OCR GCSE Biology: Cells, Enzymes and Transport (B1–B2)
OCR GCSE Biology: Cells, Enzymes and Transport (B1–B2)
Almost everything else in GCSE Biology rests on the first two topics of the OCR Gateway Science A specification (J247). B1 (Cell-level systems) and B2 (Scaling up) between them explain what living things are built from, how those building blocks do chemistry, and how substances move into, out of and around an organism. Get these ideas secure and the later topics — coordination, ecology, genetics and disease — become far easier, because they all assume you already understand cells, enzymes and transport.
This guide works through both topics at GCSE depth. For each idea you will find what you need to know, a worked example or table to make it concrete, the highest-yield exam points, and the misconceptions that cost students marks every year. The content here is examined on both the Foundation and Higher tiers; where a point is Higher only, it is flagged with [H].
If you want structured practice alongside this guide, work through the LearningBro OCR GCSE Biology: Cell-level Systems course for B1 and the Scaling Up course for B2. Both cover every idea below with exam-style questions that match the OCR format.
How B1 and B2 Are Examined on OCR J247
OCR Gateway Science A GCSE Biology (J247) is assessed by two papers, each lasting 1 hour 45 minutes and worth 90 marks. Paper 1 covers B1–B3 and Paper 2 covers B4–B6, so the cells, enzymes and transport material in B1 and B2 sits firmly on Paper 1. Each paper carries a mixture of short-answer recall, longer "describe and explain" questions, data and graph work, and at least one extended six-mark response.
A few features of the specification are worth keeping in mind as you revise:
- Working scientifically runs through everything. You are expected to handle required-practical methods, interpret data and evaluate results, not just recall facts.
- Maths skills are tested directly — magnification, surface-area-to-volume ratios, percentages and rates all appear, and a calculator is allowed.
- The command word tells you what to do. "State" or "name" wants a short fact; "describe" wants what happens; "explain" wants the reason why; "calculate" wants a number with working shown.
Keep the command words in mind throughout, because they decide how much you need to write.
B1: Cell-Level Systems
Eukaryotic and Prokaryotic Cells
All living things are made of cells, and at GCSE you must distinguish two types. Eukaryotic cells have a true nucleus enclosed in a membrane and include all animal, plant and fungal cells. Prokaryotic cells, such as bacteria, are smaller and have no nucleus; instead their genetic material is a single loop of DNA free in the cytoplasm, often with small extra rings called plasmids.
The table below summarises the structures you must be able to identify and what each one does.
| Structure | Found in | Function |
|---|---|---|
| Nucleus | Animal, plant (eukaryotic) | Controls the cell; contains DNA |
| Cytoplasm | All cells | Site of most chemical reactions |
| Cell membrane | All cells | Controls what enters and leaves |
| Mitochondria | Animal, plant | Site of aerobic respiration |
| Ribosomes | All cells | Site of protein synthesis |
| Cell wall | Plant, bacteria | Strengthens and supports the cell |
| Chloroplasts | Plant only | Site of photosynthesis; contain chlorophyll |
| Permanent vacuole | Plant only | Filled with sap; keeps the cell firm |
A classic exam point is how a plant cell differs from an animal cell: a plant cell additionally has a cell wall (made of cellulose), chloroplasts and a permanent vacuole. A bacterial cell has a cell wall too, but it is not made of cellulose, and bacteria never have chloroplasts, mitochondria or a true nucleus.
Common misconception: students write that "all cells have a cell wall". Only plant and bacterial cells do — animal cells do not.
Specialised Cells
Cells become specialised to carry out particular jobs, and their structure is adapted to their function. Be ready to link a feature to its purpose:
- Sperm cell — a tail (flagellum) for swimming and many mitochondria to release the energy needed to swim.
- Nerve cell (neurone) — long and thin to carry impulses over distances; branched endings to connect to other cells.
- Muscle cell — packed with mitochondria to power contraction.
- Root hair cell — a large surface area to absorb water and mineral ions.
- Red blood cell — no nucleus, leaving more room for haemoglobin to carry oxygen, and a biconcave disc shape for a large surface area.
In the exam, do not just name the adaptation — explain how it helps. "The root hair cell has a large surface area" earns less than "...a large surface area, which increases the rate of water absorption by osmosis."
