Cell organization

Cells are the basic building blocks of living organism. The approx.
value of cells is between 10-100 million. Not all cells are of same
types. They are divided in groups of specialized cells, each with a
special function e.g nerve cells conduct impulses, red blood cells
carry oxygen and bone cells provide support. Animal and plants
cells have a whole lot in common despite their various structure
and functions. Both have nucleus, cytoplasm and a cell
membrane. Cytoplasm is the living material that makes up most
of a cell. It has a jelly like texture and also contains many sub-
cellular structures defined as organelles. Only the largest can be
seen by a light microscope. Most of the chemical reactions take
place in the cytoplasm. The reactions are called the metabolic
reactions or cell’s metabolism. Nucleus is one of the largest
organelle visible in a cell. Most cell types have a single nucleus in
their cytoplasm. A few have no nucleus e.g red blood cells and
xylem vessels. Nucleus controls the metabolic reactions
occurring in the cell. They also contain genes within thread like
structures called chromosome. Excluding cell division,
chromosome are rarely visible in cell Genes are made up of
deoxyribose nucleic acid, and control metabolism by determining
which enzymes a cell can produce. Cell membrane is the
partially permeable, thin layer (7 millionth of a millimeter) at the
surface of cell which forms a boundary between cell contents and
the outside. It also controls what can pass in and out of cell. The
actual thickness of a cell membrane is not visible through a
microscope, just its position. Cell membrane, cytoplasm and
nucleus are found in both plant and animal cells. Cell wall is a
dead structure of a carb, cellulose and surrounds the cell
membrane of a plant cell. Cellulose is the tough material that
helps plant maintain its shape. Animal cells can change shape to
some extent because of their lack of cell wall. Plant cells have a
fixed shape. Plant cells take in water producing pressure against
the wall. The cell becomes inflates like balloon blown in a box.
This condition is the state of turgor. When all cells are turgid they
push against each other to maintain shape. Cell wall is freely
permeable to water and dissolved substances. It does not control
movement of substances. Cell wall is thicker and visible through
microscope. Fully developed cells have a large central space in
the cytoplasm called sap vacuole, a solution of sugars and ions.
The vacuole acts like a store for these solutes. Animal cells also
have vacuoles but smaller and temporary structures.
Chloroplasts are the green organelles of a plant cell and contain
chlorophyll. They too are visible through a microscope. Their
function is to absorb sunlight during photosynthesis. Root hair
cells are suited at the back of root tips. They are suited where
most of the soil water is absorbed by the root. They increase
surface area available for absorption. Each hair is a single
specialized cell of root epidermis. It is long and grows between
soil particles around the root, surrounded by soil water. It has
thin walls for water to enter easily. Water is absorbed from soil
by osmosis and minerals are taken up by diffusion or active
transport. Xylem cells are dead, hollow cells that transport water
+ minerals from root to leaves. Xylem vessels develop from
living cells and have cellulose cell wall and cytoplasm as they
mature. Then they become elongated and the end walls between
cells break down. The walls become impregnated by a woody
substance called lignin, which is impermeable to water. They
cells die forming a continuous hollow tube. Lignification of cell
walls makes the xylem cell very strong. This enables them to
carry water up through the plant without collapsing. This helps
support the stem and plant leaves. Red blood cells are most
common, 6 million rbc in 1 cubic mm of blood. They carry oxygen
around the body, taking it from the lung and transporting it to
respiring tissues. They develop from bone marrow cells. They
lose their nucleus by time and form a biconcave shape. They also
contain haemoglobin, a red pigment. Oxygen is attached to
packed haemoglobin in cells. The cells shape give it more surface
area and rbc is flexible so it can pass through narrow blood
capillaries. Plants also has organs and tissues. Leaves stem roots
and flower are organs. Photosynthetic transport and support
tissue are plant tissues. On average, an animal cell is smaller
than a plant cell. Measurements of cells are done in micrometers
(µm). One micrometer= millionth of a meter, thousandth of a
millimeter. Typical animal cell = 10-20 µm, typical plant cell =
50-100 µm.
