Cell Structure

2. Cell Structure — AQA A Level Biology 7402 Revision Notes & Topical Questions

15 min read 26 parts Lesson 1 of 47 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision

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Lesson summary

Cell Structure — notes: Describe eukaryotic, prokaryotic and viral structure, microscopy and cell fractionation, the cell cycle, membranes and transport, and the immune system including vaccines, HIV, monoclonal antibodies and ELISA.Cell theory — all organisms are made of one or…

Organelles
Levels of organisation
Prokaryotes
Magnification and resolution
Starch grains
Scientific drawings

Part 1 of 24Cell theory and eukaryotic organelles

Cell theory

Cell theory all organisms are made of one or more cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

Organelles

Cell-surface membranephospholipid bilayer with proteins controlling entry and exit.
Nucleus
- linear DNA bound to proteins as chromosomes;
  - nucleolus makes ribosomes.
Mitochondriaaerobic respiration and ATP.
Chloroplastsphotosynthesis, with chlorophyll.
Golgi apparatusmodifies and packages proteins and lipids and forms vesicles such as lysosomes.
Lysosomesvesicles with hydrolytic enzymes digesting worn-out organelles and pathogens.
Rough ER
- ribosomes, protein synthesis and transport;
  - smooth ER: lipids and carbohydrates.
Ribosomesprotein synthesis (80S in eukaryotes, 70S in prokaryotes).
Cell wallcellulose in plants, chitin in fungi.
Vacuolecell sap, helps maintain pressure.

Part 2 of 24Specialised cells and organisation

Cell	Function	Adaptations
Red blood cell	Oxygen transport	Biconcave, no nucleus
Neurone	Conducts impulses	Cell body with organelles, dendrites receive and axon transmits (up to 1 m), myelin sheath speeds conduction
Muscle cell	Contraction	Actin and myosin filaments, many mitochondria, skeletal fibres multinucleate
Sperm cell	Fertilisation	Haploid head with acrosome enzymes, mid-piece full of mitochondria, tail
Root hair cell	Absorb water and ions	Large surface area, thin wall, concentrated vacuole sap, mitochondria for active transport, no chloroplasts
Xylem vessel	Water transport	Dead, no end walls, lignin thickening
Phloem	Sugar and amino acid transport	Living sieve tube elements with sieve plates, companion cells provide ATP

Levels of organisation
- 1specialised cell2tissue3organ4organ system
- Example
  1epithelial cells2epithelial tissue3stomach4digestive system

Part 3 of 24Prokaryotic cells and viruses

Prokaryotes (bacteria) are much smaller and have no nucleus.
- A single circular DNA molecule free in cytoplasm, not bound to protein.
- 70S ribosomes and no membrane-bound organelles.
- Cell wall containing murein (peptidoglycan).
- Optionalplasmids (small circular DNA with non-essential genes such as antibiotic resistance), a capsule (prevents drying, resists immune attack) and flagella.

Feature	Prokaryotes	Eukaryotes
Size	0.5 to 5 µm	Up to 100 µm
DNA	Circular, no histones	Linear, with histones, forming chromosomes
Division	Binary fission	Mitosis or meiosis with spindle
Ribosomes	70S	80S
Organelles	Few, none membrane-bound	Many; single membrane (lysosomes, Golgi, vacuoles), double (nucleus, mitochondria, chloroplasts), none (ribosomes)
Cell wall	Murein	Cellulose or lignin in plants, chitin in fungi

Viruses are non-living: acellular, cannot reproduce alone, do not grow, have no metabolism or protein synthesis.
- Structure
  - DNA or RNA genome, a protein capsid, and attachment proteins;
    - some have a lipid envelope.
- All are parasitic and much smaller than cells.

Part 4 of 24Microscopy: light, TEM and SEM

Feature	Light	TEM	SEM
Radiation	Light	Electrons	Electrons
Resolution	About 200 nm	About 0.2 nm	About 2 nm
Magnification	Up to ×1500	Up to ×1 500 000	Up to ×500 000
Image	2D, colour	2D, black and white	3D, black and white
Specimen	Living or dead	Dead, very thin	Dead, thick allowed
Shows	Nucleus, mitochondria, chloroplasts	Internal organelles including ribosomes and ER	Surface detail

1Magnification and resolution

Magnification and resolution magnification is how many times larger the image is;
- resolution is the ability to distinguish two close points, limited by wavelength.
Light cannot resolve the 10 nm phospholipid bilayer or ribosomes.

