Effect of PH Amylase — AQA GCSE Biology

SPEC 4.2.2.1 • ORGANISATION — RP5

Effect of pH on Amylase

Required Practical 5 — the continuous-sampling method, the iodine spot test, the equilibration step, and the rate calculation the spec demands.

The 1-Mark Equilibration Trap

RP5 has a step that catches most students out. Picture this typical exam scenario:

Typical question pattern

A student places the amylase and the starch + buffer solutions in a water bath separately and leaves them for 5 minutes before mixing. Suggest why this step is necessary. 1 MARK

Most students who get this wrong don't know what the step is even for. The correct answer is equilibration — letting both solutions reach the same temperature as the water bath BEFORE the reaction starts.

Verified mark-scheme phrasings

MS Nov 2021 Paper 1 Q5.5 accepts: "to allow the solutions to reach the same temperature as the water" OR "to allow both solutions to reach [temperature]" OR "so the solutions can equilibrate with the temperature of the water" — any of these scores the mark.

In the next 10 minutes, you'll know the RP5 method end-to-end, including equilibration and the rate calculation.

Learning Objectives

RP5 investigates how pH affects the rate at which amylase breaks down starch. By the end of this lesson, you'll be able to:

Objective 1

Describe the RP5 method — using continuous sampling with iodine to find the time for complete starch digestion.

Objective 2

Identify the IV, DV, and control variables — and explain the role of the equilibration step.

Objective 3

Calculate the rate of reaction using rate = 1 ÷ time (spec MS 1a/1c), and explain the result using the denaturation chain from Lesson 2.03.

SPEC REF 4.2.2.1 • RP5 • PAPER 1 • HIGHER TIER

What RP5 Investigates

RP5 — SPEC WORDING

"Investigate the effect of pH on the rate of reaction of amylase enzyme. Students should use a continuous sampling technique to determine the time taken to completely digest a starch solution at a range of pH values. Iodine reagent is to be used to test for starch every 30 seconds . Temperature must be controlled by use of a water bath or electric heater."

The big idea

Amylase breaks down starch. We can time how long this takes at different pH values — and find the pH at which amylase works fastest (its optimum pH).

Hypothesis / prediction (WS 2.1)

Before testing, state the prediction: "Amylase activity will be highest at its optimum pH of around 7, and lower at pH values further from 7, where the amylase becomes denatured." A clear prediction frames the experiment.

Backward link — Lessons 2.03 + 2.04

You already know: amylase has an optimum pH around 7 (Lesson 2.03); iodine tests for starch — yellow/orange/brown to blue-black if starch is present (Lesson 2.04). RP5 puts both pieces together.

IV, DV and Controls

IV — Independent

pH of the buffer solution.

Test a range — e.g. pH 3, 5, 7, 9, 11 (at least 3 different values).

DV — Dependent

Time taken for all starch to be broken down (s).

Measured by when iodine first stops turning blue-black.

Controls — kept SAME

Volume + concentration of starch and amylase; temperature (water bath ~35 °C); volume of iodine on tile; time intervals (30 s).

Verified MS Q5.4 — accepted control variables (example values)

Mark schemes have credited specific volumes: "volume / 5 cm³ of starch", "volume / 1 cm³ of amylase", "volume / 1 drop of mixture added to spotting tile", "volume / 2 drops of iodine". Naming the substance scores — "same volume" alone does not. Note: these specific volumes are example values from one MS; the principle (named volume of each substance) is what matters.

Apparatus you should be able to name

test tubes + rack · spotting tile · pipettes (or syringes / measuring cylinders) · stopwatch · water bath at 35 °C · buffer solutions at each pH · starch solution · amylase solution · iodine solution. AT 1 expects you to describe what's used to measure volume, time and pH.

The Method — Continuous Sampling

Five-step method:

1

Place buffer + starch in one test tube, amylase in another. Put both in the water bath at 35 °C for 5 minutes (equilibration — next slide).

2

Set up a spotting tile with drops of iodine in each well (one per sampling time).

3

Mix amylase with starch + buffer in one tube. Start the timer immediately.

4

Every 30 seconds, transfer a drop of the mixture onto a fresh iodine well. Note the colour.

5

Record the time when iodine FIRST stays yellow-brown — all starch is broken down. Repeat at each pH.