Diffusion and osmosis
If we leave a bottle of perfume open for time, its smell would
start to spread. Molecules of perfume would evaporate from the
bottle and spread out until they reach our nose. This is because
of the random motion of molecules in gaseous state. If there are
different concentrations in two locations, after some time, they
will tend to move away from high concentration, to a less
concentrated region. In the example high concentrated region is
the bottle, and the low concentrated region is the area near
nose. Diffusion is the net movement of molecules or ions from a
region of higher concentration to a region of lower concentration,
down the concentration gradient. In it molecules will spread until
they are evenly distributed. Through diffusion substances can
pass in/out of cells. If there is a concentration gradient of a
substance across the membrane, the membrane is freely
permeable to it. Example, membrane is permeable to oxygen
and carbon dioxide. Carb dioxide is produced in cell respiration,
and as it’s concentration builds up in cytoplasm. Then CO2
diffuses through membrane, going down the concentration
gradient. Rate of diffusion is affected by, the gradient steep
(steep gradient = faster diffusion), Temperature (high temp. =
faster diffusion), Molecule size (Large molecule = slower
diffusion), Surface area (large are = overall increased diffusion).
Osmosis is special diffusion, involving water molecule movement.
It occurs when cell membrane separates two solutions with
different concentrations. The membrane allows small molecules
in/out but not larger ones. Artificial membranes also have this
property, e.g in kidney dialysis machine, the visking tubing, has
microscopic holes in it, permeable to water molecules but not to
larger ones e.g sugar sucrose. For example. Water molecule and
sucrose solution are separated by membrane. Water on left side.
Sucrose on right. Left would have high water concentration. Right
will have lower. So to diffuse, water molecule will pass through
membrane to get on other side. More water molecules will go left
to right, then right to left, because of gradient of water. Osmosis
is the net movement of water molecules from a region of their
higher concentration to an area of lower concentration through
membrane. The term used to say how freely water moves is
called water potential. Osmosis takes place in animal, plant cells.
Inside cell in cytoplasm which is a watery environment
containing a variety of solutes. If cells is surrounded by different
water potential solution then the cell would gain or lose water
through osmosis. Effect of osmosis different in animal and plant
cells. In cells, the membrane is delicate and cannot withstand a
lot of pressure and can tolerate only small water movements by
osmosis. E.g Red blood cells are surrounded by plasma in body,
with solution of 0.85% (0.85 g of salt in 100cm cube of solution),
equal to water potential of cell contents. If these cells were
placed in water, they would rapidly absorb it, swell and burst. But
if placed in 3% salt solution, they would lose all there water and
become crenated (shirk and have a crinkled appearance). An
osmotic equilibrium solution with no water loss or gain is called
isotonic solution. A solution that is less concentrated or has
higher water potential is hypotonic. More concentrated or lower
water potential is hypertonic. Most cells in body are surrounded
by isotonic solutions. These are tissue fluids that bathe cells,
acting like transfer route between blood and tissue. They contain
salt and other solute and water potential is equal to blood.
Osmosis in plant cells is done relatively differently than in animal
cells. Because of cell wall, plant cell can withstand more pressure
during osmosis. There, vacuole and cytoplasm contain various
solutes, lowering cell’s water potential. Placed in a hypotonic
solution, water will enter the cell through osmosis. In a dilute
solution, the cell swells up and its cytoplasm pushes against the
wall. A cell that develops internal pressure turns turgid. Cell wall
is inelastic. It will only expand until it exerts the pressure back on
the contents. By then no more water would enter cell and the cell
would become fully turgid. If it would be put in a hypertonic
solution, the cell would lose water by osmosis and decrease in
volume, so cytoplasm would not push against wall. This is flaccid
condition. Soon it would lose so much content that the cytoplasm
would pull away from cell wall and becomes plasmolysed. Water
moves from cell to cell by osmosis. If Cell A = high water
potential, Cell B =medium potential and Cell C = low potential,
then water will move from Cell A-B-C. This is how water moves
up a leaf. Soil water has dissolved solutes, but at lower
concentration then solutes in root hair cells. When water enters
root hair cells, it dilutes their contents, raising water potential
Water then move to next cells, the outer part of root, cortex. A
water potential gradient is set up and maintained because xylem
removes water in the middle of root and takes it to stem and
leaves. Water evaporation maintain the gradient through the
plant. Diffusion and osmosis is a passive process and does not
require external energy. Active transport is the movement of
molecules from lower to higher concentration, up and against the
gradient. It requires energy from cellular respiration in form of
ATP; adenosine triphosphate. Active transport happens in root
hair cells. Plants need mineral for growth. Usually soil water has
low concentration, so active transport is required for hair to
absorb minerals. The last part of small intestine is ileum, part of
gut where food is absorbed into blood. It has many adaptions for
this e.g villi, which would increase absorption surface area. Sugar
glucose has low concentration and active transport is needed to
absorb it.
