How are eukaryotic and prokaryotic cells built, and how does each sub-cellular structure carry out its job?
Eukaryotic and prokaryotic cells, the function of sub-cellular structures (nucleus, chromosomes, cytoplasm, cell membrane, mitochondria, ribosomes, chloroplasts, permanent vacuole, cell wall, plasmids, flagella), and how the structure of each is related to its function.
A focused answer to the OCR Gateway GCSE Biology A topic B1 on cell structures, covering eukaryotic and prokaryotic cells, the function of every sub-cellular structure, and how structure is related to function in animal, plant and bacterial cells.
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What this dot point is asking
OCR wants you to describe the structures inside eukaryotic and prokaryotic cells, state the function of each sub-cellular structure, and explain how the structure of a cell or organelle is related to the job it does. This is the foundation for every other biology topic, so the vocabulary must be precise.
Eukaryotic and prokaryotic cells
The two cell types differ in size, in where their DNA sits, and in whether they have membrane-bound organelles. A typical animal or plant cell is around to micrometres across; a bacterium is around micrometre, roughly a tenth to a hundredth of the size. Knowing this scale matters because OCR asks you to compare them and to handle the numbers in standard form.
Animal cell structures
An animal cell contains these structures, and you should be able to state what each one does:
- Nucleus. Contains the genetic material (DNA, packaged into chromosomes) and controls the cell's activities, including which proteins are made.
- Cytoplasm. A jelly-like fluid where most of the chemical reactions of the cell happen, controlled by enzymes.
- Cell membrane. Controls what moves in and out of the cell. It holds receptor molecules that let the cell respond to substances such as hormones.
- Mitochondria. The site of most aerobic respiration, which releases energy. They contain the enzymes needed for respiration, so cells that use a lot of energy (such as muscle cells) have many mitochondria.
- Ribosomes. The site of protein synthesis, where amino acids are joined in the right order to build proteins.
Plant and algal cell structures
A plant or algal cell has everything an animal cell has, plus three extra structures:
- Cell wall. Made of cellulose, it provides strength and support and stops the cell bursting when it takes in water.
- Chloroplasts. Contain the green pigment chlorophyll, which absorbs light for photosynthesis. They are found in cells that photosynthesise, such as leaf cells, but not in root cells.
- Permanent vacuole. A large sac filled with cell sap (a solution of sugars and salts). When full it pushes the contents against the cell wall and keeps the cell firm (turgid), which supports the plant.
Prokaryotic (bacterial) cell structures
A bacterial cell is far simpler and much smaller. It has:
- A cell membrane and a cell wall (but the wall is not made of cellulose like a plant wall).
- Cytoplasm containing ribosomes (so bacteria can make proteins).
- A single circular chromosome of DNA, free in the cytoplasm rather than in a nucleus.
- One or more plasmids, which are small separate rings of DNA that can carry extra genes such as antibiotic resistance.
- Some bacteria also have a flagellum (a tail-like structure) for movement, and a slime capsule for protection.
The key contrast to remember: a prokaryotic cell has no nucleus and no membrane-bound organelles (no mitochondria, no chloroplasts), but it does have a membrane, a wall, cytoplasm and ribosomes.
Relating structure to function
OCR rewards answers that link a structure to the job it does, not just the name. Useful examples to memorise:
- A sperm cell has many mitochondria to release the energy needed to swim, and a tail (flagellum) to move.
- A root hair cell has a long extension that gives a large surface area for absorbing water and mineral ions.
- A red blood cell has no nucleus (more room for haemoglobin) and a biconcave shape (large surface area for oxygen to diffuse).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksDescribe two ways the structure of a prokaryotic cell differs from the structure of a eukaryotic cell, and explain one way the structure of a red blood cell is adapted to its function. Use the command word Describe and Explain carefully.Show worked answer →
A B1 structured question that splits its marks between recall and application.
For the differences (Describe), reward any two clear structural points: a eukaryotic cell has its genetic material enclosed in a nucleus while a prokaryotic cell has no nucleus (its DNA is a single circular loop free in the cytoplasm); a eukaryotic cell has membrane-bound organelles such as mitochondria while a prokaryotic cell does not; a prokaryotic cell often has plasmids (small rings of DNA) which eukaryotic cells lack; a prokaryotic cell is much smaller.
For the adaptation (Explain), link structure to function: a red blood cell has no nucleus, leaving more room for haemoglobin to carry oxygen, and a biconcave disc shape gives a large surface area to volume ratio for fast diffusion of oxygen. Markers reward the structure and the reason it helps, not just the structure on its own.
OCR 20203 marksA plant cell and an animal cell are viewed under a microscope. Name three structures that are found in the plant cell but not in the animal cell, and state the function of one of them.Show worked answer →
A short B1 recall question. The three structures present in plant cells but not animal cells are the cellulose cell wall, chloroplasts and a permanent vacuole.
For the function, reward any correct match: the cell wall provides support and strength (stops the cell bursting and helps keep it rigid); chloroplasts contain chlorophyll and are the site of photosynthesis; the permanent vacuole contains cell sap and helps keep the cell turgid (firm). Markers do not credit "ribosomes" or "mitochondria" here, because both plant and animal cells contain them.
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