
mineral salts up the plant
There is an absence of the protoplasm in the xylem vessel, and mature xylem vessels are non-
living
Adaptation - absence of protoplasm reduce resistance to the flow of water and dissolved mineral
salts
There is a deposition of ligma lignin, which is a tough substance on the inner cellulose cell wall
Adaptation - this prevents collapse as water is pulled up the plant by transpiration and to provide
mechanical support to the plant

E.g red blood cells, another fan favourite of bio teachers

Red blood cells, or erythrocytes, contain a red pigment called haemoglobin that binds to oxygen
and enables them to function as oxygen transporters.
RBCs have a biconcave disc shape or thinner central portion
Adaptation - the biconcave disc shape increases the surface area to volume ratio for diffusion of
oxygen into and out of the cell at a higher rate.
RBCs do not have a nucleus (enucleated)
Adaptation - this enables it to carry more haemoglobin to maximise its oxygen-carrying capacity
RBCs are elastic
Adaptation - this enables it to be flexible and squeeze through narrow capillaries

VIRUSES
Currently there is no proof to back up the fact that viruses are alive; nor is there proof to verify
that they are non-living things. A viral particle is made up of a central core of nucleic acid (DNA
or RNA, ribonucleic acid), surrounded by a protein coat known as a capsid. A capsid is built up
of identical repeating protein subunits called capsomeres.

Certain viruses can have an additional layer around the capsid called an envelope which is derived
from the cell surface membrane of the host cell. Some envelopes are further covered in spikes or
coronae which project from the surface of envelopes and attach to host cells.

Viruses have several different shapes and can come in the forms of arbitrary shapes such as the
bacteriophage, which attacks bacteria

How viruses reproduce is something like uhhhhh laying eggs in another dude i guess
Oviposition

Anyways
The virus particle first attaches itself to receptor sites on the host cell’s surface
The virus may inject its genetic material (either RNA or DNA) into the host cell’s cytoplasm or
might just get succed into the host cell by endocytosis.
Once inside the host cell the animal virus’s capsid is enzymatically removed, exposing its genetic
material. For bacteriophages, uncoating is not required.
The viral dna or rna then takes over the host cell’s physiology.
The host cell then produces new capsomeres to be assembled into new viral particles.
The host cell lyses and releases these viral particles, while enveloped viruses do not lyse the host
cell but bud out from the cell surface membrane and the host cell dies from infection.

BACTERIA
A bacterium has a cell surface membrane and cytoplasm. The cytoplasm may contain granules of
glycogen, lipid or other nutrient reserves.

Surrounding the cell membrane is the peptidoglycan cell wall, not made of cellulose but a complex
mixture of proteins, sugars, and lipids. Some bacteria are encased in a layer of slime.

Each bacterial cell contains a single chromosome which is a circular strand of dna. It contains
genes that give the bacterium its characteristics and properties. The chromosome is not enclosed in
a nuclear membrane, but is instead coiled up to occupy part of the cell. Certain types of bacteria
can also harbour smaller rings of dna, or plasmids, which can act as antibiotic resistors.

Some bacteria are also mobile due to the presence of flagella which beat to enable the bacterial cell
to move about.
Bacteria can derive their nutrition through any of the following 4 methods which are classified as:
Saprotrophic - when bacteria feed on decaying organic matter by secreting enzymes on dead
organic matter so as to hydrolyse it into simple organic matter.
Parasitic - these bacteria cause diseases by living in cells of plants and animals and feeding on the
cytoplasm of cells. These are called pathogens.
Photoautotrophic - able to manufacture organic matter using light energy and inorganic
compounds such as photosynthetic bacteria, green and purple sulfur bacteria and cyanobacteria
Chemoautotrophic - able to manufacture organic matter by means of inorganic compounds such
as hydrogen sulfide as an energy source, and carbon dioxide as a carbon source

Bacteria can then be further classified as:

Obligate aerobes - which require oxygen to live
Facultative anaerobes - which can use oxygen when present but are able to continue growing via
fermentation or anaerobic respiration when oxygen is not available
Obligate anaerobes - which are unable to live in the presence of oxygen

Bacteria reproduce via binary fission where the dna replicates and the cell divides into two. Each
daughter cell then becomes an independent bacterium. This form of reproduction is asexual as it
does not involve the formation of gametes. Fission can take place up to every 20 minutes, so in a
very short time a large colony of bacterial cells can be produced.

In adverse conditions bacteria may form spores which are protected by thick walls and resistant to
the cold or heat. Under suitable conditions the spores can germinate and result in cell
multiplication.

FUNGI

Eat it

Some fungi are unicellular such as baker’s yeast (or any form of yeast in that context) in which a
thin cell wall encloses the cytoplasm, which contains a nucleus and a vacuole. Within the
cytoplasm are multiple granules of glycogen.

Other forms of fungi are multicellular and they consist of a mesh of finely branching threads
known as mycelia when in a collective environment. Each mycelium is composed of long filaments
known as hyphae that branch and intertwine. In most moulds the hyphae contain cross-walls or
septa that divide them to uninucleate cell-like units. In a few classes of fungi the hyphae contain
no cross-walls and appear as long continuous cells with many nuclei.
The hyphal cell wall may contain cellulose or chitin, a complex carbohydrate, or both. Each
hypha has a cytoplasm and central vacuole. The cytoplasm contains organelles or inclusions such
as lipid droplets and storage glycogen granules.
Like bacteria, fungi can be either saprotrophs or parasites.

