Biology 11–12 · Year 11

Effect of pH on amylase activity by timed iodine tests

Module 1: Cells as the Basis of Life (Cell Function)

PracticalLow risk

The idea

Each enzyme has a pH range in which its active site keeps the shape that fits the substrate; outside that range the rate falls.

What you need

Amylase solution 1 percent (or 0.5 percent), 2 mL per run
Starch solution 1 percent (or 0.5 percent), 2 mL per run
Buffer solutions across a pH range, for example pH 4, 5, 6, 7, 8 and 9, 1 mL per run
Iodine solution in a dropper bottle; white spotting tile; stop clock
Test tubes, 6; 2 mL syringes or graduated pipettes; water bath at 30 degrees Celsius

How to do it

Place one drop of iodine solution in each well of the spotting tile.
Put 2 mL amylase and 1 mL buffer of the chosen pH in a test tube and stand it in the water bath for 5 minutes.
Add 2 mL starch solution, mix, and start the clock; every 10 seconds transfer one drop of the mixture to the next iodine well.
Record the time at which the drop no longer turns blue-black (the iodine stays orange): this is the time to complete starch breakdown.
Repeat at each pH, three trials each; calculate rate as 1 divided by time.
Plot rate against pH and identify the optimum.

What you should see

Near the optimum (about pH 6, the usual optimum for amylase the source gives) the iodine drops stop turning blue-black within a few minutes; at pH 4 and pH 9 the starch persists for much longer or is never fully broken down within the lesson. The rate-against-pH curve peaks and falls on both sides. The learner knows it worked when the end point at the optimum is clear and reproducible and the extremes are visibly slower.

What changes

What you change: pH of the buffer
What you measure: time for starch to disappear (rate = 1/time)
What you keep the same: amylase and starch concentrations and volumes
temperature
iodine volume per well
sampling interval

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

All enzymes work best at pH 7; optima vary with where the enzyme works in the body.
A low pH only slows the enzyme; extreme pH can change its shape permanently.
The iodine test measures the enzyme; it measures the substrate that remains.

Safety card

Low riskLearners carry it out

Hazards

Iodine may irritate eyes and stains skin
Buffer solutions at pH 4 and 9 are mild irritants

Controls

Wear eye protection
Rinse splashes with water

Note

Amylase and iodine are low hazard once made up: RiskAssess (https://www.riskassess.com.au/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-8 BIO11/12-3 BIO11/12-4
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03 BI-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities