Nutrition in Plants

20 Grade 10 Biology questions on Nutrition in Plants, with worked marking schemes. 20 questions are available on this topic in full.

1. Question 1

   1 mark

   Most leaves appear green to the eye because chlorophyll

   A. is itself a green food substance manufactured during photosynthesis

   B. reflects green light while absorbing mainly red and blue light

   C. absorbs green light and uses its energy to split water molecules

   D. traps only green light, which carries the most energy in sunlight

   (1 mark)

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2. Question 2

   3 marks

   Early in the morning, as light intensity slowly increases, a potted plant reaches a point at which it neither takes in nor gives out carbon dioxide to the surrounding air. Explain this observation.

   (3 marks)

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3. Question 3

   1 mark

   In a greenhouse near Naivasha, the rate of photosynthesis of tomato plants stops rising by midday even though the sunlight keeps getting brighter and the temperature is held steady at 25 °C. The most likely explanation is that

   A. the chlorophyll has been destroyed by the very bright light

   B. the stomata have closed because the leaves are saturated with light

   C. carbon dioxide has become the limiting factor

   D. the plants have made all the glucose their leaves are able to store

   (1 mark)

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4. Question 4

   4 marks

   In a greenhouse at Mwea, the rate of photosynthesis of French beans rises steadily as the temperature rises from 15 °C to about 30 °C, but falls sharply when the temperature reaches 45 °C. Account for these observations.

   (4 marks)

   Marking scheme

   Award 1 mark per point, maximum 4. 1. The reactions of photosynthesis are controlled by enzymes. 2. From 15 °C to 30 °C, rising temperature increases the kinetic energy of enzyme and substrate molecules, so they collide and react more frequently, raising the rate. 3. The rate is highest at the optimum temperature (about 30-35 °C). 4. At 45 °C the enzymes are denatured - the shape of the active site is permanently altered so the substrate no longer fits, and the rate falls sharply. 5. (Accept) At such high temperatures excessive transpiration causes the stomata to close, cutting off the supply of carbon dioxide.

5. Question 5

   1 mark

   When testing a leaf for starch, the ethanol is heated in a beaker of hot water instead of being held directly over a Bunsen flame because

   A. ethanol catches fire easily and would be dangerous over a naked flame

   B. direct heating would break down the starch stored in the leaf

   C. hot water is needed to soften the leaf before the ethanol can act

   D. ethanol removes chlorophyll only at temperatures above 100 °C

   (1 mark)

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6. Question 6

   1 mark

   A farmer in Kakamega notices that the older, lower leaves of his maize have turned yellow between the veins while the veins themselves remain green. The mineral element most likely deficient in the soil is

   A. potassium

   B. phosphorus

   C. nitrogen

   D. magnesium

   (1 mark)

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7. Question 7

   3 marks

   Distinguish between the light-dependent stage and the light-independent stage of photosynthesis.

   (3 marks)

   Marking scheme

   Award 1 mark for each valid paired contrast, maximum 3. Both halves of the comparison must be stated. 1. Site: the light-dependent stage occurs on the grana of the chloroplast, whereas the light-independent stage occurs in the stroma. 2. Light: the light-dependent stage cannot proceed without light energy, whereas the light-independent stage does not use light directly (it uses the products of the first stage). 3. Raw material acted on: water is split (photolysis) in the light-dependent stage, whereas carbon dioxide is fixed in the light-independent stage. 4. Products: hydrogen, ATP and oxygen in the light-dependent stage, whereas sugar (carbohydrate) is formed in the light-independent stage.

8. Question 8

   3 marks

   Explain the importance of photosynthesis to living organisms other than green plants.

   (3 marks)

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9. Question 9

   1 mark

   Sundew plants growing in a waterlogged, acidic swamp near Lake Victoria trap and digest small insects on their sticky leaves. They do this mainly to obtain

   A. glucose, because their leaves contain no chlorophyll

   B. energy, because the swamp is too shaded for photosynthesis

   C. nitrogen-containing compounds, which are scarce in that soil

   D. water, which they are unable to absorb from swampy soil

   (1 mark)

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10. Question 10

    5 marks

    Describe how you would carry out an experiment, using a destarched potted plant, to show that carbon dioxide is necessary for photosynthesis.