2Artefacts

Artefacts dust, air bubbles or preparation damage can look like structures;
- different preparations are compared to check.

Part 5 of 24Using microscopes, graticules and drawings

Place the slide on the stage.

Select the lowest power objective.

Lower the lens close to the slide with the coarse focus.

Raise it slowly with the fine focus until in focus.

Move to higher power and refocus if needed.

Graticule

Graticule an eyepiece scale with no fixed units; calibrate each objective using a stage micrometer.

Starch grains

Starch grains in chloroplast stroma, potato tubers and cereal and legume seeds;
- stain with iodine in potassium iodide, which turns blue-black.
Use one or two drops and lower the coverslip at an angle to avoid bubbles.

Scientific drawings

Scientific drawings title and magnification, sharp HB pencil on plain paper, single clear lines, no shading, large and proportional, straight label lines from a ruler, no arrows.
Draw only what you see.

Part 6 of 24Magnification calculations and units

Magnification

Magnification image size ÷ actual size; total light microscope magnification = eyepiece × objective.

Units

1000 nm = 1 µm; 1000 µm = 1 mm; 1000 mm = 1 m; 1 cm = 10 mm.
Convert to the smallest unit shown in the question; magnification has no units.

Worked cases

Image 30 mm, ×3000: actual = 30 ÷ 3000 = 0.01 mm = 10 µm.
Drawing 50 mm, leaf 2000 µm = 2 mm: magnification = 50 ÷ 2 = ×25.

Part 7 of 24Cell fractionation and ultracentrifugation

Cell fractionation and ultracentrifugation

Homogenisation: blend cells in a cold, isotonic, buffered solution to release organelles; the mixture is the homogenate.

Filtration: pass through gauze to remove large debris.

Ultracentrifugation: spin the filtrate; the heaviest organelles form a pellet and the supernatant is re-spun faster.

Why the solution is

Why the solution is cold (slows enzymes), isotonic (prevents osmotic damage) and buffered (keeps pH constant).

Order of pelleting

Order of pelleting nuclei, chloroplasts, mitochondria, lysosomes, endoplasmic reticulum, ribosomes.

Part 8 of 24The cell cycle, interphase and mitosis

1Cell cycle

Cell cycle interphase, nuclear division (mitosis) and cytokinesis; cyclins trigger transitions and length varies.
- (onion root tips about 20 hours, human gut epithelium about 10 hours)

2Interphase

G1: growth and protein synthesis; a signal triggers division.
S: DNA replicates, giving two sister chromatids per chromosome.
G2: more growth, error checking of DNA, and tubulin made for the spindle.

3Importance of mitosis

Importance of mitosis growth, repair and replacement, regeneration, and asexual reproduction (strawberry runners, budding in Hydra and yeast).

Stage	Events
Prophase	Chromosomes condense, each of two chromatids; centrosomes move to poles; spindle forms; nuclear envelope breaks down
Metaphase	Chromosomes align on the metaphase plate attached to spindle fibres at the centromere
Anaphase	Spindle fibres shorten and centromeres divide; chromatids move to opposite poles
Telophase	Chromosomes decondense, nuclear envelopes reform, spindle breaks down

1Remember PMAT

; cytokinesis follows telophase and is not part of mitosis.

2Cytokinesis

Type	Description
Animal	cleavage furrow pinches the membrane.
Plant	a cell plate forms and new cell walls are built.

Part 9 of 24Practical: mitotic index in root tips

Cut about 1 cm from the tips of onion or garlic roots.

Fix in ethanoic alcohol to stop mitosis.

Warm in dilute hydrochloric acid at 60 °C to separate the cells.

Place on a slide, add acetic orcein (stains chromosomes purple).

Add a coverslip and press firmly with a paper towel to make a single layer.

View and count cells.

Mitotic index

Mitotic index cells with visible chromosomes ÷ total cells.

32 of 42 gives 0.76.
(14 + 5 + 3 + 6) ÷ 64 = 0.44.
20 of 75 gives 0.27.

Limitations

Limitations squashing and staining create artefacts, cell sizes vary between slides, and 3D cells are cut at different planes.

