unit1-biology-140328092310-phpapp01.pdf

On completion of this Module, students should:
1. understand the chemical structure of water,
carbohydrates, lipids and proteins and their roles in
living organisms;
2. understand that cells are the basic units of living
organisms, grouped into tissues and organs;
3. understand fluid mosaic model of membrane
structure and the movement of substances into and out
of cells;
4. understand the mode of action of enzymes.

 To the average person, water is a common and
ordinary substance which is often taken for
granted, that is until a drought threatens crops
and drinking water supplies or a severe flood
destroys life and property.
 Most people do not understand that without
water and its unique and unusual properties, life
as we know it on earth would not exist.

 Water molecules are charged, with the oxygen
atom being slightly negative and the hydrogen
atoms being slightly positive.
 These opposite charges attract each
other, forming hydrogen bonds.
 These are weak, long distance bonds that are
very common and very important in biology

 Water has a number of important properties
essential for life. Many of the properties below
are due to the hydrogen bonds in water.
 Solvent: water is a very good solvent (because
it is charged). Charged or polar molecules such
as salts, sugars and amino acids dissolve readily
in water and so are called hydrophilic.
Uncharged or non-polar molecules such as lipids
do not dissolve so well in water and are
called hydrophobic.

 Specific heat capacity. Water has a specific heat
capacity of 4.2 J g-1 °C-1, which means that it takes 4.2
joules of energy to heat 1 g of water by 1°C. This is
unusually high and it means that water does not
change temperature very easily. This minimizes
fluctuations in temperature inside cells, and it also
means that sea temperature is remarkably constant.
 Latent heat of evaporation. Water requires a lot of
energy to change state from a liquid into a gas, and
this is made use of as a cooling mechanism in animals
(sweating and panting) and plants (transpiration). As
water evaporates it extracts heat from around it,
cooling the organism.

 Cohesion & Surface Tension: Water molecules "stick
together" due to their hydrogen bonds that form between
them. In liquid water, these bonds constantly form and
break, each lasting for only a fraction of a second.
 Cohesion makes it easy for water to move by mass flow –
that is, a large body of water can flow in the same direction
without breaking apart.
 This explains long columns of water can be sucked up tall
trees by transpiration without breaking. It also explains
surface tension, which allows small animals to walk on
water. Cohesion is also important in the flow of blood in
mammals.

 Ionization: When many salts dissolve in water they ionize
into discrete positive and negative ions (e.g. NaCl Na+ +
Cl-). Many important biological molecules are weak acids,
which also ionize in solution (e.g. acetic acid acetate- + H+).
 pH: Water itself is partly ionized (H2O H+ + OH- ), so it is a
source of protons (H+ ions), and indeed many biochemical
reactions are sensitive to pH (-log[H+]). Pure water cannot
buffer changes in H+ concentration, so is not a buffer and
can easily be any pH, but the cytoplasms and tissue fluids of
living organisms are usually well buffered at about neutral
pH (pH 7-8).

 Density: Water is unique in that the solid state (ice) is less dense
that the liquid state, so ice floats on water. As the air
temperature cools, bodies of water freeze from the surface,
forming a layer of ice with liquid water underneath. This allows
aquatic ecosystems to exist even in sub-zero temperatures.
 Water molecules in liquid water are pulled closely together by
the hydrogen bonds between them. The density of water is 1.0
gcm-3
 Most living organisms, containing lots of water, have a density
which is quite close to that of water. This makes it easy for them
to swim. Aquatic organisms have mechanisms of for changing
their density, for example by, filling or emptying parts of their
body with air – to help them to float or sink.

 Transparency: Pure water is transparent to visible
wavelengths of light. This allows aquatic
photosynthetic organisms, such as phytoplankton
and algae, to obtain enough light for
PHOTOSYNTHESIS.
 Light only travels certain depths through
water, and so the of the oceans are totally dark.

Reactivity
PHOTOSYNTHESIS & HYDROLYSIS??

 The relationship between the structure and
function of glucose;
 The relationship between the structure and
function of sucrose;
 The molecular structure of starch, glycogen and
cellulose relate to their functions in living
organisms;

 Carbohydrates are substances whose molecules
are made up of sugar units.
 The general formula for a sugar unit is CnH2nOn
 Carbohydrates include sugars, starches and
cellulose.
 Sugars are always soluble in water and taste
sweet.
 Starches and cellulose, which are both examples
of polysaccharides, are insoluble in water and do
not taste sweet.

 Carbohydrates contain only the elements carbon,
hydrogen and oxygen. The group includes monomers,
dimers and polymers, as shown in this diagram:

 Contains one sugar unit
 Cannot be further hydrolysed
 Simplest form of sugar
 All have the formula (CH2O)n
 They can be classified according to the number of carbon
atoms in each molecule.
 Trioses have 3C, Pentose 5C and hexoses 6C.
 Some examples of monosaccharides are Glyceraldehyde,
ribose, glucose, fructose and galactose.
 Glyceraldehyde is a triose and is the first carbohydrate
formed during photosynthesis.
 Ribose (pentose) is a fundamental constituent of RNA and
a similar pentose, deoxyribose, is found in DNA.

 The most common & important monosaccharide
is glucose, which is a six-carbon sugar. It's
formula is C6H12O6.
 It’s the main respiratory substrate in many cells,
providing energy when it is oxidized.
 All sugars taste sweet, and fructose is often
found in fruits and nectar (attract insects and
other animals which assists in pollination and
seed dispersal)

The sugars are all shown in their
straight chain form. However,
pentoses and hexoses are usually
also able to flip into ring form in
which the chain links up with
itself.

 Glucose forms a six-sided ring. The six carbon atoms are
numbered as shown, so we can refer to individual carbon atoms in
the structure. In animals glucose is the main transport sugar in the
blood, and its concentration in the blood is carefully controlled.
 There are many monosaccharides, with the same chemical
formula (C6H12O6), but different structural formulae. These
include fructose and galactose.

 You can see that in both of them, carbon atom 1 joins to the
oxygen on carbon atom 5.
 The ring therefore contains oxygen, but carbon atom 6 is not part
of the ring.
 In solution, glucose (and other monaccharides) molecules
continually flip between their ring and chain forms.

 The ring can exist in two forms, known as Both
alpha – glucose and beta – glucose.
 Both of them have a six membered ring, made
up of five carbons and one oxygen.
 They differ only in the orientation of the –H and
–OH group on Carbon 1.
 The alpha position is defined as the –OH being
on the opposite side of the ring from Carbon 6.
 The beta position is defined as the – OH being
on the same side as the ring as carbon 6.

Point to note:
 Glucose has 6 –OH functional groups which
forms hydrogen bonds with the water molecules.
 The –H bonding is the reason glucose, and other
sugar monomers, are soluble in water.
 Blood Plasma is primarily water, so once in the
bloodstream it is readily absorbed into the cell,
which gives it energy to function.

 Two monosaccharide molecules can link together to
form a sugar called a disaccharide. Like
monosaccharides, disaccharides are soluble in water
and taste sweet.
 For example, two alpha – glucose molecules can react
to form maltose.
 This is a condensation reaction, in which a water
molecule is removed.
 The linkage formed between the two
monosaccharides is called a glycosidic bond. The
reaction involves the formation of a molecule of water
(H2O).
 General formula: (CnH2n O2n-2)

 Glycosidic bonds involves covalent bonds, and
are very strong.
 In maltose, the glycosidic bond formed between
carbon atom 1 of one molecule and carbon atom
4 of the other.
 Both molecules are in the alpha form.
 Hence it is termed an alpha 1 – 4 glycosidic
bond.

