Cells under the microscope — final check key

1. CK1 · Final check

Model answer. (a) A is the cell wall, B a chloroplast, C the nucleus and D the large vacuole (full of cell sap). (b) Any two of: a cell wall, chloroplasts and a large permanent vacuole; animal cells have none of these. The nucleus does not help, because animal cells have one too.

A full answer includes

• (a) A cell wall; B chloroplast; C nucleus; D vacuole.
• (b) Any two: it has a cell wall (A); it has chloroplasts (B); it has a large vacuole (D).

Also accept

• (a) A wall, B chloroplast, C nucleus, D vacuole; (b) two of wall, chloroplasts, large vacuole.

Watch for

• “Its nucleus shows it is a plant cell.” — Animal cells have a nucleus too. Only the wall, the chloroplasts and the large vacuole belong to plant cells alone.

2. CK2 · Final check

Model answer. (a) A bacterium: it has no nucleus, its DNA is a loop (circular DNA) with plasmids, and at 2 µm it is far smaller than a plant or animal cell. (b) The circular DNA carries the genes that control the cell. (c) The cell wall keeps the cell’s shape and stops it bursting.

A full answer includes

• (a) A bacterium (bacterial cell): no nucleus, circular DNA and plasmids, and tiny.
• (b) It carries the genes that control the cell.
• (c) It keeps the cell’s shape and stops it bursting.

Also accept

• (a) bacterium; (b) carries the genes / controls the cell; (c) keeps shape / stops bursting.

Watch for

• “It is a plant cell, because it has a cell wall.” — Bacteria have walls too. With no nucleus and its DNA in a loop, it is a bacterium.
• “The cell wall controls what enters the cell.” — That is the membrane’s job. The wall keeps the shape and stops the cell bursting.

3. OK1 · Final check

Model answer. (a) A neurone conducts electrical impulses. (b) It has a long thin fibre, the axon, that carries the impulse a long way through the body. (c) The brain is an organ, because several different tissues work together in it. (d) New cells are produced by the division of existing cells.

A full answer includes

• (a) It conducts electrical impulses.
• (b) Its long thin fibre, the axon, carries the impulse a long way.
• (c) An organ: several tissues working together.
• (d) From the division of existing cells.

Also accept

• (a) conducts impulses; (b) long axon carries the impulse far; (c) organ; (d) existing cells divide.

Watch for

• “The brain is a tissue, because it is made of nerve cells.” — The brain contains several different tissues working together, so it is an organ.
• “New body cells are made from the food we eat.” — Food supplies materials and energy, but every new cell comes from an existing cell that divides.

4. MK1 · Final check

Model answer. (a) 25 µm = 25 ÷ 1000 mm = 0.025 mm. (b) magnification = image size ÷ actual size = 50 mm ÷ 0.025 mm = ×2000.

Checked automatically

• (a) Actual length in mm: 0.025 mm (within ±2%)
• (b) Magnification (no unit): 2000 (within ±2%)
  • Common wrong answer 2 : You divided 50 by 25 without changing the units. Change 25 µm to 0.025 mm first.

A full answer includes

• (a) 25 µm ÷ 1000 = 0.025 mm.
• (b) 50 mm ÷ 0.025 mm = ×2000.

Also accept

• (a) 0.025 mm; (b) ×2000

Watch for

• “Divide 50 by 25 straight away.” — The sizes are in different units; change them to the same unit first.