AQA A-Level Biology: Eukaryotic and Prokaryotic Cell Structure — Revision Notes

Condensed for the final weeks. For the full explanation, use the Eukaryotic and Prokaryotic Cell Structure study guide.

Eukaryotic organelles — learn in function-linked pairs (3.2.1.1)

Organelle pair	Function link
Rough ER + ribosomes	Protein synthesis and transport
Golgi apparatus + Golgi vesicles	Protein modification, packaging, secretion
Mitochondria ↔ Chloroplasts (plants/algae)	Aerobic respiration ↔ Photosynthesis
Lysosomes	Hydrolytic enzymes — breakdown of damaged organelles/pathogens

Also required: cell-surface membrane, nucleus (chromosomes = protein-bound linear DNA, one or more nucleoli), smooth ER (lipid synthesis, no ribosomes), cell wall (plants/algae/fungi), cell vacuole (plants).

Exam questions test “explain how structure suits function,” not “name this organelle.”

Prokaryotic cells and viruses (3.2.1.2)

Feature	Eukaryotic	Prokaryotic
Membrane-bound organelles	Present	Absent
Ribosome size	Larger	Smaller
Nucleus and DNA (one specification bullet)	Nucleus present; linear DNA associated with proteins	No nucleus — a single circular DNA molecule free in the cytoplasm, NOT associated with proteins
Cell wall	Cellulose (plants, algae) or chitin (fungi); none in animal cells	Contains murein (a glycoprotein)

Many prokaryotes also have plasmids (extra circular DNA, e.g. carrying antibiotic resistance genes), a capsule, and flagella (structural detail not required).

Viruses are acellular and non-living — genetic material + capsid (protein coat) + attachment protein. Keep viruses conceptually separate from prokaryotes — a virus is not made of a cell at all, not simply a very small/simple one.

Worked example: structure-function link (mitochondria)

Structure:  double membrane, inner membrane folded into CRISTAE,
            fluid-filled MATRIX
Function:   site of the later stages of aerobic respiration
Link:       folded cristae increase surface area for the electron
            transport chain and ATP synthase; the matrix holds the
            Krebs cycle enzymes -- together allowing high ATP
            production in a compact structure

This structure → function → link pattern applies to every organelle in 3.2.1.1 and is the format mark schemes most consistently reward.

Worked example: comparing a bacterium and a plant cell

A question asks candidates to state three differences between a bacterial cell and a plant cell, beyond size.

1. Nucleus:     plant cell has a true, membrane-bound nucleus;
                bacterium has no nucleus -- DNA lies free in the
                cytoplasm as a single circular molecule
2. Organelles:  plant cell has membrane-bound organelles
                (mitochondria, chloroplasts, ER, Golgi apparatus);
                bacterial cytoplasm lacks membrane-bound organelles
                entirely
3. Cell wall:   plant cell wall is made of cellulose; bacterial
                cell wall is made of murein, a glycoprotein --
                chemically distinct materials, not just "a cell wall"
                in both cases

Naming the specific chemical difference (cellulose vs. murein), not just “both have a cell wall,” is what separates a full-mark comparison answer from a partial one.

From cells to organ systems

The specification explicitly expects you to apply eukaryotic cell structure knowledge to explain how specialised cells organise into tissues, tissues into organs, and organs into systems – this is not a separate topic but a direct extension of 3.2.1.1. A specialised cell (for example, a red blood cell, or a root hair cell) has organelles present, absent, or modified in proportion to its specific job: a red blood cell has no nucleus in its mature form, maximising space for haemoglobin; a cell specialised for secretion (such as a pancreatic cell) has an unusually extensive rough ER and Golgi apparatus for producing and packaging enzymes. Being ready to apply the organelle-function pairings above to an unfamiliar specialised cell, rather than only reciting them for a generic “typical” cell, is exactly the kind of application AQA’s assessment objectives reward at A-level.

Worked example: explaining a specialised cell’s adaptations

A question describes a pancreatic cell that secretes large quantities of digestive enzymes, and asks how its organelle content reflects this function.

Feature:      extensive rough endoplasmic reticulum and numerous
              Golgi apparatus/vesicles, more than a typical cell
Link:         rough ER (with attached ribosomes) synthesises large
              quantities of enzyme proteins; the Golgi apparatus
              modifies and packages these proteins into vesicles for
              secretion out of the cell
Conclusion:   the abundance of these two organelles directly reflects
              the cell's high demand for protein synthesis and export,
              consistent with its role in enzyme secretion

This is the same structure-function-link pattern used for mitochondria above, applied to a whole cell’s organelle profile rather than a single organelle – practise it both ways.

Key terms

Nucleolus — dense structure in the nucleus producing ribosomes; a nucleus can have one or more. Hydrolytic enzyme — breaks down molecules by adding water (found in lysosomes). Plasmid — small circular DNA in prokaryotes, separate from the main chromosome, often carrying resistance genes. Capsid — the protein coat enclosing a virus’s genetic material.

Membrane-bound vs. not: mitochondria, chloroplasts, lysosomes, ER, Golgi apparatus, nucleus are membrane-bound; ribosomes are not — this distinction comes up directly in eukaryote/prokaryote comparisons, since it’s one of the specification’s four named differences (the absence of a nucleus and the form of the DNA count as one bullet, not two).

Common mistakes

• Listing organelles by name without linking structure to function.
• Describing prokaryotic cells only by what they lack, without covering additional structures (plasmids, capsule, flagella) some possess.
• Treating viruses as a very small prokaryotic cell rather than acellular and non-living.
• Omitting that prokaryotic DNA is not protein-bound, unlike eukaryotic nuclear DNA.

Quick self-test

• Pair each of the four function-linked organelle pairs and state what each does.
• Complete the four-row eukaryote/prokaryote comparison table from memory.
• Name the three structural components of a virus.
• Explain why murein (not cellulose) is named specifically for prokaryotic cell walls.
• Apply the structure-function-link format to explain how chloroplast structure suits photosynthesis.

Related resources

Eukaryotic and Prokaryotic Cell Structure study guide | Eukaryotic and Prokaryotic Cell Structure practice questions

Official syllabus

AQA AS and A-level Biology (7401/7402) specification, Version 1.5, June 2016 exams onwards — aqa.org.uk/7402.