1.1 Cell biology

Why this matters

Everything alive is made of cells. A blade of grass, a mushroom, a person and a bacterium are all built from cells — but the cells themselves are very different. Animal and plant cells share a basic toolkit (nucleus, cytoplasm, cell membrane, mitochondria, ribosomes), but plant cells add a cellulose cell wall, chloroplasts and a permanent vacuole so they can do photosynthesis and stand upright. Edexcel goes one step further than other boards by also naming the endoplasmic reticulum (ER) and Golgi apparatus — the organelles that process and modify proteins after they leave the ribosome. Bacterial cells are smaller and much simpler — no nucleus, no membrane-bound organelles. They keep their DNA as a single loop in the cytoplasm, plus extra little rings called plasmids. Knowing which structures belong to which cell type is the foundation for almost every later topic in Edexcel GCSE Biology: how respiration releases energy (mitochondria), how plants make glucose (chloroplasts), how bacteria cause disease (cell wall + flagella), how the body builds new tissue (cell division). Get cells right and the rest of the course falls into place.

How to learn this topic

Build on what you already know

• KS3: living things are made of cells; cells are too small to see without a microscope.
• KS3: plants make their own food (photosynthesis); animals eat other organisms.
• KS3: bacteria are single-celled organisms — some cause disease, many do not.

1. Start with the difference between eukaryotic and prokaryotic cells — every Edexcel cell question hangs off this distinction.
2. Build the animal cell first (the simplest eukaryote): nucleus, cytoplasm, cell membrane, mitochondria, ribosomes.
3. Add the Edexcel-specific organelles: endoplasmic reticulum (ER) and Golgi apparatus — the protein processing/packaging pathway.
4. Then build the plant cell: animal cell toolkit + cell wall, chloroplasts, permanent vacuole.
5. Bacterial cells next: smaller, no nucleus, single DNA loop + plasmids, and (often) flagella.
6. Specialised cells: each cell type has structural features that fit its job — sperm, red blood, root hair, ciliated, nerve.
7. Finish with magnification calculations — the formula and unit conversion are easy marks if you practise them.

Key terms

eukaryotic cell

A cell with a nucleus and other membrane-bound organelles. Animal, plant and fungal cells are all eukaryotic.

prokaryotic cell

A smaller, simpler cell with no nucleus and no membrane-bound organelles. DNA is a single loop in the cytoplasm. Bacteria are prokaryotic.

nucleus

The organelle in a eukaryotic cell that controls cell activities and contains the chromosomes (DNA). (Edexcel rubric: 'nucleus contains DNA | DNA carries instructions for making proteins'. Saying a cell 'has no nucleus' is the phrase for prokaryotic cells, NOT 'has no DNA'.)

mitochondrion

Organelle where aerobic respiration takes place, releasing energy from glucose. Found in animal cells AND plant cells. (Edexcel rubric for 'why many mitochondria': mitochondria carry out respiration → respiration releases energy → active transport uses energy → more mitochondria = more active transport.)

ribosome

The site of protein synthesis (where proteins are made). Found in all cells, including bacterial cells. (Edexcel rubric for 'why many ribosomes': ribosomes make proteins → enzymes are proteins → more ribosomes = more proteins at once → more enzymes produced. Ribosomes are NOT membrane-bound.)

endoplasmic reticulum (ER)

A network of membranes where proteins (made on ribosomes) are processed. Edexcel-specific organelle. (Edexcel rubric: 'ER is where proteins are made'. Proteins move from ER → Golgi in small vesicles.)

Golgi apparatus

Organelle that modifies proteins (e.g. by adding carbohydrates) and packages them into vesicles for secretion. Edexcel-specific. (Edexcel rubric: 'Golgi modifies proteins by adding carbohydrates | proteins transported to membrane for secretion'.)

cell membrane

Selectively (partially) permeable outer boundary of a cell. Controls which substances enter and leave. (Edexcel rubric: 'selectively/partially permeable | useful substances enter (glucose, oxygen, water) | waste leaves (CO₂, urea) | maintains stable internal environment'.)

chloroplast

Green organelle in plant cells containing chlorophyll. Site of photosynthesis — where the plant makes glucose using light energy. (Chloroplasts are ONLY found in plant cells (and some protists). Not in animal cells. Not in bacterial cells.)

cell wall (plant)

