How does the internal organisation of a eukaryotic cell let it carry out its functions?
The structure of eukaryotic cells, including the structure and function of the cell-surface membrane, nucleus, mitochondria, chloroplasts, Golgi apparatus and Golgi vesicles, lysosomes, ribosomes, rough and smooth endoplasmic reticulum, cell wall and cell vacuole, and the role of these organelles in producing and secreting proteins; the importance of the cytoskeleton.
A focused answer to the AQA 3.2 dot point on eukaryotic cell structure. Covers every required organelle, the protein production and secretion pathway, and the differences between plant, animal, fungal and algal cells.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA wants you to recognise every required eukaryotic organelle from an electron micrograph or description, state its function, and explain how several organelles cooperate to make and export proteins.
The answer
A eukaryotic cell has a true membrane-bound nucleus and membrane-bound organelles. Compartmentalisation lets incompatible reactions run side by side and gives each process its own optimised environment.
The organelles you must know
- Cell-surface (plasma) membrane
- A phospholipid bilayer with proteins. Controls what enters and leaves and is the site of cell signalling and recognition.
- Nucleus
- Bounded by a double membrane (nuclear envelope) with nuclear pores. Contains chromatin (DNA plus histone proteins) and a nucleolus that makes ribosomes. Stores genetic information and controls the cell through transcription.
- Mitochondria
- Double membrane; the inner membrane is folded into cristae, surrounding a fluid matrix. Site of aerobic respiration and ATP synthesis. Cells with high energy demand (muscle, liver) have many.
- Chloroplasts
- Found in plants and algae. Double membrane plus internal thylakoid membranes stacked into grana, surrounded by stroma. Site of photosynthesis. Contain chlorophyll and their own DNA and ribosomes.
- Ribosomes
- Small structures of rRNA and protein, either free in the cytoplasm or attached to the endoplasmic reticulum. Site of translation (protein synthesis). Eukaryotic ribosomes are 80S; the 70S type is found in prokaryotes, mitochondria and chloroplasts.
- Rough endoplasmic reticulum (RER)
- A network of membranes studded with ribosomes. Folds and transports proteins destined for secretion or the membrane.
- Smooth endoplasmic reticulum (SER)
- Membranes without ribosomes. Synthesises and processes lipids and steroids.
- Golgi apparatus and Golgi vesicles
- Stacked flattened sacs. Modifies, sorts and packages proteins and lipids (for example glycosylation) into vesicles for secretion or for forming lysosomes.
- Lysosomes
- Golgi-derived vesicles containing hydrolytic (digestive) enzymes. Break down worn-out organelles, ingested material and, in white blood cells, engulfed pathogens.
- Cell wall
- A freely permeable layer outside the membrane. Made of cellulose in plants, chitin in fungi. Provides mechanical strength and resists turgor pressure.
- Cell (permanent) vacuole
- A large fluid-filled sac in plant cells bounded by the tonoplast. Maintains turgor and stores cell sap.
The cytoskeleton
The cytoskeleton is a network of protein filaments (microfilaments, intermediate filaments and microtubules) through the cytoplasm. It gives the cell mechanical support and shape, moves organelles and vesicles, and drives chromosome movement in mitosis and the beating of cilia and flagella.
The protein production and secretion pathway
This is the most heavily examined sequence in the topic.
- The gene is transcribed in the nucleus; mRNA leaves through a nuclear pore.
- Ribosomes on the RER translate the mRNA into a polypeptide, which folds in the RER.
- A vesicle buds from the RER and carries the protein to the Golgi apparatus.
- The Golgi modifies (for example adds carbohydrate) and packages the protein into a secretory vesicle.
- The vesicle moves to the cell-surface membrane, fuses with it, and releases the protein by exocytosis.
ATP from the mitochondria powers vesicle transport and exocytosis throughout.
Examples in context
Example 1. Goblet cells and mucus secretion. Goblet cells lining the airways are packed with RER and a prominent Golgi apparatus because they continuously synthesise and secrete the glycoprotein mucin. Mucin is made on the RER, glycosylated in the Golgi, packaged into vesicles and released by exocytosis, illustrating the full secretory pathway in one specialised cell.
Example 2. Palisade mesophyll cells. Palisade cells in a leaf are crammed with chloroplasts and arranged as tall columns near the upper surface to maximise light absorption for photosynthesis. Their large vacuole pushes the chloroplasts to the cell edge where light is strongest, showing how organelle number and arrangement match function.
Try this
Q1. Name three structures found in a plant cell but not an animal cell, and give the function of each. [3 marks]
- Cue. Cell wall (support and prevents bursting), chloroplast (photosynthesis), permanent vacuole (turgor and storage).
Q2. Explain why a cell that secretes large amounts of protein has many mitochondria. [2 marks]
- Cue. Mitochondria carry out aerobic respiration to make ATP; ATP is needed to power protein synthesis, vesicle transport and exocytosis.
Q3. Describe the function of the cytoskeleton. [3 marks]
- Cue. Provides shape and mechanical support; moves organelles and vesicles within the cell; moves chromosomes in mitosis and drives cilia and flagella movement.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA4 marksDescribe the role of the rough endoplasmic reticulum, the Golgi apparatus and vesicles in the production and secretion of an extracellular enzyme.Show worked answer →
A 4-mark answer needs the secretory pathway in order, each step doing a job.
- Ribosomes on the rough endoplasmic reticulum (RER) synthesise the polypeptide (protein) by translation.
- The protein is processed and folded in the RER, then pinched off in a vesicle that travels to the Golgi apparatus.
- The Golgi apparatus modifies the protein (for example adding carbohydrate to form a glycoprotein) and packages it into a secretory vesicle.
- The vesicle moves to and fuses with the cell-surface membrane, releasing the enzyme by exocytosis.
Markers reward the correct sequence, naming the organelles, and the word exocytosis.
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