2.3.6Structure and Functions in Living Organisms: Part 1
pH and Enzyme Function — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 10 of 32
Section 1 of 7Optimum pH
A typical enzyme
- A typical enzyme Most enzymes in the body work best at or near neutral conditions, around pH 7.
- This is the optimum pH for that enzyme.
- Away from the optimum, the rate of reaction drops.
Acidic conditions
- Acidic conditions Some enzymes work in acidic surroundings.
- Protease in the stomach, where hydrochloric acid is secreted, has an optimum of about pH 2.
Alkaline conditions
- Alkaline conditions Other enzymes operate in alkaline conditions.
- Enzymes released into the duodenum, such as those from the pancreas, have an optimum of around pH 8 or 9.
Section 2 of 7Why pH changes activity
1pH moves away from the optimum
- pH moves away from the optimum The solution becomes too acidic or too alkaline for that enzyme.
2Bonds are disrupted
- Bonds are disrupted The charges on the amino acids change, so the bonds holding the folded protein chain in shape are weakened or broken.
3Active site changes shape
- Active site changes shape The site is no longer complementary to the substrate, so fewer substrate molecules can bind.
4Rate falls
- Rate falls Fewer enzyme-substrate complexes form and the reaction slows down.
5Denaturation
- Denaturation If the pH is far from the optimum, the enzyme is denatured permanently and activity stops completely.
Section 3 of 7Enzymes working at different pH
| Enzyme location | Conditions | Approximate optimum pH |
|---|---|---|
| Stomach (protease) | Acidic, from hydrochloric acid | 2 |
| Mouth (amylase in saliva) | Close to neutral | About 7 |
| Small intestine (duodenum) | Slightly alkaline | 8 to 9 |
| Most cell enzymes | Neutral | About 7 |
Section 4 of 7Interpreting the pH graph
1
Read the x axis, which shows pH, and the y axis, which shows the rate of enzyme activity.
2
Find the peak of the curve and read the pH beneath it. This is the optimum pH.
3
Describe the shape on each side of the peak: rate increases as pH approaches the optimum and then decreases as the pH moves past it.
4
Explain each side using the idea of bonds breaking, the active site changing shape and the substrate no longer fitting.
5
Note where the curve reaches zero. At those pH values the enzyme has been denatured.
Section 5 of 7Investigating pH
Method outline
- Method outline Mix the enzyme and its substrate with buffer solutions of different pH values, and record how long a reaction takes or how much product is made.
- The independent variable is pH, and the dependent variable is the rate of reaction.
Control variables
- Control variables Keep the temperature, the volume and concentration of enzyme and substrate, and the time constant, so only pH changes.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Optimum pH | The pH at which an enzyme has its highest rate of activity. |
| Denaturation | The permanent loss of an enzyme's shape, which stops it working. |
| Buffer solution | A solution that holds the pH at a steady value. |
| Protease | An enzyme that breaks down proteins into amino acids. |
| Duodenum | The first section of the small intestine, where conditions are alkaline. |
Section 7 of 7Exam technique
1
Assuming every enzyme works best at pH 7. Stomach protease prefers about pH 2 and intestinal enzymes prefer pH 8 to 9.
2
Saying that an enzyme 'dies' at the wrong pH. It becomes denatured, because enzymes are chemicals, not living things.
3
Only describing the curve and not explaining it. Examiners want the link between changed bonds, a changed active site and fewer substrate fits.
4
Mixing up what is plotted. The pH is on the x axis and the rate of activity on the y axis.
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