Cells and organisation · GCSE Biology
Enzyme temperature and pH
Read enzyme graphs for GCSE Biology: rate rises with temperature to an optimum, then falls as the active site denatures, and pH peaks in a bell curve around pepsin, amylase or trypsin.
Up to the optimum, warmer means more collisions. Past the optimum, the protein denatures: active site changes shape, substrate no longer fits, rate crashes. pH has its own optimum; extremes also denature.
The important bits
What you need to know
- 1
Each enzyme has an optimum temperature. Human enzymes are often near 37 °C. As temperature rises towards the optimum, particles move faster, more successful collisions occur, and rate increases.
- 2
Above the optimum, bonds holding the enzyme’s tertiary structure break. The active site changes shape permanently: the enzyme is denatured. Rate falls steeply, often to zero.
- 3
Cooling slows collisions but does not usually denature the enzyme, so activity can return on warming. Boiling does not reverse denaturation.
- 4
Each enzyme has an optimum pH. Pepsin in the stomach works at about pH 2; amylase in the mouth at about pH 7; trypsin in the small intestine at about pH 8 (bile helps provide alkaline conditions).
- 5
Extreme pH denatures enzymes by disrupting ionic and hydrogen bonds that keep the active site’s shape. A pH graph is typically a bell curve around the optimum.
- 6
A temperature graph climbs, peaks, then falls. Do not describe the falling part as “the enzyme is killed”. Write denatured, active site shape changed, substrate no longer fits.
- 7
Q10 is not required at GCSE, but you should say that a 10 °C rise roughly doubles rate only on the rising section, before denaturation.
- 8
In investigations, use a water bath to control temperature and buffers to control pH so you test one independent variable at a time.
Quotations worth analysing
Short evidence. Real method.
“The enzyme is denatured: the active site changes shape so the substrate no longer fits.”
This sentence is worth more than “it dies”. Use it on the falling part of a temperature graph and at extreme pH.
“As temperature increases, the rate of reaction increases because particles collide more often.”
This explains only the rising section up to the optimum. After the peak, denaturation dominates.
“Pepsin’s optimum pH is about 2; amylase’s optimum pH is about 7.”
Match enzyme to organ: stomach acid versus mouth or small intestine. Do not give 37 °C as a pH.
Go deeper
How do I describe a temperature graph without waffle?
Split the line in two. From 0 °C to the peak: kinetic energy rises, substrates and enzymes collide more often and with more energy, more enzyme–substrate complexes form per second, rate rises. At the peak: optimum temperature, active site still complementary. After the peak: denaturation, tertiary structure unravels, active site no longer complementary, fewer (then no) complexes, rate falls. If two enzymes are plotted, compare their optima; do not write a generic paragraph. A flat line at zero after boiling means irreversible denaturation. A low rate at 10 °C that recovers at 37 °C means the enzyme was not denatured in the cold. That recovery-versus-irreversible split is a favourite six-mark.
Go deeper
Why is a pH graph a different shape from a temperature graph?
Temperature always has a collision-rate story on the way up. pH does not speed particles in the same way; it changes the charges on amino-acid side chains that hold the active site. Too far from the optimum, those bonds break, the site warps, and rate falls on both sides of the peak, giving a bell curve. Pepsin is adapted to pH 2 in the stomach; moved to pH 8 it denatures. Trypsin is adapted to pH 8; in the stomach it would denature. Amylase from saliva meets acid in the stomach and stops working until pancreatic amylase continues in the small intestine. Buffer solutions keep pH constant while you vary temperature, or keep temperature constant while you vary pH. Changing both at once is invalid.
Go deeper
What does “optimum” mean if the enzyme still works on either side?
Optimum is the condition where rate is highest under the test, not the only condition where the enzyme works. At 30 °C a human enzyme still catalyses, just more slowly than at 37 °C. At pH 6 salivary amylase still works, just below its peak. Exam traps include reading a graph that peaks at 40 °C and writing 37 °C from memory, and assuming all enzymes peak at 37 °C — bacterial enzymes from compost or hot springs can peak much higher. Always read the graph in front of you. If a question says the rate is low at 60 °C, the explanation is denaturation, not “there is no substrate”. Substrate is usually still there; the active sites are gone.
See the idea in action
Amylase and starch are kept at 10 °C, 37 °C and 60 °C. Iodine stays blue-black for 180 s, 40 s, and still blue-black after 300 s. Rate is fastest at 37 °C: frequent successful collisions and an intact active site (optimum). At 10 °C the enzyme is not denatured; collisions are just infrequent, so warming would speed the reaction. At 60 °C the enzyme is denatured, the active site no longer fits starch, and the iodine remains blue-black. Writing “the enzyme died at 60 °C” would lose the shape mark.
Exam technique
Turn knowledge into marks
Never write “killed”. Write denatured: active site changes shape, substrate no longer fits. For graphs, describe the rising section with collisions and the falling section with denaturation. Match pepsin, amylase and trypsin to pH 2, 7 and 8.
Common mistakes
Do not give these marks away
- 01
Saying enzymes are killed by heat, or that cooling denatures them.
- 02
Explaining the falling part of a temperature graph with “there are fewer collisions” instead of denaturation.
- 03
Giving every enzyme an optimum of pH 7, or mixing up pepsin and amylase optima.
What happens to an enzyme when it is denatured?
AIt is used up in the reaction
BThe active site changes shape so the substrate no longer fits
CIt starts catalysing a different reaction
DIt becomes a carbohydrate
Show the answer
The active site changes shape so the substrate no longer fits. High temperature or extreme pH permanently alters the protein’s tertiary structure. The unique active site is lost, so the enzyme can no longer catalyse its reaction.
Quick questions
If this is the bit you searched
Why does enzyme activity fall after the optimum temperature?
The enzyme denatures. Bonds holding the active site’s shape break, so the substrate no longer fits and the rate falls sharply.
Does a low temperature denature enzymes?
Usually not. Cold slows collisions so rate is low, but the active site shape remains and activity returns on warming. High heat denatures permanently.
What is the optimum pH of pepsin?
About pH 2, matching the acidic stomach. Amylase is near pH 7 and trypsin near pH 8 in the small intestine.
How do you keep pH constant in an enzyme experiment?
Use a buffer solution. Then you can change temperature (or enzyme concentration) as the only independent variable.