Part 10 of 24Cancer, binary fission and viral replication

Cancer

Cancer uncontrolled mitosis forming a tumour, caused by mutations in genes that control the cell cycle that cause continual division or stop apoptosis.
- (oncogenes, tumour suppressor genes)
Treatments target dividing cells: methotrexate blocks nucleotide synthesis; taxol stops spindle breakdown.

Binary fission

Binary fission replicate the circular DNA and plasmids, divide the cytoplasm, giving two identical cells.
Cells that miss the circular DNA or a plasmid die.
Division can take 20 minutes, allowing fast colonisation and quick spread of resistance.

Viral replication

Attachment proteins bind complementary host receptors.
Viral nucleic acid enters the cell.
The host machinery makes viral proteins and nucleic acid.
New particles assemble.
Release by lysis or budding (taking the host membrane as an envelope).

Part 11 of 24Fluid mosaic model of membranes

Fluid mosaic model of membranes

Structure

Structure a phospholipid bilayer with intrinsic and extrinsic proteins, plus cholesterol (animals), glycoproteins and glycolipids.
Fluid because lipids and some proteins move laterally; mosaic because proteins are scattered.

Phospholipids

Phospholipids hydrophilic heads outwards and hydrophobic tails inwards;
- the bilayer blocks polar substances and ions.

Cholesterol

Cholesterol stops close packing at low temperature and stabilises the membrane at high temperature, and reduces ion leakage.

Glycoproteins and glycolipids

Glycoproteins and glycolipids receptors for hormones and neurotransmitters, endocytosis and cell adhesion, and antigens such as ABO blood groups.

Transport proteins

Transport proteins channel and carrier proteins, each specific.

Fluidity

Less fluidmore saturated tails, lower temperature.
More fluidunsaturated bent tails, higher temperature.

Part 12 of 24Diffusion, facilitated diffusion and osmosis

Diffusion

Diffusion net movement of molecules down a concentration gradient due to kinetic energy.
Rate rises with a steeper gradient, higher temperature, larger surface area and smaller molecules;
- non-polar molecules cross faster.

Facilitated diffusion

Facilitated diffusion large polar molecules and ions need proteins.
- (glucose, amino acids)
Channel proteins are water-filled pores, often gated.
Carrier proteins change shape between two forms.

Osmosis

Osmosis net movement of water from higher to lower water potential through a partially permeable membrane.
Pure water has a water potential of 0 kPa; solutions are negative, and more negative means lower.

Plant cells

Plant cells hypertonic: plasmolysis and wilting; hypotonic: turgid; isotonic: no net change.

Animal cells

Animal cells hypertonic: shrink; hypotonic: swell and burst (cytolysis).

Part 13 of 24Practical: water potential with potato cylinders

Cut potato cylinders of equal length and blot dry.

Record the initial mass.

Place one in each of at least five sucrose solutions (for example in a 30 °C water bath for 30 minutes).

Remove, blot and record the final mass.

Calculate percentage change = change ÷ initial mass × 100.

Interpretation

Interpretation positive change: water entered, solution had higher water potential;
- negative: water lost;
- zero: isotonic.

Graph

Graph plot percentage change against concentration; where the line crosses the x-axis gives the potato's sucrose concentration.
- (water potential)

Graph skills

Graph skills independent variable on x, labelled axes with units, suitable scales and a smooth best-fit line;
- bar charts for qualitative or discrete data, line or scatter graphs for continuous data.

Part 14 of 24Active transport and co-transport

Active transport

Active transport moves molecules or ions from low to high concentration through specific carrier proteins using energy from ATP hydrolysis.

Exampleskidney reabsorption, digestion products, loading sugar into phloem and ion uptake by root hairs.
Carrier proteins, not channel proteins, are used.

Co-transport

Co-transport one substance moving down its gradient pulls another against its gradient through a shared carrier.

In the ileum

Active transport moves Na⁺ out of the epithelial cell into the blood, lowering cell sodium.
Na⁺ diffuses in from the ileum through a co-transporter carrying glucose with it.
Glucose then leaves into the blood by facilitated diffusion.

