Answers · Limits on photosynthesis

How a leaf is built for photosynthesis — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. LQ1 · Practice

Options as numbered on the pupil sheet: 1 — palisade mesophyll; 2 — phloem; 3 — spongy mesophyll; 4 — upper epidermis; 5 — lower epidermis; 6 — air space; 7 — cuticle.

Model answer. A is the cuticle, the thin waxy layer on the outside. B is the upper epidermis, one row of clear cells. C is the palisade mesophyll, tall cells packed side by side with many chloroplasts. D is the spongy mesophyll, rounded, loosely packed cells. E is an air space between the spongy mesophyll cells.

A full answer includes

I matched A to the cuticle.
I matched B to the upper epidermis.
I matched C to the palisade mesophyll.
I matched D to the spongy mesophyll.
I matched E to an air space.
I did not match C to the spongy mesophyll: the tall, packed cells are the palisade.

Also accept

A cuticle; B upper epidermis; C palisade mesophyll; D spongy mesophyll; E air space

Watch for

“C is the spongy mesophyll.” — The tall cells packed side by side are the palisade mesophyll; the spongy cells are rounded and loose.
“A is the upper epidermis.” — A is the thin layer on the outside of the epidermis: the waxy cuticle.

2. LQ2 · Practice

Options as numbered on the pupil sheet: 1 — guard cell; 2 — xylem; 3 — vascular bundle; 4 — spongy mesophyll; 5 — stoma; 6 — phloem; 7 — lower epidermis; 8 — chloroplast; 9 — cuticle.

Model answer. F is the xylem, the large cells with thick walls in the upper part of the vascular bundle. G is the phloem, the smaller cells in its lower part. H is a guard cell, one of the pair of bean-shaped cells with chloroplasts. I is a stoma, the pore between the two guard cells. J is the lower epidermis, the row of clear cells along the bottom. K is a chloroplast in a palisade cell. L is the whole vascular bundle, a vein.

A full answer includes

I matched F to xylem.
I matched G to phloem.
I matched H to a guard cell.
I matched I to a stoma.
I matched J to the lower epidermis.
I matched K to a chloroplast.
I matched L to a vascular bundle.
I did not match H to the stoma: the stoma is the gap, not a cell.

Also accept

F xylem; G phloem; H guard cell; I stoma; J lower epidermis; K chloroplast; L vascular bundle

Watch for

“F is the phloem.” — In this leaf the xylem, with its large thick-walled cells, is on the upper side of the bundle; the phloem is below it.
“H is the stoma.” — The stoma is the pore, I; the cells on either side of it are guard cells.

3. LQ3 · Practice

Model answer. (a) The 1st layer passes on half, 50%, to the 2nd layer, and the 2nd passes on half of that, 25%, to the 3rd. (b) Light reaching the 6th layer has passed through five layers above it, so it has been halved five times: 100% ÷ 2⁵ = 100% ÷ 32 = 3.125%, which is 3.1% to 2 significant figures. So in a thick leaf the cells deep inside would get almost no light and could photosynthesise very little. Being thin, a leaf has only a few layers of cells, and light reaches all the cells with chloroplasts.

Checked automatically

(a) Light reaching the 3rd layer: 25 % (exact value)
- Common wrong answer 50 : That is the light reaching the 2nd layer. Halve it once more.
- Common wrong answer 12.5 : You halved it three times. Only two layers lie above the 3rd.
(b) Light reaching the 6th layer: 3.1 % (2 significant figures; within ±0.001 %)
- Common wrong answer 1.6 : You halved it six times. Only five layers lie above the 6th.
- Common wrong answer 6.3 : You halved it four times. Five layers lie above the 6th.
- Common wrong answer 17 : You divided 100 by 6. Each layer halves the light that reaches it.

A full answer includes

(a) I halved the light once for each layer above the 3rd: 100% → 50% → 25%.
(a) I wrote 25%.
(b) I halved it five times, once for each layer above the 6th: 100% ÷ 32.
(b) I wrote 3.1%.
I explained that cells many layers deep would get almost no light, so they would photosynthesise very little.
I explained that in a thin leaf the light reaches all the cells with chloroplasts.
I did not halve the light six times for the 6th layer.

