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2.1.4Cell Structure

Eukaryotic Cell Organisation — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

2.1.4 Eukaryotic Cell Organisation — AQA A Level Biology 7402 notes: Explain how organelles cooperate in the production and secretion of proteins and describe the role of the cytoskeleton and cell compartments.A eukaryotic cell is divided into compartments by membranes.Compartmen…

  • transport vesicles
  • Golgi apparatus
  • secretory vesicles
  • exocytosis
  • Mitochondria
  • microfilaments

Section 1 of 8Compartmentalisation

A eukaryotic cell is divided into compartments by membranes.

Compartmentalisation
Compartmentalisation has several advantages: enzymes and substrates can be kept at high concentration; conditions such as pH can be controlled (for example in lysosomes); harmful substances are kept apart from the rest of the cell;
  • and incompatible reactions can occur at the same time.
Membranes also provide surfaces on which reactions can occur, such as the cristae of mitochondria.

Section 2 of 8Making and secreting a protein

The production of an extracellular enzyme, such as amylase from a pancreatic cell, shows how the organelles work together:

Nucleus
The gene is transcribed in the nucleus to produce mRNA, which leaves through a nuclear pore.
Ribosome
mRNA binds to a ribosome on the rough ER, where translation occurs and the polypeptide is threaded into the lumen of the RER.
Transport vesicles
The protein is folded and may be modified in the RER, then packaged into transport vesicles that bud off from the ER.
Cytoskeleton
Vesicles move along the cytoskeleton to the Golgi apparatus and fuse with its cis (receiving) face.
Secretory vesicles
The Golgi modifies the protein (e.g. adding carbohydrate, folding it, trimming) and sorts it into secretory vesicles at the trans face.
Exocytosis
The vesicles move to the cell surface membrane and fuse with it, releasing the protein by exocytosis.

Mitochondria provide the ATP needed at all steps, particularly for vesicle movement. Cells that secrete large amounts of protein have a large amount of rough ER, a large Golgi and many mitochondria.

Section 3 of 8Lysosomes and the cytoskeleton

Lysosomes are formed from the Golgi.

They contain hydrolytic enzymes held in a vesicle so that they cannot digest the cell.
They fuse with phagocytic vesicles to digest bacteria, and break down worn-out organelles (autolysis).
White blood cells such as neutrophils contain many lysosomes.
Cytoskeleton
The cytoskeleton is a network of protein fibres - microfilaments (actin), intermediate filaments and microtubules (tubulin).
It supports the shape of the cell, anchors organelles, moves vesicles and chromosomes (using motor proteins and ATP),
  • and is involved in cell movement and in cytokinesis.
Cilia and flagella
Microtubules make up cilia and flagella in the 9+2 arrangement, and centrioles.

Section 4 of 8Which cells have which organelles?

Cell typeProminent organellesReason
Pancreatic acinar cellRER, Golgi, secretory vesiclesMakes and secretes digestive enzymes
Liver cell (hepatocyte)SER, mitochondria, Golgi, glycogen granulesDetoxification, metabolism and storage
Palisade cellChloroplasts, vacuolePhotosynthesis
Sperm cellMitochondria, flagellumMovement
MacrophageLysosomes, cytoskeletonPhagocytosis and digestion

Section 5 of 8Interpreting micrographs and exam tips

A question may show an electron micrograph and ask you to identify the cell's function from its organelles.

Look for abundant structures: many mitochondria means high energy demand; abundant RER and Golgi means protein secretion; many vesicles at the surface means exocytosis; lots of SER means lipid or steroid synthesis; lots of chloroplasts means photosynthesis.
Always say the organelle and give the reasoning, for example "large amounts of rough ER suggest synthesis of protein for export".
Pulse-chase experiments
Link to pulse-chase experiments: if radioactive amino acids are supplied for a short time, the label first appears in the RER,
  • then the Golgi,
  • then vesicles and finally outside the cell, confirming the pathway.

Further detail on vesicle transportthe vesicles are moved along microtubules by motor proteins such as kinesin, which use ATP to "walk" along the tubule.

Interpreting micrographs and exam tips
When a vesicle reaches the cell surface membrane, its membrane fuses with it,
  • which adds to the surface area of the membrane, and the contents are released.
This also explains why a cell that secretes a lot of protein must recycle membrane by endocytosis.
Questions may ask why the surface membrane does not simply keep growing; membrane recycling by pinocytosis balances membrane gained during exocytosis.

Also compare this process with prokaryotesa bacterial cell lacks ER and Golgi, so proteins are made by free ribosomes and secreted through the membrane by transport systems.

Interpreting micrographs and exam tips
In eukaryotes, the extra compartments allow extensive processing, such as glycosylation and folding, which gives more complex and varied proteins.
You should be able to explain this difference in terms of organisation of the cell.

Always name the organelle, say what it does at that stage, and say where the product goes next, so that your answer reads as a continuous pathway.

Section 6 of 8Exam-style questions

  1. Describe the route taken by a protein from its synthesis to its secretion from a cell. [5]5 marks

  2. Cells in the pancreas that secrete enzymes contain many mitochondria. Explain why. [2]2 marks

  3. Explain one advantage of compartmentalisation in eukaryotic cells. [2]2 marks

Section 7 of 8Key terms

TermMeaning
CompartmentalisationDivision of the cell into membrane-bound regions with different conditions.
ExocytosisRelease of substances from a cell by fusion of vesicles with the surface membrane.
Secretory vesicleVesicle carrying a product to the cell surface.
CytoskeletonProtein network that supports the cell and moves structures.
MicrotubuleHollow protein tube made of tubulin.
AutolysisDigestion of a cell's own parts by its lysosomes.
Pulse-chaseTechnique using a brief supply of radioactive label to track a pathway.

Section 8 of 8Exam technique

1
Saying that proteins go directly from RER to the cell surface. They pass through the Golgi.
2
Saying that vesicles are carried by diffusion. They move along the cytoskeleton using ATP.
3
Writing that the Golgi makes the protein; it modifies and packages it.
4
Describing lysosomes as having no membrane.
5
Giving 'to make energy' for mitochondria without saying ATP.

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