Microscopy and Magnification
Cells are too small to see with the naked eye, so we use microscopes. A light microscope magnifies up to around ×1500 and lets us see cells and larger structures such as the nucleus. An electron microscope has a far higher magnification and resolution (the ability to distinguish two close points as separate), which is why it can reveal tiny sub-cellular structures like ribosomes.
You must use the magnification formula confidently:
magnification=actual sizeimage size
Rearrange it as needed: actual size=magnificationimage size and image size=actual size×magnification.
Worked example: calculating actual size
An image of a cell measures 50 mm across. The magnification is ×500. Work out the actual width of the cell in micrometres.
First find the actual size in millimetres: actual size=50050=0.1 mm. Then convert to micrometres, remembering that 1 mm=1000μm, so 0.1×1000=100μm. The cell is 100 μm wide.
The single most common error here is unit mismatch — measuring the image in millimetres but giving the answer as if it were micrometres. Always check that the image size and actual size are in the same unit before dividing, then convert at the end.
DNA and the Genome
The instructions for building and running an organism are carried by DNA, a polymer made of two strands coiled into a double helix. A length of DNA that codes for a particular protein is a gene, and the entire set of genetic material in an organism is its genome. At GCSE you should know that understanding the human genome helps scientists search for genes linked to disease, trace human migration and develop better treatments.
Enzymes: the Lock-and-Key Model
Enzymes are biological catalysts — proteins that speed up reactions without being used up. Each enzyme has a region called the active site with a specific shape that fits one particular substrate, much as a key fits one lock. This is the lock-and-key model, and it explains why enzymes are specific: an enzyme that breaks down starch will not act on a protein.
When the substrate binds, the enzyme converts it into products, which are released, leaving the enzyme free to repeat the process.
Factors Affecting Enzyme Activity
Enzyme reactions speed up or slow down depending on conditions, and three factors are examined.
- Temperature. Raising the temperature increases activity up to an optimum, because molecules collide more often and with more energy. Beyond the optimum the enzyme denatures: the active site changes shape and can no longer bind the substrate, so activity falls sharply.
- pH. Each enzyme has an optimum pH. Too far either side and the enzyme denatures. Stomach protease works best in acidic conditions, whereas enzymes in the small intestine prefer slightly alkaline conditions.
- Substrate concentration. More substrate means more frequent collisions and a faster rate, until every active site is working as fast as it can; after that the rate levels off.
Common misconception: a denatured enzyme is not "killed" — enzymes were never alive. The correct phrasing is that the active site changes shape so the substrate no longer fits.
Respiration
Respiration releases energy from glucose in every living cell, all the time. It is an exothermic reaction and must not be confused with breathing.
Aerobic respiration uses oxygen and releases the most energy:
glucose+oxygen→carbon dioxide+water
Anaerobic respiration happens without oxygen and releases far less energy. In animal cells it produces lactic acid, which builds up during hard exercise. In yeast and plant cells it produces ethanol and carbon dioxide — the basis of fermentation in brewing and baking.
A reliable exam point is why aerobic respiration is preferred: it transfers much more energy per molecule of glucose because the glucose is fully broken down, whereas anaerobic respiration only partially breaks it down.
Photosynthesis and Limiting Factors
Photosynthesis is how plants and algae make their own food. It is endothermic — it takes in energy, transferred from light:
carbon dioxide+water→glucose+oxygen
The rate of photosynthesis depends on three limiting factors: light intensity, carbon dioxide concentration and temperature. A limiting factor is whichever one is in shortest supply and is therefore capping the rate. On a graph, the rate rises as you increase a factor and then plateaus once a different factor becomes limiting.
Worked example: reading a limiting-factor graph
A graph shows the rate of photosynthesis rising steeply as light intensity increases, then levelling off. Explain the shape.
At low light intensity, light is the limiting factor, so as you add more light the rate rises in proportion. Where the line levels off, light is no longer limiting — adding more makes no difference because something else, such as carbon dioxide concentration or temperature, has become the limiting factor instead. A good answer names a specific alternative factor; it does not simply say "the plant got tired."
This is one of the most heavily examined ideas in B1, and it links straight back to enzymes, because temperature affects photosynthesis partly because the reactions are enzyme-controlled.