DECOMPOSITION

Decomposition can be carried out via bacteria or fungi. It doesnt really matter ig

Decomposers secrete enzymes onto their food source that catalyses hydrolysis of complex organic
compounds such as carbohydrates and proteins into simple soluble organic compounds. These
simple compounds then diffuse into their bodies to be used as substrates for respiration

MOVEMENT OF SUBSTANCES

Introduction yaaayyyyy
The cell surface membrane is selectively permeable and allows some substances to pass through,
but not others

The molecules of a gas such as oxygen are moving about all the time. So are the molecules of a
liquid, or a substance such as sugar dissolved in water. As a result of this movement the molecules
spread themselves out evenly to fill all the available space given.

This is called diffusion and is defined as the net movement of particles from a region of their
higher concentration to that of lower concentration. Diffusion is a passive process as it is non-
energy requiring. Diffusion can also occur in liquids, such as if you add a drop of red ink to a
beaker of water you’ll find that even without stirring, the ink spreads slowly throughout the
water. Eventually all the water in the beaker will become evenly red. Thus dissolved particles or
solutes can diffuse through a liquid down the concentration gradient.

The rate of diffusion depends upon 4 factors:

The concentration gradient - the greater the difference in concentration between the two regions
of a substance, the higher the rate of diffusion. I.e. the steeper the concentration gradient for a
substance, the greater the rate of diffusion.
The distance over which diffusion takes place - the shorter the distance between two regions of
different concentrations, the higher the rate of diffusion
The area over which diffusion takes place - the larger the surface area, the higher the rate of
diffusion. Diffusion surfaces frequently have structures for increasing their surface area and
hence the rate at which they exchange materials. These structures include microvilli on cells
lining the small intestines.
The size and nature of the diffusing molecule - small molecules diffuse at a higher rate than
larger ones.

Diffusion is an important way to move substances in and out of cells. In both plant and animal
cells gases are exchanged through the process of diffusion. The presence of millions of alveoli in
the lungs provides a very large surface area for gaseous exchange, and oxygen thus diffuses from
the alveoli into the blood capillaries and carbon dioxide diffuses from the blood capillaries into the
alveoli.

Charged particles or ions and relatively large and polar molecules such as glucose do not readily
pass through the plasma membrane. In the plasma membrane channel and carrier proteins assist
such particles to diffuse in and out of the cell.

SIMPLE AND FACILITATED DIFFUSION

Simple diffusion does not always involve the passage of substances through a membrane. If a
membrane is present it must be permeable to the molecules involved. Small, uncharged, lipid-
soluble substances are able to diffuse via the plasma membrane.

facilitated diffusion is applied to the use of certain membrane proteins to assist charged particles
and hydrophilic molecules, such as glucose to easily pass through the membrane

There are two types of transport proteins used in facilitated diffusion:

Channel proteins which form a water-filled pore in the membrane. As the lining of the channel is
hydrophilic so that water-soluble substances can pass through the membrane easily. Most cells
have water channels made from a protein called aquaporin.

Carrier proteins contain binding sites for the diffusing substance. The carrier protein alternates
between two conformations, moving a solute through the membrane as the shape of the carrier
protein molecule changes.

OSMOSIS

When water molecules move passively from the dilute solution into the more concentrated
solution through the selectively permeable membrane, it is called osmosis.

The water potential of a solution is a measure of whether it is likely to gain or lose water
molecules from another solution. A dilute solution contains more water molecules per unit volume
than a concentrated solution so it has a less negative water potential than a concentrated solution
which has a more negative water potential. Pure water has the highest water potential. When
two solutions of different water potential are separated by a selectively permeable membrane, a
water potential gradient is established.

Osmosis is thus defined as the net movement of water molecules from a region of less negative
water potential to a region of more negative water potential, down a water potential gradient
through a selectively permeable membrane.

E.g. in plants, the soil solution consists of water molecules and dissolved mineral salts that are not
bound tightly to soil particles. The soil solution is usually less concentrated than the cell sap in
the vacuole of the root hair cell, thus the soil solution has a less negative water potential than that
of the root hair cell. Water molecules move into the root hair cell by osmosis down the water
potential gradient through the selectively permeable plasma membrane of the root hair cell. Water
molecules pass from one cell to another by osmosis down a water potential gradient in the root
cortex until reaching the xylem vessel.
Consider two solutions a with 20% sucrose solution and b with 10% sucrose solution. Solution b
has a less negative water potential than a, thus solution b is hypotonic with regard to solution a.
Solution a has a more negative water potential than b, thus solution a is hypertonic with regard
to solution b. When solutions a and b are separated by a selectively permeable membrane, water
molecules move from the more dilute or hypotonic solution to the more concentrated or
hypertonic solution. This movement continues until dynamic equilibrium is reached. At this point
there is no net movement of water molecules. Both solutions are of equal concentration or
isotonic to each other.

EFFECT OF OSMOSIS ON PLANT & ANIMAL CELLS

When a plant cell is placed in a solution with less negative water potential than that of the cell,
water molecules first enter the cell by osmosis through the selectively permeable cell membrane.
The vacuole increases in size, and the cell contents are pushed against the cellulose cell wall.
The cellulose cell wall prevents the over expansion of the cell by exerting an opposing pressure
preventing the entry of more water. The cell becomes turgid.

The pressure exerted by the water on the cellulose cell wall is called turgor pressure but it does
not burst due to the presence of the tough cellulose cell wall.

