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Cell structure and microscopy

Identify cell structures and confidently answer microscopy calculation questions (magnification, real size, image size).

Teach (quick)

Eukaryotes vs prokaryotes

  • Eukaryotic cells (plants/animals): have a nucleus.
  • Prokaryotic cells (bacteria): no nucleus; DNA is a single circular loop in the cytoplasm.
  • Bacteria can have plasmids (extra small rings of DNA).

Structures to recognise

Animal cells: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes.

Plant cells: everything in animal cells plus cell wall, chloroplasts, permanent vacuole.

Bacterial cells (prokaryote): cytoplasm, cell membrane, cell wall, circular DNA, plasmids (sometimes).

Microscopy calculations

Formula:

  • magnification = image size ÷ real size

Be careful with units:

  • 1 mm = 1000 µm

Worked examples

  1. An image of a cell is 20 mm long. The real cell is 50 µm long. Calculate magnification.
  • Convert 50 µm to mm: 50 µm = 0.05 mm
  • magnification = 20 ÷ 0.05 = 400×
  1. A microscope image is 60 mm. The magnification is 300×. Calculate the real size.
  • real size = image size ÷ magnification = 60 mm ÷ 300 = 0.2 mm = 200 µm

Practice

MCQ

  1. Which structure is found in plant cells but not animal cells?
  • A Mitochondria
  • B Cell membrane
  • C Chloroplasts
  • D Ribosomes
  1. Which organelle is the site of aerobic respiration?
  • A Ribosome
  • B Mitochondrion
  • C Nucleus
  • D Vacuole
  1. Which statement about bacteria is correct?
  • A They have a nucleus
  • B Their DNA is a single circular loop
  • C They have chloroplasts
  • D They are always larger than animal cells
  1. If magnification = image size ÷ real size, then real size =
  • A image size × magnification
  • B image size ÷ magnification
  • C magnification ÷ image size
  • D image size + magnification

Short answer

  1. Give two differences between prokaryotic and eukaryotic cells. (2)

  2. State the function of mitochondria. (1)

  3. A cell image is 12 mm long. The real cell is 30 µm long. Calculate the magnification. (3)

Mark scheme (condensed)

  • Prokaryotes: no nucleus, DNA is circular (can mention plasmids)
  • Eukaryotes: nucleus present
  • Mitochondria: aerobic respiration / releases energy
  • Calculation: convert units correctly; use magnification equation