What the plant does with its sugar — answer key

1. YQ1 · Practice

Options as numbered on the pupil sheet: 1 — an energy store; 2 — builds cell walls; 3 — provides energy for the cells; 4 — is transported in the phloem; 5 — attracts insects for pollination.

Model answer. Starch is an insoluble energy store. Cellulose builds cell walls. Glucose is used in respiration to provide energy. Sucrose is the form in which sugar is transported in the phloem. Nectar is a sugary liquid that attracts insects for pollination.

A full answer includes

• Starch: an energy store.
• Cellulose: builds cell walls.
• Glucose in respiration: provides energy.
• Sucrose: transported in the phloem.
• Nectar: attracts insects for pollination.

Also accept

• starch: store; cellulose: walls; respiration: energy; sucrose: phloem; nectar: pollination

Watch for

• “Cellulose is an energy store.” — Cellulose builds cell walls; the store is starch.
• “Glucose travels in the phloem.” — Glucose is turned into sucrose for transport in the phloem.

2. YQ2 · Practice

Model answer. Starch is insoluble. A cell full of dissolved glucose would draw water in by osmosis, but starch does not, so a cell can store a great deal of it. When energy is needed, the starch is broken back down into glucose.

A full answer includes

• Starch is insoluble, so it does not draw water into the cells by osmosis; glucose would.

Also accept

• starch is insoluble, so it does not affect water movement by osmosis

Watch for

• “Starch contains more energy than the glucose it is made from.” — Starch is made of glucose units; the reason to store it is that it is insoluble.
• “Glucose cannot be stored because it is used up at once.” — Glucose could be kept, but dissolved it would draw water into the cells.

3. YQ3 · Practice

Model answer. Nitrate ions supply the nitrogen that the plant combines with sugar to make amino acids, which are joined to make proteins. Magnesium ions are needed to make chlorophyll.

A full answer includes

• g1: amino acids.
• g2: proteins.
• g3: chlorophyll.

Also accept

• amino acids; proteins; chlorophyll

Watch for

• “Nitrate is used to make chlorophyll only.” — The main use of the nitrogen from nitrate is to make amino acids, and so proteins. Magnesium is the ion that goes into chlorophyll.
• “Magnesium makes proteins.” — Magnesium is needed for chlorophyll.

4. YQ4 · Practice

Model answer. The leaves above ground make glucose by photosynthesis. The glucose is turned into sucrose, which is carried down the stem in the phloem to the growing potato. There the sucrose is turned back into glucose, and the glucose is joined into starch, an insoluble energy store that the plant can use later.

A full answer includes

• Glucose is made by photosynthesis in the leaves.
• It is turned into sucrose and carried in the phloem to the potato.
• In the potato it is turned back into glucose and built into starch, an insoluble store.

Also accept

• leaves make glucose; it travels as sucrose in the phloem; in the potato it is made into starch

Watch for

• “The potato makes its own starch from the soil.” — A potato has no chlorophyll and no light; its sugar comes from the leaves.
• “Glucose is carried in the phloem.” — Sugar travels as sucrose.

5. YQ5 · Practice

Model answer. Glucose is used in respiration in every living cell, day and night. It is turned into sucrose for transport in the phloem, built into cellulose for cell walls, and used to make nectar, which attracts insects for pollination. Plants respire all the time, not only at night, and a potato stores starch, not glucose.

A full answer includes

• Correct: respiration in every living cell, day and night; sucrose in the phloem; cellulose for cell walls; nectar to attract insects.
• Not correct: respiration only at night; potatoes store glucose.

Also accept

• respiration day and night; sucrose in phloem; cellulose; nectar

Watch for

• “Plants respire only at night.” — Every living cell respires all the time; in bright light photosynthesis is faster, which hides it.
• “A potato stores glucose.” — A potato stores starch, which is insoluble.

6. YQ6 · Practice

Model answer. Every chlorophyll molecule contains magnesium, so without magnesium ions the plant cannot make enough chlorophyll. Its leaves turn yellow, between the veins of the older leaves first. With less chlorophyll, the leaves absorb less light and make less glucose by photosynthesis, so the plant has less sugar for growth.

A full answer includes

• Magnesium is needed to make chlorophyll, so less chlorophyll is made and leaves turn yellow (between the veins).
• Less chlorophyll means less light absorbed, less photosynthesis, less glucose, so less growth.

Also accept

• magnesium is needed for chlorophyll; less chlorophyll, yellow leaves, less photosynthesis, less growth

Watch for

• “Magnesium is needed to make proteins.” — Magnesium is needed to make chlorophyll: there is one magnesium atom in every chlorophyll molecule.
• “The plant grows less because magnesium is its food.” — Its food is sugar; magnesium lets it make the chlorophyll that makes the sugar.

7. YQ7 · Practice

Model answer. Glucose contains only carbon, hydrogen and oxygen. To make amino acids, the plant needs nitrogen too, which comes from nitrate ions. Amino acids are joined into proteins, which are needed to build new cells and to make enzymes. Without enough nitrate, the plant cannot make enough protein, so it grows slowly and stays small and weak, however much glucose it makes.

A full answer includes

• Nitrate supplies nitrogen, needed (with sugar) to make amino acids.
• Amino acids make proteins, needed for new cells and enzymes, so growth is limited.

Also accept

• no nitrogen for amino acids, so not enough protein for new cells and enzymes

Watch for

• “Without nitrate the plant has no energy.” — It can still make glucose and respire; it lacks nitrogen for proteins.
• “Nitrate is the plant's food.” — The food is the sugar it makes; nitrate supplies nitrogen.