When a plant cell is placed in a solution with more negative water potential than that of the cell,
Water molecules leave the cell by osmosis across the selectively permeable cell membrane. The
vacuole decreases in size and the cytoplasm stops pushing outwards on the cellulose cell wall.
The cell loses its turgor and becomes flaccid, and as more water is lost, the cytoplasm shrinks
further into the centre of the cell but the cellulose cell wall is too stiff to shrink much. When the
cell membrane surrounding the cytoplasm has just begun to pull away from the cellulose cell
wall, the cell is at incipient plasmolysis. The cell becomes plasmolysed.

Fer a’imal cells iz much aezier innit bruv yer go’a beleev’ me mat’ me’s got tha best sci’nce no’tes
i’ tha whole entiyah werld

The effect of osmosis on animal cells can be demonstrated by putting red blood cells in water, or a
concentrated salt solution that has a more negative water potential than the cytoplasm of the red
blood cell

When placed in pure or distilled water, water molecules will enter the red blood cell by osmosis
across the selectively permeable plasma membrane, resulting in the cell to swell and burst due to
an absence of the cell wall. When placed in salt solution, water molecules leave the cell by
osmosis across the selectively permeable membrane, resulting in the cell to shrink and shrivel,
also known as crenation.
ACTIVE TRANSPORT

Cells are able to take in substances which are only present in small quantities around them even
though these substances are of higher concentration inside the cell. They do so by a process
known as active transport, defined as the transport of substances against a concentration gradient
from a region of lower concentration to that of a higher concentration. As active transport
requires energy, it only occurs in living cells as only living cells respire to release energy in the
form of adenosine triphosphate molecules. Cells and tissues carrying out active transport are
characterised by the presence of numerous mitochondria, a high concentration of atp and a high
respiratory rate.

E.g. root hair cells, which take in nitrate ions from the soil even though the concentration of
nitrate ions inside the root hair cell is higher than the concentration in the soil
Glucose and amino acids are actively transported from the lumen of the intestine into the cells of
the villi.

BULK TRANSPORT

Bulk transport is defined as the transport of material into or out of a cell by enclosing it within a
vacuole or vesicle. A vacuole is a fluid-filled membrane-bound sac whereas a vesicle is a small
vacuole. In bulk transport materials are released from the cell by exocytosis or taken into the cell
by endocytosis. Both are active and energy-requiring processes. Exocytosis allows large
molecules to be released from cells, such as the secretion of extracellular enzymes and hormones.
vesicles containing the material are pinched off from the golgi apparatus and move towards the
surface of the cell, fuse with the cell membrane and release the contents inside the cell. Whereas
exocytosis provides a means by which large molecules are taken into the cell. In phagocytosis, a
solid is taken into the cell, e.g. macrophages engulfing bacteria, amoeba engulfing food particles;
while in pinocytosis, a liquid is taken into the cell.

SURFACE AREA TO VOLUME RATIO

In a large cell the rates of movement of materials across cell membranes needed to meet the needs
of the cells may be inadequate because most of the cytoplasm is relatively far from the cell
membrane. By dividing the large cell into many small cells a high surface area to volume ratio
can be restored, serving each cell’s need for acquiring nutrients and expelling waste products.
This explains why larger organisms do not generally have larger cells than smaller organisms, but
more cells.

ENZYMES

Yaaaaaaayyyyy
Within any living organism chemical reactions are always present. These reactions are known as
metabolic reactions. Reactions in which large molecules are built up from smaller molecules are
called anabolic reactions while reactions that split large molecules into smaller ones are called
catabolic reactions. The speed of reaction can be increased, or catalysed, by raising the
temperature, but it would kill the organism. However the chemical reactions of metabolism occur
rapidly, even at the relatively low temperatures in living things, made possible by the action of
enzymes.

CATALYSIS OF CHEMICAL REACTIONS

Enzymes can be defined as biological catalysts made up of proteins. A catalyst is a substance

which speeds up a chemical reaction, but remains unchanged at the end. Therefore enzymes alter
the rates of chemical reactions without themselves being chemically changed at the end of the
reaction. They speed up a chemical reaction by lowering the activation energy barrier required to
start a reaction. Enzymes can either function inside cells, or are secreted out of cells through
extracellular enzymes.

E.g.
Anabolic reactions - amino acids taken into the cells may be used to build up a cell’s proteins.
In plant cells, hundreds of glucose molecules may be joined together to form cellulose to be added
to the cell wall.

Catabolic reactions - digestion

Starch is digested to sugar by amylase
Protein is digested to amino acids by protease
Fats are digested to fatty acids and glycerol by lipase
In the alimentary canal, large water-insoluble molecules are hydrolysed to smaller ones in the
process of digestion, catalysed by the above enzymes. Simpler and smaller substances are soluble
in water and can pass through the wall of the small intestine into the bloodstream.

Catabolic reactions - cellular respiration

Glucose is oxidised to release energy in the form of adenosine triphosphate molecules, forming
carbon dioxide and water. This process involved a chain of chemical reactions and a series of
enzymes, each catalysing one reaction.

Catabolic reactions - breakdown of hydrogen peroxide

Sometimes in chemical reactions in cells hydrogen peroxide which is a toxic substance is produced.
The cells produce an enzyme called catalase which catalyses the breakdown of hydrogen peroxide
into water and oxygen. This enzyme is abundant in liver cells and potato cells.
Germination of seeds - the seed takes in water and complex insoluble food stores are digested by
enzymes. Simple and soluble digested food is taken to the regions of growth, and new cells are
made at the growing tips of the root and shoot. During this anabolism or synthesis, amino acids
catalyse the breakdown of proteins to form the protoplasm, while glucose is broken down to form
the cellulose cell wall.