 This kind of reaction, where water is formed, is called a condensation reaction.
The reverse process, when bonds are broken by the addition of water (e.g. in
digestion), is called a hydrolysis reaction.

There are three common disaccharides:
 Maltose (or malt sugar) is glucose & glucose. It is formed
on digestion of starch by amylase, because this enzyme
breaks starch down into two-glucose units. Brewing beer
starts with malt, which is a maltose solution made from
germinated barley. Maltose is the structure shown above.
 Sucrose (or cane sugar) is glucose & fructose. It is common
in plants because it is less reactive than glucose, and it is
their main transport sugar. It's the common table sugar
that you put in tea.
 Lactose (or milk sugar) is galactose & glucose. It is found
only in mammalian milk, and is the main source of energy
for infant mammals.

 When carbohydrates are digested, glycosidic bonds
are broken down by carbohydrase enzymes.
 The enzymes that breaks maltose apart is called
maltase.
 Sucrose, ordinary table sugar, is a disaccharide that
occurs naturally in the plant kingdom especially in
sugar cane, where it makes up 20% by weight, and in
beets, where it makes up 15% by weight.
 As a result sucrose is often known as cane sugar or
beet sugar.
 Sucrose is the main form in which carbohydrates are
transported in plants.

 A sucrose molecule is made up of an alpha glucose molecule linked to a beta
fructose molecule by a 1 – 2 glycosidic bond. This is therefore an alpha 1 – beta
2 bond.

Sucrose can be an
important source of
energy for the human
body.
The enzyme sucrase
hydrolyses sucrose to
glucose and fructose.
The fructose molecule
is then rearranged
(isomerised) to form
glucose.
Hence, every sucrose
molecule produce two
glucose molecules
that can be used in
respiration.

 Polysaccharides are long chains of many
monosaccharides joined together by glycosidic
bonds.There are three important polysaccharides:
 Starch is the plant storage polysaccharide.
 It is insoluble and forms starch granules inside many
plant cells.
 Being insoluble means starch does not change the
water potential of cells, so does not cause the cells to
take up water by osmosis (more on osmosis later).
 It is not a pure substance, but is a mixture
of amylose and amylopectin.

 Starch is how plants store the carbohydrates that they
make in photosynthesis.
 Starch is metabolically inactive, so it does not interfere
with chemical reactions inside the cell.
 However it can be easily broken down by the enzyme
amylase, which breaks the glycosidic bonds and releases
glucose that can be used in respiration.
 Starch is stored in organelles called plastids. The starch
forms grains or granules, and these have distinctive forms
in different plant species.
 Animal cells never contain starch, but they may contain a
substance that is very similar to amylopectin , called
glycogen.
 Like starch, glycogen forms an insoluble and unreactive
energy reserve in animal cells.

 Amylose is simply poly-(1-4) glucose, so is a
straight chain. In fact the chain is floppy, and it
tends to coil up into a helix.

 Amylopectin is poly(1-4) glucose with about 4%
(1-6) branches. This gives it a more open
molecular structure than amylose. Because it has
more ends, it can be broken more quickly than
amylose by amylase enzymes.

 Glycogen is similar in structure to amylopectin. It is poly (1-4)
glucose with 9% (1-6) branches. It is made by animals as their
storage polysaccharide, and is found mainly in muscle and liver.
Because it is so highly branched, it can be mobilised(broken down
to glucose for energy) very quickly.

 Glycogen tends to have even more branches than
amylopectin, and so is even less able to form spirals,
making it less dense than amylopectin.
 Like starch, glycogen forms an insoluble and
unreactive energy reserve in animal cells.
 It is even easier to break down to glucose than
amylose or amylopectin, because its branching
shape allows many enzymes to act simultaneously
on one glycogen molecule.
 This may be related to the fact that most animals
have higher metabolic rates than plants, and so may
need energy reserves such as glycogen to be
mobilised more rapidly.

 In the human body, glycogen stores are found in the
liver and in muscles, where little dark granules of
glycogen can often be seen in photomicrographs.
 They are broken down to form glucose by an
enzyme called glycogen phosphorylase, which is
activated by the hormone insulin when blood
glucose levels are low.
 The presence of starch can be indicated using
iodine. Iodine molecules can fit neatly inside the
coiled molecules of amylose, and this produces a
blue – black compound.
 Glycogen and amylopectin give a faint red colour
with iodine.

 Cellulose is only found in plants, where it is the main
component of cell walls. It is poly (1-4) glucose, but with a
different isomer of glucose. Cellulose contains beta-
glucose, in which the hydroxyl group on carbon 1 sticks up.
This means that in a chain alternate glucose molecules are
inverted.
 This allows hydrogen bonds to be formed between – OH
groups of neighbouring molecules.
 This produces bundles of molecules lying sided by side, all
held together by thousands of hydrogen bonds.
 Although each hydrogen bond is weak, there are so many of
them that they collectively for a strong structure.

 While the alpha 1-4 glucose polymer in starch
coils up to form granules, the beta 1-4 glucose
polymer in cellulose forms straight chains.

 These bundles are fibrils and they themselves form
larger bundles called fibers.
 They are structurally very strong, with high tensile
strength. This makes cell walls able to expand
without breaking when a plant cell takes up
water, enabling the cell to become turgid.
 Cellulose is difficult to digest, because very few
animals have an enzyme can break its beta 1 – 4
glycosidic bonds.
 It is therefore a good source of dietary fiber in the
human diet, remaining undigested as it passes
through the alimentary canal and stimulating
peristalsis.

 The beta-glycosidic bond cannot be broken by amylase, but requires a
specific cellulase enzyme. The only organisms that possess a cellulase
enzyme are bacteria, so herbivorous animals, like cows and termites
whose diet is mainly cellulose, have mutualistic bacteria in their guts
so that they can digest cellulose

Other polysaccharides that you may come across
include:
 Chitin (poly glucose amine), found in fungal cell
walls and the exoskeletons of insects.
 Pectin (poly galactose uronate), found in plant cell
walls.
 Agar (poly galactose sulphate), found in algae and
used to make agar plates.
 Murein (a sugar-peptide polymer), found in bacterial
cell walls.
 Lignin (a complex polymer), found in the walls of
xylem cells, is the main component of wood.

 describe the molecular structure of a triglyceride
and its role as a source of energy;
 describe the structure of phospholipids and their
role in membrane structure and function;

 Lipids are a group of substances that – like
Carbohydrates – are made up of carbon,
hydrogen and oxygen.
 They have a much higher proportion of hydrogen
than carbohydrates. They do not have dipoles and
so are insoluble in water.
 Lipids include fats, which tend to be solid at room
temperatures, and oils, which tend to be liquid.
 In general, animals produce mostly fats and
plants mostly oils, although they are many
exceptions to this rule.

 Glycerol is a small, 3-carbon molecule with
three hydroxyl groups.

 Triglycerides are made up of three fatty acids
attached to a glycerol molecule.

