B1.1 Cell structures

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 cell wall, chloroplasts and a permanent vacuole so they can photosynthesise and stand upright. Bacterial cells are smaller and much simpler — no nucleus, no membrane-bound organelles. They keep their DNA in a nucleoid region of the cytoplasm, plus extra little rings called plasmids. In OCR Gateway B1.1, knowing which structures belong to which cell type is the foundation for almost every later topic: how respiration releases energy (mitochondria), how plants make glucose (chloroplasts), how bacteria cause disease (cell wall), 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 cell question hangs off this distinction.
2. Build the animal cell first (the simplest eukaryote): nucleus, cytoplasm, cell membrane, mitochondria, ribosomes.
3. Then add to it for a plant cell: cell wall, chloroplasts, permanent vacuole. Same toolkit + 3 plant-only extras.
4. Bacterial cells next: smaller, no nucleus, DNA in a nucleoid region + plasmids.
5. Specialised cells: each cell type has structural features that fit its job — palisade mesophyll, root hair, ciliated, sperm, red blood, nerve.
6. Finish with light vs electron microscopy and the difference between magnification and resolution.

Key terms

eukaryotic cell

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

prokaryotic cell

A smaller, simpler cell with no nucleus and no membrane-bound organelles. DNA sits in a nucleoid region of the cytoplasm, plus extra plasmids. Bacteria are prokaryotic.

nucleus

The organelle in a eukaryotic cell that controls the cell and contains the chromosomes (DNA). (Saying a cell 'has no nucleus' is the marking phrase for prokaryotic cells, NOT 'has no DNA' — bacteria do have DNA, just not inside a nucleus.)

mitochondrion

Organelle where aerobic respiration takes place. Found in animal cells AND plant cells (because plants also respire), and in yeast. (Plant cells have BOTH mitochondria AND chloroplasts. Don't write 'only chloroplasts' — plants respire as well as photosynthesise.)

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 algae). Not in animal cells. Not in fungal cells (yeast). Not in bacterial cells.)

cell wall (plant)

A tough outer layer made of cellulose that surrounds a plant cell. Provides strength and support and prevents the cell bursting in water. (Plant cell walls are made of CELLULOSE. Bacterial cell walls are peptidoglycan; fungal cell walls are chitin.)

permanent vacuole

A large central sac in plant cells filled with cell sap. Helps keep the cell rigid (turgor pressure). (The contents are 'cell sap', not 'water' — writing 'water' often loses the mark.)

ribosome

The site of protein synthesis (where proteins are made). Found in all cells, including bacterial cells. (Ribosomes are NOT membrane-bound, so they are found in both prokaryotic AND eukaryotic cells.)

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.)

nucleoid region

The area of cytoplasm in a bacterial (prokaryotic) cell where the single circular chromosome of DNA sits. NOT a true nucleus — there is no membrane around it. (OCR uses 'nucleoid region' for bacteria. Don't call this a nucleus — that's the marking trap.)

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 convert image (mm) and object (μm) to the same unit.)

resolution

The smallest distance at which two points can still be seen as separate. Electron microscopes have much higher resolution than light microscopes. (Examiners distinguish resolution from magnification — magnifying a blurry image doesn't reveal more detail; you need higher resolution for that.)

specialised cell

A cell adapted for a particular function — with structural features that fit the job. E.g. palisade mesophyll cells are packed with chloroplasts for photosynthesis; red blood cells have no nucleus to make room for haemoglobin. (Always LINK the structural feature to the function — 'lots of chloroplasts' alone doesn't score; 'lots of chloroplasts so the cell can do plenty of photosynthesis' 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 other membrane-bound organelles like mitochondria and (in plants) chloroplasts. Animal, plant and fungal cells (including yeast) are all eukaryotic.
• Prokaryotic cells are smaller (typically 0.2–5 micrometres — about a tenth the size of an animal cell). They have no nucleus — the DNA sits in a nucleoid region of the cytoplasm as a single circular chromosome. 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 animal cell contains:

• Nucleus — controls the cell and contains the chromosomes (DNA) that hold the genetic information.
• Cytoplasm — jelly-like fluid where most chemical reactions happen.
• Cell membrane — the partially-permeable outer boundary that controls movement of substances in and out of the cell.
• Mitochondria — site of aerobic respiration, where glucose and oxygen react to release the energy the cell needs.
• Ribosomes — tiny structures where proteins are made (protein synthesis).

That's it for animal cells. Five structures, five jobs — examiners want you to know all five.

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. Provides strength and support, stops the cell bursting when it takes in water by osmosis, and gives the plant its shape.
• 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.

Fungal cells (e.g. yeast) are also eukaryotic and have a cell wall (made of chitin, not cellulose) and a vacuole, but they do NOT have chloroplasts — fungi don't photosynthesise.

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 — also contains the respiratory enzymes (bacteria respire too, just not inside mitochondria).
• Ribosomes — protein synthesis still happens, just on smaller (70S) ribosomes than in eukaryotic cells.
• Nucleoid region — the area of cytoplasm containing the single circular DNA chromosome. NOT a true nucleus (no membrane around it).
• Plasmids — small extra circles of DNA that can carry useful genes (e.g. antibiotic resistance).

No nucleus. No mitochondria. No chloroplasts. The bacterium does all its chemistry — including aerobic respiration — in the cytoplasm.

Specialised cells — structure fits function

Multicellular organisms have many different cell types, each adapted for a specific job. OCR favourites:

• Palisade mesophyll cell (plant) — tall, column-shaped cell packed near the upper surface of the leaf; full of chloroplasts to maximise light absorption for photosynthesis; thin walls so CO₂ can diffuse in easily.
• Root hair cell (plant) — 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.
• 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.
• 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 exam phrase is 'structure fits function' — for every specialised cell, you must link a structural feature to what it lets the cell do.

Microscopy — light vs electron

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. They use visible light, are cheap, and can image living cells — but their resolution is limited by the wavelength of light (~200 nm).

Electron microscopes use a beam of electrons (much shorter wavelength than light) so they have much higher resolution (down to about 0.1 nm) and can magnify over ×500,000 usefully. They reveal the internal structure of organelles. The cost is that samples must be dehydrated and placed in a vacuum — you can't image living cells.

Resolution is the smallest distance at which two points can still be seen as separate. Magnifying a blurry image just makes it bigger and blurry — more magnification only helps if you also have higher resolution.

Exam tips

• When asked 'differences between prokaryotic and eukaryotic cells', give four clear differences and contrast BOTH cell types in each one. e.g. 'prokaryotic has no nucleus; eukaryotic has a nucleus.'
• 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 specialised cells, always LINK structure to function: 'feature X is for purpose Y'. Listing features alone usually doesn't score with OCR examiners.
• 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.
• Use OCR's preferred phrasing for plant cell walls: 'made of cellulose, provides strength and/or support'.
• Remember yeast is a eukaryote (fungus). It has mitochondria but no chloroplasts — yeast does not photosynthesise.
• Magnification vs resolution: examiners often ask why electron microscopes are better — the answer is HIGHER RESOLUTION, not just higher magnification.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in OCR GCSE Biology A: Gateway Science (J247)

• B1.2 What happens in cells (and what do cells need)
• B1.3 Respiration
• B1.4 Photosynthesis