Enzymes
If a chemist wants to increase the rate of chemical reactions in
can do so by changing the reaction’s concentration, increasing
the temperature, or by using a catalyst, which is a substance
that speeds up a chemical reaction, but remains unchanged at
the end of reaction. A used catalyst can be reused. But different
reactions require different catalysts. At once a thousand
reactions can take place in a cell. These are known as metabolic
reactions. These are catalyzed by a series of proteins call
enzymes. They are proteins which function as biological catalyst.
A cell can contain thousands of enzymes type, each catalyzing a
different type of reaction. The temperature of cells is low, (37 C)
and if not for enzymes, the metabolic reactions would go too
slow to sustain life. A cell can only carry out reactions fast
enough by producing the right type of enzyme. Enzymes control
cell metabolism. Production of cell’s enzymes is under control of
nucleus genes. Most enzymes remain inside the cell as
intracellular enzymes. Some are secreted out and are called
extracellular enzymes. Some of these extra cellular enzymes are
involved in food digestion in gut. The molecules that the
enzymes act on is enzyme subtract. Protein molecule that forms
enzyme, has z small region of its surface called active site, where
substrate bind themselves to enzymes and product is formed.
Shape of active site = shape of substrate. The substrate fits itself
in the site, like key in a lock. This action is called lock and key
hypothesis. Substrate and enzymes are in random motion due to
kinetic energy. When substrate hit active site they fit in and bind
themselves to it. Then the catalyzed reaction takes place and
product is formed. Then the products leave and enzymes
catalyse another reaction. Each type of enzyme has a different
protein structure and different active site shape, so it will bind to
a particular substrate only. Enzymes increase rate of reaction as
a result of binding of enzyme and substrate at site. Enzyme
activity is affected by temperature. Rate of most reaction is
increased by temp. As reactant molecules have more kinetic
energy, they move faster and collide more frequently. Usually
after a certain temperature, the rate decreases. The peak rate of
temperature is the optimum temperature. Decrease in enzyme
activity is due to their protein structures, which are broken down
by heat. Higher than the optimum temperature, heat can
damage the structure and become denatured, by heat. Then a
molecule can no longer bind itself with the active site. The rate of
reaction is decreased. Denaturing can be seen while boiling egg.
Egg white is made up of albumin protein. Heat denatures the
white runny liquid in to solid shape. But some organisms have
high optimum temperature, with heat resistant enzymes e.g
bacteria in hot volcanic springs. Even though, temperature does
not affects the rate of overall amount of product formed. Most
environments in body have close to 7 pH. More or less pH would
cause a rapid decline in enzyme activity and denature it. 7 is the
optimum pH for enzymes. pH affect is because it is a measure of
concentration of hydrogen ions in a solution. They interact with
enzyme protein, altering the structure and active site. At
optimum pH, the enzyme exposure to hydrogen ion is just right.
To maintain correct structure of protein molecules. Still some
enzymes have different optimum temperatures. Human stomach
secretes hydrochloric acid, to kill bacteria in food. And pepsin, an
enzyme has pH of 2. To find pH effect on enzyme activity we
need to vary the pH of a mixture of enzyme and substrate and
measure the rate of disappearance of substrate or product
formation. We can do this by adding acid or alkaline to enzyme.
It would change but not control the pH. To keep pH constant we
use buffer solution, pH-change-resistant-salts added to substrate
before adding enzymes. Different buffer solutions for maintaining
different pH value. Catalyse is used in experiments too, to break
metabolism waste product, a toxic, hydrogen peroxide, in water
and oxygen.