CLASSIFICATION OF ENZYMES

Enzymes are classified according to the chemical reactions they catalyse. The names of enzymes
usually end in “-ase” and are named according to the substance on which they act or the reaction
they promote

E.g. hydrolases
During the breakdown of large, complex molecules to small, simple products, water molecules
are required, known as hydrolysis. Enzymes that cause hydrolytic reactions are called hydrolases.
E.g. carbohydrases digest carbohydrates, amylase digests starch, cellulase digests cellulose,
protease digests proteins (e.g. pepsin), lipase digests fats, or lipids

HOW THEY WORK

Enzymes are highly specific in terms of their action, with each enzyme only catalysing a certain
molecule type. E.g. amylase will only act on starch and not on proteins or fats.

Only a restricted region of the enzyme molecule actually binds to the substrate. This region is
called the active site, and is usually formed by only a few of the enzyme’s amino acids. The
specificity of an enzyme is due to its very precise 3d conformation that allows a complementary fit
between the substrate and the active site. The lock and key hypothesis states that there is an
exact fit between the substrate “key” and enzyme “lock”. According to the induced fit hypothesis,
when a substrate molecule fits into an enzyme molecule, the active site alters its shape slightly so
that it fits more tightly around the substrate molecule to facilitate the chemical reaction.

The action of enzymes are as follows:

1. Binding of substrates at the active site of the enzyme molecules as the 3d conformation of
the enzyme active site is complementary to that of the substrate.
2. Formation of the enzyme-substrate complex
3. Chemical reactions that convert the substrate molecules into product molecules
4. Separation of product molecules from the active site
5. Free, unchanged enzyme molecules combine with more substrate molecules

Enzymes are sensitive to changes in temperature and pH. Each enzyme has its own optimum
temperature and pH at which they work most efficiently. Enzymes may require cofactors to be
bound to them before they can catalyse reactions, such as inorganic ions like zn2+ and mg2+ , or
coenzymes which are non-protein organic compounds. Most enzymes can also catalyse reversible
reactions.

FACTORS AFFECTING THE RATE OF AN ENZYME-CATALYSED REACTION

The rate of reaction of enzymes can be determined by measuring 1) the amount of product formed
per unit time or 2) the amount of substrate that disappeared per unit time. Any factor that affects
the enzyme activity will alter the rate of enzyme-catalysed reaction.

a) Temperature

At low temperatures enzymes are inactivated. As the temperature increases its activity also
increases. The rate of reaction too increases. This is due to an increase in kinetic energy of
enzyme-substrate molecules, and as they move about more rapidly the frequency of effective
collisions between substrate molecules and the enzyme's active site increases. The rate of enzyme
substrate complex formation then increases. The enzyme is about twice as active for every 10 deg
C rise in temperature until the optimum temperature is reached, with the temp coefficient being

[Rate of enzyme-catalysed reaction at (x+10)deg c]/[rate of enzyme-catalysed reaction at x deg

c]

The optimum temperature is the temperature at which the rate of reaction occurs fastest, or at
which enzyme activity is the greatest. For most enzymes this lies at around 40-45 deg c. high
temperatures (thermal agitation) disrupts the non covalent bonds such as hydrogen bonds, ionic
bonds and hydrophobic interactions that stabilise the 3d conformation of the enzyme molecule.
This specific 2d conformation of the enzyme active site is altered and the active site is no longer
complementary to the shape of the substrate. No enzyme substrate complex is formed, resulting in
denaturation. As the enzyme denatures, its activity decreases until it completely loses catalytic
ability.

b) pH

Enzymes are affected by the acidity or alkalinity of the solutions in which they act; the optimum
pH, like temperature, is that at which the enzyme works best - pepsin at pH 2, amylase at pH 7,
and intestinal enzymes at pH 10. Although small changes in pH affect the activity of enzymes
these effects are usually reversible, that is, an enzyme which is inactivated by a low pH will
resume normal activity when its optimum pH is restored. Extreme changes in pH may denature
some enzymes irreversibly by changing the 3d conformation of the enzyme molecule and altering
the shape of the active site.
 c) Enzyme concentration

At low enzyme concentrations there are not enough enzyme active sites available to bind to the
substrate molecules at any given moment. Enzyme concentrations limit the rate of reaction; in
which a limiting factor is any factor that is in the shortest supply so it directly affects the rate of
reaction. The value of this factor has to be increased in order to increase the rate of reaction. As
the enzyme concentration increases the rate of reaction also increases up to a point when any
further addition of enzymes has no effect on the rate. This is represented by the plateau area of
the graph which corresponds to the maximum rate of reaction. At low concentrations, however, it
is more likely that a substrate will bind to an empty active site on an enzyme.

At any given moment there are not enough substrate molecules to occupy all the enzyme sites.
And at high enzyme concentrations the substrate concentration becomes limiting.

d) Substrate concentration

At low substrate concentrations the rate of reaction increases proportionally with an increase in
substrate concentration. There are not enough substrate molecules to occupy all enzyme active
sites at any given moment. The rate of reaction is limited by the substrate concentration and the
limiting factor is thus the substrate concentration. A point is reached where all sites are being
used; the enzyme active sites at any given moment are saturated with substrates. Additional
substrates have to wait until the complex releases the products before entering the active site.
Increasing the substrate concentration cannot increase the rate of reaction as the enzyme
concentration is the limiting factor. However when the enzyme concentration is increased the
rate of reaction increases. But when it reaches the plateau the rate remains constant again as
enzyme concentrations become limiting.