Part 15 of 24Adaptations for rapid transport

Adaptation	Effect	Example
Increased surface area	More crossing at once	Microvilli, root hairs
More channel proteins	Faster facilitated diffusion	Na⁺/K⁺ channels in neurones
More carrier proteins	Faster facilitated diffusion and active transport	Glucose carriers in kidney and ileum
Thin exchange surface	Short diffusion distance	Alveoli and capillaries
Rich blood supply	Steep gradient	Alveoli, villi
Many mitochondria	More ATP	Root hair cells

Specialised cells
- Root hair cellslong projections, thin wall, vacuole sap.
- Ileum epitheliummicrovilli, capillary network, co-transport proteins.
- Collecting duct cellsmany aquaporins (water channels) for water reabsorption.
- Neurones and muscle cellsNa⁺, K⁺ and Ca²⁺ channels.

Part 16 of 24Practical: membrane permeability with beetroot

Cut five equal beetroot pieces and rinse away released pigment.

Put each in 5 cm³ of water in a tube.

Place tubes in water baths at different temperatures (for example 10 to 50 °C) for about 30 minutes.

Remove the beetroot and measure absorbance of each liquid with a colorimeter.

Higher absorbance means more pigment leaked and higher permeability.

Results

Results permeability rises with temperature because phospholipids move more and pack less tightly; very high temperatures melt the bilayer and denature proteins,
- and freezing can damage membranes.

Solvents

Solvents alcohol or acetone dissolve lipids and increase permeability.

Limitations

Limitations cuvettes of different thickness, uneven beetroot pieces, so repeat and calculate a mean.
- (use the same one)

Part 17 of 24Cell recognition, antigens and phagocytosis

Self and non-self

Self and non-self surface glycoproteins and glycolipids (antigens) let the body recognise pathogens, abnormal cells, toxins and cells from other individuals.
- (transplant rejection)

Antigen variability

Antigen variability mutation changes antigens, so memory cells no longer match; vaccines such as flu are updated each year.
- (cold and flu viruses)

Phagocytosis

(neutrophils and macrophages, non-specific)

Chemotaxischemicals attract phagocytes.
Receptors bind non-self antigens.
The membrane engulfs the pathogen into a phagocytic vacuole (phagosome).
A lysosome fuses with it to form a phagolysosome.
Lysozymes digest the pathogen.
Macrophages present antigens to activate the specific response.

Part 18 of 24T lymphocytes and B lymphocytes

Lymphocytes

Lymphocytes are made in bone marrow: T cells give the cellular response, B cells the humoral response.

Antigen-presenting cells

Antigen-presenting cells show antigens on MHC molecules to activate T cells with complementary receptors.

Helper T cells

Helper T cells divide to form clones and release cytokines that

Mature B cells into plasma cells.
Produce memory cells.
Increase phagocytosis.
Activate cytotoxic T cells.

Cytotoxic T cells

Cytotoxic T cells bind infected cells and release perforins that make holes so toxins kill them.

B cells

B cells have antibody receptors.

Clonal selectionthe B cell with the complementary receptor is chosen.
Clonal expansionit divides by mitosis.
Plasma cells secrete antibodies; memory cells remain.

Part 19 of 24Antibodies and the immune responses

Antibodies and the immune responses

Antibody structure

Antibody structure globular glycoprotein with quaternary structure, Y-shaped, with two heavy and two light chains joined by disulfide bonds.
Constant region sets the class and mode of destruction.
Variable region forms the antigen-binding site complementary to an epitope.
A hinge gives flexibility.

Antigen-antibody complex

Antigen-antibody complex two binding sites cause agglutination, which neutralises pathogens or marks them for phagocytes.

Primary response

Primary response slow, as it takes time to select lymphocytes, divide and make antibody;
- plasma cells are short-lived.

Secondary response

Secondary response memory cells divide and differentiate quickly into plasma cells,
- so antibody comes faster and in greater amounts, stopping disease before symptoms.

Part 20 of 24Vaccines, herd immunity and types of immunity

Vaccines, herd immunity and types of immunity

Vaccines

Vaccines introduce antigens (weakened pathogens or genetic material coding for them) to produce memory cells and active immunity.

Herd immunity

Herd immunity enough vaccinated people stop spread and protect unvaccinated people; more infectious diseases need a higher proportion,
- and measles outbreaks (Swansea 2012) occurred when coverage fell.

