Complete revision notes for Topic 5 of the CIE IGCSE Biology course: Enzymes. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
Define the term catalyst as a substance that increases the rate of a chemical reaction and is
not changed by the reaction
Define enzymes as proteins that function as biological catalysts
Describe why enzymes are important in all living organisms in terms of reaction speed
necessary to sustain life
Enzymes are necessary to all living organisms as they maintain reaction speeds of all
metabolic reactions (all the reactions that keep an organism alive) at a rate that can sustain
life.
For example, if we did not produce digestive enzymes, it would take around 2-3 weeks to
digest one meal; with enzymes, it takes around 4 hours.
Explain enzyme action with reference to the active site, enzyme-substrate complex,
substrate and product
Enzymes are specific to one particular substrate (molecule/s that get broken down or joined
together in the reaction) as the enzyme is a complementary shape to the substrate.
The product is made from the substrate(s) and is released.
Explain the specificity of enzymes in terms of the complementary shape and fit of the active
site with the substrate
Lock and key hypothesis:
Enzymes are specific to one particular substrate(s) as the active site of the enzyme,
where the substrate attaches, is a complementary shape to the substrate
This is because the enzyme is a protein and has a specific 3-D shape
When the substrate moves into the enzyme’s active site, they become known as
the enzyme-substrate complex.
After the reaction has occurred, the products leave the enzyme’s active site as they no
longer fit it and it is free to take up another substrate.
Enzyme action:
Enzymes and substates randomly move about in solution
When an enzyme and its complementary substrate randomly collide – with the
substrate fitting into the active site of the enzyme – an enzyme-substrate complex
forms, and the reaction occurs
, A product (or products) forms from the substrate(s) which are then released from
the active site. The enzyme is unchanged and will go on to catalyse further reactions
Explain the effect of changes in temperature on enzyme activity in terms of kinetic energy,
shape and fit, frequency of effective collisions and denaturation
Above the optimum
Heating to high temperatures (beyond the optimum) will break the bonds that hold the
enzyme together and it will lose its shape – this is known as denaturation.
Substrates cannot fit into denatured enzymes as the shape of their active site has been lost.
Denaturation is irreversible – once enzymes are denatured, they cannot regain their proper
shape and activity will stop.
Below the optimum
Increasing the temperature from 0oC to the optimum increases the activity of enzymes as
the more energy the molecules have, the faster they move and the number of collisions
with the substrate molecules increases (-> more effective collisions), leading to a faster rate
of reaction.
This means that low temperatures do not denature enzymes, they just make them work
more slowly.
Investigating temperature and enzyme activity
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