The michaelis constant, or Km is the substrate concentration at which the rate of reaction is half
the maximum rate. The Km value is a characteristic of an enzyme and varies from enzyme to
enzyme. A low Km indicates a high affinity between an enzyme and a substrate, while a high Km
means there 6 a low affinity between enzyme and substrate.

ENZYME INHIBITORS

Irreversible inhibitors bind tightly and permanently to enzymes and destroy their catalytic
properties. These effects occur at very low concentrations of inhibitors. Examples include heavy
metal ions e.g. mercury (Hg), silver (Ag), and lead (Pb). reversible inhibitors, however, bind less
tightly to an enzyme and can be classified as competitive or non-competitive inhibitors. A
competitive inhibitor bears close structural resemblance to the substrate of the enzyme. It
competes with the substrate molecule for the active site of the enzyme. If the substrate
concentration is raised to a high level, the inhibitor is progressively replaced at active sites by the
substrate. At very high concentrations of substrate, the maximum rate of reaction in the presence
of the inhibitor can be very close or similar to that of a reaction in the absence of a competitive
inhibitor. A non-competitive inhibitor, however, bears no structural resemblance to the substrate
molecule and binds at a site other than the active site, resulting in a change in the conformation of
the enzymes so that the active site is no longer complementary to the substrate. As a proportion of
enzyme molecules are rendered out of action the effective enzyme concentration is decreased.
Increasing the substrate concentration has no effect on the rate and the maximum rate of reaction
in the presence of a non-competitive inhibitor is thus less than that of reaction in the absence of an
inhibitor.

Examples of IRL applications of enzymes include:

Biological washing powders where proteases help break down proteins, and help with removal of
stains caused by proteins such as bloodstains. Lipases break down fats and can thus remove
greasy stains.
Food industries where in brewing, sugar-fermenting enzymes in yeast convert sugar into alcohol
and carbon dioxide, and the enzyme rennin extracted from calves’ stomachs clots milk in the
manufacturing of cheese.
Estimation of glucose concentration where the enzymes oxidase and peroxidase are used in
dipsticks to estimate the concentration of glucose in blood or urine.

IMMOBILISED ENZYMES

An immobilised enzyme is an enzyme that has been attached to an insoluble support. It can follow
the following techniques for immobilisation:

Chemical attachments of enzymes to a solid support (e.g. cellulose fibres)

Entrapment of enzymes in beads of jelly-like substance called sodium alginate. The pores are
large enough to let the substrate in but not the enzyme out
Adsorption of enzymes to various surfaces, where attachment is not permanent

The immobilised enzyme beads are added to a reaction tank and are later recovered by
sedimentation, or are set up in a reactor column so that liquid flows continuously past the beads.

Adv. of immobilised enzymes:

1) The enzymes are easy to recover and can be used over and over again
2) It prevents losses due to the flushing of enzymes
3) The product does not contain enzymes so it does not need expensive purification
4) The immobilisation of the enzyme often makes them more resistant to temperature and pH
changes
Disadv. of immobilised enzymes:
a) Losses of enzyme activity can occur during the preparation of beads
b) Diffusion of substrates and products may be hampered, so the rate of catalysis might be
lowered.
c) The enzyme may have a more constrained conformation in the immobilised state, giving it
a lower catalytic activity.
d) There may be a high initial investment for immobilisation compared to free enzymes

NUTRITION IN HUMANS
the title should really be “digestion and nutrition” but who cares. the same thing applies to plants
anyways.

there are two types of digestion = physical and chemical

Physical digestion is the physical breakdown of large pieces of food into smaller pieces.
As the identities of food substances remain unchanged, no new products are created.

Physical digestion serves to increase the surface area to volume ratio of food to speed up
chemical digestion.
It takes place in the mouth (chewing), oesophagus (peristalsis), stomach (churning action) and
small intestine (emulsification).

Chemical digestion is the process where large and complex food substances are broken down into
smaller and simpler substances.
The identities of food substances change in chemical digestion, thus new products are formed.

Physical digestion allows food substances to enter the bloodstream through the intestinal wall,
and is carried out by enzymes.

Chemical digestion takes place in the mouth (starch), stomach (proteins), and small intestine
(carbohydrates, fats and proteins).

The human digestive system is made up of an 8-m long tube called the alimentary canal.
Associated glands include the liver, gallbladder, and the pancreas.

- The alimentary canal is a tube running from the mouth to the anus.
- Peristalsis moves food along the gut
- Mucus along the gut acts as a lubricant
- Digestion takes place outside of the living cells within the guts
- Secretes substances used in digestion

MOUTH - The site of intake of food, or ingestion.

- Salivary glands produce saliva and the tongue aids to mix the food with it in the mouth.
- Saliva consists of a mixture of salivary amylase and mucus.
- Saliva in the mouth provides a neutral environment for salivary amylase to function
effectively
- Food is chewed with the help of teeth to cut and grind them into smaller pieces. This
increases their surface area to volume ratio and allows enzymes to digest them faster.
- Starch in the mouth is digested into maltose by salivary amylase.
 - A bolus, a food mass of partially digested food mixed with mucus for swallowing, is
formed.
- Food then travels from the mouth to the oesophagus via the pharynx.

