AQA A-Level Biology: Eukaryotic and Prokaryotic Cell Structure (7402)

This guide covers 3.2.1.1 Structure of eukaryotic cells and 3.2.1.2 Structure of prokaryotic cells and of viruses, from AQA AS and A-level Biology (7401/7402), AS and A-level exams June 2016 onwards, Version 1.5. These are the first two sub-topics of 3.2.1 Cell Structure, within Topic 2 (Cells), examined at both AS and A-level.

Scope of this guide

3.2.1 Cell Structure has three sub-topics: 3.2.1.1 (eukaryotic cell structure), 3.2.1.2 (prokaryotic cell structure and viruses), and 3.2.1.3 (methods of studying cells — microscopy and cell fractionation). This resource covers the first two, which together build the structural knowledge the rest of the specification depends on; the practical and technique-based content of 3.2.1.3 is left for a separate resource.

Syllabus coverage

AQA AS AND A-LEVEL BIOLOGY (7401/7402) — 3.2.1.1 STRUCTURE OF EUKARYOTIC CELLS

The structure of eukaryotic cells is restricted to the structure and function of: the cell-surface membrane; the nucleus (containing chromosomes, consisting of protein-bound, linear DNA, and one or more nucleoli); mitochondria; chloroplasts (in plants and algae); Golgi apparatus and Golgi vesicles; lysosomes (a membrane-bound organelle that releases hydrolytic enzymes); ribosomes; rough and smooth endoplasmic reticulum; the cell wall (in plants, algae and fungi); and the cell vacuole (in plants). Students should be able to apply this knowledge to explain adaptations of eukaryotic cells, and to understand how specialised cells organise into tissues, tissues into organs, and organs into systems.

3.2.1.2 STRUCTURE OF PROKARYOTIC CELLS AND OF VIRUSES

Prokaryotic cells are much smaller than eukaryotic cells and differ in having: cytoplasm that lacks membrane-bound organelles; smaller ribosomes; no nucleus, instead a single circular DNA molecule free in the cytoplasm and not associated with proteins; and a cell wall that contains murein, a glycoprotein. Many prokaryotic cells also have one or more plasmids, a capsule surrounding the cell, and one or more flagella (structural details of these are not required). Viruses are acellular and non-living; their structure includes genetic material, a capsid, and an attachment protein.

How to approach it

Learn eukaryotic organelles in function-linked pairs rather than as an isolated list: rough endoplasmic reticulum with ribosomes attached (protein synthesis and transport) pairs with the Golgi apparatus (protein modification, packaging and secretion via Golgi vesicles); mitochondria (aerobic respiration) pairs with chloroplasts in plant cells (photosynthesis); lysosomes pair with the breakdown of damaged organelles and pathogens via hydrolytic enzymes. This mirrors how exam questions typically probe understanding — not “name this organelle” but “explain how this organelle’s structure suits its function.”

For 3.2.1.2, build a direct comparison table between eukaryotic and prokaryotic cells across the specification’s four named differences — cytoplasm lacking membrane-bound organelles, smaller ribosomes, no true nucleus (a single circular DNA molecule free in the cytoplasm and not associated with proteins), and a cell wall containing murein — since exam questions frequently ask candidates to “compare and contrast” the two cell types directly rather than describe either in isolation. Keep viruses conceptually separate from prokaryotic cells — a virus is acellular, meaning it is not made of a cell at all, which is a distinct point from being a small or simple cell.

Worked example: explaining an organelle’s structure-function link

A question asks candidates to explain how the structure of mitochondria relates to their function.

Structure:   double membrane, with the inner membrane folded into
             cristae, surrounding a fluid-filled matrix
Function:    site of the later stages of aerobic respiration
Link:        the folded cristae increase the surface area available for
             the electron transport chain and ATP synthase, while the
             matrix contains the enzymes for the Krebs cycle, allowing
             a high rate of ATP production in a compact structure

This structure-function-link pattern applies across every organelle in 3.2.1.1 and is the format most consistently rewarded in mark schemes.

Key terms to define precisely

Nucleolus — a dense structure within the nucleus responsible for producing ribosomes; a nucleus can contain one or more nucleoli. Hydrolytic enzyme — an enzyme that breaks down molecules by adding water, the type of enzyme contained within lysosomes for breaking down waste material and damaged organelles. Rough endoplasmic reticulum — endoplasmic reticulum studded with ribosomes, involved in the synthesis and processing of proteins, as distinct from smooth endoplasmic reticulum, which lacks ribosomes and is involved in lipid synthesis. Plasmid — a small, circular piece of DNA found in many prokaryotic cells, separate from the main circular chromosome, often carrying genes such as antibiotic resistance. Capsid — the protein coat that encloses a virus’s genetic material. Being precise about which structures are membrane-bound (mitochondria, chloroplasts, lysosomes, the endoplasmic reticulum, the Golgi apparatus, the nucleus) versus not (ribosomes) is a distinction that comes up directly when comparing eukaryotic and prokaryotic cells, since the presence or absence of membrane-bound organelles is one of the specification’s own four named differences between the two cell types (the absence of a nucleus and the form of the DNA are one bullet, not two).

Common mistakes

Listing organelles by name without linking structure to function. Describing prokaryotic cells only in terms of what they lack, without also covering the additional structures (plasmids, capsule, flagella) some of them possess. Treating viruses as a very small type of prokaryotic cell rather than as acellular and non-living. Omitting that prokaryotic DNA is not associated with proteins, unlike the protein-bound, linear DNA found in a eukaryotic nucleus.

Quick revision checklist

• Learn each named eukaryotic organelle with its structure and function paired together.
• Build a four-point comparison table between eukaryotic and prokaryotic cells.
• Know the three defining features of virus structure: genetic material, capsid, attachment protein.
• Practise the structure-function-link answer format for at least three different organelles.

Related resources

Eukaryotic and Prokaryotic Cell Structure revision notes | 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.