 Fatty acids are acids because they contain a carboxyl
group – COOH.
 The carboxyl groups of fatty acids are able to react with
the – OH (hydroxyl) groups of glycerol, forming ester
bonds. These linkages involve covalent bonds, and so are
very strong.
 As with the formation of peptide bonds and glycosidic
bonds, this is a condensation reaction.
 The breakage of an ester bond is a hydrolysis reaction.
 Triglycerides are insoluble in water.
 This is because none of their atoms carries an electrical
charge, and so they are not attracted to water molecules.
They are said to be hydrophobic.

 Fatty acids, and the fats in which they are found, can
be classified as saturated and unsaturated.
 A saturated fat is one in which the fatty acids all
contain as much hydrogen as they can.
 Each carbon atom in the fatty acid ‘tail’ is linked to its
neighbouring carbon atom by single bonds, while the
other two bonds are linked to hydrogen atoms.
 An unsaturated fat, however, has one or more fatty
acids in which at least one carbon atom is using two
of its bonds to link to a neighbouring carbon
atom, so it has one bond to spare link to hydrogen.
 This double carbon – carbon bond forms a ‘kink’ in
the chain.

 If there are no C=C double bonds in the hydrocarbon
chain, then it is a saturated fatty acid (i.e. saturated
with hydrogen). These fatty acids form straight
chains, and have a high melting point.
 If there are C=C double bonds in the hydrocarbon
chain, then it is an unsaturated fatty acid (i.e.
unsaturated with hydrogen). These fatty acids form
bent chains, and have a low melting point. Fatty acids
with more than one double bond are called poly-
unsaturated fatty acids (PUFAs).

 Triglycerides are rich in energy, and they are often
used as energy stores in living organisms.
 One gram of triglycerides can release twice as much
energy as one gram of carbohydrate when it is
respired, so they make compact and efficient stores.
 In humans, cells in a tissue called adipose tissue are
almost filled with globules of triglycerides, and they
make very good thermal insulators.
 Animals that live in cold environments, such as whales
and polar bears, often have especially thick layers of
adipose tissue beneath the skin.
 Stored triglycerides also provide a place in which fat
soluble vitamins, especially vitamin D and vitamin A,
can be stored.

 Carbohydrates can be mobilized more quickly, and
glycogen is stored in muscles and liver for immediate
energy requirements.
 Triglycerides containing saturated fatty acids have a
high melting point and tend to be found in warm-
blooded animals. At room temperature they are
solids (fats), e.g. butter, lard.
 Triglycerides containing unsaturated fatty acids have
a low melting point and tend to be found in cold-
blooded animals and plants. At room temperature
they are liquids (oils), e.g. fish oil, vegetable oils.

 Phospholipids have a similar structure to
triglycerides, but with a phosphate group in place of
one fatty acid chain.
 There may also be other groups attached to the
phosphate.
 Phospholipids have a polar hydrophilic "head" (the
negatively-charged phosphate group) and two non-
polar hydrophobic "tails" (the fatty acid chains).
 This mixture of properties is fundamental to biology,
for phospholipids are the main components of cell
membranes.

The phosphate group has a
negative electrical charge on it,
which is attracted to the tiny
positive electrical charge on the
hydrogen atoms in a water
molecule.
When it is in water, the two ends of
a phospholipids molecule do
different things.
The phosphate is drawn towards
water molecules and dissolves in
them.
The fatty acids are repelled by
water molecules and avoid them.
In water the phospholipids
molecules arrange themselves in a
sheet called a
Bilayer

 A phospholipid bilayer like this is the basic
structure of a cell membrane.
 Phospholipids are one of the most important
molecules in a cell.
 Without them, there could be no plasma
membrane and the cell would simply cease to
exist.

 When mixed with
water, phospholipids
form droplet spheres
with the hydrophilic
heads facing the water
and the hydrophobic
tails facing each other.
This is called a micelle.

 Alternatively, they may
form a double-
layered phospholipid
bilayer.This traps a
compartment of water in
the middle separated
from the external water
by the hydrophobic
sphere.This naturally-
occurring structure is
called a liposome, and is
similar to a membrane
surrounding a cell.

 Cholesterol and other substances with similar
structures, which are formed from it, are called
steroids.

 They are a huge number of different kinds of
steroids in the body.
 Many of them are hormones – for
example, testosterones and oestrogen.
 Cholesterol itself is a major constituent of cell
membranes where it helps to regulate the
fluidity of the membrane.

 describe the generalized structure of an amino
acid, and the formation and breakage of a
peptide bond;
 explain the meaning of the terms: primary,
secondary, tertiary and quaternary structures of
proteins;
 outline the molecular structure of haemoglobin, as
an example of a globular protein, and of collagen,
as an example of a fibrous protein;

Proteins are the most complex and most diverse group of biological
compounds.They have an astonishing range of different functions, as this
list shows.
 structure e.g. collagen (bone, cartilage, tendon), keratin (hair), actin
(muscle)
 enzymes e.g. amylase, pepsin, catalase, etc (>10,000 others)
 transport e.g. haemoglobin (oxygen), transferrin (iron)
 pumps e.g. Na+K+ pump in cell membranes
 motors e.g. myosin (muscle), kinesin (cilia)
 hormones e.g. insulin, glucagon
 receptors e.g. rhodopsin (light receptor in retina)
 antibodies e.g. immunoglobulins
 storage e.g. albumins in eggs and blood, caesin in milk
 blood clotting e.g. thrombin, fibrin
 lubrication e.g. glycoproteins in synovial fluid
 toxins e.g. diphtheria toxin
 antifreeze e.g. glycoproteins in arctic flea
 and many more!

 Proteins are made of amino acids. Amino acids
are made of the five elementsC H O N S.There is
a central carbon atom (called the "alpha carbon"),
with four different chemical groups attached to
it:
 a hydrogen atom
 a basic amino group
 an acidic carboxyl group
 a variable "R" group (or side chain)

Amino acids are so-called
because they have both amino
groups and acid groups, which
have opposite charges. At
neutral pH (found in most living
organisms), the groups are
ionized as shown right, so there
is a positive charge at one end of
the molecule and a negative
charge at the other end. The
overall net charge on the
molecule is therefore zero. A
molecule like this, with both
positive and negative charges is
called a zwitterion.
ZWITTERION HUH!!!!!

 The charge on the amino acid changes with pH:
LOW PH (ACID) NEUTRAL PH HIGH PH (ALKALI)
Charge = +1 Charge = 0 Charge = -1
It is these changes in charge with pH that explain the effect of pH on enzymes.

 A solid, crystallized amino acid has the uncharged structure;
 However this form never exists in solution, and therefore does
not exist in living organisms (although it is the form usually given
in textbooks).
 There are 20 different R groups, and so 20 different amino acids.
Since each R group is slightly different, each amino acid has
different properties, and this in turn means that proteins can
have a wide range of properties.

 The following table shows the 20 different R groups,
grouped by property, which gives an idea of the range of
properties.
 You do not need to learn these, but it is interesting to see
the different structures, and you should be familiar with
the amino acid names.
 You may already have heard of some, such as the food
additive monosodium glutamate, which is simply the
sodium salt of the amino acid glutamate.
 Be careful not to confuse the names of amino acids with
those of bases in DNA, such as cysteine (amino acid) and
cytosine (base), threonine (amino acid) and thymine
(base). There are 3-letter and 1-letter abbreviations for
each amino acid.

