1. 1
   A commercial salad grower uses LED grow lights above trays of lettuce seedlings grown indoors with no natural daylight. She measures the rate of photosynthesis of a cut shoot by counting the number of small gas bubbles released into the water per minute, at three light intensities, keeping carbon dioxide concentration and temperature constant throughout. At low light intensity the shoot released 4 bubbles per minute; at medium light intensity, 15 bubbles per minute; at high light intensity, only 16 bubbles per minute, a much smaller increase than between low and medium intensity.

   (a)

   The gas being counted as bubbles is released from the shoot as a product of photosynthesis. Which gas is this?

   [1 mark]

   • ACarbon dioxide
   • BOxygen
   • CWater vapour
   • DNitrogen

   (b)

   Between low and medium light intensity, the rate of bubble release increased sharply. Between medium and high intensity, it increased only slightly. What does this suggest about medium-to-high light intensity?

   [1 mark]

   • APhotosynthesis has stopped completely
   • BThe shoot has started respiring instead of photosynthesising
   • CLight intensity is still the limiting factor
   • DAnother factor, such as carbon dioxide concentration, has become the limiting factor

   (c)

   Explain, using the idea of limiting factors, why increasing the light intensity from low to medium caused a much bigger increase in the rate of photosynthesis than increasing it further from medium to high.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A biology class is designing a revision poster for younger students that summarises photosynthesis using the word equation, the balanced chemical equation, and a short explanation of the role of chlorophyll, without copying any diagram.

   (a)

   Which pair of substances are the raw materials combined in photosynthesis?

   [1 mark]

   • AGlucose and oxygen
   • BOxygen and water
   • CCarbon dioxide and water
   • DGlucose and carbon dioxide

   (b)

   What is the role of chlorophyll in photosynthesis?

   [1 mark]

   • AIt transfers energy from light into energy in chemicals used to make carbohydrates
   • BIt absorbs mineral ions from the soil
   • CIt breaks down starch into glucose for transport
   • DIt carries sucrose from the leaf to the roots

   (c)

   Write the balanced chemical equation for photosynthesis, and state in which part of the plant cell this reaction takes place.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An orchard owner is explaining to an apprentice what happens to the glucose made by photosynthesis in the leaves of an apple tree once summer arrives and the developing apples on the tree begin to grow and sweeten.

   (a)

   Describe three different fates of the glucose produced by photosynthesis in the leaf cells of the apple tree.

   [3 marks]

   (b)

   Explain how glucose made in the leaves is converted and transported to the developing apples to make them sweet, and explain why this transported form is more suitable for movement through the plant than glucose itself.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A greenhouse tomato business manager wants to increase her crop's yield and asks a plant scientist to explain how light intensity, carbon dioxide concentration and temperature each affect the rate of photosynthesis, so she can decide whether to invest in supplementary lighting, a carbon dioxide generator, or heating for her tomato plants.

   (a)

   Explain what is meant by a limiting factor, and explain, in terms of the availability of light, carbon dioxide and warmth, why the rate of photosynthesis in the greenhouse is likely to be low on a cold, overcast winter day even though the tomato plants are otherwise healthy.

   [6 marks]

   (b)

   The scientist explains that on a bright, warm summer day the greenhouse's carbon dioxide concentration falls well below normal atmospheric levels by midday, because the many tomato plants are photosynthesising rapidly and using it up faster than it can enter through the vents. Using this information, explain which single investment, supplementary lighting, a carbon dioxide generator, or heating, would most increase yield in summer, and explain why the same investment would probably not be the most useful one to make for winter.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A gardener notices that the roses in one bed have unusually pale, yellowish leaves and much smaller flowers than roses in a bed that was fertilised earlier in the season. A garden centre tests the soil and finds it is low in both nitrate ions and magnesium ions.

   (a)

   Nitrate ions absorbed from the soil are mainly used by the plant to make which substance?

   [1 mark]

   • AAmino acids
   • BChlorophyll
   • CCellulose
   • DStarch

   (b)

   Magnesium ions absorbed from the soil are mainly needed by the plant to make which substance?

   [1 mark]

   • AAmino acids
   • BCellulose
   • CStarch
   • DChlorophyll

   (c)

   Explain why a shortage of magnesium ions in the soil is likely to be linked to the pale, yellowish colour of the roses' leaves, and explain why a shortage of nitrate ions is likely to be linked to their smaller flowers and generally poorer growth.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   During a hot, dry afternoon, a horticulturist notices that many of the stomata on the lower surface of the leaves of her potted basil plants have closed, even though the plants are still receiving bright sunlight.

   (a)

   Which leaf structures control the opening and closing of the stomata?

   [1 mark]

   • APalisade mesophyll cells
   • BGuard cells
   • CXylem vessels
   • DEpidermal hair cells

   (b)

   What is the main function of stomata in a basil leaf?

   [1 mark]

   • ATo store starch made in photosynthesis
   • BTo transport sucrose out of the leaf
   • CTo allow gases to be exchanged between the leaf and the surrounding air
   • DTo absorb mineral ions directly from the soil

   (c)

   Explain one disadvantage and one advantage to the basil plant of its stomata closing on this hot, dry afternoon.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A plant ecologist compares two leaves from the same species of tree: one grown high up in full sunlight at the top of the canopy, and one grown lower down in the shade of the branches above it. The sun leaf is noticeably thicker than the shade leaf.

   (a)

   Explain how both leaves being broad and thin, whatever their exact thickness, helps carbon dioxide and light reach the photosynthesising cells efficiently.

   [3 marks]

   (b)

   Suggest why it is an advantage for the sun leaf to be thicker, with a deeper layer of tightly packed photosynthesising cells near its upper surface, while the shade leaf benefits instead from being thinner overall.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A houseplant enthusiast is comparing two plants for a bright windowsill: a broad-leaved tropical houseplant with many thin, flat leaves, and a succulent with thick, fleshy leaves and far fewer stomata, adapted to a hot, dry climate with occasional intense sunshine.

   (a)

   Explain how the broad-leaved houseplant's leaves are adapted, through their large surface area, thinness, internal air spaces and numerous stomata, for an efficient rate of photosynthesis on the windowsill.

   [6 marks]

   (b)

   Explain why the succulent's leaves having far fewer stomata and being thick and fleshy, rather than thin, might still be a successful set of adaptations for its hot, dry native climate, even though these features would reduce its rate of photosynthesis compared with a leaf like the houseplant's.

   [6 marks]

   Total for question 8: 12 marks

End of questions