Animal Nutrition
Carbohydrates, proteins, lipids, vitamins and mineral salts, fiber
and water, are the 7 substances that keep our body healthy
through life. A healthy diet is a balanced diet, which contains the
correct proportion of each nutrient. A person without a balanced
diet suffers from malnutrition or lack any particular nutrients,
resulting in dietary diseases. Over eating on the other hand
causes obesity. The three main solid body components are carbs,
protein and lipids. Carbohydrates are made up of carbon, oxygen
and hydrogen. Atoms of hydrogen to oxygen ratio = 2:1. Glucose
is a carbohydrate under the heading of simple sugars, or
monosaccharides. Formula is C6H12O6. Other simple sugars
include fructose in fruit. Monosaccharides form building blocks of
complex carbs. To simple sugars compile to form a disaccharide.
Glucose and fructose disaccharide to form sucrose. A substance
made from a chain of many monosaccharides is called a
polysaccharide, e.g starch, composed of glucose chains. Carbs
make 5% of body mass but act as fuel for cells during
respiration. During respiration glucose is oxidized to release
energy for metabolism processes. Starch is also broken down
during digestion. Sugars are sweet and water soluble. Their
solubility allows monosaccharides like glucose to be absorbed by
digestive system and transported around the body in blood.
Starch is large and insoluble and found as a storage carb in
plants e.g cereal potato etc. Human body contains 17% protein
on average. Protein are vital for cells and tissue growth and
repair. Cells produce a wide range of protein e.g structural
protein and haemoglobin. Proteins are large molecules made
from smaller chains of amino acids, which can be up to 20. They
are arranged in different sequence to produce different proteins
in body. All amino acids contain four elements, oxygen,
hydrogen, carbon and nitrogen. 2/20 acids also have Sulphur in
them. Long chains of acids are twisted and coiled with cross
linking bonds for holding them in place. They are most complex
3D. Structure of each proteins defines its function. Most protein
are insoluble and large. During digestion, they are broken into
their constituent amino acids for absorption. Food rich is protein
are meat, eggs, pulses, cheese. Lipids are fats / oils composed of
hydrogen, oxygen and carbon. They are water insoluble and act
as a store of energy in human and animal bodies. Body fats
stores around internal organs. Lipids digestion products can be
used in respiration alternative to glucose. Fat is a good insulator
against heat loss, and protects organs from mechanical damage.
Fats are solid and oils are liquid at room temperature. Oils are
more found in plant tissues e.g olive oil, corn oil. Plant oils, meat,
oily fish, cheese, eggs are rich is lipid as are oil fried food.
Glycerol and fatty acids are the chemical building blocks of lipids.
Glycerol is an oily liquid used in cosmetics. In lipids, it is joined to
3 chains of fatty acids. Vitamins are substances needed in small
amounts to keep the body healthy. Vitamins are given letters
each with a specific amount. Lack of any vitamin can cause a
particular disease. Vitamin A is needed for a light sensitive
chemical in retina. Without it we can have night blindness. The
recommended daily amount RDA of Vitamin A is 0.8mg. Vitamin
D helps bone absorb calcium phosphate. Its deficiency would
cause poor bone growth, a disease called rickets. In it the bones
remain soft and unable to support weight of body. Its RDA is 5ug.
Vitamin D is found in eggs, milk and sunlight. Vitamin C is need
for making fibers of connective tissue to bind cells together. Its
RDA is 60mg. Without vitamin C scurvy would take palace where
tissue damage and bleeding on wound occurs, especially on
gums. Vit C sources are fresh fruit and vegetable, lemon and
oranges. Minerals are elements needed in microgram quantity,
e.g calcium and phosphorus. Calcium in body is about 1 kg, most
in bones and teeth. Calcium ions are also required in cells for
them to function properly. Dairy products are good calcium
sources. Lack of calcium also causes rickets. Iron is another
mineral. There is about 3g iron in body most in haemoglobin, in
which iron combines with oxygen molecules to carry oxygen
around body through blood. Lack of iron causes anaemia.
