🧬 Biology · Energy

Photosynthesis practice questions

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Answer key with explanations

1. Which pair of substances does a plant take in from its surroundings as the starting materials for photosynthesis?

   Answer: Carbon dioxide and water. The reactants are carbon dioxide (from the air) and water (from the soil, in a land plant), plus light energy. Oxygen and glucose are the products, so ‘oxygen and glucose’ swaps the two sides of the equation. The choices ‘oxygen and water’ and ‘glucose and carbon dioxide’ each pair one reactant with one product.

2. In a plant cell, which organelle is the site of photosynthesis?

   Answer: Chloroplast. Chloroplasts contain chlorophyll and the structures where both stages of photosynthesis happen. Mitochondria are where much of cellular respiration takes place. The nucleus stores DNA, and the cell wall gives support.

3. A healthy leaf looks green. Which statement explains why?

   Answer: Chlorophyll absorbs red and blue light best, so more green light is reflected back to our eyes. The color we see is the light that comes back to our eyes. Chlorophyll absorbs red and blue best, so more green light is reflected. The choice that says chlorophyll absorbs green best reverses this. Leaves do not give off light of their own, and they do absorb some green light.

4. Which pair of molecules is made in the light-dependent reactions and then used by the Calvin cycle?

   Answer: ATP and NADPH. ATP and NADPH carry the energy that was captured in stage 1 to the Calvin cycle. Glucose and oxygen are products of the whole process, and carbon dioxide and water are starting materials. ADP and NADP⁺ move the other way: the Calvin cycle makes them when it uses up ATP and NADPH, and they go back to the thylakoids to be recharged.

5. Algae are grown in water whose oxygen atoms carry a detectable tag, and they are given ordinary carbon dioxide. In the light, where does the tag mainly turn up?

   Answer: In the oxygen gas the algae release. Stage 1 splits water and releases its oxygen atoms as O₂, so the tag mainly appears in the oxygen gas. The sugar gets its oxygen atoms mostly from carbon dioxide, so ‘only in the glucose’ is wrong. The carbon dioxide was supplied untagged, so it is not where the tag mainly turns up. And water is a starting material, so the last choice is wrong.

6. A plant is in bright light at a comfortable temperature, in ordinary air. The grower doubles the brightness, and the rate of photosynthesis does not change. What is the most reasonable explanation?

   Answer: Another factor, such as the carbon dioxide supply, is now limiting the rate. If more light no longer raises the rate, something else is in shorter supply and has become the limiting factor. The carbon dioxide supply is a common example. Light does affect the rate in dimmer conditions. A plant does not switch off photosynthesis because the light gets brighter. And chlorophyll works better, not worse, in brighter light up to a point.

7. A young tree grows in a pot for several years. It is watered but given no extra soil. Where does most of the tree’s dry mass come from?

   Answer: Carbon dioxide from the air and water. Most of the dry mass is carbon, hydrogen and oxygen. The carbon and the oxygen built into sugar come from carbon dioxide, and the hydrogen comes from water. The soil in the pot loses only a little. Minerals are needed, but they are a small share of the mass. Oxygen gas is released by photosynthesis, so it is not what the sugar is built from.

8. Which statement correctly places a process inside a chloroplast?

   Answer: Water is split in the thylakoid membranes. Stage 1, including the splitting of water, happens in the thylakoid membranes, where chlorophyll is built in. Sugar is built in the stroma by the Calvin cycle. So the other three choices each put a process or pigment in the wrong place.

9. A student writes: ‘Plants do photosynthesis in the day and cellular respiration at night.’ Which correction is best?

   Answer: Plants carry out cellular respiration day and night, and in the light they also photosynthesize. Plant cells need ATP all the time, so respiration goes on in the light and in the dark. Photosynthesis needs light, so it does not run at night. Plants do carry out respiration, and it is not limited to roots.

10. Which statement about matter and energy in photosynthesis and respiration is correct?

    Answer: The atoms of carbon, hydrogen and oxygen cycle, while energy flows through, arriving as light and eventually leaving as heat. Carbon, hydrogen and oxygen atoms are reused: they move between air, water, plants and animals. Energy does not cycle. It enters as light, is transformed along the way, and is eventually lost as heat. The other choices get one or both of these wrong.