From root hair to leaf — answer key

1. HQ1 · Practice

Options as numbered on the pupil sheet: 1 — A; 2 — B; 3 — C; 4 — D.

Model answer. A is a root hair cell: a cell of the outer layer with a long, thin outgrowth, its root hair. B is the cortex, the wide band of cells between the outer layer and the middle of the root. C, the cross in the middle, is xylem, and D, between its arms, is phloem.

A full answer includes

• Root hair cell: A, the long, thin outgrowth on the outside.
• Root cortex cells: B, the wide band of cells inside the outer layer.

Also accept

• root hair cell A; root cortex cells B

Watch for

• “C is the root hair cell, because it is in the middle.” — Root hair cells are on the outside of the root, where they touch the soil.
• “A is a whole small root.” — A root hair is part of one cell: a thin outgrowth of a root hair cell.

2. HQ2 · Practice

Model answer. Root hair cells take in water and mineral ions from the soil. They have no chloroplasts and get no light, so they cannot photosynthesise. Each hair is far too thin to store much water, and sucrose is carried by the phloem, not by root hair cells.

A full answer includes

• Functions: taking in water; taking in mineral ions.
• Not functions: photosynthesis (no chloroplasts, no light); storing water for dry days; carrying sucrose to the leaves.

Also accept

• taking in water; taking in mineral ions

Watch for

• “Root hair cells photosynthesise.” — They grow underground, in the dark, and have no chloroplasts.
• “Root hair cells store water.” — They are far too thin to store much; their job is to take water in.

3. HQ3 · Practice

Model answer. Water goes in by osmosis. The water in the soil is a more dilute solution than the cell sap inside the root hair cell, so water moves from the soil into the cell through its partially permeable cell membrane.

A full answer includes

• By osmosis: the soil water is more dilute than the cell sap, so water moves into the cell through its partially permeable cell membrane.

Also accept

• by osmosis, from the more dilute soil water into the cell

Watch for

• “Water moves in because the soil water is more concentrated.” — In osmosis water moves from a more dilute solution into a more concentrated one. The soil water is the more dilute.
• “The root hair sucks water in through an open end, like a straw.” — Water moves in by osmosis, across the cell membrane; the hair has no open end.

4. HQ4 · Practice

Model answer. Root hairs give the root a large surface area, which increases the uptake of water and mineral ions.

A full answer includes

• g1: surface.
• g2: uptake (taking up, absorption).

Also accept

• surface; uptake

Watch for

• “Root hairs make the root heavier, so it holds more water.” — It is their surface area, in contact with the soil water, that matters.
• “A large surface area slows uptake down.” — More surface touching the soil water means more water and mineral ions taken up in the same time.

5. HQ5 · Practice

Model answer. Pulling the first seedling roughly out of the soil tore off most of its root hairs, which stayed behind in the soil. With few root hairs, its roots had a much smaller surface area, so they took up water more slowly than the leaves lost it, and the seedling wilted. The second seedling kept its root hairs inside the ball of soil, so its roots still had a large surface area and took up enough water.

A full answer includes

• Pulling it out roughly left most of its root hairs torn off in the soil.
• With fewer root hairs the surface area for uptake is much smaller, so it takes up water too slowly and wilts.
• The ball of soil protects the root hairs of the second seedling, so it keeps a large surface area.

Also accept

• root hairs torn off, so a smaller surface area and too little water taken up; the ball of soil keeps the root hairs

Watch for

• “The ball of soil is a store of food for the seedling.” — The soil protects the root hairs, so the roots keep their large surface area for taking up water.
• “The first seedling wilted because it had too much water.” — It wilted because it could not take up water fast enough.

6. HQ6 · Practice

Options as numbered on the pupil sheet: 1 — Root hair cells; 2 — Root cortex cells; 3 — Xylem; 4 — Mesophyll cells.

Model answer. Water goes into the root hair cells from the soil, crosses the root cortex cells, enters the xylem in the middle of the root, is carried up through the root, stem and leaf, and then moves out of the xylem into the mesophyll cells.

A full answer includes

• Root hair cells, then root cortex cells, then xylem, then mesophyll cells.

Also accept

• root hair cells; root cortex cells; xylem; mesophyll cells

Watch for

• “Root hair cells, then xylem, then root cortex cells.” — The xylem is in the middle of the root, so water crosses the cortex first.
• “Water goes up the stem in the phloem.” — Water goes up in the xylem.

7. HQ7 · Practice

Model answer. Mistake 1: water does not go straight from the root hairs into the xylem. The xylem is in the middle of the root, so water first crosses the root cortex cells. Mistake 2: water goes up the stem in the xylem, not the phloem. The correct pathway is root hair cells, root cortex cells, xylem, mesophyll cells.

A full answer includes

• The root cortex cells are missing: water crosses them between the root hair cells and the xylem.
• Water goes up the stem in the xylem, not the phloem.

Also accept

• missing root cortex cells; xylem, not phloem

Watch for

• “The only mistake is "phloem".” — The root cortex cells are missing too.
• “Water goes through the phloem in the root and the xylem in the stem.” — Water travels in the xylem all the way from the root to the leaf.

8. HQ8 · Practice

Options as numbered on the pupil sheet: 1 — A; 2 — B; 3 — C; 4 — D.

Model answer. In a root the xylem is in the middle, shaped like a star or a cross: that is C. The phloem is in small groups between the arms of the xylem: that is D.

