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LearnYBI11WBI12 · Unit 2: Cells, Development, Biodiversity and ConservationCell structure, reproduction and developmentEukaryotic and prokaryotic cell structure
Cell structure, reproduction and development · Subtopic 2 of 5

Eukaryotic and Prokaryotic Cell Structure

Eukaryotic cells have a nucleus (double envelope with pores, chromatin, nucleolus that makes ribosomes) and membrane-bound organelles: rough ER (ribosomes; makes and transports proteins), smooth ER (lipids, steroids), Golgi body (modifies, packages proteins into vesicles), mitochondria (aerobic respiration, ATP; folded cristae), lysosomes (digestive enzymes), 80S ribosomes, centrioles and microtubules. Plant cells also have a cellulose cell wall, chloroplasts and a large vacuole. Prokaryotic cells are smaller, with no nucleus (circular DNA, plasmids), 70S ribosomes, a peptidoglycan (murein) wall and sometimes a capsule and flagellum. Viruses are non-living: nucleic acid in a protein capsid, sometimes an envelope.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI12AQA 7402 Paper 1≈ 45 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Identify organelles and describe their functions.
  • Describe how proteins are made, modified and secreted.
  • Compare prokaryotic and eukaryotic cells.
  • Describe the structure of viruses.
Pause and think

Cells in the pancreas that make digestive enzymes have huge amounts of rough ER and Golgi. Why?

Section 2 of 14

Core idea

Start hereA factory in a cell

A pancreas cell makes and exports digestive enzymes all day. Every organelle has a job on that production line.

The big idea

Eukaryotic cells divide work between membrane-bound organelles. Prokaryotes do the same jobs without them, and viruses borrow a host's machinery entirely.

1. The protein production line

Making and secreting a protein

Each step happens in a different organelle.

Worked exampleSurface area to volume

Compare SA : V for cube-shaped cells of side 1 µm (bacterium) and 20 µm (animal cell).

Goal
Two ratios.
Strategy
SA : V = 6/x for a cube.
  1. 61=6 μm−1\frac{6}{1}=6\ \mu\text{m}^{-1}
    Small cell.
  2. 620=0.3 μm−1\frac{6}{20}=0.3\ \mu\text{m}^{-1}
    Large cell: 20 times smaller ratio.
Answer6 and 0.3 µm⁻¹.
✓ CheckSmaller cells have relatively more surface ✓.
Key takeaway70S vs 80S Prokaryotes, mitochondria and chloroplasts have 70S ribosomes; eukaryotic cytoplasm has 80S.

Summary

  • RER → Golgi → vesicle → membrane.
  • Mitochondria, chloroplasts: double membranes.
  • Prokaryotes: no nucleus, plasmids, 70S.
  • Viruses: nucleic acid + capsid.
Key words
Organelle
A structure in a cell with a specific function.
Plasmid
Small circular DNA in prokaryotes.
Capsid
Protein coat of a virus.
HistorySeeing the invisible. The first electron microscopes (1930s) revealed organelles such as the ER and Golgi body that light microscopes could never resolve.
Fun factMitochondria were once bacteria. The endosymbiotic theory: mitochondria and chloroplasts have their own circular DNA and 70S ribosomes because they descend from free-living prokaryotes.
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