A tough outer layer made of cellulose that surrounds a plant cell. Prevents the cell bursting in water and provides support. (Plant cell walls are CELLULOSE. Bacterial cell walls are made of peptidoglycan.)

permanent vacuole

A large central sac in plant cells filled with cell sap. Helps keep the cell rigid (turgor pressure). (Edexcel wants 'cell sap' as the contents — writing 'water' or 'liquid' often loses the mark.)

plasmid

A small extra circle of DNA found in bacterial cells. Can carry useful genes such as antibiotic resistance. (Plasmids are a bacteria-only feature. Naming them is one of the reliable mark-points in 'differences between' questions.)

stem cell

An unspecialised cell that can divide and develop into different types of cell. Embryonic stem cells can become any cell type; adult stem cells can only become a limited range. (Edexcel distinguishes embryonic (pluripotent) from adult stem cells — know which is which.)

magnification

How many times bigger the image is than the real object. Calculated as image size ÷ real object size. Has no units. (Always convert units to match before calculating. Common slip: forgetting to divide image (mm) by object (μm) after converting.)

specialised cell

A cell adapted for a particular function — with structural features that fit the job. E.g. sperm cells have a flagellum to swim; red blood cells have no nucleus to make room for haemoglobin. (Always LINK the structural feature to the function — 'long tail' alone doesn't score; 'long tail to swim to the egg' does.)

Notes

Eukaryotic vs prokaryotic — the master distinction

Every cell on Earth is either eukaryotic or prokaryotic.

• Eukaryotic cells are larger (typically 10–100 micrometres across) and have a nucleus that holds the DNA. They also have membrane-bound organelles like mitochondria, chloroplasts, the endoplasmic reticulum and the Golgi apparatus. Animal, plant and fungal cells are all eukaryotic. The Edexcel mark scheme phrase is: eukaryotic cells have a nucleus | nucleus controls cell activities | eukaryotic cells have membrane-bound organelles such as mitochondria | each organelle carries out a specific function.
• Prokaryotic cells are smaller (typically 0.2–5 micrometres — about a tenth the size of an animal cell). They have no nucleus — the DNA is a single circular loop floating in the cytoplasm. They also have no membrane-bound organelles. Bacteria are prokaryotic.

The critical exam phrasing is 'no membrane-bound organelles' — bacteria still have ribosomes (which aren't membrane-bound), so saying 'no organelles' will lose a mark.

The animal cell — the basic eukaryotic toolkit

An Edexcel animal cell contains:

• Nucleus — controls cell activities and contains the chromosomes (DNA). The Edexcel rubric: nucleus contains DNA | DNA carries instructions for making proteins | DNA is replicated before division | new cells would have no DNA and could not function.
• Cytoplasm — jelly-like fluid where most chemical reactions happen.
• Cell membrane — selectively (partially) permeable outer boundary. The Edexcel rubric: selectively/partially permeable | useful substances (glucose, oxygen, water) enter | waste (CO₂, urea) leaves | maintains stable internal environment.
• Mitochondria — site of aerobic respiration. The Edexcel rubric for 'many mitochondria': mitochondria carry out respiration | respiration releases energy | active transport uses energy | more mitochondria produce more energy so more active transport.
• Ribosomes — site of protein synthesis. The Edexcel rubric for 'many ribosomes': ribosomes are where proteins are made/synthesised | enzymes are proteins | more ribosomes = more proteins at once | so more enzymes produced.
• Endoplasmic reticulum (ER) — Edexcel-specific. A network of membranes where proteins made on the ribosomes are processed.
• Golgi apparatus — Edexcel-specific. Modifies proteins (e.g. by adding carbohydrates) and packages them into vesicles for secretion. The Edexcel rubric: ER is where proteins are made | Golgi modifies proteins by adding carbohydrates | proteins transported ER → Golgi in vesicles | proteins transported to membrane for secretion.

That's the seven-structure Edexcel animal-cell toolkit.

The plant cell — animal cell plus three

A plant cell has everything an animal cell has, plus three plant-specific structures:

• Cell wall — a tough outer layer made of cellulose. Stops the cell bursting when it takes in water by osmosis, and gives the plant its shape and support.
• Chloroplasts — green organelles containing chlorophyll. The site of photosynthesis — where the plant makes glucose from CO₂ and water using light energy.
• Permanent vacuole — a large central sac filled with cell sap (a sugary water solution). Helps maintain turgor pressure to keep the cell rigid.

