B1.2 What happens in cells (and what do cells need)

Why this matters

Every living cell is a tiny chemical factory that must constantly take in raw materials, run controlled chemical reactions, and remove waste — all without overheating, exploding, or running out of energy. The two reactions that matter most are photosynthesis and respiration: photosynthesis makes glucose from CO₂ and water using light energy, and respiration breaks glucose down to release energy as ATP. Plants do both. Animals can only respire — they rely on plants (or organisms that ate plants) for their glucose. Inside the cell, the reaction is not one big bang but a long chain of carefully controlled steps, each one catalysed by a specific enzyme. Enzymes are protein molecules with a precise active-site shape that matches a single substrate, like a key fits a lock. They speed up reactions enormously — cellular respiration without enzymes would take days per glucose molecule, not seconds — but they are also delicate: heat them too far above body temperature, expose them to extreme pH, and the active site changes shape permanently. This is why every cell tightly regulates its internal conditions, and why understanding enzymes is the foundation for understanding digestion, photosynthesis, respiration, and almost every other process you will meet in GCSE Biology.

How to learn this topic

Build on what you already know

• OCR B1.1: cells contain mitochondria (aerobic respiration), chloroplasts (photosynthesis in plant cells), cytoplasm and a cell membrane.
• KS3 Chemistry: word equations, reactants and products, the idea of a catalyst.
• OCR B2.1: substances enter and leave cells by diffusion, osmosis and active transport — active transport needs energy.
• KS3 Biology: plants make their own food using sunlight; animals get their food by eating; both need oxygen.
• OCR B1.4 (later): full detail on photosynthesis limiting factors; this topic gives the overview only.

1. Establish what every cell needs (glucose + oxygen) and what it produces (CO₂, water, energy as ATP).
2. Photosynthesis: word + symbol equation, takes place in chloroplasts, endothermic — links to B1.4.
3. Aerobic respiration: word equation, takes place in mitochondria, releases ATP — links to B1.3.
4. Anaerobic respiration: in muscle (lactic acid + fatigue) and in yeast/plants (ethanol + CO₂); much less energy.
5. Enzymes: define as biological catalysts with an active site; substrate fits like a key.
6. Effect of temperature: rate rises with temperature up to the optimum, then drops as the active site denatures.
7. Apply the chain to the OCR rubric questions — plant darkness, cyclist fatigue, mould amylase.

Key terms

cell

The smallest unit of a living organism. Needs a constant supply of glucose and oxygen to release ATP for its essential processes. (OCR exam chain: cells need glucose + oxygen → respiration → ATP → essential processes (muscle contraction, active transport, protein synthesis, growth).)

glucose

A simple sugar (C₆H₁₂O₆) that is the main fuel for cellular respiration. Made in plants by photosynthesis; obtained in animals from food. (Without glucose, respiration cannot happen, so no ATP can be released and the cell cannot carry out essential processes.)

oxygen

A gas used by cells in aerobic respiration to release energy from glucose. Diffuses into cells from the blood (animals) or air spaces (plants). (When oxygen is used faster than it is delivered, cells switch to anaerobic respiration.)

ATP

Adenosine triphosphate — the energy-carrying molecule released by respiration. ATP is used directly by the cell for every active process. (OCR phrasing: 'energy released as ATP'. Examples of ATP-using processes: muscle contraction, active transport, protein synthesis, growth.)

photosynthesis

The endothermic reaction in plant and algal chloroplasts that uses light energy to convert carbon dioxide and water into glucose and oxygen. Equation: carbon dioxide + water → glucose + oxygen. (Endothermic — energy is transferred from the environment to the chloroplasts by light.)

aerobic respiration

Exothermic reaction in mitochondria that uses oxygen to break glucose down completely to carbon dioxide and water, releasing ATP. Occurs continuously in every living cell. (Word equation must include BOTH products (CO₂ AND water) for the mark.)

anaerobic respiration

Respiration that does NOT use oxygen. In muscle: glucose → lactic acid. In yeast and plants: glucose → ethanol + carbon dioxide. Releases much less ATP per glucose than aerobic respiration. ('Glucose broken down incompletely' is a marked phrase. Lactic acid is the MUSCLE product only; ethanol + CO₂ is the YEAST/PLANT product.)

lactic acid

Waste product of anaerobic respiration in animal muscle cells. Accumulates in muscles during vigorous exercise and causes fatigue. (OCR rubric: 'oxygen used faster than delivered → anaerobic respiration → glucose → lactic acid → lactic acid accumulates causing fatigue'.)

mitochondria

Subcellular organelles where aerobic respiration takes place. Cells with high energy demand (muscle, liver, sperm) have many mitochondria. (Aerobic respiration → mitochondria. Anaerobic respiration → cytoplasm.)

