Photosynthesis & Respiration

Sun as primary energy source. Requirements for photosynthesis: water, light, CO₂ → produces sugar and oxygen. Respiration releases energy from food.

photosynthesisrespirationSun as energy source
Photosynthesis — the equation to know
Carbon dioxide
+
Water
light energy
(from Sun)
→
Sugar (food)
+
Oxygen
Plants obtain energy by...
Making their own food through photosynthesis using sunlight, then releasing energy from that food through respiration
Animals obtain energy by...
Eating plants or other animals to get food, then releasing energy from that food through respiration
Key distinction: Both plants and animals release energy through respiration. Only plants can make food through photosynthesis. The Sun is the ultimate source of energy for all living things.
Part A — Multiple choice (Booklet A style · 2 marks each)
Question 1 — MCQ (2 marks) Booklet A Requirements for photosynthesis

Which row correctly shows all the requirements for photosynthesis?

AOxygen, water and light energy
BCarbon dioxide, oxygen and light energy
CCarbon dioxide, water and light energy
DCarbon dioxide, water and sugar

The three requirements are CO₂, water and light energy. Oxygen is a product of photosynthesis, not a requirement. Sugar is also a product. Option D confuses inputs with outputs.

Question 2 — MCQ (2 marks) Booklet A Plants vs animals — energy source

Which statement correctly describes how plants and animals are different in the way they obtain energy?

APlants use respiration to make food; animals use photosynthesis to release energy.
BBoth plants and animals make their own food using sunlight.
CPlants obtain energy by eating other organisms; animals make food using sunlight.
DPlants make their own food using light energy through photosynthesis; animals must consume food from other organisms.

The core distinction: plants are producers — they manufacture food from raw materials using sunlight. Animals are consumers — they must obtain food by eating. Both then release energy from that food through respiration.

Part B — Structured questions (Booklet B style)
Question 3 — Structured (4 marks) Booklet B Investigating light & photosynthesis · fair test

A student places three identical potted plants in different light conditions for one week. All other conditions (water, temperature, soil) are kept the same. The table shows the results.

Plant Light condition Leaf colour after 1 week 1 Bright sunlight Dark green 2 Dim light Pale green 3 Complete darkness Yellow

(a) What is the changed variable in this experiment? (1 mark)

(b) What conclusion can be drawn from the results? (1 mark)

(c) Plant 3 turned yellow after one week in darkness. Using your knowledge of photosynthesis, explain why. (2 marks)

(a)The amount (intensity) of light the plants receive.
(b)The more light a plant receives, the more photosynthesis it can carry out, and the greener its leaves. Without light, photosynthesis cannot occur.
(c)In complete darkness, Plant 3 cannot carry out photosynthesis because light energy is required. Without photosynthesis, the plant cannot make sugar (food). The plant uses up its stored food through respiration but cannot replenish it. Without enough food and energy, the plant's cells break down and the green colour (chlorophyll) is lost, causing the leaves to turn yellow.
No light → no photosynthesis → no food made → plant deteriorates → yellowing

Part (c) requires a chain of reasoning: no light → condition for photosynthesis not met → no food produced → plant cannot sustain itself → leaves yellow. Each link in the chain is a step the examiner expects to see.

Question 4 — Structured (4 marks) Booklet B Photosynthesis vs respiration · linked systems Distinction level

The diagram shows the gases exchanged by a plant during the day and at night.

DAY (sunlight present) Takes in CO₂ Releases O₂ (net effect) NIGHT (no sunlight) Takes in O₂ Releases CO₂ (respiration only) Both processes produce water vapour

(a) During the day, the plant appears to only take in CO₂ and release O₂. However, the plant is also carrying out respiration. Explain why respiration is not detectable during the day. (2 marks)

(b) A student says: "Plants only need to breathe at night." Is this correct? Explain your answer. (2 marks)

(a)During the day, the plant carries out both photosynthesis and respiration simultaneously. Photosynthesis uses CO₂ and produces O₂. Respiration uses O₂ and produces CO₂. During daylight, the rate of photosynthesis is greater than the rate of respiration, so photosynthesis uses up all the CO₂ produced by respiration — and produces more O₂ than respiration uses. The net effect is CO₂ taken in and O₂ released.
(b)No, this is incorrect. Plants carry out respiration 24 hours a day, both day and night, because all living cells need energy at all times. It only appears that plants "breathe" at night because photosynthesis stops (no sunlight), so the gases from respiration — CO₂ released and O₂ taken in — become detectable.
Respiration: 24/7 · Photosynthesis: daylight only · Day = both happening simultaneously
The most common misconception in this topic: students think plants only respire at night. In fact, respiration never stops — it is just masked during the day because photosynthesis is faster and consumes the CO₂ produced by respiration.
Question 5 — Structured (5 marks) Booklet B Photosynthesis + food chains + energy flow Distinction level

The diagram below shows a food chain in a garden.

Rose bush Caterpillar Garden spider ...

(a) Where does the rose bush get its energy from? Explain how it obtains this energy. (2 marks)

(b) The caterpillar eats the rose bush. Explain how the energy from the Sun eventually reaches the caterpillar. (2 marks)

(c) If the rose bushes in the garden were completely removed, predict and explain what would happen to the caterpillar and spider populations over time. (1 mark)

(a)The rose bush gets its energy ultimately from the Sun (light energy). It absorbs light energy and uses it to carry out photosynthesis — converting carbon dioxide and water into sugar (food). The plant then releases energy from this sugar through respiration to carry out life processes.
(b)The Sun's light energy is captured by the rose bush during photosynthesis and stored as chemical energy in the sugar (food) produced. When the caterpillar eats the rose bush, it takes in this food. The caterpillar then releases energy from the food through respiration, making the Sun's energy available to the caterpillar.
(c)Without rose bushes, caterpillars would have no food source and their population would decrease. As caterpillar numbers fall, spiders would also have less food and their population would decrease too. The entire food chain is disrupted because the rose bush is the only producer providing energy to the chain.
Sun → photosynthesis → sugar in plant → eaten by animal → respiration → energy released

Part (b) requires tracing the energy pathway explicitly: Sun → photosynthesis → stored in sugar → caterpillar eats → respiration releases energy. Skipping any step loses marks. The arrow in a food chain shows the direction of energy flow, not "who eats whom."