The word 'permanent' matters — animal cells can have small temporary vacuoles, but only plant cells have a permanent one.

The bacterial cell — small, simple, prokaryotic

A bacterial cell is much smaller and simpler:

• Cell wall — but made of a different material (peptidoglycan) than the plant cell wall (cellulose).
• Cell membrane — same role as in any cell.
• Cytoplasm.
• Ribosomes — protein synthesis still happens, just on smaller (70S) ribosomes than in eukaryotic cells.
• Single loop of DNA — the bacterium's main chromosome, free in the cytoplasm.
• Plasmids — small extra circles of DNA that can carry useful genes (e.g. antibiotic resistance).
• Flagellum (some bacteria) — a long thin tail for swimming.

No nucleus. No mitochondria. No chloroplasts. No endoplasmic reticulum. No Golgi. The bacterium does all its chemistry in the cytoplasm.

Specialised cells — structure fits function

Edexcel asks you to know five specialised cells. For each, you must link a structural feature to what it lets the cell do.

• Sperm cell — long flagellum (tail) to swim toward the egg; many mitochondria to power swimming; an acrosome at the head with enzymes to break through the egg membrane; haploid (23 chromosomes) so fertilisation produces a normal cell.
• Red blood cell (erythrocyte) — biconcave disc to maximise surface area for oxygen exchange; no nucleus to leave more room for haemoglobin (the oxygen-carrying protein); flexible to squeeze through narrow capillaries.
• Root hair cell — long thin extension to increase surface area for water + mineral absorption; thin cell wall for faster movement; many mitochondria to power active transport of minerals against their concentration gradient.
• Ciliated epithelial cell — covered in tiny hair-like cilia that beat in unison; lines the airways and sweeps mucus (with trapped dust/microbes) up toward the throat.
• Nerve cell (neurone) — very long axon to carry impulses over long distances; many dendrites to connect to many other cells; insulated myelin sheath to speed conduction.

The Edexcel marking phrase is 'structure fits function' — listing features alone usually doesn't score; the link to the job does.

Magnification — the calculation Edexcel loves

Under a microscope, the image you see is bigger than the real object. The relationship is:

magnification = size of image ÷ size of real object

Three rules to remember:

1. Always convert to the same unit first. If the image is in mm and the object in μm, convert one. Conversions: 1 mm = 1000 μm; 1 μm = 1000 nm.
2. Magnification has no units — it's just 'how many times bigger', so write '×400' (or '400×').
3. You can rearrange the formula: image size = magnification × real size, or real size = image size ÷ magnification.

Light microscopes magnify up to about ×1500. Electron microscopes can go much further (over ×500,000) and also have higher resolution — they can distinguish smaller details. (Resolution = the smallest distance at which two points can still be seen as separate — set by the wavelength of light or the electron beam.)

Exam tips

• When asked 'differences between prokaryotic and eukaryotic cells', give FOUR clear differences and contrast BOTH cell types in each — Edexcel marks comparison points, not one-sided lists.
• Always specify 'no membrane-bound organelles' for bacteria, not just 'no organelles' — bacteria have ribosomes.
• On vacuole questions, write 'cell sap' for the contents, not 'water'.
• For 'why many mitochondria' questions, walk through the full Edexcel chain: respiration → releases energy → active transport uses energy → more mitochondria = more active transport.
• For 'why many ribosomes' questions, walk through: ribosomes make proteins → enzymes are proteins → more ribosomes = more enzymes produced.
• On cell membrane questions, the magic word is 'selectively (partially) permeable' — and name specific substances that cross (glucose, oxygen, water in; CO₂, urea out).
• For specialised cells, always LINK structure to function: 'feature X is for purpose Y'.
• On magnification calculations, convert both numbers to the same unit FIRST. If asked 'in micrometres', remember 1 mm = 1000 μm.
• Plants have mitochondria AS WELL AS chloroplasts — don't fall into the 'only chloroplasts' trap.
• When the question mentions secretion of a protein (e.g. an enzyme or hormone), describe the FULL pathway: ribosome → ER → vesicle → Golgi → vesicle → cell membrane → outside.

Mark-scheme phrasing

Common misconceptions

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Worked example

Question:

Answer:

Frequently asked questions

Related topics in Edexcel GCSE Biology (1BI0)

• 1.2 Levels of organisation
• 1.5 Transport in cells