B2: Scaling Up
B2 builds on B1 by asking how single cells become whole organisms, and how those organisms move substances around once they are too big to rely on simple diffusion alone.
Mitosis and the Cell Cycle
Mitosis is cell division that produces two genetically identical daughter cells, used for growth, repair and asexual reproduction. It is one stage of the larger cell cycle, in which the cell first grows and copies its DNA and sub-cellular structures, then divides.
The key facts to bank are that each new cell has the same number of chromosomes as the parent and is genetically identical to it. Contrast this with meiosis (covered in B5), which halves the chromosome number and produces variation.
Common misconception: mitosis does not create variation. Identical daughter cells are the whole point; variation comes from meiosis and sexual reproduction.
Cell Differentiation and Stem Cells
As an organism develops, unspecialised cells differentiate — they switch on particular genes to become specialised for a job, such as a muscle cell or a nerve cell. In animals, most cells differentiate early and lose the ability to change, whereas many plant cells keep the ability to differentiate throughout life.
A stem cell is an undifferentiated cell that can keep dividing and can become different cell types:
- Embryonic stem cells can become almost any type of cell.
- Adult stem cells (for example in bone marrow) form a more limited range, such as blood cells.
- Meristem tissue in plants contains stem cells found in the growing tips of roots and shoots.
Stem cells offer medical potential — for example, replacing damaged cells in conditions such as diabetes or paralysis, or in therapeutic cloning, where cells genetically matched to a patient reduce the risk of rejection. Be ready to discuss the benefits and the ethical and practical concerns (the use of embryos, the risk of viral infection, and cost), because evaluation questions are common.
Diffusion, Osmosis and Active Transport
Substances move across cell membranes by three processes you must keep clearly separate.
| Process | What moves | Direction | Energy needed? |
|---|---|---|---|
| Diffusion | Any particles (e.g. oxygen, CO₂) | High to low concentration | No |
| Osmosis | Water only | High to low water concentration, across a partially permeable membrane | No |
| Active transport | Dissolved particles (e.g. mineral ions) | Low to high concentration (against the gradient) | Yes |
Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, down a concentration gradient. Osmosis is the special case for water: water moves across a partially permeable membrane from a dilute solution (high water concentration) to a more concentrated one (low water concentration). Active transport moves substances the "wrong" way, from low to high concentration, which requires energy from respiration — this is how root hair cells absorb mineral ions from very dilute soil water.
Common misconception: osmosis is "water moving from low to high concentration". Be precise — it is from high water concentration (dilute) to low water concentration (concentrated). Talking about the solute the wrong way round is the classic trap.
Surface Area to Volume Ratio and Exchange Surfaces
Diffusion alone is fast enough only over very short distances, so this idea explains why large organisms need transport systems. As an object gets bigger, its volume grows faster than its surface area, so the surface-area-to-volume ratio falls. A single small cell has a high ratio and can exchange substances by diffusion across its surface; a large multicellular organism has a low ratio and cannot, so it needs specialised exchange surfaces and a transport system.
Worked example: surface-area-to-volume ratio
Compare the surface-area-to-volume ratio of a cube of side 1 cm with a cube of side 2 cm.
For the 1 cm cube: surface area =6×(1×1)=6 cm2 and volume =1×1×1=1 cm3, so the ratio is 6:1. For the 2 cm cube: surface area =6×(2×2)=24 cm2 and volume =2×2×2=8 cm3, so the ratio is 24:8=3:1. The larger cube has the smaller ratio, showing that as size increases the surface area available per unit of volume drops — which is exactly why bigger organisms cannot rely on diffusion across the body surface.
Good exchange surfaces share features you should be able to list: a large surface area, a thin membrane (short diffusion distance), and — in animals — a good blood supply and ventilation to maintain steep concentration gradients. The alveoli in the lungs and the villi in the small intestine are the standard examples.
The Human Circulatory System
Humans use a double circulatory system: the heart pumps blood to the lungs and back in one circuit, and to the rest of the body and back in the other. This keeps oxygenated and deoxygenated blood separate and delivers oxygen at high pressure.
