Plant and animal cells — answer key

1. CQ1 · Practice

Model answer. c. The cell membrane controls what enters and leaves the cell. Plant cells also have a cell wall outside the membrane, but water and dissolved substances pass through the wall easily: it keeps the cell’s shape and stops it bursting.

A full answer includes

• I chose c, the cell membrane.

Also accept

• c; the cell membrane.

Watch for

• “The cell wall controls what enters the cell.” — The cellulose wall is like a strong net: water and dissolved substances pass through it. The thin membrane inside it is the layer that controls entry and exit.
• “The nucleus decides what enters the cell.” — The nucleus controls the cell’s activities through its DNA, but substances actually cross the cell membrane.

2. CQ2 · Practice

Options as numbered on the pupil sheet: 1 — Contains the genetic material that controls the cell; 2 — Photosynthesis; 3 — Aerobic respiration: releases energy; 4 — Makes proteins; 5 — Stores cell sap; keeps the cell firm.

Model answer. Nucleus: contains the genetic material (DNA) that controls the cell. Chloroplast: photosynthesis. Mitochondrion: aerobic respiration, which releases energy from food. Ribosome: protein synthesis. Vacuole: stores cell sap, which presses outwards and keeps a plant cell firm.

A full answer includes

• Nucleus → contains the genetic material that controls the cell.
• Chloroplast → photosynthesis.
• Mitochondrion → aerobic respiration (releases energy).
• Ribosome → makes proteins.
• Large vacuole → stores cell sap and keeps the cell firm.

Also accept

• nucleus–genetic material/control; chloroplast–photosynthesis; mitochondrion–respiration; ribosome–proteins; vacuole–cell sap/firmness.

Watch for

• “Mitochondria make energy.” — Energy cannot be made. Aerobic respiration in the mitochondria releases energy that was stored in food.
• “Chloroplasts are in every cell.” — Only plant cells in green parts have chloroplasts; onion skin and root cells have none, and animal cells never do.

3. CQ6 · Practice

Options as numbered on the pupil sheet: 1 — Keeps the cell’s shape and stops it bursting; 2 — Controls what enters and leaves; 3 — Carries the genes that control the cell; 4 — Carries a few extra genes, such as one for surviving an antibiotic; 5 — Makes proteins.

Model answer. Cell wall: keeps the cell’s shape and stops it bursting (it is not made of cellulose). Cell membrane: controls what enters and leaves the cell. Circular DNA: a single loop that carries the genes controlling the cell, since there is no nucleus. Plasmid: a small extra ring of DNA with a few genes, for example one that lets the bacterium survive an antibiotic. Ribosome: makes proteins.

A full answer includes

• Cell wall → keeps the cell’s shape and stops it bursting.
• Cell membrane → controls what enters and leaves.
• Circular DNA → carries the genes that control the cell.
• Plasmid → carries a few extra genes, such as one for surviving an antibiotic.
• Ribosome → makes proteins.

Also accept

• wall–shape/stops bursting; membrane–controls entry and exit; circular DNA–genes that control the cell; plasmid–a few extra genes (e.g. antibiotic resistance); ribosome–proteins.

Watch for

• “The circular DNA is the bacterium’s nucleus.” — A bacterium has no nucleus: its DNA lies loose in the cytoplasm as one loop.
• “Plasmids are where a bacterium makes proteins.” — Proteins are made on ribosomes. Plasmids are small rings of DNA that carry a few extra genes.

4. CQ3 · Practice

Model answer. Cell membrane (the thin outline of each cell), nucleus (the dark blue body) and cytoplasm (the pale material around it). There is no cell wall, no large vacuole and no chloroplasts: these are animal cells. Mitochondria and ribosomes are too small to see at ×400.

A full answer includes

• I chose cell membrane, nucleus and cytoplasm.
• I did not choose a cell wall, a large vacuole or chloroplasts: animal cells have none of them.

Also accept

• cell membrane, nucleus, cytoplasm (not cell wall, large vacuole or chloroplasts).

Watch for

• “The outline of each cheek cell is a cell wall.” — Animal cells have no wall. The outline you see is the cell membrane, the thin layer round every cell.
• “An animal cell has no membrane, because you cannot see a wall.” — Every cell has a membrane. In a cheek cell it is the outline itself; in a plant cell it is pressed against the inside of the wall.

5. CQ4 · Practice

Model answer. (a) Both have a cell membrane, cytoplasm, a nucleus, mitochondria and ribosomes. (b) Only plant cells have a cellulose cell wall, a large permanent vacuole containing cell sap and, in green parts, chloroplasts.

A full answer includes

• (a) I named at least two structures in both: cell membrane, cytoplasm, nucleus, mitochondria or ribosomes.
• (b) I named the three found only in plant cells: cell wall, large (permanent) vacuole, chloroplasts.

Also accept

• (a) membrane, cytoplasm, nucleus, mitochondria, ribosomes (any two); (b) cell wall, large vacuole, chloroplasts.

Watch for

• “Animal cells have no membrane, because they have no wall.” — Every cell has a membrane. A plant cell has a wall as well, outside its membrane.
• “Animal cells have no nucleus.” — Nearly all animal cells have a nucleus; your cheek cells each had one. (Red blood cells are a special exception.)

6. CQ5 · Practice

Model answer. b. A bacterial cell has no nucleus: its DNA is a single loop, the circular DNA, in the cytoplasm, and many bacteria also have small rings of DNA called plasmids. It has a cell wall, a cell membrane, cytoplasm and ribosomes. It has no chloroplasts and no mitochondria.

A full answer includes

• I chose b.
• I can name the other parts: cell wall, membrane, cytoplasm, ribosomes, plasmids.

Also accept

• b; the DNA is a loop in the cytoplasm, not in a nucleus.

Watch for

• “Bacteria have no cell wall, like animal cells.” — Bacteria do have a cell wall (not made of cellulose), outside their membrane.
• “Bacteria are just very small animal cells.” — They are built differently: no nucleus, circular DNA and plasmids, and a cell wall.

7. CT1 · Apply

Model answer. X is a plant cell: only plant cells have a cell wall with a large permanent vacuole; it has no chloroplasts because it is not from a green part (for example a root). Y is an animal cell: it has a nucleus and a membrane but no cell wall. Z is a bacterium: it has a wall and ribosomes but no nucleus, and at 2 µm it is far smaller than plant and animal cells.

A full answer includes

• X is a plant cell: it has a cell wall and a large vacuole (no chloroplasts is fine: cells away from the light have none).
• Y is an animal cell: it has a nucleus but no cell wall.
• Z is a bacterium: it has no nucleus (and it is tiny, only 2 µm).

Also accept

• X plant (wall and large vacuole); Y animal (nucleus, no wall); Z bacterium (no nucleus; tiny).

Watch for

• “X cannot be a plant cell, because it has no chloroplasts.” — Only cells of green parts, which get light, have chloroplasts. A root cell or onion skin cell has none but is still a plant cell.
• “Z has a cell wall, so it must be a plant cell.” — Bacteria have a wall too. Z has no nucleus, so it is a bacterium.