Anaemia victim tires easily. Shellfish, liver and red meat are
good iron source. Dietary fiber is made up of cellulose, which is
carbohydrate and a polysaccharide for glucose. But our gut
cannot break don cellulose, so it is not a nutrient. Still it has
important role in digestion. Food is pushed through gut by
peristalsis. Fiber provides the gut something to push against
aiding peristalsis. It also protects gut against diseases e.g bowel
cancer and colitis. Best sources are fresh fruit and vegetables.
Water is essential for life. Even food contains quantities of water.
Even dry food like bread is 30% water. Fresh vegetable are 90%
water. People water mass/ body mass = 55-75%. Water is major
component in blood, tissue fluid, dissolved substances and
cytoplasm in body. It also takes part in hydrolysis reactions
where substance is broken down by adding water. Water also has
many physical properties to maintain constant body conditions,
like it is main component of sweat and maintains body
temperature. Unit for chemical food energy is kilojoules. Certain
foods contain more energy than others. A gram of carbs contains
17 Kj of energy. A gram of fats contain 37Kj of energy. Protein
contains about 18Kj/g. An adult office worker needs 9500-10500
Kj for one day to stay healthy. I need 11000 Kj energy/day. It is
not just requirement that varies, but also nutrients. Malnutrition
is under-nourishment due to lack of food and has a poor diet.
Carbs supply our body with most of energy. Carbs supply our
body with most of the energy. When starvation, body uses
protein because fats and carbs ae used up. Soon the muscle
proteins start to metabolise as a source of energy. This is called
Protein Energy Metabolism (PEM) and is affecting millions of
children. This causes in poor development and the child becomes
inclined to be affected by more diseases.. Kwashiorkor is a
disease caused by PEM. It causes distended belly due to tissue
fluid, an osmotic problem. It is caused because of nutrient lack
and can cause general weakness, skin damage, and swollen
liver. A person whose weight is 20% more than his age group is
known to be obese, occurring when people eat more than
recommended DET and does not do exercise. Being overweight
also adds strain to heart and causes coronary heart diseases and
diabetes. Famine is the scarcity of food due to crop failure, over
population and environmental disasters plus man-made
disasters. Famines cause malnutrition and starvation, killing
millions of weakened people. In last 30 years, food
production/person has grown 17% and world population 70%. We
have enough food to feed each person on earth but the problem
is unequal distribution. Some people suffer from obesity and
others from undernourishment. Rich countries have hoard of
foods. Poorer countries have less food supplies. Also even in
countries, the rich fools are well fed, while the poor people are
famished. One billion people are under nourished, mostly in
African countries.When we eat food, it must first be broken down
into their molecule building blocks, to be absorbed by gut and
blood. This is called digestion. Products are monosaccharides
from carbs, amino acid from proteins, fatty acid and glycerol
from lipids. Others can be transported easily. Digestion has two
parts, chemical and mechanical. Mechanical digestion stats at
mouth, physical breakdown of food by teeth. Then the food travel
down to the gut where it is churned by gut walls. Chemical
digestion is when enzymes are secreted in the gut to speed up
breakage of food. We have 4 type of teeth. Incisors and canines,
sharp chisel shaped for biting of food pieces. Molars and
premolars, flat top for grinding and chewing food. Crown is the
teeth’s exposed part, covered in enamel, which is nonliving and
hardest substance in body. Below it is softer dentine and in the
epicenter is the pulp cavity, which contains nerves and vessels.
Channels with living cytoplasm connect pulp to dentine for
exchange of nutrients and oxygen. Below gum is tooth’s root,
anchored to jaw’s bone with fiber and cement secreted by
dentine, holding it in place. After a few months, a newborn baby
teeth emerge from gums. These are milk/ deciduous teeth. After
the age of 7 milk teeth began to fall and are replaced by
permanent teeth. An adult has 32 teeth. Tooth decay / dental
caries is caused by bacteria feeding on sugar in mouth, from the
eaten food. Bacteria breaks down sugar and produces acids
which dissolve in the outer enamel. Soon the acids will penetrate
the dentine, dissolving that too forming tooth cavity. Then, if not
stopped the decay will enter the pulp and pain starts. This is a
periodontal disease in which gums become inflamed and start to
bleed and sometimes, tooth loss takes place. The bacteria and
sugar forms a layer on teeth surface called plague, which is
removed by brushing and takes 24 hours to reform. Small brush
head ensures that all teeth are brushed. Dental floss should also
be used to remove food between teeth. Plague evolves into
tartar if not removed, encouraging decay. Tartar cannot be
removed by brushing, but by dentist. It is recommend to go to
dentist once every 6 months, for clean ups. Fluoride prevents
tooth decay and is added to water supply and toothpaste. A
balanced diet is extremely effective too. It ensures that we are
getting calcium and other nutrients for our teeth to stay healthy.