You should know the three blood vessels and how their structure suits their function:
- Arteries carry blood away from the heart at high pressure; they have thick, muscular, elastic walls and a narrow lumen.
- Veins carry blood back to the heart at low pressure; they have thinner walls, a wide lumen, and valves to stop backflow.
- Capillaries are one cell thick, giving a short diffusion distance for exchanging substances with the tissues.
Blood itself is made of red blood cells (carry oxygen using haemoglobin), white blood cells (defence against pathogens), platelets (help clotting) and plasma (the liquid that carries cells, carbon dioxide, glucose and other dissolved substances).
A frequent recall slip is the artery/vein exception: in general arteries carry oxygenated blood and veins deoxygenated, but the pulmonary artery (to the lungs) carries deoxygenated blood and the pulmonary vein carries oxygenated blood. The defining feature is direction relative to the heart, not the oxygen content.
Transport in Plants: Xylem, Phloem and Transpiration
Plants have their own transport tissues. Xylem carries water and dissolved mineral ions from the roots upward to the leaves, in one direction only; its cells are dead, hollow and strengthened with lignin. Phloem carries dissolved sugars (made in the leaves) to wherever they are needed for growth or storage, and can move them in either direction — a process called translocation.
Transpiration is the loss of water vapour from the leaves, mainly through tiny pores called stomata, which are opened and closed by guard cells. The continuous loss of water from the leaves pulls a column of water up through the xylem — the transpiration stream. Four factors increase the transpiration rate:
- Higher temperature — water evaporates faster.
- Higher light intensity — stomata open wider for photosynthesis, so more water escapes.
- Greater air movement (wind) — blows away humid air, keeping the gradient steep.
- Lower humidity — drier surrounding air steepens the concentration gradient.
Common misconception: xylem and phloem get swapped constantly. A memory hook: xylem carries water up (think "xy = sky", going up), while phloem carries food ("ph = food") both ways.
Common Mistakes Across B1 and B2
The same errors recur every year. Knowing them in advance is half the battle.
- Unit slips in magnification. Mixing millimetres and micrometres ruins an otherwise correct calculation. Convert at the end.
- "Enzymes are killed." They denature; the active site changes shape. Enzymes are not alive.
- Osmosis described by solute, the wrong way round. State it in terms of water concentration: high to low, across a partially permeable membrane.
- Confusing breathing with respiration. Respiration is the chemical release of energy in cells; breathing (ventilation) moves air in and out.
- Saying mitosis "creates variation." It produces genetically identical cells. Variation is a meiosis idea.
- Swapping xylem and phloem, or claiming phloem moves only upward. Phloem translocates both ways.
- Naming an adaptation without explaining it. Always link structure to function and to a rate of exchange.
Exam Technique for B1 and B2 on OCR J247
These topics sit on Paper 1, so prepare for a mix of recall, calculation and extended explanation.
- Answer the command word. "Describe" wants what happens; "explain" wants why. A description where an explanation is asked for caps your marks.
- Show working in calculations. Magnification and surface-area-to-volume questions carry method marks; a wrong final number with correct working still scores.
- Plan six-mark answers. Extended responses on, say, exchange surfaces or limiting factors are marked on a levels basis for clear, logical, joined-up reasoning. Jot three or four bullet points first so your answer flows.
- Use the data given. Graph and table questions reward you for quoting figures and trends from the source, not for general knowledge alone.
- Learn the required-practical methods. Microscopy, osmosis in potato chips, and the rate of photosynthesis are all examinable as method, variables and evaluation, not just results.
Prepare with LearningBro
The LearningBro OCR GCSE Biology: Cell-level Systems course covers all of B1 — cells, microscopy, DNA, enzymes, respiration and photosynthesis — while the Scaling Up course covers all of B2 — mitosis, stem cells, transport across membranes, exchange surfaces and the circulatory and plant transport systems. Each lesson includes worked examples and exam-style questions that mirror the format and difficulty of the real OCR papers, with immediate feedback.
To rehearse whole-paper strategy and the command words, work through the OCR GCSE Biology Exam Prep course. And for the wider picture of the whole subject, start with our OCR GCSE Biology complete revision guide.
Cells, enzymes and transport are the foundation of the entire course. Secure them first, and every topic that follows will make more sense. Good luck with your revision.