OESOPHAGUS
- The oesophagus is approximately 25 cm long and it leads to the stomach. Food travels to
the stomach because of peristalsis combined with gravity.
- The muscular walls of the oesophagus consist of circular muscles and longitudinal
muscles. Alternating waves of contraction and relaxation in the lining of muscles of the
alimentary canal that push food along the canal.
- To push the bolus forward, the circular muscles contract, the longitudinal muscles relax,
and this thus causes the gut to become longer and narrower to squeeze the bolus forward.
- Near the other end of the bolus, the circular muscles relax, and the longitudinal muscles
contract. This causes the gut to become short and wider, allowing the bolus to move to a
new position in the lumen.

STOMACH
- The stomach acts as a food storage which can hold the food for a few hours.
- It can store almost 5 dm^3 of food when fully distended.
- Stretch receptors in the stomach signal the hypothalamus when full.
- Protein digestion occurs in the stomach to form chyme, a thin watery liquid consisting of
partially digested food.
- Strong muscular walls and peristaltic action churn the food and bring it into close contact
with gastric juices.
- The mucous lining in the stomach has numerous pits bearing mucus-secreting epithelial
cells.
- Gastric glands in walls of the pits secrete gastric juice. This consists of hydrochloric acid,
rennin and pepsin, which are collectively called proteases.
- The enzymes are initially produced as inactive prorenin and pepsinogen.
- The important functions of HCl in the stomach include converting prorenin and pepsinogen
into active forms of rennin and pepsin.
- HCl also provides an acidic environment in the stomach to allow the proteases to function
efficiently in this optimum condition. It also kills germs and parasites, and it stops the
action of salivary amylase.
- Two sphincter muscles prevent the uncontrolled exit of food from the stomach: the pyloric
sphincter and the cardiac sphincter.
- Food remains in the stomach for around 3 to 4 hours.
- The partially digested food becomes liquefied to form chyme, which then passes into the
duodenum when the pyloric sphincter relaxes.
- The churning action by the muscular wall of the stomach causes food to break up further
into smaller pieces. It also helps food to mix with gastric juices.
 - The churning action by the muscular wall of the stomach causes food to break up further
into smaller pieces.
- Pepsin acts on proteins to form polypeptides, also known as peptones.
- Rennin acts on caseinogen with calcium (Ca2+ ions) to form insoluble casein.

SMALL INTESTINE
- The small intestine consists of 3 major parts - the duodenum, jejunum and ileum.
- The duodenum is the area where the most digestion takes place, while the jejunum and the
ileum are mainly involved in the absorption of digested food.
- The lining of the intestinal walls contain glands that secrete digestive enzymes.
- The digestion in the small intestine involves 3 different secretions - pancreatic juice,
intestinal juice and bile.
- All secretions are alkaline, and neutralise the acidic chyme.
- These alkaline secretions also maintain a suitable pH of 7 to 8 for optimum enzyme action.
- Physical digestion takes place through emulsification and chemical digestion takes place
by various enzymes.
- Bile is produced by the liver and stored in the gallbladder. It is transported to the small
intestine via the bile duct.
- Bile is NOT an enzyme; it is a substance used to break up fat molecules into smaller fat
globules.
- Emulsification increases the surface area of the fat molecules so that they can be digested
by lipase faster.
- Pancreatic juice is secreted by the pancreas and is transported to the small intestine by the
pancreatic duct. This juice contains 3 enzymes - amylase, lipase, and trypsinogen.
- Amylase converts starch into maltose, while lipase converts fats into fatty acids and
glycerol.
- Trypsinogen, in its inactive form, is broken down by enterokinase into trypsin. Trypsin
further breaks down proteins into peptones.
- Intestinal juice is secreted by intestinal glands which line the intestinal walls, which
contain 6 enzymes:
- Maltase, which converts maltose into glucose.
- Sucrase, which converts sucrose into glucose and fructose.
- Lactase, which converts lactose into glucose and galactose.
- Erepsin, or protease, which converts peptones into amino acids.
- Enterokinase (mentioned earlier)

LARGE INTESTINE
- The large intestine has an average length of around 1.5 m.
- In the colon of the large intestine, water and mineral salts are absorbed into the body.
- Certain bacteria in the large intestine can also produce vitamins B and K.
- Undigested food, along with unabsorbed minerals, form faces.
 - Faeces contains dead bacteria, cellulose, mucus, cholesterol, bile pigment derivatives and
water.
- Faeces is then expelled through the anus by a process called egestion.