Also accept

(a) 25%; (b) 3.1%; deep cells get almost no light, so a thin leaf lets light reach all its cells

Watch for

“Each layer takes the same share, so the 6th layer gets 100 ÷ 6, about 17%.” — Each layer passes on half of what reaches it, so the light is halved again at every layer and falls much faster.
“A thick leaf is better, because it has more cells to photosynthesise.” — The extra cells deep inside get almost no light, so they add very little photosynthesis.

4. LQ4 · Practice

Options as numbered on the pupil sheet: 1 — carries sucrose away from the leaf; 2 — brings water to the leaf; 3 — absorbs most of the light; 4 — opens and closes the stomata; 5 — lets carbon dioxide diffuse to the mesophyll cells.

Model answer. The palisade mesophyll, near the top and packed with chloroplasts, absorbs most of the light. The air spaces let carbon dioxide diffuse from the stomata to the mesophyll cells. The xylem brings water to the leaf, and the phloem carries the sucrose made from glucose away. The guard cells open and close the stomata.

A full answer includes

I matched the palisade mesophyll to absorbing most of the light.
I matched the air spaces to letting carbon dioxide diffuse to the mesophyll cells.
I matched the xylem to bringing water to the leaf.
I matched the phloem to carrying sucrose away.
I matched the guard cells to opening and closing the stomata.
I did not swap the xylem and the phloem.

Also accept

palisade: light; air spaces: carbon dioxide; xylem: water; phloem: sucrose; guard cells: stomata

Watch for

“Xylem carries sugar.” — Xylem carries water and mineral ions; phloem carries sucrose.
“Guard cells absorb most of the light.” — They have chloroplasts, but the palisade cells absorb most light; guard cells open and close stomata.

5. LQ5 · Practice

Model answer. Row B. V marks the palisade mesophyll, the tall cells just under the clear top row. In the drawing each of them has 12 chloroplasts, more than any other lettered cell, and they lie near the upper surface, where the light is brightest, so they absorb the most light and carry out the most photosynthesis. W marks the upper epidermis: the light reaches it first, but its cells have no chloroplasts. T marks a spongy mesophyll cell, with air spaces around it but only 4 chloroplasts. U marks a guard cell beside the stoma: it has 3 chloroplasts, and it lies on the lower surface, where little light is left.

A full answer includes

I chose row B.
I counted 12 chloroplasts in each cell V, more than in any other lettered cell.
I checked that cells V lie just under the clear top row, where the light is brightest.
I identified the cells V as the palisade mesophyll.
I did not choose row A: the cells W have no chloroplasts.

Also accept

Watch for

“Row A.” — The light does reach the cells W first, but count again: they have no chloroplasts, not 12. They are the clear upper epidermis, which lets the light through.
“Row C.” — T does have air spaces around it, but each T cell has only 4 chloroplasts, not 12. It is a spongy mesophyll cell.
“Row D.” — U does have 3 chloroplasts and sits beside the gap, but 3 is far fewer than 12, and U lies on the lower surface, where little light is left. It is a guard cell.

6. LQ11 · Practice

Model answer. (a) A leaf catches light and takes in carbon dioxide over its surface. With half the area left, each leaf absorbs less light and has less surface, with fewer stomata, through which carbon dioxide can diffuse in, so the plant photosynthesises more slowly, even in bright light. (b) The light a leaf catches depends on its area, not on its number of cells: half the area catches about half the light, however thick the leaf is. And in a leaf twice as thick, the cells would be stacked deeper. The upper layers would absorb most of the light, so little would reach the cells underneath, and carbon dioxide would have further to diffuse to the cells in the middle. Many of the cells would photosynthesise slowly. A thin leaf lets light reach all its cells, and carbon dioxide has only a short distance to diffuse.

A full answer includes

(a) I wrote that a smaller surface area absorbs less light.
(a) I wrote that there is less surface, with fewer stomata, for carbon dioxide to diffuse in.
(b) I wrote that the light a leaf catches depends on its area, not on its number of cells.
(b) I wrote that in a thick leaf little light reaches the cells deep inside.
(b) I wrote that carbon dioxide would have further to diffuse to the inner cells.
I did not write that a thicker leaf is better because it has more chloroplasts.