THE TWENTY AMINO ACID R-GROUPS (FOR INTEREST ONLY NO
KNOWLEDGE REQUIRED)
SIMPLE R GROUPS BASIC R GROUPS
Glycine
Gly G
Lysine
Lys K
Alanine
Ala A
Arginine
Arg R
Valine
Val V
Histidine
His H
Leucine
Leu L
Asparagine
Asn N
Isoleucine
Ile I
Glutamine
Gln Q
HYDROXYL R GROUPS ACIDIC R GROUPS
Serine
Ser S
Aspartate
Asp D
Threonine
Thr T
Glutamate
Glu E
SULPHUR R GROUPS RINGED R GROUPS
Cysteine
Cys C
Phenylalanine
Phe F

Methionine
Met M
Tyrosine
Tyr Y
CYCLIC R GROUP
Proline
Pro P
Tryptophan
Trp W

Polypeptides
Amino acids are
joined together
by peptide
bonds. The
reaction involves
the formation of a
molecule of water
in another
condensation
polymerization
reaction as shown
right.

 When two amino acids join together a dipeptide is
formed.Three amino acids form a tripeptide. Many
amino acids form a polypeptide. e.g.:
 +NH3-Gly — Pro — His — Leu —Tyr — Ser —Trp —
Asp — Lys — Cys-COO-
 In a polypeptide there is always one end with a free
amino (NH2) (NH3 in solution) group, called the N-
terminus, and one end with a free carboxyl (COOH)
(COO in solution) group, called the C-terminus.

 Polypeptides are just a string of amino acids, but they
fold up to form the complex and well-defined three-
dimensional structure of working proteins.
 To help to understand protein structure, it is broken
down into four levels:
1. Primary
2. Secondary
3. Tertiary
4. Quaternary

1. Primary Structure
 This is just the sequence of amino acids in the
polypeptide chain, so is not really a structure at all.
 The primary structure does determine the rest of the
protein structure.
 Finding the primary structure of a protein is
called protein sequencing, and the first protein to
be sequenced was the protein hormone insulin, by
the Cambridge biochemist Fredrick Sanger, for
which work he got the Nobel prize in 1958.

2. Secondary Structure
 This is the most basic level of protein folding, and
consists of a few basic motifs that are found in all
proteins. The secondary structure is held together by
hydrogen bonds between the carboxyl groups and the
amino groups in the polypeptide backbone. The two
secondary structures are the a-helix and the b-sheet

The a-helix. The polypeptide
chain is wound round to form a
helix. It is held together by
hydrogen bonds running
parallel with the long helical
axis.There are so many
hydrogen bonds that this is a
very stable and strong
structure. Helices are common
structures throughout biology.

 The b-sheet. The
polypeptide chain zig-zags
back and forward forming a
sheet. Once again it is held
together by hydrogen
bonds.

3.Tertiary Structure
 This is the 3 dimensional structure formed by the folding up of a
whole polypeptide chain. Every protein has a unique tertiary
structure, which is responsible for its properties and function.
For example the shape of the active site in an enzyme is due to
its tertiary structure.The tertiary structure is held together by
bonds between the R groups of the amino acids in the
protein, and so depends on the sequence of amino acids in the
protein structure.There are three kinds of bonds involved:
 hydrogen bonds, which are weak.
 ionic bonds between R-groups with positive or negative charges, which
are quite strong.
 sulphur bridges - covalent S-S bonds between two cysteine amino
acids, which are strong

4. Quaternary Structure
 This structure is found only in proteins containing more than one
polypeptide chain, and simply means how the different
polypeptide chains are arranged together. The individual
polypeptide chains are usually globular, but can arrange
themselves into a variety of quaternary shapes. e.g.:
Haemoglobin, the oxygen-carrying
protein in red blood cells, consists of
four globular subunits arranged in a
tetrahedral (pyramid) structure. Each
subunit contains one iron atom and
can bind one molecule of oxygen

The final three-dimensional shape of a protein can be classified
as globular or fibrous.
globular structure fibrous (or filamentous) structure
The vast majority of proteins are globular, including enzymes, membrane proteins,
receptors, storage proteins, etc. Fibrous proteins look like ropes and tend to have
structural roles such as collagen (bone), keratin (hair), tubulin (cytoskeleton) and actin
(muscle). They are usually composed of many polypeptide chains. A few proteins have
both structures: the muscle protein myosin has a long fibrous tail and a globular head,
which acts as an enzyme.

This diagram shows part
of a molecule of collagen,
which is found in bone
and cartilage. It has a
unique, very strong
triple-helix structure. It is
a fibrous protein

 All living things are made of cells, and cells are the
smallest units that can be alive.
 Life on Earth is classified into five kingdoms, and they
each have their own characteristic kind of cell.
 However, the biggest division is between the cells of the
prokaryote kingdom (the bacteria) and those of the
other four kingdoms (animals, plants, fungi and
protoctista), which are all eukaryotic cells.
 Prokaryotic cells are smaller and simpler than
eukaryotic cells, and do not have a nucleus
 Prokaryote = without a nucleus
 Eukaryote = with a nucleus

EukaryoticCell
Cytoplasm (or Cytosol).This is the
solution within the cell membrane. It
contains enzymes for metabolic reactions
together with sugars, salts, amino
acids, nucleotides and everything else
needed for the cell to function.
Nucleus.This is the largest organelle.
Surrounded by a nuclear envelope, which
is a double membrane with nuclear pores -
large holes containing proteins that
control the exit of substances such as RNA
from the nucleus.The interior is called the
nucleoplasm, which is full of chromatin- a
DNA/protein complex containing the
genes. During cell division the chromatin
becomes condensed into discrete
observable chromosomes.The nucleolus is
a dark region of chromatin, involved in
making ribosomes.
Mitochondrion (pl. Mitochondria). This is
a sausage-shaped organelle (8µm
long), and is where aerobic respiration
takes place in all eukaryotic cells.
Mitochondria are surrounded by a double
membrane: the outer membrane is
simple, while the inner membrane is
highly folded into cristae, which give it a
large surface area.The space enclosed by
the inner membrane is called the
matrix, and contains small circular strands
of DNA.The inner membrane is studded
with stalked particles, which are the site of
ATP synthesis.

Chloroplast. Bigger and fatter than mitochondria,
chloroplasts are where photosynthesis takes place,
so are only found in photosynthetic organisms
(plants and algae). Like mitochondria they are
enclosed by a double membrane, but chloroplasts
also have a third membrane called the thylakoid
membrane. The thylakoid membrane is folded into
thylakoid disks, which are then stacked into piles
called grana. The space between the inner
membrane and the thylakoid is called the stroma.
The thylakoid membrane contains chlorophyll and
stalked particles, and is the site of photosynthesis
and ATP synthesis. Chloroplasts also contain starch
grains, ribosomes and circular DNA.
Ribosomes. These are the smallest and most
numerous of the cell organelles, and are the sites of
protein synthesis. They are composed of protein and
RNA, and are manufactured in the nucleolus of the
nucleus. Ribosomes are either found free in the
cytoplasm, where they make proteins for the cell's
own use, or they are found attached to the rough
endoplasmic reticulum, where they make proteins
for export from the cell. They are often found in
groups called polysomes. All eukaryotic ribosomes
are of the larger, "80S", type.
Smooth Endoplasmic Reticulum (SER). Series of
membrane channels involved in synthesising and
transporting materials, mainly lipids, needed by the
cell.
Rough Endoplasmic Reticulum (RER). Similar to
the SER, but studded with numerous ribosomes,
which give it its rough appearance. The ribosomes
synthesise proteins, which are processed in the RER
(e.g. by enzymatically modifying the polypeptide
chain, or adding carbohydrates), before being
exported from the cell via the Golgi Body.

