Eukaryotes took 1.5 billion years to evolve from prokaryotes. The result: a cell so complex it can become a neuron, a muscle fibre, a root hair, or a cancer cell — all from the same DNA.
Before reading on, make a prediction:
You know from Lesson 03 that prokaryotes lack membrane-bound organelles. Eukaryotes have them. Predict: which three organelles do you think are most important for a eukaryotic cell's survival, and why? Think about what a cell fundamentally needs to do.
Come back to this at the end of the lesson.
Core Content
For the first 2 billion years of life, every organism was prokaryotic — small, simple, and without internal compartments. Then, approximately 1.5–2 billion years ago, a new kind of cell appeared: the eukaryote. Its defining innovation was compartmentalisation — using internal membranes to divide the cell into specialised zones, each optimised for a different function.
This was not a small upgrade. Membrane compartmentalisation allowed eukaryotic cells to become orders of magnitude more complex than any prokaryote. It also allowed cells to specialise — to become neurons, muscle fibres, or photosynthetic leaf cells — while still carrying identical DNA.
These structures are found in every eukaryotic cell — animal, plant, fungal, and protist:
Double membrane-bound organelle containing the cell's DNA as linear chromosomes. Controls gene expression and therefore all cell activity. Contains the nucleolus where rRNA is made.
All eukaryotes
Phospholipid bilayer with embedded proteins. Selectively permeable — controls what enters and exits. Site of cell signalling and transport.
All eukaryotes
Gel-like fluid (cytosol) plus all organelles. Site of many metabolic reactions. Contains the cytoskeleton — a network of protein filaments providing structure and enabling movement.
All eukaryotes
Double membrane. Inner membrane folded into cristae — increases surface area for ATP production. Site of aerobic cellular respiration. Contains its own circular DNA (evidence of endosymbiotic origin).
All eukaryotes
Site of protein synthesis. Found free in cytoplasm or bound to rough ER. Eukaryotic ribosomes are larger (80S) than prokaryotic (70S) — the basis of antibiotic selectivity.
All eukaryotes
Network of membrane-bound tubes and sacs continuous with the nuclear envelope. Rough ER: studded with ribosomes; folds and processes proteins. Smooth ER: no ribosomes; synthesises lipids and detoxifies chemicals.
All eukaryotes
Stack of flattened membrane sacs. Receives proteins from rough ER, modifies them (e.g. adds sugar chains), packages them into vesicles, and directs them to their destination — secretion, lysosomes, or cell membrane.
All eukaryotes
Small vesicles containing digestive enzymes at low pH. Break down worn-out organelles, food particles, and pathogens. Dysfunction linked to Tay-Sachs disease and other lysosomal storage disorders.
Primarily animal cells
Plant cells have all the organelles above, plus three additional structures not found in animal cells:
Double membrane. Inner membrane system (thylakoids) stacked into grana — site of light reactions. Stroma surrounds thylakoids — site of Calvin cycle. Contains its own circular DNA (endosymbiotic origin, like mitochondria). Site of photosynthesis.
Plant cells only
Rigid outer layer of cellulose fibres outside the cell membrane. Provides structural support, prevents over-expansion, and gives plant cells their fixed shape. Unlike bacterial cell walls (peptidoglycan), plant cell walls are made of cellulose.
Plant cells only
Single large membrane-bound sac occupying up to 90% of cell volume in mature plant cells. Stores water, nutrients, and waste products. Turgor pressure from the vacuole pushing against the cell wall keeps plants rigid. Also stores pigments (e.g. anthocyanins giving red/purple colours).
Plant cells (large central vacuole); small vacuoles in animal cells
| Feature | Animal cell | Plant cell |
|---|---|---|
| Cell wall | Absent | Present — cellulose |
| Cell membrane | Present | Present (inside cell wall) |
| Nucleus | Present — often central | Present — often peripheral (pushed aside by vacuole) |
| Chloroplasts | Absent | Present — in photosynthetic cells |
| Vacuole | Small, temporary vacuoles | Large central vacuole (up to 90% of cell volume) |
| Mitochondria | Present — many | Present — fewer (chloroplasts also produce ATP) |
| Lysosomes | Common | Rare — vacuole performs similar digestive role |
| Shape | Irregular, flexible | Regular, fixed (cell wall) |
| Centrioles | Present — for cell division | Absent in most plant cells |
Misconception: Plant cells don't have mitochondria because they have chloroplasts.
