Photosynthesis: 18 MCQs with answers

The process by which green plants make their own food using carbon dioxide, water and sunlight.

Photosynthesis is how green plants make their own food using carbon dioxide, water and sunlight, releasing oxygen. Absorbing minerals is not photosynthesis, releasing energy from glucose is respiration, and losing water vapour is transpiration.

Chlorophyll

Chlorophyll is the green pigment in chloroplasts that absorbs light energy. Starch and glucose are products, and iodine is used in the starch test.

Stomata

Carbon dioxide diffuses into the leaf through the stomata, which are tiny pores mostly on the lower surface. Roots absorb water, the petiole holds the leaf to the stem, and xylem transports water.

Roots absorb water and minerals, not food; the plant makes its own food in the leaves.

Roots absorb water and dissolved minerals, not food. The plant makes its own food through photosynthesis in the leaves. This is one of the most commonly tested misconceptions.

Carbon dioxide + water → glucose + oxygen

Photosynthesis uses carbon dioxide and water to make glucose and oxygen in the presence of sunlight and chlorophyll. The reverse equation describes respiration.

True

Photosynthesis converts light energy into chemical energy stored in glucose, using carbon dioxide and water, and releases oxygen.

It will stop because there is no light energy to absorb.

Photosynthesis mainly occurs in light. Without light, chlorophyll cannot capture light energy, so the process stops. Water alone cannot drive photosynthesis without light.

Chlorophyll reflects green light and absorbs blue and red light.

Chlorophyll absorbs mainly blue and red light and reflects green light, which is why leaves appear green. The other options reverse the absorption or invent a cause.

The colour of the flower on the plant

Light intensity, CO₂ concentration, temperature, water availability and chlorophyll all affect photosynthesis. The colour of the flower is unrelated to the rate of photosynthesis in the leaves.

Starch is present, so photosynthesis has occurred.

Iodine turns blue-black in the presence of starch. Since glucose made in photosynthesis is stored as starch, a blue-black result confirms photosynthesis occurred.

The white patches lack chlorophyll, so they cannot trap light energy.

White patches contain no chlorophyll, so they cannot absorb light energy for photosynthesis. The green areas photosynthesise normally, but the white areas do not.

Photosynthesis mainly occurs in light, while respiration occurs all the time.

Photosynthesis mainly occurs in light and takes in CO₂ while releasing O₂. Respiration occurs all the time in all living cells, taking in O₂ and releasing CO₂.

Oxygen released during photosynthesis

The bubbles are oxygen, which is released as a by-product when the plant photosynthesises in sunlight. Glucose is not a gas and stays in the plant.

Both A and R are true, and R correctly explains A.

Water is one of the three raw materials for photosynthesis, absorbed by the roots. If the soil is dry, water supply is limited, so photosynthesis slows down even when light is plentiful. R directly explains A.

Increase light, raise CO₂ concentration and maintain a suitable temperature

Photosynthesis is limited by light intensity, CO₂ concentration and temperature. Increasing these within suitable ranges raises the rate and improves yield. Blocking stomata or removing chlorophyll would reduce photosynthesis.

I, II and III are all correct.

Photosynthesis converts light energy into chemical energy stored in glucose (I). Respiration releases that stored energy (II). Together they form a complementary pair — photosynthesis makes food and oxygen that respiration uses (III).

The plant cannot take in carbon dioxide, which is essential for glucose synthesis.

The correct option is B. Stomata are responsible for the exchange of gases; if they are blocked, carbon dioxide cannot enter the leaf, halting the photosynthesis process and preventing glucose production, which is vital for the plant's growth.

Photosynthesis occurred due to sufficient light and chlorophyll, indicating healthy leaf function.

The correct option is A. The blue-black color from the starch test indicates that photosynthesis occurred, which requires both sufficient light and chlorophyll. This implies that the leaf is functioning well and is healthy.