A full answer includes

• Xylem: C, the star or cross in the middle.
• Phloem: D, the small groups between the arms of the xylem.

Also accept

• xylem C; phloem D

Watch for

• “The xylem is in a ring near the outside, as in a stem.” — In a root the xylem is in the middle.
• “D is xylem, because it is in the middle too.” — D is the small groups between the arms: phloem. The cross itself is xylem.

9. HQ9 · Practice

Options as numbered on the pupil sheet: 1 — A; 2 — B; 3 — C; 4 — D.

Model answer. In each vein of a leaf, the xylem is towards the upper surface: C, the wide cells with thick walls. The phloem is towards the lower surface: B, the small cells. A is the upper epidermis and D the spongy mesophyll.

A full answer includes

• Xylem: C, in the upper half of the vein (towards the upper surface).
• Phloem: B, in the lower half of the vein.

Also accept

• xylem C; phloem B

Watch for

• “B is the xylem, because water vapour leaves through the lower surface.” — In each vein the xylem, with its wide tubes, is in the upper half; the phloem is in the lower half.
• “A is the xylem, because water goes to the top of the leaf.” — A is the upper epidermis. The xylem is inside the vein.

10. HQ10 · Practice

Options as numbered on the pupil sheet: 1 — on the inner side of each bundle, in a ring near the outside; 2 — in the middle, like a star or a cross; 3 — in each vein, towards the upper surface.

Model answer. In a stem the xylem is on the inner side of each bundle, in a ring of bundles near the outside. In a root it is in the middle, shaped like a star or a cross. In a leaf it is in each vein, towards the upper surface.

A full answer includes

• Stem: on the inner side of each bundle, in a ring near the outside.
• Root: in the middle, like a star or a cross.
• Leaf: in each vein, towards the upper surface.

Also accept

• stem: inner side of bundles in a ring; root: middle; leaf: towards the upper surface of each vein

Watch for

• “The xylem is in the middle of a stem, as in a root.” — In a stem the bundles form a ring near the outside.
• “In a leaf the xylem is towards the lower surface.” — It is towards the upper surface; the phloem is towards the lower surface.

11. HT1 · Apply

Model answer. Seedling P will take up mineral ions faster. Its many root hairs give its roots a much larger surface area in contact with the solution, so more mineral ions can be taken up in the same time. Q has lost most of that surface area.

A full answer includes

• P takes up mineral ions faster.
• Its many root hairs give a larger surface area in contact with the solution, so more ions can be taken up in the same time.

Also accept

• P: more root hairs, larger surface area, faster uptake

Watch for

• “Q is faster, because damaged roots let ions leak in.” — The root hairs are the large surface for uptake; losing them slows uptake.
• “Root hairs take in only water, not mineral ions.” — Root hair cells take in both water and mineral ions.

12. HT2 · Apply

Model answer. (a) The xylem carries the water up the trunk; the wood of a tree is mostly xylem. (b) Mesophyll cells use water as a raw material for photosynthesis. Much of the water also evaporates from their surfaces into the air spaces and diffuses out through the stomata, which is transpiration, so it must be replaced all the time.

A full answer includes

• (a) Xylem.
• (b) Mesophyll cells use water in photosynthesis; much of the water also evaporates from them and leaves through the stomata, so it must be replaced.

Also accept

• (a) xylem; (b) for photosynthesis, and to replace water lost through the stomata

Watch for

• “The phloem carries water up the trunk.” — Water goes up in the xylem; phloem carries sucrose and amino acids.
• “The mesophyll cells make their own water.” — Water comes up from the roots in the xylem.

13. HT3 · Apply

Model answer. Long roots reach down to water deeper in the soil, where the sand is still damp. The many root hairs on them give a very large surface area in contact with the thin films of water, which increases the uptake of water and mineral ions, so the plant takes up as much as it can of the little water there is.

A full answer includes

• Long roots reach water deeper in the soil.
• Many root hairs give a large surface area, which increases the uptake of the little water (and mineral ions) there is.

Also accept

• long roots reach deeper water; root hairs give a large surface area for uptake

Watch for

• “The root hairs store water for the dry season.” — They are too thin to store much; their job is uptake.
• “Long roots make food in the dry soil.” — Roots have no chloroplasts and get no light.

14. HT4 · Apply

Model answer. The dye travels with the water, in the xylem, so the red part of each vein is the xylem. In each vein of a leaf, the xylem is towards the upper surface and the phloem towards the lower surface. So (a) X, the surface nearer the red xylem, is the upper surface. (b) The pale part of each vein, nearer Y, the lower surface, is the phloem: it carries sucrose and amino acids, not the dyed water, so it stays pale.

A full answer includes

• (a) X is the upper surface.
• (b) The dye travels with the water in the xylem, so the red part of each vein is the xylem, and in a leaf the xylem is towards the upper surface. The pale part, towards the lower surface (Y), is the phloem.

Also accept

• (a) X; (b) the red part is the xylem, which is towards the upper surface; the pale part is the phloem

Watch for

• “Y is the upper surface, because the phloem is always on the outer side of a bundle.” — That is a stem. In each vein of a leaf, the xylem is towards the upper surface and the phloem towards the lower surface.
• “The pale part is more xylem that the dye has not reached yet.” — The dye is in every vein. The pale part is a different tissue, the phloem, which does not carry the water.