Also accept

(a) less surface area, so less light absorbed and less carbon dioxide taken in; (b) the light caught depends on the area, not the number of cells; light would not reach the inner cells, and carbon dioxide would have further to diffuse

Watch for

“Thick leaves are better, because they hold more chloroplasts.” — Chloroplasts deep in a thick leaf get little light, so they add little photosynthesis.
“A smaller leaf photosynthesises just as fast, because the light on it is just as bright.” — Each square centimetre gets the same light, but a smaller area catches less light in all.

7. LQ6 · Practice

Options as numbered on the pupil sheet: 1 — less water reaches the mesophyll cells; 2 — less carbon dioxide gets into the leaf; 3 — carbon dioxide inside the leaf reaches the mesophyll cells more slowly; 4 — the light that reaches the cells is hardly absorbed; 5 — less light reaches the palisade cells.

Model answer. Soot on the upper surface blocks the light before it gets into the leaf, so less light reaches the palisade cells. In the pale patch the light still passes through the clear epidermis, but with very few chloroplasts the palisade cells hardly absorb it. Closed guard cells shut the stomata, so less carbon dioxide gets into the leaf. In the flooded leaf the stomata are open, but gases diffuse far more slowly through water than through air, so carbon dioxide inside the leaf reaches the mesophyll cells more slowly. None of the faults cuts the water supply: that comes up the xylem.

A full answer includes

I matched the soot to less light reaching the palisade cells.
I matched the pale patch to the light that reaches the cells being hardly absorbed.
I matched the closed guard cells to less carbon dioxide getting into the leaf.
I matched the flooded air spaces to carbon dioxide reaching the mesophyll cells more slowly.
I did not match the flooded air spaces to less carbon dioxide getting in: the stomata are still open.

Also accept

soot: less light reaches the palisade cells; pale patch: light hardly absorbed; closed guard cells: less carbon dioxide gets in; flooded air spaces: carbon dioxide reaches the cells more slowly

Watch for

“The soot means less carbon dioxide gets into the leaf.” — Most stomata are on the lower surface. The soot on top blocks the light.
“The pale patch means less light reaches the palisade cells.” — Light still passes through the clear epidermis. With few chloroplasts the cells cannot absorb it.
“The closed guard cells mean carbon dioxide reaches the mesophyll cells more slowly.” — The gas cannot get into the leaf at all, because the pores are shut.
“The flooded air spaces mean less carbon dioxide gets into the leaf.” — The stomata are open. Gases diffuse far more slowly through water than through air.
“One of the faults means less water reaches the mesophyll cells.” — Water comes up the xylem in the veins. None of these faults touches the veins.

8. LQ7 · Practice

Options as numbered on the pupil sheet: 1 — a guard cell in the lower epidermis; 2 — the spongy mesophyll; 3 — the upper epidermis; 4 — the cuticle; 5 — the palisade mesophyll.

Model answer. The light passes through the transparent, waxy cuticle and the clear cells of the upper epidermis, then through the palisade mesophyll, where most of it is absorbed, and the spongy mesophyll with its air spaces. Most stomata are in the lower epidermis, so the guard cells around them get only the light that is left.

A full answer includes

First: the cuticle.
Second: the upper epidermis.
Third: the palisade mesophyll.
Fourth: the spongy mesophyll.
Last: a guard cell in the lower epidermis.
I did not end the path in the upper epidermis: most stomata, and so most guard cells, are in the lower epidermis.

Also accept

cuticle; upper epidermis; palisade mesophyll; spongy mesophyll; guard cell in the lower epidermis

Watch for

“The light reaches the spongy mesophyll before the palisade.” — The palisade mesophyll lies just under the upper epidermis, so the light reaches it first.
“The guard cells are in the upper epidermis, so the light reaches them second.” — Most stomata, with their guard cells, are in the lower epidermis, so the light reaches them last.

