Cells under the microscope — return questions key

1. CR1 · Return

Model answer. In every cell, the cell membrane controls what enters and leaves, and most chemical reactions happen in the cytoplasm. The nucleus contains the genetic material. Only plant cells have a cell wall made of cellulose. A bacterium has no nucleus: its genes are on a loop of circular DNA in the cytoplasm.

A full answer includes

• cell membrane; cytoplasm; nucleus; cell wall; circular DNA.

Also accept

• cell membrane; cytoplasm; nucleus; cell wall; circular DNA

Watch for

• “The cell wall controls what enters.” — That is the membrane. The wall keeps the shape and stops the cell bursting.

2. CR2 · Return

Options as numbered on the pupil sheet: 1 — Cell wall; 2 — Cell membrane; 3 — Circular DNA; 4 — Plasmid; 5 — Ribosome; 6 — Nucleus.

Model answer. A is the cell wall, B the cell membrane just inside it, C the circular DNA (a loose loop: there is no nucleus), D a plasmid (a small extra ring of DNA) and E a ribosome.

A full answer includes

• A cell wall; B cell membrane; C circular DNA; D plasmid; E ribosome.
• I did not choose Nucleus: bacteria have none.

Also accept

• A cell wall; B cell membrane; C circular DNA; D plasmid; E ribosome

Watch for

• “C is the nucleus.” — A bacterium has no nucleus. C is its circular DNA, lying loose in the cytoplasm.
• “A bacterium has no cell wall, like an animal cell.” — Bacteria have a wall (not made of cellulose) outside the membrane: A is the wall, B the membrane.

3. OR1 · Return

Model answer. New cells are produced by division of existing cells. A group of similar cells working together is a tissue. Cells that carry electrical impulses are neurones. Cells that move mucus in the airways have cilia on their surface. Sperm and egg cells are specialised for reproduction.

A full answer includes

• division; tissue; neurones; cilia; reproduction.

Also accept

• division; tissue; neurones; cilia; reproduction

Watch for

• “A group of similar cells is an organ.” — An organ is made of several tissues; a group of similar cells is a tissue.

4. MR1 · Return

Model answer. actual size = 120 mm ÷ 400 = 0.3 mm = 0.3 × 1000 µm = 300 µm.

Checked automatically

• Actual length: 300 µm (within ±2%)
  • Common wrong answer 48000 : You multiplied by 400. Divide the image size by the magnification.

A full answer includes

• 120 ÷ 400 = 0.3 mm.
• 0.3 × 1000 = 300 µm.

Also accept

• 300 µm

Watch for

• “Give the answer as 0.3 because that is what the division gives.” — 0.3 is in mm. The question asks for µm: multiply by 1000.

5. MR2 · Return

Model answer. Change the actual size to millimetres: 4 µm = 0.004 mm. magnification = image size ÷ actual size = 36 mm ÷ 0.004 mm = ×9000. The photograph was enlarged again when it was printed, so it is not ×400.

Checked automatically

• Magnification (no unit): 9000 (within ±2%)
  • Common wrong answer 400 : ×400 is the microscope. Work out the photograph’s magnification: image size ÷ actual size.
  • Common wrong answer 9 : You divided 36 by 4 without changing the units. Change 4 µm to 0.004 mm first.

A full answer includes

• 4 µm = 4 ÷ 1000 = 0.004 mm.
• magnification = 36 mm ÷ 0.004 mm = ×9000.

Also accept

• ×9000 (9000)

Watch for

• “The photograph is ×400, because the microscope was set at ×400.” — The magnification of a picture is image size ÷ actual size. A photo can be printed at any size, so work it out.