chloroplast

The organelle in plant and algal cells where photosynthesis takes place. Contains the green pigment chlorophyll. (Chloroplast = organelle. Chlorophyll = pigment inside the chloroplast that absorbs the light. Don't muddle them.)

enzyme

A biological catalyst — a protein molecule that speeds up a specific chemical reaction in a living cell, without being used up itself. Examples: amylase (starch → sugar), lipase, catalase. (Always say 'protein' and 'speeds up reactions' for the basic definition mark.)

active site

The specific region on the surface of an enzyme that has a shape complementary to its substrate. The substrate binds to the active site so the reaction can be catalysed. (OCR phrase: 'active site complementary in shape to substrate'.)

substrate

The molecule that an enzyme acts on. The substrate fits into the active site of the enzyme like a key in a lock. (Each enzyme only acts on a specific substrate — this is enzyme specificity.)

denaturation

The permanent change of shape of an enzyme's active site, caused by high temperature or extreme pH. Once denatured, the substrate can no longer bind, so the enzyme can no longer catalyse the reaction. (OCR phrase chain: 'active site changes shape permanently → substrate no longer fits → enzyme denatures'. NEVER say 'the enzyme dies' — enzymes are not alive.)

lock and key model

A simple model of enzyme action: the substrate (key) fits exactly into the enzyme's active site (lock) because the two have complementary shapes. (Use this term in any answer explaining why enzymes are specific to one substrate.)

optimum temperature

The temperature at which an enzyme has its maximum rate of activity. For most human enzymes this is around 37 °C. Above the optimum, the enzyme denatures and the rate falls sharply. (On a rate-vs-temperature graph the curve rises to the optimum, then drops — be ready to label denaturation on the falling side.)

Notes

What every cell needs and what every cell produces

Every living cell — whether it is in a leaf, a liver, a muscle or a mould — needs a constant supply of two things: glucose (fuel) and oxygen (for aerobic respiration). It also needs water, mineral ions and the right temperature and pH. In return, the cell produces ATP, the universal energy currency that powers every cellular process: muscle contraction, active transport, protein synthesis (joining amino acids together) and growth. ATP is made inside mitochondria during aerobic respiration, and as soon as it is made the cell uses it. There is no big stockpile — cells make ATP continuously, every second of every day, for as long as they are alive.

This is why a plant kept in darkness eventually dies. No light means no photosynthesis. No photosynthesis means no glucose is made. No glucose means cellular respiration stops. No respiration means no ATP — and without ATP the cell cannot carry out the essential processes that keep it alive. This is a favourite OCR exam chain: trace it from light → photosynthesis → glucose → respiration → ATP → essential processes.

Photosynthesis — making the food

Photosynthesis is the reaction that builds glucose from inorganic raw materials. It happens inside chloroplasts in plant and algal cells, using the green pigment chlorophyll to absorb light energy.

Word equation:

> carbon dioxide + water → glucose + oxygen

Balanced symbol equation:

> 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The key features:

• Photosynthesis is endothermic — energy is transferred from the environment to the chloroplasts by light.
• Plants use the glucose for respiration, store some as starch, build cellulose for cell walls, make lipids for seed energy stores, and combine it with nitrate ions from the soil to make amino acids and then proteins.
• The oxygen released is a waste product — but it is essential for every aerobic organism on the planet.

Fuller detail on the four limiting factors (light intensity, CO₂, temperature, chlorophyll) is in B1.4; here you only need the equation, location, and the link to respiration.

Aerobic respiration — releasing the energy

Aerobic respiration is the reaction that releases energy from glucose, using oxygen. It happens mainly in the mitochondria of every living cell, and runs continuously, day and night.

Word equation:

> glucose + oxygen → carbon dioxide + water (+ energy as ATP)

Symbol equation:

> C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Key features:

• Aerobic respiration is exothermic — it releases energy.
• Glucose is broken down completely, so a large amount of ATP is released per glucose molecule.
• The ATP powers all of the cell's active processes: muscle contraction, active transport, building bigger molecules (proteins, starch, glycogen), keeping warm in mammals and birds, and transmitting nervous impulses.

Anaerobic respiration — when oxygen runs short

When oxygen is used faster than it can be delivered — for example in a sprinting cyclist's leg muscles — cells switch to anaerobic respiration, which does NOT use oxygen.

In animal muscle cells:

> glucose → lactic acid (+ a little energy)

Lactic acid accumulates in the muscles and causes fatigue and the burning ache you feel during hard exercise. The breakdown of glucose is incomplete, so anaerobic respiration releases much less ATP per glucose than aerobic respiration.