Golgi Body (or Golgi Apparatus). Another series of
flattened membrane vesicles, formed from the
endoplasmic reticulum. Its job is to transport proteins
from the RER to the cell membrane for export. Parts of the
RER containing proteins fuse with one side of the Golgi
body membranes, while at the other side small vesicles
bud off and move towards the cell membrane, where they
fuse, releasing their contents by exocytosis.
Vacuoles. These are membrane-bound sacs containing
water or dilute solutions of salts and other solutes. Most
cells can have small vacuoles that are formed as required,
but plant cells usually have one very large permanent
vacuole that fills most of the cell, so that the cytoplasm
(and everything else) forms a thin layer round the outside.
Plant cell vacuoles are filled with cell sap, and are very
important in keeping the cell rigid, or turgid. Some
unicellular protoctists have feeding vacuoles for digesting
food, or contractile vacuoles for expelling water.
Lysosomes. These are small membrane-bound vesicles
formed from the RER containing a cocktail of digestive
enzymes. They are used to break down unwanted
chemicals, toxins, organelles or even whole cells, so that
the materials may be recycled. They can also fuse with a
feeding vacuole to digest its contents.
Cytoskeleton. This is a network of protein fibres
extending throughout all eukaryotic cells, used for
support, transport and motility. The cytoskeleton is
attached to the cell membrane and gives the cell its shape,
as well as holding all the organelles in position. There are
three types of protein fibres (microfilaments, intermediate
filaments and microtubules), and each has a
corresponding motor protein that can move along the
fibre carrying a cargo such as organelles, chromosomes or
other cytoskeleton fibres. These motor proteins are
responsible for such actions as: chromosome movement in
mitosis, cytoplasm cleavage in cell division, cytoplasmic
streaming in plant cells, cilia and flagella movements, cell
crawling and even muscle contraction in animals.

Centriole. This is a pair of short microtubules
involved in cell division.
Cilium and Flagellum. These are flexible tails
present in some cells and used for motility. They
are an extension of the cytoplasm, surrounded by
the cell membrane, and are full of microtubules
and motor proteins so are capable of complex
swimming movements. There are two kinds:
flagella (pl.) (no relation of the bacterial flagellum)
are longer than the cell, and there are usually only
one or two of them, while cilia (pl.) are identical in
structure, but are much smaller and there are
usually very many of them.
Microvilli. These are small finger-like extensions of
the cell membrane found in certain cells such as in
the epithelial cells of the intestine and kidney,
where they increase the surface area for
absorption of materials. They are just visible
under the light microscope as a brush border.
Cell Membrane (or Plasma Membrane). This is a
thin, flexible layer round the outside of all cells
made of phospholipids and proteins. It separates
the contents of the cell from the outside
environment, and controls the entry and exit of
materials. The membrane is examined in detail
later.
Cell Wall. This is a thick layer outside the cell
membrane used to give a cell strength and
rigidity. Cell walls consist of a network of fibres,
which give strength but are freely permeable to
solutes (unlike membranes). Plant cell walls are
made mainly of cellulose, but can also contain
hemicellulose, pectin, lignin and other
polysaccharides. There are often channels through
plant cell walls called plasmodesmata, which link
the cytoplasms of adjacent cells. Fungal cell walls
are made of chitin. Animal cells do not have a cell
wall.

ProkaryoticCell
Cytoplasm:Contains all the enzymes
needed for all metabolic reactions,
since there are no organelles
Ribosomes.:The smaller (70 S) type.
NuclearZone:The region of the
cytoplasm that contains DNA. It is
not surrounded by a nuclear
membrane.
DNA:Always circular, and not
associated with any proteins to form
chromatin.
Plasmid:Small circles of DNA, used
to exchange DNA between bacterial
cells, and very useful for genetic
engineering.
Cell membrane: made of
phospholipids and proteins, like
eukaryotic membranes.
Mesosome: A tightly-folded region
of the cell membrane containing all
the membrane-bound proteins
required for respiration and
photosynthesis.

CellWall: Made of murein, which is a
glycoprotein (i.e. a
protein/carbohydrate complex).
There are two kinds of cell
wall, which can be distinguished by a
Gram stain: Gram positive bacteria
have a thick cell wall and stain
purple, while Gram negative bacteria
have a thin cell wall with an outer
lipid layer and stain pink.
Capsule (or Slime Layer): A thick
polysaccharide layer outside of the
cell wall. Used for sticking cells
together, as a food reserve, as
protection against desiccation and
chemicals, and as protection against
phagocytosis.
Flagellum: A rigid rotating helical-
shaped tail used for propulsion.The
motor is embedded in the cell
membrane and is driven by a H+
gradient across the membrane.
Clockwise rotation drives the cell
forwards, while anticlockwise
rotation causes a chaotic spin.This is
an example of a rotating motor in
nature.

PROKARYOTIC CELLS EUKARYOTIC CELLS
small cells (< 5 mm) larger cells (> 10 mm)
always unicellular often multicellular
no nucleus or any membrane-bound organelles always have nucleus and other membrane-bound organelles
DNA is circular, without proteins DNA is linear and associated with proteins to form chromatin
ribosomes are small (70S) ribosomes are large (80S)
no cytoskeleton always has a cytoskeleton
cell division is by binary fission cell division is by mitosis or meiosis
reproduction is always asexual reproduction is asexual or sexual

 Prokaryotic cells are far older and more diverse
than eukaryotic cells.
 Prokaryotic cells have probably been around for
3.5 billion years - 2.5 billion years longer than
eukaryotic cells.
 It is thought that eukaryotic cell organelles like
mitochondria and chloroplasts are derived from
prokaryotic cells that became incorporated inside
larger prokaryotic cells.

 This idea is called endosymbiosis, and is supported
by these observations:
 organelles contain circular DNA, like bacteria cells.
 organelles contain 70S ribosomes, like bacteria cells.
 organelles have double membranes, as though a
single-membrane cell had been engulfed and
surrounded by a larger cell.
 Mitochondria and chloroplast reproduce by binary
fission, like bacteria, before the whole eukaryotic
cell divides by mitosis.
 Mitochondria and chloroplasts are often very similar
in size to prokaryotic cells.

 We can see examples of symbiosis between
prokaryotes and eukaryotes happening today.
For example, nitrogen fixing bacteria, such as
Rhizobium, live inside the root cells of
leguminous and other plants. They are still
clearly bacteria, and we can also find these cells
living freely in the soil. Perhaps one day they
will have evolved to look less like cells and more
like organelles within the eukaryotic cell.

 In a multicellular organism such as an animal or
plant, different cells are specialized to carry out
particular functions.
 Each cell in the organism has an identical set of
genes, but only some of these genes are
expressed (‘switched on’) in any one cell.

 The cell membrane (or plasma membrane)
surrounds all living cells.
 It controls how substances can move in and out of
the cell and is responsible for many other
properties of the cell as well.
 The membranes that surround the nucleus and
other organelles are almost identical to the cell
membrane.
 Membranes are composed of
phospholipids, proteins and carbohydrates
arranged in a fluid mosaic structure.