9. LQ8 · Practice

Model answer. A waxy layer is waterproof, so the thick wax cuts the water lost from the leaf surface, and P wilts less in the hot, dry week. But a cuticle helps photosynthesis only when it is transparent: thick, cloudy wax lets less light through to the palisade cells. In the sunny week there is light to spare, so this costs P little. In the dull week light is the limiting factor, so P, with less light reaching its palisade cells, photosynthesises more slowly than Q.

A full answer includes

I chose: the thick wax cuts water loss, but it lets less light through to the palisade.
I explained that the wax is waterproof, so P loses less water and wilts less in the dry week.
I explained that cloudy wax lets less light through than a thin, clear cuticle.
I explained that in a dull week light is the limiting factor, so P, with less light reaching its palisade cells, photosynthesises more slowly.
I did not choose the stomata statement: most stomata are in the lower epidermis, not the upper.

Also accept

the thick wax cuts water loss, but lets less light through to the palisade: that matters when light is limiting

Watch for

“The thick wax blocks the stomata in the upper epidermis.” — The upper epidermis has few stomata; most are in the lower epidermis. The wax on top changes what passes through the top surface: water vapour and light.
“The thick wax absorbs light for photosynthesis, so P needs bright sun to use it.” — Wax has no chlorophyll and cannot photosynthesise. Light it stops is lost to the palisade cells below.
“The thick wax stops rain soaking in, so P gets less water in dull, wet weather.” — Leaves do not take in rain through their surface. Water comes from the roots, up the xylem.

10. LQ9 · Practice

Model answer. The midrib is the largest of the leaf's vascular bundles, and its xylem brings water from the roots. Beyond the cut no more water arrives, so the cells lose water and that part of the leaf wilts. Water is a raw material of photosynthesis; a leaf short of water also closes its stomata, which lets in less carbon dioxide. So photosynthesis slows beyond the cut. The phloem carries sucrose out of the leaf, and the gases move through the stomata and air spaces, not along the veins.

A full answer includes

I chose: the xylem can no longer bring water to the cells.
I checked that the wilting shows the cells beyond the cut are short of water.
I checked that water is a raw material of photosynthesis.
I did not choose the sucrose statement: the phloem carries sucrose out of a leaf, not into it.

Also accept

the cut xylem can no longer bring water, a raw material, to the cells beyond the cut

Watch for

“Beyond the cut, the phloem can no longer bring sucrose to the cells.” — The phloem carries sucrose away from a leaf, not into it. Sucrose is a product of photosynthesis, not a raw material.
“Beyond the cut, the veins can no longer bring carbon dioxide to the cells.” — Carbon dioxide diffuses in through the stomata and the air spaces, not along the veins. The cut does not touch that path.
“Beyond the cut, the phloem can no longer carry the oxygen away.” — Oxygen diffuses out through the air spaces and the stomata. The phloem carries sucrose.

11. LQ10 · Practice

Model answer. (a) Water vapour escapes through open stomata. The desert is hot in the day and cool at night, so opening the stomata only at night saves water: far less is lost. (b) Photosynthesis needs light, which comes only in the day, and carbon dioxide, which enters through open stomata. In the day the stomata are closed, so no carbon dioxide can diffuse in while there is light to use it. (c) The cactus could take in carbon dioxide through its open stomata at night and store it in its cells, then use the stored carbon dioxide for photosynthesis in the day.

A full answer includes

(a) I wrote that water vapour escapes through open stomata.
(a) I wrote that the desert night is cool, so less water is lost through stomata open at night than it would be in the hot day.
(b) I wrote that with the stomata closed, carbon dioxide cannot diffuse into the cactus.
(b) I wrote that the day is when there is light for photosynthesis.
(c) I suggested that it takes in carbon dioxide at night and stores it in its cells until the day.
I did not write that the cactus photosynthesises at night: there is no light then.

Also accept

(a) less water lost through stomata open in the cool night; (b) no carbon dioxide can enter in the day, when there is light; (c) take in carbon dioxide at night and store it for the day

Watch for

“The cactus photosynthesises at night, when its stomata are open.” — Photosynthesis needs light. At night the stomata let carbon dioxide in, but the light comes only in the day.
“The stomata are closed in the day to keep the carbon dioxide in.” — They are closed to save water. Closed stomata also stop carbon dioxide getting in from the air.

