Brown's Biology AQA GCSE Additional Science-BIOLOGY 2
Biology Unit B2.5 Proteins – their functions and uses
for GCSE Additional Science or GCSE Biology
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AQA GCSE Science BIOLOGY 2 Unit
B2.5 Proteins – their functions and uses
Know that proteins have many functions,
both inside and outside the cells of living organisms.
Know that proteins, as enzymes, are now
used widely in the home and in industry.
You should be able to use your
skills, knowledge and understanding to evaluate the advantages and
disadvantages of using enzymes in the home and in industry.
AQA GCSE Science BIOLOGY Unit B2.5.1 Proteins
a) Know that protein molecules are made up
of long chains of amino acids.
Know and understand that proteins act as:
b) Know and understand
that catalysts increase the
rate of chemical reactions.
Know that biological catalysts are called enzymes
and they enable the vast majority of reactions in cells, hence all the
essential chemical reactions of any living organism.
Know that enzymes
AQA GCSE Science BIOLOGY Unit
a) Know that the shape of an enzyme is
vital for the enzyme’s function.
The typical graph of speed/rate of reaction
versus the temperature of the solution for an enzyme.
The diagram above illustrates
what is known as the 'key and lock mechanism'. The molecule being processed
must fit 'snugly' into the right shaped enzyme protein molecule otherwise it
won't be changed into the
Know that high temperatures change the shape
- enzymes become denatured and ineffective, the heat interferes with the
bonding within the protein molecule.
Enzymes work best at their
optimum temperature eg ~37oC for enzymes in the human body.
As the temperature increase the
rate of catalysis increases (normal effect on the speed of reaction as the
average kinetic energy of the molecules increases), but at high temperatures the protein structure
of the enzyme is destroyed, so the active site on the enzyme is damaged and
won't work correctly because the shape of the protein molecule has changed
and the substrate molecule can't 'dock in'.
So, a graph of rate versus
temperature rises to a maximum (optimum temperature) and then falls away as
the enzyme becomes thermally denatured and destroyed and ceases to function
at high temperatures (see diagram above on right).
This is illustrated by the graph
above, which shows what happens to the speed of the enzyme catalysed process
of photosynthesis as the temperature is increased.
b) Know that different enzymes work best
at different pH values.
The typical graph of speed/rate of reaction
versus the pH of the solution for an enzyme.
Different enzymes have different
optimum pHs (diagrams on right and below).
The pH ie how acid or how
alkaline the aqueous medium is, affects the protein structure of the enzyme,
so like the temperature graph, the graph rises to a maximum for the optimum
Increase in acidity or
alkalinity creating a pH well away from the optimum, can affect the protein
structure of the enzyme and so affecting the active site and the substrate
molecule can no longer readily lock into place into the active site and
cannot be transformed into the product molecules.
The optimum is often around pH 7
neutral, which is the most common pH region inside cells.
The graph of speed/rate of reaction versus the
pH of the solution for several different enzymes.
The graph above shows a variety of optimum pH values for three different enzymes -
pepsin, urease and glycine oxidase.
Note that pepsin works best in
the acid conditions of the stomach.
Carbonic hydrase works best in
nearly neutral media like blood plasma or cell cytoplasm.
Trypsin is most effective in
mildly alkaline conditions.
Extra note on substrate
- The graph of speed/rate of reaction
versus substrate concentration.
- The higher the enzyme concentration,
the faster the reaction, and likewise, the higher the substrate
concentration, the faster the reaction.
- For a given enzyme concentration, when the concentration of substrate is high,
all the active sites on the enzymes are occupied and the rate of reaction
reaches a maximum and stays constant no matter how much more concentrated
the substrate concentration is..
c) Know that some enzymes work
outside the body cells.
Know that the digestive enzymes are produced by specialised
cells in glands and in the lining of the gut.
The enzymes then pass out of
the cells into the gut where they come into contact with food molecules.
They catalyse the breakdown of large molecules into smaller molecules.
d) Know that the enzyme amylase is
produced in the salivary glands, the pancreas and the small intestine.
Know that this
enzyme amylase in saliva catalyses the breakdown of starch into sugars in the mouth and small
Starch molecules are too big to
pass through the cell membranes of the walls of the digestive system.
e) Know that protease enzymes are produced
by the stomach, the pancreas and the small intestine.
Know that protease enzymes catalyse
the breakdown of proteins into amino acids in the stomach and the small
Protein molecules are too big to
pass through the cell membranes of the walls of the digestive system and
must be broken down by protease enzymes into amino acids first.
f) Know that lipase enzymes are
produced by the pancreas and small intestine.
Know that these enzymes catalyse the
breakdown of lipids (fats and oils) into fatty acids and glycerol in the
Again, fat and oil molecules are
too big to be absorbed through the intestine walls and must be first broken
down into smaller molecules, such as fatty acids and glycerol, by lipase
g) Know that the stomach also produces
h) Know that the liver produces bile,
which is stored in the gall bladder before being released into the small
Bile neutralises the acid that was added to food in the stomach,
the stomach fluids are too acid for most digestive processes.
This provides alkaline conditions in which enzymes in the small intestine
work most effectively.
Bile also emulsifies fats,
breaking them into smaller droplets which are broken down more rapidly
because of the increased surface area - typical rates of reaction factor.
i) Know that some microorganisms produce
enzymes that pass out of the cells.
j) Know and understand that in industry, enzymes are used to bring about reactions
at normal temperatures and pressures that would
Enzymes are excellent catalysts
for speeding up reactions at relatively low temperatures and pressures
without the need for increased energy costs of using high
temperatures and more costly high pressure engineering in chemical
They have the advantage of
catalysing a specific reaction (no other reaction, no waste chemicals),
lower energy costs, they can be recycled over and over again and any waste
enzymes are biodegradable.
There are a few disadvantages eg
they are easily inhibited or denatured by changes in temperature and pH, so
reaction conditions must be carefully controlled.
keywords: amino acids, shape, structural
components of tissues muscles, hormones, antibodies, biological catalysts fat
digesting proteases lipases carbohydrases isomerase
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