Plant cells have both. Chloroplasts produce ATP via photosynthesis in light, but all cells need ATP continuously — including at night. Mitochondria provide ATP through cellular respiration at all times.
Misconception: The nucleus is the most important organelle.
The nucleus controls gene expression, but a cell can survive for a period without it (mature red blood cells have no nucleus and live ~120 days). "Most important" depends on the function — mitochondria are equally critical for energy supply.
Misconception: The cell wall and cell membrane are the same thing in plant cells.
They are distinct structures. The cell membrane (phospholipid bilayer) controls what enters and exits the cell. The cell wall (cellulose) sits outside the membrane and provides structural support. Both are present in plant cells.
Activities
In your book, draw and label two diagrams: one animal cell and one plant cell. Each diagram must include all organelles covered in this lesson with annotations in the format:
Organelle name → key structural feature → function
Then, circle in green any structures that appear in your plant cell diagram but not your animal cell diagram. Below your diagrams, write one sentence explaining why plant cells need a large central vacuole but animal cells do not.
Write your one sentence here.
A researcher examines a cell from an unknown organism under TEM. The cell has the following features:
Write your responses here or in your book.
Assessment
1. Which of the following organelles is found in plant cells but NOT in animal cells?
2. A cell is observed to have a large central vacuole, a cell wall, and chloroplasts. Which of the following can be concluded?
3. Which statement best explains why plant cells still require mitochondria despite having chloroplasts?
4. A protein is synthesised on a ribosome attached to the rough ER. What is the correct order of organelles the protein passes through before being secreted from the cell?
5. The Warburg effect in cancer cells refers to which of the following?
1. Compare the structure and function of the rough endoplasmic reticulum and the Golgi body. In your answer, explain how these two organelles work together. (3 marks)
1 mark rough ER structure+function; 1 mark Golgi structure+function; 1 mark how they cooperate
2. Explain why mitochondria and chloroplasts are thought to have originated from ancient prokaryotes. Include two pieces of structural evidence in your answer. (3 marks)
1 mark for endosymbiotic theory statement; 1 mark per structural evidence piece
3. Cancer is sometimes described as "a disease of organelle dysfunction." Using your knowledge of eukaryotic cell structure, evaluate this statement with reference to at least two specific organelles. (3 marks)
1 mark per organelle correctly linked to cancer mechanism; deduct if no evaluation
Answers
SA1: The rough ER is a network of membrane-bound sacs studded with ribosomes on its outer surface. Proteins synthesised on these ribosomes enter the ER lumen, where they are folded into their correct three-dimensional shape and undergo initial processing. The Golgi body is a stack of flattened membrane sacs that receives proteins from the rough ER via transport vesicles. It further modifies proteins (e.g. adding carbohydrate chains), sorts them, and packages them into vesicles directed to their final destination — secretion, the cell membrane, or lysosomes. Together, the rough ER and Golgi form the cell's protein processing and distribution system.
SA2: The endosymbiotic theory proposes that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by a larger cell approximately 1.5–2 billion years ago. Two pieces of structural evidence: (1) both organelles have their own circular DNA, resembling prokaryotic chromosomes rather than eukaryotic linear chromosomes; (2) both have double membranes — the outer membrane is thought to be the remnant of the host cell's engulfing vesicle, while the inner membrane is the original prokaryote's cell membrane.
SA3: The statement is well supported. The nucleus is central to cancer development — mutations in DNA within the nucleus damage tumour suppressor genes (such as p53, which triggers cell death in damaged cells) and proto-oncogenes that regulate division, allowing cells to proliferate without control. Mitochondria are also implicated — cancer cells frequently exhibit the Warburg effect, switching to aerobic glycolysis and producing lactate rather than fully oxidising glucose in mitochondria. This metabolic reprogramming is thought to redirect carbon molecules toward biosynthesis needed for rapid growth. The statement is slightly limited because cancer also involves dysfunction at the cell membrane level (disrupted signalling) and in the cytoskeleton (enabling metastasis) — so "organelle dysfunction" alone does not capture the full picture.
You predicted the three most important organelles for a eukaryotic cell's survival. What's the verdict?
There's no single correct answer, but the strongest case goes to: nucleus (controls all gene expression — without it the cell cannot respond to anything), mitochondria (without ATP production the cell dies within seconds), and ribosomes (without protein synthesis nothing else can be built or maintained).
Interestingly, red blood cells lack a nucleus and survive for 120 days — but they can't repair themselves or divide, and they die. The nucleus isn't needed for moment-to-moment survival, but for long-term cell function it's essential.