8. YQ8 · Practice

Model answer. Complete: (22 + 25 + 24 + 23 + 26) ÷ 5 = 120 ÷ 5 = 24.0 cm. Without nitrate: (9 + 8 + 10 + 9 + 9) ÷ 5 = 45 ÷ 5 = 9.0 cm. Without magnesium: (17 + 19 + 18 + 16 + 20) ÷ 5 = 90 ÷ 5 = 18.0 cm.

Checked automatically

• Mean height, complete solution: 24.0 cm (exact value)
  • Common wrong answer 120 : That is the total. Divide by the number of seedlings, 5.
  • Common wrong answer 24.5 : Check your adding: the five heights add up to 120.
• Mean height, without nitrate: 9.0 cm (exact value)
  • Common wrong answer 45 : That is the total. Divide by 5.
  • Common wrong answer 9.2 : Check your adding: the five heights add up to 45.
• Mean height, without magnesium: 18.0 cm (exact value)
  • Common wrong answer 90 : That is the total. Divide by 5.
  • Common wrong answer 18.2 : Check your adding: the five heights add up to 90.

A full answer includes

• Complete: 120 ÷ 5 = 24.0 cm.
• Without nitrate: 45 ÷ 5 = 9.0 cm.
• Without magnesium: 90 ÷ 5 = 18.0 cm.

Also accept

• 24.0 cm; 9.0 cm; 18.0 cm

Watch for

• “The mean is the middle value.” — The middle value is the median. The mean is the total divided by the number.
• “Divide by 3 because there are three groups.” — Each mean uses the five seedlings of one group.

9. YQ9 · Practice

Model answer. (a) The complete solution is the control. It shows how the seedlings grow when nothing is missing, so any difference in another group can be put down to the one ion left out. (b) Without nitrate, the mean height was 9.0 cm against 24.0 cm in the control, and the older leaves were yellow: nitrate is needed for growth, because it supplies nitrogen for amino acids and proteins. Without magnesium, the mean height was 18.0 cm and the older leaves were yellow between the veins: magnesium is needed to make chlorophyll. (c) Seedlings vary, so one could be unusually tall or short; the mean of five gives a more reliable comparison.

A full answer includes

• (a) The complete solution: it shows how the seedlings grow with every ion, so each difference can be put down to the missing ion.
• (b) Without nitrate: much shorter (9.0 vs 24.0 cm), older leaves yellow, so nitrate is needed for growth (proteins). Without magnesium: shorter (18.0 cm), older leaves yellow between the veins, so magnesium is needed for chlorophyll.
• (c) Seedlings vary; a mean of five reduces the effect of one unusual seedling.

Also accept

• (a) complete solution, to compare with; (b) nitrate: much less growth, yellow older leaves; magnesium: less growth, yellow between veins; (c) seedlings vary, a mean is more reliable

Watch for

• “Without magnesium the plants grew best, because they still grew 18 cm.” — Compare with the control: 18.0 cm is less than 24.0 cm.
• “Yellow leaves prove which ion is missing.” — The comparison with the control, with one ion different, shows it; colour alone does not.

10. YT1 · Apply

Model answer. Nectar is a sugary liquid that attracts insects, which carry pollen between flowers. Grass flowers are pollinated by the wind, so they do not need to attract insects. Making nectar would use up sugar that the plant can use instead for growth, storage or respiration.

A full answer includes

• Nectar attracts insects for pollination; wind-pollinated flowers do not need insects.
• So making nectar would waste sugar that can be used for growth, storage or respiration.

Also accept

• nectar attracts insects; wind pollination needs no insects, so nectar would waste sugar

Watch for

• “Grasses cannot make sugar.” — Grasses photosynthesise like other plants; they simply do not spend sugar on nectar.
• “Nectar feeds the flower.” — Nectar feeds insects, to attract them for pollination.

11. YT2 · Apply

Model answer. (a) Magnesium is the most likely: older leaves yellow between the veins is the sign of too little magnesium, which is needed to make chlorophyll. Adding nitrate made no difference, which fits. (b) Grow two groups of similar lettuces, one in the complete solution with magnesium and one in the same solution without it (or with extra magnesium added to her solution), with everything else the same. If the leaves with magnesium stay green and the others turn yellow between the veins, magnesium was the problem.

A full answer includes

• (a) Magnesium: yellowing between the veins of older leaves matches magnesium shortage (less chlorophyll), and adding nitrate did not help.
• (b) Grow similar lettuces with and without added magnesium, everything else the same (control: complete solution), and compare leaf colour and growth.

Also accept

• (a) magnesium, yellow between veins, nitrate did not help; (b) groups with and without magnesium, all else the same, compare

Watch for

• “Yellow leaves always mean too little nitrate.” — Yellowing has many causes; the pattern and the nitrate result point to magnesium.
• “One test plant is enough.” — Use several plants in each group, so one unusual plant does not mislead.

12. YT3 · Apply

Model answer. (a) The parent plant stored starch in the seed. The seed breaks the starch down into glucose and uses it in respiration, which provides energy for growth. (b) Glucose from the same store is built into cellulose to make the walls of the new cells. Only when the leaves reach the light can the seedling make its own glucose.

A full answer includes

• (a) Starch stored in the seed is broken down to glucose, which is used in respiration to provide energy.
• (b) Glucose from the stored starch is built into cellulose for the new cell walls.

Also accept

• (a) stored starch → glucose → respiration; (b) glucose → cellulose

Watch for

• “The seed photosynthesises underground.” — There is no light underground; the seed uses its store.
• “The seed takes its food from the soil.” — The soil gives water and mineral ions; the energy store is in the seed.