AQA Biology (8464) 4.2.2.1 · Required practical
Enzymes
Chew a cracker long enough and it turns sweet. Heat it, or change the pH, and that slows or stops. What is doing it, and how do you measure it the way AQA expects?
Combined Science · Foundation tier
Start here
Chew a plain cracker for a minute. It starts to taste sweet.
A cracker is mostly starch, and starch does not taste sweet. Keep a piece in your mouth without swallowing, and after a minute or so it tastes sugary.
If you had to guess, is the sugar made by the chewing, or by something in your saliva?
Enzymes are biological catalysts: proteins that speed up reactions in living things without being used up. Each enzyme has an active site, a region with a specific shape. Only a substrate whose shape fits the active site can bind to it, which is why amylase digests starch but not protein or fat. The lock and key model is a simplified picture of this: the active site is the lock, the substrate is the key.
The lock and key model
Which molecule is the key?
37 °C
Three molecules collide with amylase. Which one is its substrate?
Enzyme: 1 · unchanged · products made: 0
Think again · hot enzymes
“At 60 °C the amylase is killed, so the reaction stops.”
Spot the flaw
ExplainWhat should the answer say instead?
Sketch it
Which sketch shows the rate of an enzyme reaction against temperature?
pH changes the shape of the active site too. Each enzyme has an optimum pH, matched to where it works. The protease made in the stomach works best in strong acid; enzymes in the small intestine work best near neutral, because bile, which is alkaline, neutralises the stomach acid. Far from its optimum pH, an enzyme is denatured. AQA's required practical measures this for amylase.
Required practical · RP4 Combined Science · RP5 Biology
Change the pH. Time how long the starch lasts.
- Put one drop of iodine solution in each well of a spotting tile.
- Put 2 cm³ of amylase solution and 1 cm³ of a pH buffer in one test tube, and 2 cm³ of starch solution in another. Stand both in a water bath at 35 °C for 5 minutes.
- Mix the two and start the stopwatch.
- Every 30 seconds, put one drop of the mixture into a fresh well of iodine.
- Stop when a drop leaves the iodine orange-brown: the starch is all digested. Record the time.
- Repeat with buffers of other pH values, and repeat each pH to calculate a mean.
Risks. Iodine solution irritates the eyes and stains: wear eye protection. Amylase can irritate skin and eyes, so avoid contact. Keep the water bath below 40 °C so it cannot scald.
Run the practical
Call the graph first.
0 of 5 pH values
Commit first. Plot rate against pH. Which graph will your results make?
Variables
Independent, dependent or control?
For the amylase investigation.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Equation · processing the data
rate = 1 ÷ time
time in seconds (s); rate in per second (s⁻¹).
A shorter time is a faster rate. Plotted as rate against pH, the optimum shows as a peak; plotted as time, it would be a dip.
Command words in this lesson
- Describe a method
- Equipment, what you change, what you measure and how often, what you keep the same, how you stay safe.
- Explain
- Linked steps: the active site changes shape, so the substrate no longer fits, so the rate falls.
- Sketch
- The right shape on labelled axes. No exact values needed.
- Calculate
- Rate = 1 ÷ time, with the time in seconds, and the unit s⁻¹.
Key fact
An enzyme works fastest at its optimum temperature and optimum pH. Too hot, or too far from its optimum pH, and the active site changes shape: the enzyme is denatured, and the substrate no longer fits.
Key note · AQA 4.2.2.1 (8464) · RP4
Enzymes
- Enzymes are biological catalysts: proteins that speed up reactions without being used up.
- Required practical (RP4 Combined / RP5 Biology): amylase + starch + pH buffer at 35 °C; a drop into iodine every 30 s; time until it stays orange-brown.
- Rate = 1 ÷ time. The shortest time is the fastest rate: the optimum pH.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure why a denatured enzyme stops working?
The lock and key bench is a simplified model: real active sites are not rigid outlines, and the molecules are not drawn to scale. The practical is simulated. End-point times come from a model amylase with its optimum near pH 6 at 35 °C, with a small random scatter, and sampling every 30 seconds rounds each time up to the next sample. Groups B and C are invented results, and one of them is anomalous on purpose.