Temperature choice + AT 2 safety

The spec says temperature must be controlled by a water bath. 35 °C is the conventional choice for RP5 (matches the Revision Guide); some setups use 37 °C — both valid as long as the same temperature is held throughout. Safety: iodine solution is a skin and eye irritant — wear safety goggles and avoid skin contact; clean up spills promptly. The water bath itself is warm (not hot) so the burn risk is low.

The Equilibration Step — Why 5 Minutes?

EQUILIBRATION

Leaving the starch + buffer and amylase solutions in the water bath separately for 5 minutes before mixing — so that both solutions reach the same temperature as the water bath.

If you mix cold solutions and drop them in a 35 °C bath, the temperature is changing during the reaction — which confounds the result. Temperature affects rate, so it has to be CONSTANT (not the IV here — pH is). Equilibration ensures the only variable changing between trials is pH.

Verified MS Nov 2021 Q5.5 — phrases that score

All three are accepted (use any one):

1. "To allow the solutions to reach the same temperature as the water"

2. "To allow both solutions to reach [the target temperature]"

3. "To allow the solutions to equilibrate with the temperature of the water"

Phrases that DON'T score

"To make the experiment work" (vague), "so they don't react too fast" (wrong — temperature, not rate, is the issue), "to mix them properly" (irrelevant). When asked WHY a step is done, name the specific scientific reason.

Reading the Iodine — Two Colours, Two Meanings

This is the OPPOSITE direction from the iodine test in Lesson 2.04. There you were detecting starch presence; here you're detecting when starch disappears.

Iodine stays BLUE-BLACK

Starch is still present. The reaction is not yet complete — amylase hasn't broken it all down. Keep sampling.

Iodine stays YELLOW-BROWN

Starch is gone — all broken down by amylase. This is the endpoint. Record the time.

Verified MS Nov 2021 Q5.7 phrasing

When asked to explain why iodine still turns blue-black, the mark scheme awards: "(iodine is not yellow-brown because) starch is still present" OR "starch has not been broken down." Both phrasings score.

Forward link — RP5 mark schemes

In some questions (like MS Nov 2021 Q5.7), iodine "did not become yellow-brown" at extreme temperatures or pH — meaning amylase was denatured and never broke down the starch. Always interpret iodine colour in context of whether the enzyme is working.

Rate = 1 ÷ Time — Spec MS 1a/1c

RP5 doesn't just need the time data — the spec explicitly requires rate calculations. The formula is short:

RATE OF REACTION

rate = 1 ÷ time

Why use 1 ÷ time?

The DV measured is time taken for the reaction to complete. A shorter time means a faster reaction. To convert time into a true rate (where larger = faster), we use the reciprocal.

Example calculation

At pH 7, all starch was broken down in 50 seconds.

Rate = 1 ÷ 50 = 0.02 s⁻¹ (or 0.02 per second)

Unit reminder

If time is in seconds, the rate unit is s⁻¹ (per second). If time is in minutes, the rate unit is min⁻¹. Always include the unit — mark schemes deduct for missing units.

Plotting Results — The Bell Curve

Plot rate (1 ÷ time) on the y-axis against pH on the x-axis. For amylase, the result is a bell curve peaking at pH 7.

What the graph shows

Rate is highest at pH 7 — the optimum for salivary amylase. Above or below pH 7, rate falls because the amylase is denatured — active site changes shape — starch no longer fits (the chain from Lesson 2.03).

Making the Results Reliable

Repeats

Repeat the experiment at each pH at least 3 times. Calculate the mean time from your repeats. Plot the mean rate on the graph.

Anomalies

Identify any anomalous results (clearly different from the others) and exclude them from the mean. Note them in your conclusion.

Range

Use a wide pH range (e.g. 3 to 11) with several values either side of the expected optimum — so the curve can be properly drawn.

Sources of error

Sampling interval (30 s) limits accuracy — the actual endpoint could be up to 30 seconds before you sampled it. Subjective colour judgement — what looks "yellow-brown" varies between students. Use the same student to read all samples, or use a colorimeter for more precision.

PEE Worked Example — 4-Mark Explain

In an RP5 experiment, a student tested amylase at pH 3, 7 and 11. At pH 3, the iodine continued to turn blue-black throughout the entire experiment. Explain this observation. 4 MARKS

Step 1 — Read the command word. "Explain" — give the reasoning, not just describe what happened.

Step 2 — Identify the pattern. 4-mark explain question on RP5 results = the denaturation chain plus the iodine interpretation.

Step 3 — Write the answer.

