CIE IGCSE Biology

Revision Notes

5.1 How Enzymes Work

What Are Enzymes?

Enzymes:

  • Are catalysts that speed up the rate of a chemical reaction without being changed or used up in the reaction
  • Are proteins
  • Are biological catalysts (biological because they are made in living cells, catalysts because they speed up the rate of chemical reactions without being changed)
  • 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

How Do Enzymes Work?

Enzyme substrate specificity, IGCSE & GCSE Biology revision notesEnzyme substrate specificity

 

  • 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

 

Lock and key model of enzyme activity, IGCSE & GCSE Biology revision notesEnzyme specificity: lock and key model of enzyme activity

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Enzyme Specificity

  • 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
  • This is known as the lock and key hypothesis
  • 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

 

How enzymes work, IGCSE & GCSE Biology revision notesHow enzymes work

 

1. Enzymes and substates randomly move about in solution
2. 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.
3. 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.

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Effect of Temperature on Enzyme Function

  • Enzymes are proteins and have a specific shape, held in place by bonds
  • This is extremely important around the active site area as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed
  • Enzymes work fastest at their ‘optimum temperature’ – in the human body, the optimum temperature is 37⁰C
  • 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

 

Effect of temperature on enzyme activity, IGCSE & GCSE Chemistry revision notesEffect of temperature on enzyme activity

 

  • Increasing the temperature from 0⁰C 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, leading to a faster rate of reaction
  • This means that low temperatures do not denature enzymes, they just make them work more slowly

 

Graph showing the effect of temperature on rate of enzyme activity, IGCSE & GCSE Biology revision notesGraph showing the effect of temperature on the rate of enzyme activity

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Effect of pH on Enzyme Function

  • The optimum pH for most enzymes is 7 but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
  • If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be destroyed
  • This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity
  • Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

 

Effect of pH on enzyme activity, IGCSE & GCSE Biology revision notesEffect of pH on enzyme activity

 

Graph showing effect of pH on rate of activity for an enzyme from duodenum, IGCSE & GCSE Biology revision notesGraph showing the effect of pH on rate of activity for an enzyme from the duodenum

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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