 The phospholipids are arranged in a bilayer, with their
polar, hydrophilic phosphate heads facing outwards, and
their non-polar, hydrophobic fatty acid tails facing each
other in the middle of the bilayer.
 This hydrophobic layer acts as a barrier to all but the
smallest molecules, effectively isolating the two sides of
the membrane.
 Different kinds of membranes can contain phospholipids
with different fatty acids, affecting the strength and
flexibility of the membrane, and animal cell membranes
also contain cholesterol linking the fatty acids together
and so stabilizing and strengthening the membrane.

 The proteins usually span from one side of the
phospholipid bilayer to the other (intrinsic proteins), but
can also sit on one of the surfaces (extrinsic proteins).
 They can slide around the membrane very quickly and
collide with each other, but can never flip from one side to
the other.
 The proteins have hydrophilic amino acids in contact with
the water on the outside of membranes, and hydrophobic
amino acids in contact with the fatty chains inside the
membrane.
 Proteins comprise about 50% of the mass of
membranes, and are responsible for most of the
membrane's properties.

 The carbohydrates are found on the outer surface of all
eukaryotic cell membranes, and are usually attached to
the membrane proteins. Proteins with carbohydrates
attached are called glycoproteins.
 The carbohydrates are short polysaccharides composed
of a variety of different monosaccharaides, and form
a cell coat or glycocalyx outside the cell membrane.
 The glycocalyx is involved in protection and cell
recognition, and antigens such as the ABO antigens on
blood cells are usually cell-surface glycoproteins.
 Remember that a membrane is not just a lipid bilayer, but
comprises the lipid, protein and carbohydrate parts.

 Cell membranes are a barrier to most substances, and this
property allows materials to be concentrated inside cells,
excluded from cells, or simply separated from the outside
environment. This is compartmentalization is essential for
life, as it enables reactions to take place that would
otherwise be impossible. Eukaryotic cells can also
compartmentalize materials inside organelles. Obviously
materials need to be able to enter and leave cells, and
there are five main methods by which substances can
move across a cell membrane:
1. Simple Diffusion
2. Osmosis
3. Facilitated Diffusion
4. ActiveTransport
5. Vesicles

A few substances can diffuse directly through the lipid bilayer part of the membrane.
The only substances that can do this are lipid-soluble molecules such as steroids, or
very small molecules, such as H2O, O2 and CO2. For these molecules the membrane is
no barrier at all. Since lipid diffusion is (obviously) a passive diffusion process, no
energy is involved and substances can only move down their concentration gradient.
Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.

 Osmosis is the diffusion of water across a membrane.
 It is in fact just normal lipid diffusion, but since water is
so important and so abundant in cells (its
concentration is about 50 M), the diffusion of water has
its own name - osmosis.
 The contents of cells are essentially solutions of
numerous different solutes, and the more
concentrated the solution, the more solute molecules
there are in a given volume, so the fewer water
molecules there are.
 Water molecules can diffuse freely across a
membrane, but always down their concentration
gradient, so water therefore diffuses from a dilute to a
concentrated solution.

Water Potential.Osmosis can be
quantified using water potential, so
we can calculate which way water will
move, and how fast. Water potential
(Y, the Greek letter psi, pronounced
"sy") is a measure of the water
molecule potential for movement in a
solution. It is measured in units of
pressure (Pa, or usually kPa), and the
rule is that water always moves by
osmosis from less negative to more
negative water potential (in other
words it's a bit like gravity potential
or electrical potential). 100% pure
water hasY = 0, which is the highest
possible water potential, so all
solutions haveY < 0 (i.e. a negative
number), and you cannot getY > 0.

 Cells and Osmosis.The concentration (or OP) of the solution that surrounds a
cell will affect the state of the cell, due to osmosis.There are three possible
concentrations of solution to consider:
 Isotonic solution a solution of equal OP (or concentration) to a cell
 Hypertonic solution a solution of higher OP (or concentration) than a cell
 Hypotonic solution a solution of lower OP (or concentration) than a cell
 The effects of these solutions on cells are shown in this diagram:

 The diagram below shows what happens when 2 fresh raw eggs with their
shells removed with acid are placed into sucrose solution (hypertonic) and
distilled water (hypotonic).Water enters the egg in water (endosmosis) causing
it to swell and water leaves the egg in sucrose causing it to shrink (exosmosis).

 Facilitated diffusion is the transport of substances across a membrane by a
trans-membrane protein molecule. The transport proteins tend to be
specific for one molecule (a bit like enzymes), so substances can only cross
a membrane if it contains the appropriate protein. As the name suggests,
this is a passive diffusion process, so no energy is involved and substances
can only move down their concentration gradient. There are two kinds of
transport protein:
 Channel Proteins form a water-filled pore or channel in the membrane.
This allows charged substances (usually ions) to diffuse across membranes.
Most channels can be gated (opened or closed), allowing the cell to control
the entry and exit of ions.
 Carrier Proteins have a binding site for a specific solute and constantly flip
between two states so that the site is alternately open to opposite sides of
the membrane. The substance will bind on the side where it at a high
concentration and be released where it is at a low concentration.

The rate of diffusion of a substance across a membrane increases as its concentration
gradient increases, but whereas lipid diffusion shows a linear relationship, facilitated
diffusion has a curved relationship with a maximum rate. This is due to the rate being
limited by the number of transport proteins.

 Active transport is the pumping of substances across a
membrane by a trans-membrane protein pump molecule.
 The protein binds a molecule of the substance to be
transported on one side of the membrane, changes shape, and
releases it on the other side.
 The proteins are highly specific, so there is a different protein
pump for each molecule to be transported.
 The protein pumps are also ATPase enzymes, since they
catalyze the splitting of ATP into ADP + phosphate (Pi), and use
the energy released to change shape and pump the molecule.
 Pumping is therefore an active process, and is the only
transport mechanism that can transport substances up their
concentration gradient.

 This transport protein is present in the cell membranes of all
animal cells and is the most abundant and important of all
membrane pumps.

 Enzymes are protein molecules that can be defined as
biological catalysts. A catalyst is a substance that speeds up a
chemical reaction, but remains unchanged at the end of the
reaction.There are about 40,000 different enzymes in human
cells, each controlling a different chemical reaction.
 Metabolism is defined as all the chemical reactions that take
place within an organism’s body. Some metabolic reactions are
anabolic reactions – they involve the joining together of small
molecules to produce large ones. An example of an anabolic
reaction is the synthesis of protein molecules on ribosomes, by
the formation of peptide bonds between amino groups.
 Anabolic reactions require input of energy. Other metabolic
reactions are catabolic reactions – they involve breaking large
molecules down to produce smaller molecules.

 An example of a catabolic reaction is respiration, in
which glucose is broken down to carbon dioxide and
water in a series of many small steps. Catabolic reactions
release energy. Both anabolic and catabolic reactions are
catalyzed by enzymes.
 They increase the rate of reactions by a factor of
between 106 to 1012 times, allowing the chemical
reactions that make life possible to take place at normal
temperatures.
 They were discovered in fermenting yeast in 1900 by
Buchner, and the name enzyme means "in yeast".As well
as catalyzing all the metabolic reactions of cells (such as
respiration, photosynthesis and digestion), they also act
as motors, membrane pumps and receptors.

 Enzymes are proteins, and their function is determined by
their complex structure.
 The reaction takes place in a small part of the enzyme
called the active site, while the rest of the protein acts as
"scaffolding".
 This is shown in this diagram of a molecule of the
enzyme amylase, with a short length of starch being
digested in its active site.
 The amino acids around the active site attach to the
substrate molecule and hold it in position while the
reaction takes place.This makes the enzyme specific for
one reaction only, as other molecules won't fit into the
active site.