Model answer

At pH 3, the amylase is far from its optimum pH (~7), so the amylase is denatured. This causes the active site to change shape. The starch (substrate) no longer fits the active site, so the amylase cannot break down the starch. Because the starch is still present throughout the experiment, the iodine continues to turn blue-black.

Mark-scheme tick

✅ Amylase denatured (1) · ✅ Active site changes shape (1) · ✅ Starch no longer fits (1) · ✅ Starch still present → iodine stays blue-black (1). 4 / 4 marks.

Examiner Alerts

Equilibration step has a SPECIFIC reason

"To allow the solutions to reach the same temperature as the water" — verbatim mark-scheme phrasing. Don't write "to make sure the experiment works" or "to warm up the enzyme" — both vague and unscored.

Iodine colour direction matters here

In RP4 (Lesson 2.04), iodine going to blue-black was the POSITIVE result. In RP5, blue-black means starch is still there — the reaction is NOT yet complete. The endpoint is when iodine first stays yellow-brown. Read the question context carefully.

Denaturation chain — still applies

From Lesson 2.03: denatured → active site changes shape → substrate (starch) no longer fits. Use it for EVERY question explaining why rate slows at extreme pH. MS Nov 2021 Q5.7 explicitly: "do not accept the amylase is killed / has died."

Always include units on rate

Rate = 1 ÷ time. Units: s⁻¹ if time in seconds, min⁻¹ if minutes. A bare number ("0.02") loses the units mark.

Common Errors

WrongRight

"to make the experiment work" (equilibration)"to allow both solutions to reach the same temperature as the water bath "

"iodine turns blue-black so reaction complete""iodine staying blue-black means starch is still present ; endpoint is when iodine stays yellow-brown "

"the amylase is killed at pH 3""the amylase is denatured at pH 3 — active site changes shape"

"rate = time" or just quoting time"rate = 1 ÷ time " — and include units (s⁻¹ or min⁻¹)

"same volume" (no detail)"same volume of starch ; same volume of amylase ; same volume of iodine " (name the substance)

"buffer is acid" (mechanism wrong)"buffer maintains a constant pH throughout the experiment"

Action

Screenshot this table — every row is a documented examiner-flagged error from RP5-type questions.

Practice Question

A student carried out RP5 at five pH values. They recorded the time taken for iodine to first stay yellow-brown:

pHTime taken (s)

3did not stay yellow-brown

5180

740

990

11did not stay yellow-brown

(a) State the optimum pH for this amylase. Give a reason for your answer. 2 MARKS

(b) Calculate the rate of reaction at pH 7. Give the unit. 2 MARKS

(c) Suggest why the iodine did not stay yellow-brown at pH 3. 2 MARKS

a

Optimum pH is pH 7. (1 mark)

Reason: at pH 7 the reaction took the shortest time (40 s) — so the rate of reaction was highest at pH 7. (1 mark)

b

Rate = 1 ÷ time = 1 ÷ 40 = 0.025. (1 mark)

Unit: s⁻¹ (per second). (1 mark)

c

At pH 3, the amylase is denatured (far below its optimum pH). (1 mark)

The active site has changed shape, so starch can no longer fit — the starch is not broken down, so iodine continues to turn blue-black throughout the experiment. (1 mark)

Total: 6/6. Note for (a): the "shortest time = highest rate" reasoning is what mark schemes look for. For (b): the rate is a reciprocal — without unit, only 1 mark. For (c): the denaturation chain explains the observation.

Recap — Three Takeaways

1. The method — continuous sampling

Mix amylase and starch + buffer at the test pH. Sample onto iodine every 30 seconds. Record the time when iodine first stays yellow-brown = all starch broken down.

2. Equilibration is essential

5-minute wait BEFORE mixing the solutions, "to allow the solutions to reach the same temperature as the water bath." This is the verified mark-scheme phrasing — a 1-marker every student should win.

3. Rate = 1 ÷ time (with units)

Convert time to rate using the reciprocal. Plot rate vs pH — bell curve, peak at pH 7 (amylase optimum). At extreme pH: denatured → active site changes shape → starch no longer fits.

You're set

Method, equilibration, rate calculation, denaturation chain. You'll handle any RP5 question on Paper 1.

Next Up

Lesson 2.06 — The Heart, Blood Vessels & Blood

You've finished digestion. Next comes the circulatory system — the next organ system on the spec.

Anatomy of the heart, the double-circulation pump, the three types of blood vessel (artery, capillary, vein), and the four components of blood — with the mark-scheme intelligence on each.

SEE YOU IN LESSON 2.06.