    (5 marks)

    Marking scheme

    Award 1 mark per point, maximum 5. 1. Enclose one attached leaf in a clear polythene bag containing sodium hydroxide solution (or soda lime) to absorb carbon dioxide, and seal the neck of the bag. 2. Enclose a second attached leaf of the same plant in an identical bag containing sodium hydrogen carbonate solution, which supplies carbon dioxide, to act as the control. 3. Place the whole plant in sunlight for about four to six hours. 4. Detach both leaves and test each for starch: boil in water, decolourise in ethanol in a hot water bath, rinse in warm water, then add iodine solution. 5. Expected result: the leaf from the sodium hydroxide bag remains brown/yellow, showing no starch, while the control leaf turns blue-black. 6. Conclusion: since the only difference between the two leaves was the supply of carbon dioxide, carbon dioxide is necessary for photosynthesis.

11. Question 11

    3 marks

    Explain three ways in which a palisade mesophyll cell is adapted to carry out photosynthesis efficiently.

    (3 marks)

    Marking scheme

    Any THREE, 1 mark each; the mark is given only where the adaptation is linked to its function. 1. Contains very many chloroplasts - traps the maximum amount of light energy. 2. Cells are long/cylindrical and packed upright just beneath the upper epidermis - light passes into them with least obstruction. 3. Cell wall is thin and kept moist - carbon dioxide dissolves and diffuses in over a short distance. 4. Cells border the intercellular air spaces - ready supply of carbon dioxide. 5. Large central vacuole pushes the chloroplasts to the edge of the cell - shortens the diffusion path for gases and light.

12. Question 12

    4 marks

    Explain why mineral salts continue to enter the root hairs of a bean plant even when their concentration in the soil solution is lower than that in the root hair cell sap, and suggest why this uptake slows down when the field becomes waterlogged.

    (4 marks)

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13. Question 13

    1 mark

    The feature of a leaf that most improves the supply of carbon dioxide to the palisade cells is the

    A. tight, upright packing of the palisade cells

    B. waxy cuticle covering the upper surface

    C. network of veins containing xylem vessels

    D. large air spaces between the spongy mesophyll cells

    (1 mark)

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14. Question 14

    4 marks

    Describe the route followed by a molecule of carbon dioxide from the air outside a maize leaf until it reaches a chloroplast inside a palisade cell.

    (4 marks)

    Marking scheme

    Award 1 mark per point, maximum 4; the sequence must be in order. 1. Carbon dioxide diffuses from the air through an open stoma, down a concentration gradient, into the sub-stomatal air chamber. 2. It diffuses through the intercellular air spaces between the spongy mesophyll cells. 3. It dissolves in the film of moisture on the surface of the mesophyll cell wall. 4. It diffuses in solution across the cell wall and the cell membrane into the cytoplasm, and then into the chloroplast. 5. The gradient is maintained because carbon dioxide is continuously used up in the chloroplast, keeping its concentration inside low.

15. Question 15

    1 mark

    The oxygen given off by a water plant kept in bright light comes from

    A. air that was already dissolved in the surrounding water

    B. water molecules split during the light-dependent stage

    C. the breakdown of glucose inside the chloroplast

    D. carbon dioxide molecules that have been split apart

    (1 mark)

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16. Question 16

    2 marks

    State the balanced chemical equation for photosynthesis, showing the two conditions required for the reaction.

    (2 marks)

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17. Question 17

    1 mark

    A variegated leaf (green with white patches) was taken from a destarched plant that had stood in sunlight for six hours and was tested for starch. Only the parts that had been green turned blue-black. The conclusion from this result is that

    A. iodine solution stains only cells that contain chlorophyll

    B. chlorophyll is necessary for photosynthesis

    C. the white parts of the leaf are unable to take in carbon dioxide

    D. the green parts of the leaf received more sunlight than the white parts

    (1 mark)

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18. Question 18

    1 mark

    Glucose made in a leaf during the day is quickly converted into starch. This is an advantage to the plant because starch

    A. travels through the phloem far more easily than glucose does

    B. contains more chemical energy per gram than glucose does

    C. can be broken down again without the use of enzymes

    D. is insoluble and therefore does not upset the osmotic balance of the cell

    (1 mark)

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19. Question 19

    1 mark

    Before testing a leaf for starch during a practical lesson, the potted plant is first kept in a dark cupboard for two days. The purpose of this is to

    A. destroy the chlorophyll so that the iodine colour can be seen clearly

    B. stop the leaves from respiring while the experiment is running

    C. close the stomata so that carbon dioxide cannot enter the leaf

    D. make sure any starch already present in the leaves is used up or moved away

    (1 mark)

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20. Question 20

    3 marks

    A maize farmer in Trans Nzoia top-dresses his crop with a nitrogenous fertiliser about six weeks after planting. Explain how this practice raises the yield of grain.

    (3 marks)

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