Feature	Active	Passive
Antibodies made by	The body	Another organism
Onset	1 to 2 weeks	Immediate
Memory cells	Yes	No
Duration	Long-term	Short-term
Natural	Infection	Placenta, colostrum, breast milk
Artificial	Vaccination	Injection of antibodies or antitoxin (tetanus)

Part 21 of 24HIV and AIDS

1HIV structure

HIV structure a retrovirus with RNA genome, reverse transcriptase, a capsid, lipid envelope from the host and attachment proteins;
- spread by body fluids.

2Replication in helper T cells

Viral RNA enters the cell.
Reverse transcriptase makes a DNA copy.
DNA is inserted into host chromosomes and copied at each division.
Viral proteins are made and new particles assembled.
Thousands of particles are released, killing the cell, and infect more T cells.

3Consequences

Consequences fewer helper T cells mean less B cell activation, antibody production and phagocytosis,
- leading to AIDS and opportunistic infections such as TB.

4Treatment

Treatment antivirals slow replication, allowing a normal life expectancy; antibiotics do not work on viruses.

5HIV is the virus; AIDS is the disease.

Part 22 of 24Monoclonal antibodies, ethics and evaluating studies

Monoclonal antibodies, ethics and evaluating studies

Monoclonal antibodies

Monoclonal antibodies are identical antibodies from one B cell clone, binding one antigen.

Drug targetinga drug attached to the antibody is delivered to cancer cells.
Diagnosislabelled antibodies detect antigens in pregnancy tests (hCG), HIV, Streptococcus and tumours.

Ethical issues with vaccines

Ethical issues with vaccines animal testing, risks and vulnerability of trial volunteers, small side effects versus herd immunity benefits, parents refusing vaccines,
- and unequal global access.

Ethical issues with monoclonal antibodies

Ethical issues with monoclonal antibodies mice are used, treatments carry side effects, informed consent is needed, and costs limit access.

MMR example

A 1998 study of 12 children linking MMR to autism was discredited because of conflict of interest.
A Yokohama study of 30 000 children found autism cases kept rising after MMR stopped, so no link.
Larger samples, peer review, replication and transparency increase confidence.

Evaluating studies

Evaluating studies repeatability, reproducibility and validity.

Part 23 of 24ELISA

ELISA uses an antibody linked to an enzyme; a coloured product shows that the target is present.
- Direct ELISA uses one antibody; indirect ELISA uses primary and secondary antibodies.

Fix HIV antigens to the bottom of the well.

Add the patient's plasma; HIV-specific antibodies (primary) bind.

Wash to remove unbound antibodies.

Add enzyme-linked secondary antibodies that bind the primary antibodies.

Wash again to avoid false positives.

Add substrate; a colour change shows the patient has HIV antibodies.

Part 24 of 24Worked examples

Explain why a TEM is better than a light microscope for studying ribosomes. [3]3 marks
An image of a cell is 45 mm wide at ×900. Calculate the actual width in µm. [2]2 marks
A root tip had 120 cells, 30 with visible chromosomes. Calculate the mitotic index. [2]2 marks
1. Mitotic index = 30 ÷ 120.
2. = 0.25.
Explain how glucose is absorbed from the ileum by co-transport. [4]4 marks
Explain why the secondary immune response is faster and stronger. [4]4 marks

Key terms

Term	Meaning
Resolution	The ability to distinguish two separate points.
Homogenate	The mixture of broken cells and organelles after homogenisation.
Mitotic index	The proportion of cells in a sample undergoing mitosis.
Fluid mosaic model	Membrane model with a moving phospholipid bilayer and scattered proteins.
Co-transport	Coupled movement of two substances across a membrane via a carrier protein.
Antigen	A molecule on a cell or virus that triggers an immune response.
Plasma cell	A B cell derivative that secretes large amounts of antibody.
Herd immunity	Protection of a population when enough people are immune to stop the spread.
ELISA	An enzyme-linked immunosorbent assay detecting antigens or antibodies by colour change.

Avoid these mistakes

Saying cytokinesis is part of mitosis — it follows telophase.

Mixing up channel and carrier proteins — active transport and co-transport use carriers that change shape.

Saying antibiotics treat HIV — viruses lack the targeted structures.

Confusing clonal selection and clonal expansion — selection is choosing the lymphocyte, expansion is its division.

Giving the definition of antigen generally — define it as a non-self antigen when discussing immunity.

Last updated: 14 October 2026