 Many enzymes need cofactors (or coenzymes)
to work properly.These can be metal ions (such
as Fe2+, Mg2+, Cu2+) or organic molecules (such
as haem, biotin, FAD, NAD or coenzymeA).
Many of these are derived from dietary
vitamins, which is why they are so important.
 The complete active enzyme with its cofactor is
called a holoenzyme, while just the protein part
without its cofactor is called the apoenzyme.

 Some enzymes act inside cells, and are known as intracellular
enzymes. Examples include hydrolases found inside
lysosomes, which hydrolyze (break down) substances that a cell
has taken in by phagocytosis. ATPases are also intracellular
enzymes found, for example, inside mitochondria where they are
involved in the synthesis of ATP during aerobic respiration.
 Some enzymes act outside cells, and are known as extracellular
enzymes. These includes the digestive enzymes in the alimentary
canal, such as amylase, which hydrolyses starch to maltose.
 Enzymes are globular proteins. Like all globular
proteins, enzymes molecules are coiled into a precise three –
dimensional shape – their tertiary structure – with hydrophilic R
groups (side chains) on the outside e of the molecules, making
them soluble in water.

 Enzyme molecules also have a special feature that
in that they posses an active site. The active site of
an enzyme is a region, usually a cleft or
depression, to which another particular molecule
can bind.
 This molecule is the substrate of the enzyme. The
shape of the active site allows the substrate to fit
perfectly, and to be held in place by temporary
bonds that form between the substrate and some
of the R groups of the enzyme’s amino acids.
 This combined structure is called the enzyme –
substrate complex.

 Each type of enzyme will usually act on one type of
substrate molecule. This is because the shape of the
active site will allow only one shape of molecule to fit, like
a key fitting into a lock.
 In most enzymes, when the substrate fits into the active
site, the shape of the whole enzyme changes slightly so
that it can accommodate and hold the substrate in the
right position for the reaction to occur.
 This is called induced fit – the arrival of the substrate
molecule causes a change in the shape of the enzyme.
Whether it works by a simple lock-and-key mechanism,
or by induced fit, the enzyme and its substrate must be a
perfect match. The enzyme is said to be specific for this
substrate.

 The enzyme may catalyze a reaction in which the
substrate molecule is split into two or more
molecules.
 Alternatively, it may catalyze the joining together
of two molecules – for example, when linking
amino acids to for polypeptides during protein
synthesis on ribosomes.
 Interaction between the R groups of the enzyme
and the atoms of the substrate can break, or
encourage formation of, bonds in the substrate
molecule. As a result, one, two or more products
are formed.

 When the reaction is complete, the product(s)
leave the active site. The enzyme is unchanged by
the process, so it is now available to receive
another substrate molecule.
 The rate at which substrate molecules can bind to
the enzyme active site, be formed into products
and leave can be very rapid. A molecule of the
enzyme catalase for example, can bind with
hydrogen peroxide molecules, split them into
water and oxygen and release these products at a
rate of 107 molecules per second.

 In contrast, one of the enzymes involved in
photosynthesis called rubisco, can only deal
with three molecules per second. The maximum
number of substrate molecules that can be
converted to products per minute is know as the
enzyme’s turnover number.

 There are three ways of thinking about enzyme
catalysis. They all describe the same
process, though in different ways, and you
should know about each of them.
1. Reaction Mechanism
 In any chemical reaction, a substrate (S) is
converted into a product (P):
 S P

 (There may be more than one substrate and more than one
product, but that doesn't matter here.) In an enzyme-catalyzed
reaction, the substrate first binds to the active site of the
enzyme to form an enzyme-substrate (ES) complex, then the
substrate is converted into product while attached to the
enzyme, and finally the product is released.This mechanism
can be shown as:
E + S ES EP E + P
 The enzyme is then free to start again.The end result is the
same (S P), but a different route is taken, so that the S P
reaction as such never takes place. In by-passing this step, the
reaction can be made to happen much more quickly.

2. Molecule Geometry
 The substrate molecule fits into the active site of the
enzyme molecule like a key fitting into a lock (in fact it is
sometimes called a lock and key mechanism).
 Once there, the enzyme changes shape slightly, distorting
the molecule in the active site, and making it more likely
to change into the product.
 For example if a bond in the substrate is to be broken, that
bond might be stretched by the enzyme, making it more
likely to break.Alternatively the enzyme can make the
local conditions inside the active site quite different from
those outside (such as pH, water concentration, charge),
so that the reaction is more likely to happen.

 It's a bit more complicated than that though. Although
enzymes can change the speed of a chemical reaction, they
cannot change its direction, otherwise they could make
"impossible" reactions happen and break the laws of
thermodynamics.
 So an enzyme can just as easily turn a product into a substrate
as turn a substrate into a product, depending on which way the
reaction would go anyway.
 In fact the active site doesn't really fit the substrate (or the
product) at all, but instead fits a sort of half-way house, called
the transition state. When a substrate (or product) molecule
binds, the active site changes shape and fits itself around the
molecule, distorting it into forming the transition state, and so
speeding up the reaction.This is sometimes called the induced
fit mechanism.

3. Energy Changes
 The way enzymes work can also be shown by
considering the energy changes that take place during a
chemical reaction. We shall consider a reaction where
the product has a lower energy than the substrate, so
the substrate naturally turns into product (in other
words the equilibrium lies in the direction of the
product).

Before it can change into
product, the substrate must
overcome an “energy barrier” called
activation energy (EA) . The larger
the activation energy, the slower the
reaction will be because only a few
substrate molecules will by chance
have sufficient energy to overcome
the activation energy barrier.
Imagine pushing boulders over a
hump before they can roll down
hill, and you have the idea. Most
physiological reaction have large
activation energies, so they simply
don’t happen on a useful time scale.
Enzymes dramatically reduce the
activation energy of a reaction, so
that most molecules can easily get
over the activation energy barrier
and quickly turn into product.
For example for the catalase reaction
(2H2O2 2H2O + O2) the activation
energy is 86 kJ mol-1 with no
catalyst, 62 kJ mol-1 with an inorganic
catalyst of iron filings, and just 1 kJ
mol-1 in the presence of the enzyme

 The activation energy is actually the energy
required to form the transition state, so
enzymes lower the activation energy by
stabilizing the transition state, and they do this
by changing the conditions within the active site
of the enzyme. So the three ideas above are
really three ways of describing the same
process.

1. Temperature
 Enzymes have an optimum temperature at which they work
fastest. For mammalian enzymes this is about 40°C, but
there are enzymes that work best at very different
temperatures, e.g. enzymes from the arctic snow flea work at
-10°C, and enzymes from thermophilic bacteria work at 90°C.
 Up to the optimum temperature the rate increases
geometrically with temperature (i.e. it's a curve, not a
straight line). The rate increases because the enzyme and
substrate molecules both have more kinetic energy so collide
more often, and also because more molecules have sufficient
energy to overcome the (greatly reduced) activation energy.

 The increase rate at which temperature can be
quantified as Q10, which is the relative increase for a
10 degrees rise in temperature.
 Q10 is usually 2-3 for enzyme-catalyzed reactions (i.e.
the rate doubles every 10°C) and usually less than 2
for non-enzyme reactions.
 The rate is not zero at 0°C, so enzymes still work in
the fridge (and food still goes off), but they work
slowly. Enzymes can even work in ice, though the
rate is extremely slow due to the very slow diffusion
of enzyme and substrate molecules through the ice
lattice.