Sugary foods deprive the bacteria too. From mouth the food
enters the gut, due to peristalsis, and involuntary process
coordinated by nervous system. The gut wall has 2 muscle sets.
Circular inner muscle layer, made of muscle fibers, running in
rings. The outer layer is called longitudinal muscle layer, which
has fibers running down the wall’s length. Both layers work
together to push the food to the lumen, central space of gut.
Behind the food, inner muscles contract and outer relax, pushing
food further. Ahead of food it is vice versa. The gut is also called
alimentary canal. So food travel through the canal in following
order. Mouth-Oesophagus-Stomach-Small intestine-Large
intestine-Anus. Small intestine and large intestine are divided
into different parts. Small intestine’s first part is duodenum
where digestive enzymes are added to food. Then comes ileum
where food products are absorbed by blood. Food is digested by
carbohydrases (break poly-dis accharides), proteases break
proteins into amino acids and lipase digest lipids in fatty oil and
glycerol. In mouth, teeth break food in pieces, increasing surface
area for enzymes. Salivary glands secrete saliva (containing
amylase) in buccal cavity (mouth). Then food goes down by
oesophagus. Food travels to stomach, released there are
intervals by splinter muscles. The walls secrete Hydrochloric acid
to kill bacteria and Pespin to break down protein. Then food
travels to duodenum where pancreatic juice and bile, from
pancreas and bile duct. Bile is stored is gall bladder. It has bile
slats and emulsion, which increases surface area of lipid to be
broken down easily by lipase. Both bile and Gut neutralize the
acid contents from stomach before they pass to the gut. In small
intestine, semi-digested food passes and more enzymes are
added to break down contents until they form soluble end
products. Then they are absorbed into blood by ileum. Ileum has
many adaptions making it highly efficient to absorbing digestion
products. It is many meters in length and has villi on internal
surface, both increase surface area for absorption. Villi have
network to blood capillaries to shorten distance between lumen
of ileum and blood. The surface cells, epithelium lining the villi
absorb the digestion products and pass them across capillaries.
Glucose is taken by active transport. Villi also have lacteal, a
blind ending tube, making human’s lymphatic system. Lipids and
its products enter the lacteal and are transported around body
by blood. By the end of ileum, digestion products and food water
has been absorbed by blood. Only the indigestible fiber is left,
with some water and bacteria. In the first part of large intestine,
the colon, absorb water and minerals from the remaining faeces,
and are later stored in rectum, the last part of large intestine,
until passed out of body (egested) by anus. Blood capillaries from
villi join to form larger vessels and eventually form hepatic portal
vein. This vein transport from ileum to liver. The liver acts like a
metabolic processing works, breaking down some molecules why
storing others. Digestion products go through same operation
before entering blood circulation. Liver stores digestion glucose
as polysaccharide, glycogen, which is similar to starch structure.
A healthy person has 200g glycogen in liver. Glycogen can be
down into glucose when required. This inter conversion is under
some body hormones. Hormones, adrenaline and glucagon cause
glycogen to be broken down into glucose, increasing its
concentration in blood. Insulin hormone causes glucose to be
taken up from blood to liver cells, lowering concentration. Liver
also controls metabolism reaction of amino acids, and break
down excess proteins/other unwanted substances e.g ethanol.
Part of each acid is used for energy and rest is converted to
ammonia, further converted into urea/nitrogenous excretory
product as it contains nitrogen from amino acids. It travels from
liver to kidney, and is extracted as urine. After blood passes liver,
it enters hepatic vein, taking the blood back to the heart and
pumps it around the body. In cells, digestion products are
assimilated into cells components. Amino acids are built up into
proteins and fatty acids/glycerol are converted into lipids e.g fats
and stored ion deposits under skin and internal organ.