ABSORPTION OF FOOD SUBSTANCES

- Absorption is the process whereby digested food materials are taken into the body cells.
- The digested foods (simple sugars, amino acids, fatty acids and glycerol) are absorbed by
the villi of the ileum and the small intestine.
- Both digestion and absorption take place in the ileum.
- The complete process of digestion in the ileum produces simple food substances such as
simple sugars (glucose, fructose and galactose), amino acids, glycerol, and fatty acids.
- The ileum contains several folds covered with millions of finger-like projections called
villi.
- Each villus has a network of blood capillaries and a lymphatic vessel known as lacteal in
its centre.
- Blood capillaries and lacteal are involved in the absorption of different types of digested
food.
- The small intestine is very long (7 metres) and is folded. This helps to increase the surface
area to volume ratio for absorption to take place rapidly.
- The presence of villi and microvilli increases the surface area to volume ratio further.
- A one-cell thick epithelium ensures the rapid diffusion of digested food substances into the
blood capillaries.
- The intestinal wall and villi are richly supplied with blood vessels to transport sugars and
amino acids away from the small intestine. As the food substances are transported away, a
concentration gradient is maintained to facilitate diffusion. Similarly, villi contain lacteals
which transport fats away from the small intestine.
- The small intestine also has smooth muscles that constantly relax and contract in a wave-
like pattern, or the aforementioned peristalsis. This is to allow food to be brought into close
contact with the intestinal walls.
- Blood capillaries absorb simple sugars and amino acids by diffusion. Active transport is
also involved in the absorption of these food substances.
- Active transport occurs when the concentration of simple sugars and amino acids in the
small intestines are lower as compared to the concentration of these food substances in the
blood capillaries.
- However they are still being absorbed against the concentration gradient and into the
blood capillaries using energy in the form of adenosine triphosphate.
- These simple sugars and amino acids absorbed through the villi walls pass through the
blood capillaries and converge into the hepatic portal vein in the liver.
- Fatty acids and glycerol diffuse into the epithelium of the villus and recombine to form fat
droplets. The fat droplets will then enter the lacteals.
 - The individual lacteals merge to form larger lymphatic vessels. The lymphatic vessels
carry the fat droplets away from the ileum into the bloodstream via the left shoulder vein
before they are distributed to the rest of the body through the circulatory system.
- The schematic diagram below shows the route of the absorption of fatty acids and glycerol.
- After the absorption into the bloodstream from the lumen of the small intestine simple
sugars and amino acids are transported to the liver by the hepatic portal vein.
- They are then distributed to the rest of the body cells by the circulatory system for
utilisation.
- Assimilation is a process where some of the absorbed food materials are converted into
protoplasm or used as an energy source.
- Simple sugars are transported to the liver from the small intestine by the hepatic portal
vein. In the liver, the simple sugars are converted into glycogen for storage.
- Some glucose leaves the liver and is transported to cells. These glucose molecules are
oxidised during respiration. The excess glucose is transported back to the liver for storage
in a form of glycogen.
- The conversion of the excess glucose into glycogen is stimulated by the presence of insulin.
Insulin is a type of hormone produced by the islets of langerhans in the pancreas.
- When the body requires more energy, the liver converts the glycogen back into glucose.
This process can be stimulated by the hormones adrenaline or glucagon.
- Amino acids are transported to the liver from the small intestine by the hepatic portal vein.
They are essential substances to form new protoplasms, and for the growth and repair of
tissues as well as to form enzymes and hormones.
- The excess amino acids are broken down in the liver through demaination, where they are
converted to urea, which is removed in urine.
- Amino groups are converted into nh3 ammonia, which is further converted into urea.
Carboxyls, however, are converted into glucose which is then further converted into
glycogen.

LIVER

- The organ used most in transplants in soap operas (apart from the kidney) has the
following functions:
- The blood glucose concentration is metabolised and regulated at 70-90 mg
glucose/100ml of blood. Excess glucose is then converted into glycogen.
- The liver is responsible for the production of bile, which is stored in the gallbladder
and released when required for the digestion of fats.
- Excess amino acids undergo deamination in the liver to form urea for excretion.
The liver also synthesises plasma proteins from dietary amino acids. These proteins
include albumins, globulins, and fibrinogen, which is essential for blood clotting.
 - Enzymes in the liver can break down or transform substances like metabolic waste,
drugs, alcohol and chemicals so that they can be excreted. Alcohol is broken down
by alcohol dehydrogenase into carbon dioxide and water.
- The liver stores the vitamins a, d, k, b12 and folate.
- Worn-out red blood cells are destroyed and their remaining haemoglobin content is
sent to the liver. The liver then breaks down this haemoglobin and stores the iron
released in the process.
- Heat is produced when the liver carries out its various chemical reactions. This
heat is transported by blood to other parts of the body so as to maintain the body
temperature.

ALCOHOL
- “Beer; helping ugly people have sex for 2000 years”
- Alcoholic drinks are beverages containing ethanol. Ethanol is produced by fermentation of
grains or fruits. Excessive alcohol consumption can have adverse biological and social
effects on the individual drinker, his or her environment and society as a whole.
- Excessive alcohol can cause liver damage by replacing fresh liver tissue with scar tissue
resulting in cirrhosis.
- Some other effects on drinkers include the following:
- Short term effects include dizziness, slowed reaction time, slurred speech, mood
swings, decreased coordination and concentration, confusion, blurred vision, nausea
and vomiting.
- Long term effects can include high blood pressure, irregular heartbeat, brain
injury, amnesia, liver damage (e.g. liver cancer, hepatitis, etc.), hallucinations, a
greater chance of infection, and loss of muscle tissue.
PLANT NUTRITION

PHOTOSYNTHESIS

- Photosynthesis is the process by which green plants take in carbon dioxide and water, and
in the presence of light, to manufacture glucose.
- Photosynthesis occurs in two stages: the light-dependent stage and the light-independent
stage.
- In the light-dependent stage, light is trapped by chlorophyll. It is also converted into
chemical energy and also is used to split water molecules. The photolysis of water releases
oxygen gas and hydrogen atoms. This light-dependent stage then drives the light-
independent stage of photosynthesis.
- In the light-independent stage, these reactions are catalysed by a series of soluble enzymes.
Glucose is formed using chemical energy and hydrogen atoms released during the light
stage. The formation of glucose is a reduction reaction because carbon dioxide gains
hydrogen atoms.
- The glucose that is formed is then stored temporarily as starch in the leaf.
- It is a source of potential chemical energy to be used by the plant to make complex
compounds.

FACTORS CONTROLLING PHOTOSYNTHESIS

A limiting factor is any factor that directly affects a process if its quantity is changed. When it
comes to photosynthesis, some limiting factors include:

- LIGHT INTENSITY
- When light intensity increases, the rate of photosynthesis increases.
- The rate of photosynthesis will be maximum at its light saturation point.
- After its light saturation point, the rate of photosynthesis remains constant even when the
light intensity increases.
- Plants respire simultaneously when they photosynthesise, the light intensity at which net
gaseous exchange equals zero is known as the compensation point.