 Above the optimum temperature the rate decreases as
more and more of the enzyme molecules denature.The
thermal energy breaks the hydrogen bonds holding the
secondary and tertiary structure of the enzyme
together, so the enzyme (and especially the active site)
loses its shape to become a random coil.The substrate
can no longer bind, and the reaction is no longer
catalysed. At very high temperatures this is irreversible.
Remember that only the weak hydrogen bonds are
broken at these mild temperatures; to break strong
covalent bonds you need to boil in concentrated acid for
many hours.

 The Q10 temperature coefficient is a measure
of the rate of change of a biological or chemical
system as a consequence of increasing the
temperature by 10 °C.
 There are many examples where the Q10 is
used, one being the calculation of the nerve
conduction velocity and another being
calculating the contraction velocity of muscle
fibers. It can also be applied to chemical
reactions and many other systems

 The Q10 is calculated as:
where
R is the rate
T is the temperature in Celsius degrees or kelvins.
Q10 is a unitless quantity, as it is the factor by which a rate changes, and is a
useful way to express the temperature dependence of a process.
For most biological systems, the Q10 value is ~ 2 to 3

Enzymes have an optimum pH at which they work fastest. For
most enzymes this is about pH 7-8 (physiological pH of most
cells), but a few enzymes can work at extreme pH, such as
protease enzymes in animal stomachs, which have an optimum of
pH 1.The pH affects the charge of the amino acids at the active
site, so the properties of the active site change and the substrate
can no longer bind. For example a carboxyl acid R groups will be
uncharged a low pH (COOH), but charged at high pH (COO-).

 Extreme pH levels will produce denaturation
 The structure of the enzyme is changed
 The active site is distorted and the substrate
molecules will no longer fit in it
 At pH values slightly different from the enzyme’s
optimum value, small changes in the charges of the
enzyme and it’s substrate molecules will occur
 This change in ionisation will affect the binding of
the substrate with the active site.

As the enzyme concentration increases the rate of the
reaction increases linearly, because there are more
enzyme molecules available to catalyze the reaction. At
very high enzyme concentration the substrate
concentration may become rate-limiting, so the rate
stops increasing. Normally enzymes are present in cells
in rather low concentrations.

The rate of an enzyme-catalysed reaction shows a
curved dependence on substrate concentration.
As the substrate concentration increases, the rate
increases because more substrate molecules can
collide with enzyme molecules, so more
reactions will take place. At higher concentrations
the enzyme molecules become saturated with
substrate, so there are few free enzyme
molecules, so adding more substrate doesn't
make much difference (though it will increase the
rate of E-S collisions).
The maximum rate at infinite substrate
concentration is called vmax, and the substrate
concentration that give a rate of half vmax is
called KM. These quantities are useful for
characterizing an enzyme. A good enzyme has a
high vmax and a low KM.

 The increase in velocity is proportional to the
substrate concentration
Reaction
velocity
Substrate concentration

 Faster reaction but it reaches a saturation point when all the
enzyme molecules are occupied.
 If you alter the concentration of the enzyme then Vmax will
change too.
Reaction
velocity
Substrate concentration
Vmax

 The activity of some enzymes is controlled by
other enzymes, which modify the protein chain
by cutting it, or adding a phosphate or methyl
group.
 This modification can turn an inactive enzyme
into an active enzyme (or vice versa), and this
is used to control many metabolic enzymes
and to switch on enzymes in the gut) e.g.
hydrochloric acid in stomach activates pepsin
activates rennin.

 Inhibitors inhibit the activity of
enzymes, reducing the rate of their reactions.
They are found naturally, but are also used
artificially as drugs, pesticides and research
tools.There are two kinds of inhibitors.
 They are:
1. Competitive Inhibitors
2. Non-Competitive Inhibitors

 Inhibitors are chemicals that reduce the rate
of enzymic reactions.
 The are usually specific and they work at low
concentrations.
 They block the enzyme but they do not
usually destroy it.
 Many drugs and poisons are inhibitors of
enzymes in the nervous system.

 Irreversible inhibitors: Combine with the
functional groups of the amino acids in the
active site, irreversibly.
Examples: nerve gases and pesticides, containing
organ phosphorus, combine with serine residues
in the enzyme acetylcholine esterase.

 Reversible inhibitors:These can be washed out
of the solution of enzyme by dialysis.

 A competitive inhibitor molecule has a similar
structure to the normal substrate molecule, and it
can fit into the active site of the enzyme.
 It therefore competes with the substrate for the
active site, so the reaction is slower.
 Competitive inhibitors increase KM for the enzyme,
but have no effect on Vmax, so the rate can approach
a normal rate if the substrate concentration is
increased high enough.
 The sulphonamide anti-bacterial drugs are
competitive inhibitors.

1. Competitive: These
compete with the
substrate molecules for
the active site.
The inhibitor’s action is
proportional to its
concentration.
Resembles the substrate’s
structure closely.
Enzyme inhibitor
complex
Reversible
reaction
E + I EI

Succinate Fumarate + 2H++ 2e-
Succinate dehydrogenase
CH2COOH
CH2COOH CHCOOH
CHCOOH
COOH
COOH
CH2
Malonate

158
Enzyme
Competitive inhibitor
Substrate
. Competitive inhibitors: are chemicals that resemble an
enzyme’s normal substrate and compete with it for the
active site.

 A non-competitive inhibitor molecule is quite different in
structure from the substrate molecule and does not fit into the
active site.
 It binds to another part of the enzyme molecule, changing the
shape of the whole enzyme, including the active site, so that it
can no longer bind substrate molecules.
 Non-competitive inhibitors therefore simply reduce the amount
of active enzyme (just like decreasing the enzyme
concentration), so they decrease Vmax, but have no effect on
KM.
 Inhibitors that bind fairly weakly and can be washed out are
sometimes called reversible inhibitors, while those that bind
tightly and cannot be washed out are called irreversible
inhibitors. Poisons like cyanide, heavy metal ions and some
insecticides are all non-competitive inhibitors.

161
Non-competitive inhibitors:
Inhibitors that do not enter
the active site, but bind to
another part of the enzyme
causing the enzyme to change
its shape, which in turn
alters the active site.
Enzyme
active site
altered
Noncompetitive
Inhibitor
Substrate

2. Non-competitive:These are not influenced by the
concentration of the substrate. It inhibits by binding
irreversibly to the enzyme but not at the active site.
Examples
 Cyanide combines with the Iron in the enzymes
cytochrome oxidase.
 Heavy metals, Ag or Hg, combine with –SH groups.
These can be removed by using a chelating agent such
as EDTA.

 Negative feedback: end point or end product
inhibition
 Poisons snake bite, plant alkaloids and nerve
gases.
 Medicine antibiotics, sulphonamides,
sedatives and stimulants

The activity of some enzymes is controlled by certain molecules
binding to a specific regulatory (or allosteric) site on the enzyme,
distinct from the active site. Different molecules can inhibit or
activate the enzyme, allowing sophisticated control of the rate.
Only a few enzymes can do this, and they are often at the start of a
long biochemical pathway. They are generally activated by the
substrate of the pathway and inhibited by the product of the
pathway, thus only turning the pathway on when it is needed.

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