In yeast and plant cells (called fermentation in yeast):

> glucose → ethanol + carbon dioxide (+ a little energy)

Fermentation in yeast is the basis of brewing (the ethanol is the alcohol; the CO₂ gives beer its fizz) and bread-making (the CO₂ bubbles make dough rise).

Enzymes — biological catalysts

Enzymes are biological catalysts: protein molecules that speed up chemical reactions in living cells without being used up themselves. Photosynthesis, respiration, digestion, DNA replication — every one of them is controlled by enzymes.

Each enzyme has a specific shape, and a region called the active site which is complementary in shape to its substrate (the molecule the enzyme acts on). The substrate fits into the active site like a key fits a lock — this is the lock-and-key model. Only one substrate fits one enzyme: amylase only breaks down starch, lipase only breaks down lipids, catalase only breaks down hydrogen peroxide. This is called enzyme specificity.

How the reaction proceeds:

1. The substrate collides with the enzyme and binds to the active site.
2. The enzyme catalyses the reaction — for example, splitting the substrate into smaller products.
3. The products leave the active site, and the enzyme is free to bind another substrate molecule.

Factors affecting enzyme activity

Temperature. Up to an optimum, raising the temperature increases the rate of an enzyme-catalysed reaction because the substrate and enzyme molecules have more kinetic energy and collide more often. For most human enzymes the optimum is around 37 °C. Above the optimum, the rate falls sharply because the enzyme denatures.

pH. Each enzyme has an optimum pH where its active site is the right shape. Stomach enzymes (like pepsin) work best at very acidic pH ~2; saliva amylase prefers neutral pH ~7; intestinal enzymes prefer slightly alkaline pH ~8. Move away from the optimum pH and the active site changes shape and the rate falls.

Substrate concentration. Higher substrate concentration means more successful collisions per second, so a faster rate — until every active site is occupied and the rate plateaus.

Denaturation — what actually happens

When the temperature rises too far (above about 40 °C for human enzymes; above the optimum for any enzyme), the bonds that hold the enzyme's 3D shape together break. The active site changes shape, so the substrate can no longer bind to it. The enzyme is said to be denatured, and this change is permanent — cooling the enzyme back down does not return it to its original shape. The reaction it used to catalyse will now happen incredibly slowly, or not at all.

A mark-scheme-perfect description of denaturation reads: the high temperature causes the active site to change shape permanently, so the substrate no longer fits and the enzyme can no longer catalyse the reaction. Drop that sentence into any 4-mark enzyme denaturation question and you have most of the marks already.

How it all fits together — interdependence

Photosynthesis and respiration are mirror images of each other. Photosynthesis takes CO₂ and water and uses light energy to make glucose and oxygen. Respiration takes glucose and oxygen and breaks them down to CO₂ and water, releasing the energy as ATP. The two reactions cycle the same atoms back and forth, with light as the input and ATP as the cellular output. Without photosynthesis there is no glucose; without glucose there is no ATP; without ATP a cell cannot survive. And every step of both reactions is controlled by enzymes — which is why even a few degrees of temperature change can shut a cell down.

Exam tips

• On the plant-in-darkness question, write the full chain: no light → no photosynthesis → no glucose → no cellular respiration → no ATP → cell cannot carry out essential processes. That chain is worth every mark on the question.
• Always state that respiration occurs IN ALL LIVING CELLS, CONTINUOUSLY, and releases energy AS ATP. Those three phrases bag three separate marks on OCR mark schemes.
• For the cyclist-fatigue question, name the full sequence: oxygen used faster than delivered → anaerobic respiration → glucose → lactic acid → lactic acid accumulates causing fatigue. Five marking points in one chain.
• When defining an enzyme, say 'biological catalyst' AND 'protein' AND 'speeds up a reaction without being used up'. Three marks in one sentence.
• For denaturation, the magic phrase is 'active site changes shape PERMANENTLY so the substrate no longer fits' — the word 'permanently' is what separates a 2-mark answer from a 4-mark one.
• Name specific examples in enzyme answers: amylase breaks down starch into simple sugars; lipase breaks down lipids; pepsin works in the stomach. Specifics score; vague answers don't.
• Don't say 'the enzyme dies' or 'the enzyme is killed' — examiners delete the mark every time. Enzymes are protein molecules, not living organisms.
• If asked what ATP is used for, give specific named processes: muscle contraction, active transport, protein synthesis, keeping warm. One mark each.

Mark-scheme phrasing

Common misconceptions

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Worked example

Question:

Answer:

Frequently asked questions

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

• B1.1 Cell structures
• B1.3 Respiration
• B1.4 Photosynthesis