IGCSE · Practice questions · Investigating photosynthesis

Evidence, controls and the limiting factor.

Six original Cambridge-style questions on testing leaves for starch, designing fair rate investigations, interpreting indicator colours and explaining limiting factors from graphs and greenhouse data.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Change one, control the rest.

Starch test: destarch, expose, boil, hot ethanol (water bath), rinse, iodine: blue-black = starch.
Every experiment needs a control that differs only in the factor being tested.
Indicator: purple = less carbon dioxide, red = normal, yellow = more carbon dioxide.
Extended: on a plateau, the x-axis factor is no longer limiting; name another factor.

Recall

[4 marks]

Describe how to test a leaf for starch, giving the reason for each step.

Boil the leaf in water for about a minute to kill the cells and stop reactions, making it permeable to iodine. ✓
Place it in hot ethanol in a water bath to remove the chlorophyll (ethanol is flammable, so no flame). ✓
Rinse in water to soften the brittle leaf, then spread it on a white tile. ✓
Add iodine solution: blue-black shows starch is present, orange-brown shows it is absent. ✓

Explain

[4 marks]

A destarched plant with variegated leaves was left in bright light for six hours. A leaf was then tested for starch. Describe the expected result and explain what it shows. Identify the control.

The parts of the leaf that were green turn blue-black; the white parts stay orange-brown. ✓
Only the green parts contain chlorophyll, so only they could photosynthesise and make starch. ✓
This shows chlorophyll is needed for photosynthesis. ✓
The white parts of the same leaf (which had the same light and carbon dioxide) act as the comparison / control. ✓

Analysis

[5 marks]

Plan an investigation into the effect of light intensity on the rate of photosynthesis of pondweed.

Place a piece of pondweed in a test-tube of water containing sodium hydrogencarbonate (a source of carbon dioxide). ✓
Put a lamp at a measured distance, for example 10, 20, 30, 40 and 50 cm. ✓
Allow a few minutes to adjust, then count the bubbles of oxygen released in one minute (or collect the gas and measure its volume). ✓
Keep carbon dioxide concentration and temperature constant (use a water bath or a heat shield between lamp and tube); use the same piece of pondweed. ✓
Repeat each distance and calculate a mean; work in a dark room so the lamp is the only light source. ✓

Data

[4 marks]

The table shows the rate of oxygen production by pondweed at two carbon dioxide concentrations.

Light intensity / arbitrary units	1	2	4	6	8
Rate at 0.01% carbon dioxide	8	15	20	20	20
Rate at 0.04% carbon dioxide	8	16	30	38	38

Extended. Identify the limiting factor at light intensity 1 and at light intensity 8 for 0.01% carbon dioxide, and explain your answers.

At light intensity 1: light is limiting. ✓
Increasing carbon dioxide makes no difference there (both rates are 8), but increasing light increases the rate. ✓
At light intensity 8 (0.01%): carbon dioxide is limiting. ✓
Increasing light from 4 to 8 has no effect (rate stays 20), but raising carbon dioxide to 0.04% increases the rate to 38. ✓

Explain

[4 marks]

Three sealed tubes of red hydrogencarbonate indicator were set up: A with an aquatic plant in light, B with the plant in a light-proof cover, C with no plant in light. Predict the colour of each after several hours and explain.

A turns purple: photosynthesis is faster than respiration, so the plant removes carbon dioxide from the water. ✓
B turns yellow: no light, so no photosynthesis; respiration releases carbon dioxide. ✓
C stays red: there is no organism to change the carbon dioxide concentration. ✓
C is the control, showing that the colour changes in A and B are caused by the plant. ✓

Explain

[3 marks]

Extended. A tomato grower heats a greenhouse in winter but finds the crop still grows slowly on dull days. Explain why, and suggest what the grower could do.

On dull days light intensity is low, so light is the limiting factor. ✓
Raising the temperature does not increase the rate when another factor is limiting. ✓
The grower could use artificial lighting to raise light intensity (and add carbon dioxide once light is no longer limiting). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.