AQA GCSE Biology (8461) · Paper 1

🔬 Cell Biology

Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.

Cell structure

Every living thing is built from cells, the smallest units that can be called alive. Some organisms consist of a single cell, while the human body is made up of roughly 37 trillion cells working together.

The two types of cell

EukaryoticA eukaryotic cell has a nucleus that holds the genetic material, along with cytoplasm and a cell membrane. Animal, plant and fungal cells are all eukaryotic, and they are the larger, more complex type of cell.

ProkaryoticA prokaryotic cell has no nucleus, so its DNA floats free in the cytoplasm as a single loop, often together with extra small rings called plasmids. Bacteria are prokaryotes, and they are much smaller than eukaryotic cells.

🎯 Examiner tip: Examiners often focus on this difference. It is incorrect to say that bacteria have no genetic material, since they do have DNA, it is just not enclosed in a nucleus.

Structures inside an animal cell

NucleusThe nucleus contains the cell's DNA and controls its activities.

CytoplasmThe cytoplasm is the jelly-like substance where most chemical reactions in the cell happen.

Cell membraneThe cell membrane controls what enters and leaves the cell.

MitochondriaMitochondria are where aerobic respiration takes place, transferring energy for the cell to use.

RibosomesRibosomes are tiny structures where proteins are made.

What a plant cell has in addition

Cell wallThe cell wall is made of cellulose and keeps the cell rigid, giving it shape.

ChloroplastsChloroplasts contain chlorophyll and carry out photosynthesis.

Permanent vacuoleThe permanent vacuole is a sap-filled space that keeps the cell firm, or turgid.

Algal cells are built like plant cells, with a wall, chloroplasts and a vacuole. Fungal cells also have a wall, but it is made of chitin rather than cellulose, and fungal cells have no chloroplasts.

Specialised cells

As an organism develops, its cells differentiate: they switch on the genes they need and grow the features suited to one job. A cell that has developed in this way for a particular job is called a specialised cell.

A sperm cell has a tail for swimming, many mitochondria to provide energy, and enzymes in its head to help it enter the egg.
A nerve cell is long and has branched connections, allowing it to carry electrical impulses.
A muscle cell is packed with mitochondria and protein fibres that contract.
A root hair cell has a huge surface area to absorb water and minerals, and it has no chloroplasts, since it lives underground where there is no light.
Xylem cells are hollow, dead tubes strengthened with lignin, and they carry water upwards.
Phloem cells are living tubes with few organelles, and they move dissolved sugars.

Microscopy

Light microscopes are cheap and can be used to look at living cells, but their resolution (the ability to see two points as separate) is limited. Electron microscopes use beams of electrons instead of light, giving far higher magnification and resolution, which is why the fine detail of organelles was only discovered after they were invented.

magnification = image size ÷ real size

🎯 Examiner tip: This can be rearranged, using the formula triangle, into real size = image size ÷ magnification. Watch the units: 1 mm = 1000 µm = 1,000,000 nm. Most marks are lost on unit conversions, not on the formula itself.

🧪 Required practical: To look at cells, peel a thin layer from an onion and place it flat on a slide, then add a drop of iodine stain so the structures show up, and lower the cover slip at an angle to avoid trapping air bubbles. View the slide at low power first, then increase the magnification.

Culturing microorganisms (Triple)

Bacteria are grown on agar jelly using aseptic technique, which keeps unwanted microorganisms out. Equipment is sterilised in an autoclave, the inoculating loop is flamed, and the dish lid is lifted only at an angle and taped rather than sealed, to let air in while keeping contamination out. In schools, cultures are incubated at 25°C rather than body temperature, since body temperature would encourage the growth of pathogens. Bacteria reproduce by binary fission, and in ideal conditions they can divide as often as every 20 minutes.

area of a clear zone = π r²

🎯 Examiner tip: In the antibiotic-disc practical, a bigger clear zone around a disc means more bacteria have been killed, showing a more effective antibiotic. Always compare results against the control disc (sterile water) to prove that the antibiotic, rather than something else, is responsible.