- CARBON DIOXIDE CONCENTRATION

- Carbon dioxide makes up 0.03% of air, so it is an important limiting factor.
- When the concentration of carbon dioxide is increased under the experimental conditions,
the rate of photosynthesis increases dramatically.

- TEMPERATURE
 - When the temperature increases, the rate of photosynthesis increases. However, beyond
40 deg C, the rate drops rapidly as enzymes involved in photosynthesis are being
denatured.

IMPORTANCE OF PHOTOSYNTHESIS

- Since plants undergo autotrophic nutrition, the carbohydrates which are formed are
converted into fats, proteins and other organic compounds. They become the food for
animals as animals undergo heterotrophic nutrition and cannot make food for themselves.
- Photosynthesis converts light energy into chemical energy, which is stored within the
carbohydrate molecules. This becomes a source of energy for animals.
- Coals, one of the fossil fuels, are formed from trees. When coal is burnt, stored energy
from photosynthesis is released. This energy can be used to drive machinery and generate
other forms of energy.
- Photosynthesis removes carbon dioxide from the air and releases oxygen. In this way the
air is constantly purified and the earth’s temperature can be maintained. Carbon dioxide in
the air contributes to the greenhouse effect and raises the atmospheric temperature.

DIGESTION OF STORED FOOD IN PLANTS

- Excess proteins, fats and starch are stored as insoluble products in storage organs.
- Plants have no digestive system but their digestive processes are similar to animals.
- Cells in storage organs produce enzymes to digest the stored food.

Starch -(diastase)-> maltose -(maltase)-> glucose

proteins-(pepsin)-> peptones-(erepsin)-> amino acids
fats-(lipase)-> fatty acids and glycerol

EXPERIMENTS TO INVESTIGATE FACTORS AFFECTING THE RATE OF

PHOTOSYNTHESIS

- Factor tested: light intensity

- Objective: to investigate the effect of light intensity on the rate of photosynthesis
- Manipulation of factors being investigated: vary the distance, x cm, of the lamp (source of
light) from the plant for each investigation, e.g. 5 cm, 10 cm, 15 cm etc. the shorter the
distance, the stronger the light intensity.
- Factors kept constant: concentration of carbon dioxide + temperature + pH

- Factor tested: carbon dioxide

- Objective: to investigate the effect of different carbon dioxide concentrations on the rate of
photosynthesis
 - Manipulation of factors being investigated: use different concentrations, x M, of sodium
hydrogencarbonate solutions, e.g. 0.1M, 0.2M, 0.3M etc. for each investigation. Sodium
hydrogencarbonate is a source for carbon dioxide gas.
- Factors kept constant: light intensity + temperature + pH
- Checking the effect of the manipulated factor on the rate of photosynthesis: count the
number of bubbles per unit time
- Factor tested: temperature
- Objective: to investigate the effect of different temperatures on the rate of photosynthesis
- Manipulation of factor being investigated
- Use different temperatures, x deg C, of the water within the water bath, e.g. 10 deg C, 20
deg C, 30 deg C etc. for each investigation.
- Factors kept constant: light intensity + concentration of carbon dioxide + pH

LEAF

- The lamina, of the leaf blade, has a large surface area to volume ratio to trap maximum
amounts of sunlight and allow for the rapid diffusion of carbon dioxide.
- The petiole, the leaf stalk, holds the lamina away from the stem, and may or may not be
present in all varieties of leaves.
- The vein network transports water and mineral salts to cells in the lamina. It transports
manufactured food from the lamina to other parts of the plant.
- The veins may be branching (reticulate venation) or parallel (parallel venation)
- The cross-section of the internal structure of a leaf makes up the following parts: the upper
epidermis, the mesophyll layer consisting of a palisade mesophyll layer and a spongy
mesophyll layer, as well as the lower epidermis where guard cells and stoma are commonly
found
- The upper epidermis contains a single uppermost layer of cells that protects the inner layer
of cells, and is covered by a layer of waxy cuticle to reduce the evaporation of water from
the leaf and prevent the entry of disease-causing microbes. The upper epidermal cells do
not contain chloroplasts and are transparent, allowing light to penetrate into the inner
layers
- The mesophyll layer, specifically the palisade mesophyll layer, contains long and cylindrical
cells packed vertically, in which the vertical arrangement exposes many cells to light rays.
It also contains numerous chloroplasts, allowing for photosynthesis to occur.
- The spongy mesophyll layer, however, is irregularly shaped, with loosely arranged cells
with many air spaces in between, allowing for the rapid diffusion of co2 and o2 in and out
of the mesophyll cells. The cells are surrounded by a thin film of water, allowing for gases
entering the cells to dissolve inside.
- The veins, or the vascular bundle, extend throughout the left, within a short distance of
every mesophyll cell. Not only does this allow for the diffusion of water and mineral salts
absorbed by the roots, this also supports the leaf and keeps it flat to ensure maximum
 exposure to light for photosynthesis, as well as the maximum surface area to volume ratio
in contact with the surroundings in order for transpiration to occur.
- In the lower epidermis, the lowest layer of cells protect the inner tissues, and the waxy
cuticle slows down the loss of water. Guard cells surrounding the stomata regulate the
opening and closing of the stomata for the diffusion of co2 and o2 in and out of the leaf.
- The guard cells surround the stomata, and have uneven thickening of walls.