IGCSE · Practice questions · Photosynthesis

Carbon dioxide + water → glucose + oxygen.

Six original Cambridge-style questions on the photosynthesis equations, chlorophyll, how a plant uses the glucose it makes, and why it needs nitrate and magnesium ions.

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

Light in, sugar out.

Word equation: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
Extended: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Glucose becomes: starch (store), cellulose (walls), sucrose (phloem), nectar (insects), or is respired.
Nitrate for amino acids; magnesium for chlorophyll.

Recall

[3 marks]

(a) State the word equation for photosynthesis. (b) State where chlorophyll is found in a leaf cell and describe its role.

(a) carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. ✓
(b) Chlorophyll is a green pigment found in chloroplasts. ✓
It absorbs light and transfers the energy from light into energy in chemicals, for the synthesis of carbohydrates. ✓

Recall

[2 marks]

Extended. Write the balanced chemical equation for photosynthesis.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ ✓
One mark for correct formulae, one mark for correct balancing. ✓

Explain

[5 marks]

Outline how a plant uses and stores the carbohydrates it makes in photosynthesis. Give five uses.

Converted to starch as an energy store, in leaves, roots, tubers and seeds. ✓
Converted to cellulose to build cell walls. ✓
Glucose is used in respiration to provide energy. ✓
Converted to sucrose for transport in the phloem. ✓
Converted to nectar to attract insects for pollination. ✓

Explain

[3 marks]

Suggest why sucrose, not starch, is the form in which carbohydrate is carried in the phloem.

Sucrose is soluble, so it can be carried dissolved in the phloem sap. ✓
Starch is insoluble, so it cannot be carried in solution. ✓
Starch is therefore used for storage, where it does not affect osmosis, and sucrose for transport. ✓

Data

[4 marks]

Seedlings were grown for four weeks in three solutions. The table shows the results.

Solution	Mean height / cm	Leaf colour
all mineral ions	18	dark green
no nitrate	7	small leaves, older leaves yellow
no magnesium	14	yellow between the veins

(a) Calculate the percentage reduction in height caused by the lack of nitrate. (b) Explain the results for the two deficient solutions.

(a) (18 − 7) ÷ 18 × 100 = 61% (to 2 significant figures). ✓
(b) Nitrate ions are needed to make amino acids and so proteins; without them few new cells can be made, so growth is stunted. ✓
Magnesium ions are needed to make chlorophyll; without them leaves turn yellow between the veins. ✓
With less chlorophyll, less light is absorbed, so less photosynthesis happens and growth is reduced, though less severely. ✓

Explain

[3 marks]

A potato plant photosynthesises in its leaves, yet starch is found in the tubers underground. Explain how the starch gets there.

Glucose made in the leaves is converted into sucrose. ✓
Sucrose is carried down to the tubers in the phloem. ✓
In the tuber cells sucrose is converted back to glucose and then into starch for storage. ✓

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.