Cell division

The nucleus holds DNA bundled into chromosomes. Humans have 23 pairs of chromosomes, one of each pair inherited from each parent. A gene is a short section of a chromosome that codes for one protein.

The cell cycle and mitosis

Body cells copy themselves through the cell cycle. In the long growth stage, the cell grows, makes more organelles, and replicates its DNA. Then in mitosis, one set of chromosomes is pulled to each end of the cell and the nucleus splits. Finally, the cytoplasm and membranes divide.

The result is two identical daughter cells, clones of the original cell with the full set of chromosomes.
Mitosis is used for growth, for repair of damaged tissue, and for asexual reproduction.

🎯 Examiner tip: When describing mitosis, state both that the cells are identical and that they have the same number of chromosomes, since examiners want both points. Do not confuse mitosis, which produces identical body cells, with meiosis, which produces gametes and is covered in Topic 6.

Stem cells

A stem cell is an undifferentiated cell that can keep dividing and turn into other cell types.

Embryonic stem cellsEmbryonic stem cells come from early embryos and can become any cell type, making them the most flexible kind of stem cell.

Adult stem cellsAdult stem cells are found in places such as bone marrow, and they are limited to producing certain cell types, such as blood cells.

Meristem cellsMeristem cells are found in plant shoot and root tips and can differentiate into any plant cell throughout the plant's life. They are used to clone rare plants quickly and cheaply.

Stem cells could be used to treat conditions such as diabetes and paralysis. In therapeutic cloning, an embryo is made using the patient's own genes, so the resulting stem cells are not rejected by the immune system. The use of stem cells is debated: some object on ethical grounds because embryos are destroyed, and there is a risk that transferred cells could carry a viral infection.

Transport in cells

Cells constantly move substances in and out. There are exactly three transport processes to know, and exam questions often test whether students can tell them apart.

DiffusionDiffusion is the movement of particles from a high to a low concentration, down the concentration gradient. It is a passive process, needing no energy, and it is how oxygen and carbon dioxide move throughout the body.

OsmosisOsmosis is the movement of water only, from a dilute solution to a more concentrated one, across a partially permeable membrane. It is also a passive process.

Active transportIn active transport, particles are pumped against the concentration gradient, from a low to a high concentration, which requires energy from respiration. This is how roots absorb scarce minerals from the soil and how the gut absorbs the last of the glucose from digested food.

🎯 Examiner tip: It helps to link one idea to each word: diffusion is spreading, osmosis is water, and active transport is pumping uphill using energy. If a question mentions movement against the concentration gradient, the answer always involves active transport and respiration.

Diffusion speeds up with a bigger concentration difference, higher temperature (faster particles) and more surface area.

Why size matters: surface area to volume

A single-celled organism has a large surface area compared to its volume, so diffusion alone can supply everything it needs. Large multicellular organisms have a much smaller surface-area-to-volume ratio, so substances cannot diffuse in fast enough to meet demand. This is why they need dedicated exchange surfaces and transport systems.

The alveoli in the lungs are millions of tiny air sacs, giving a massive surface area, with thin walls and a rich blood supply.
The villi in the small intestine are finger-like projections that absorb digested food.
Gills in fish have filaments and lamellae, with water flowing one way and blood flowing the other.
Root hairs in plants absorb water and minerals from the soil.
Leaves are flat and thin, with air spaces inside them for gases to move through.

Every effective exchange surface shares the same features: a large surface area, thin walls that keep the diffusion distance short, and a way of maintaining the concentration gradient, such as a blood supply or ventilation.

🧪 Required practical: In the osmosis practical, equal-sized potato cylinders are cut and weighed, then left in different sugar concentrations before being weighed again. In dilute solutions, water enters the potato and its mass rises; in concentrated solutions, water leaves and its mass falls. Plotting the percentage change in mass shows that where the line crosses zero, the solution's concentration matches that inside the potato.

% change in mass = (change ÷ starting mass) × 100

🎯 Examiner tip: Percentage change should be used rather than the raw change in grams, because the cylinders never start at exactly the same mass, and percentages make the results comparable. Stating this reasoning is what earns the evaluation mark.

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