12. LT1 · Apply

Model answer. The lower surface of a floating leaf lies on the water, so stomata there could not exchange gases with the air. The upper surface is open to the air, so stomata there let carbon dioxide diffuse in from the air and oxygen diffuse out.

A full answer includes

I wrote that the lower surface lies on the water, so stomata there could not take in carbon dioxide from the air.
I wrote that the upper surface is in the air, so stomata there let carbon dioxide diffuse in.
I did not write that the stomata take in water from the pond.

Also accept

the lower surface is on water; the upper surface is in the air, where the carbon dioxide is

Watch for

“Its stomata are on top to take in water from the pond.” — Stomata exchange gases; a water lily takes in water through its roots.
“The upper surface gets more light, so stomata are there to catch light.” — Stomata let gases in and out; chloroplasts absorb the light.

13. LT2 · Apply

Model answer. In the shade the light is dim. A larger leaf has a bigger surface to catch more of the little light there is. Being thinner, with fewer palisade layers, means the dim light still reaches all the cells: extra layers deeper down would get almost no light and would only cost the plant materials to build.

A full answer includes

I wrote that in the shade the light is dim.
I wrote that a larger surface area catches more of the little light there is.
I wrote that a thinner leaf, with fewer palisade layers, lets the dim light reach all its cells.
I did not write that shade leaves are bigger because they get more light.

Also accept

larger area catches more of the dim light; thinner so the light reaches all the cells

Watch for

“Shade leaves are bigger because they get more light.” — They get less light, so a bigger surface helps to catch more of it.
“Shade leaves need more palisade layers to absorb light.” — In dim light, deeper layers would get almost no light; fewer layers is enough.

14. LT3 · Apply

Model answer. Photosynthesis in that leaf slows down greatly. Most of the stomata are in the lower epidermis, and the jelly blocks them, so carbon dioxide can hardly diffuse into the leaf and the air spaces run short of it. Carbon dioxide is a raw material, so photosynthesis slows.

A full answer includes

I predicted that photosynthesis in that leaf slows down greatly.
I wrote that most stomata are in the lower epidermis, so the jelly blocks them.
I wrote that carbon dioxide can no longer diffuse into the leaf.
I wrote that carbon dioxide is a raw material, so with little of it the rate falls.
I did not write that water can no longer get into the leaf.

Also accept

it slows: stomata on the lower surface are blocked, so little carbon dioxide gets in

Watch for

“Photosynthesis is not affected, because light still reaches the upper surface.” — Light is not the only need; without carbon dioxide getting in, the rate falls.
“Photosynthesis stops because water cannot get in.” — Water arrives through the xylem; the blocked stomata stop carbon dioxide getting in.

15. LT4 · Apply

Model answer. (a) The bottom edge of her view is the upper surface of the leaf, so the section is upside down. (b) The tall cells crowded with chloroplasts are palisade mesophyll cells. The bean-shaped green cells are guard cells, and the gap between them is a stoma. The wide, thick-walled tubes are xylem; the small cells are phloem. (c) In a leaf the right way up, the upper epidermis is on top, the palisade mesophyll lies just under it, the spongy mesophyll with its air spaces lies below it, and most stomata are in the lower epidermis. In the vascular bundle the xylem is on the upper side. Here every one of these is the other way round.

A full answer includes

(a) I identified the bottom edge of her view as the upper surface of the leaf.
(b) I identified the tall cells as palisade mesophyll.
(b) I identified the bean-shaped cells as guard cells.
(b) I identified the gap between them as a stoma.
(b) I identified the wide tubes as xylem.
(c) I gave two features: for example the palisade lies just under the upper epidermis, and the stoma is in the lower epidermis.
I did not take the top of the view to be the upper surface.

Also accept

(a) the bottom edge; (b) palisade mesophyll, guard cells, stoma, xylem; (c) e.g. the palisade is next to the upper epidermis, and the stoma is in the lower epidermis

Watch for

“The top of the view is always the upper surface of the leaf.” — A section can be put on the slide either way up. Use the layers: the palisade lies just under the upper epidermis.
“The wide, thick-walled tubes are phloem.” — Wide tubes with thick walls are xylem. The phloem is the smaller cells.