4.4.1 Photosynthesis

Why this matters

Photosynthesis is the single most important chemical reaction on Earth for life as we know it. Every food chain ultimately starts with a producer — almost always a green plant or alga — converting light energy into the chemical store of energy locked inside glucose. Without that conversion, no animal would have anything to eat, and the atmosphere would not contain the ~21% oxygen that aerobic respiration needs. For AQA GCSE Biology, photosynthesis sits in unit 4 (Bioenergetics) right next to respiration, and the examiners deliberately compare and contrast the two. You are expected to know the word equation, the balanced symbol equation (higher-tier and triple), and to describe the reaction as endothermic — meaning energy is transferred from the environment to the chloroplasts by light. The 'limiting factors' part of the topic is one of the most frequently examined areas in the whole specification, because it tests both biology recall and graph interpretation. Examiners commonly ask 4- and 6-mark questions where you must explain why the rate plateaus, why CO₂ is the limiting factor at high light, or how a commercial greenhouse maximises yield by raising CO₂ and using lamps. The 'uses of glucose' sub-topic gives a neat 4-mark structured answer that comes up most years. Get fluent with the five fates and the role of nitrate ions, and you essentially have those marks banked. This page bakes in the exact mark-scheme phrasings used by AQA so your answers read like the official rubric — that is what scores the marks.

How to learn this topic

Build on what you already know

• Plant cell structure including chloroplasts (from 4.1 Cell biology)
• Word and balanced symbol equations from KS3 chemistry
• Idea of energy transfer and the difference between exo- and endothermic reactions
• Reading line graphs and identifying when a curve plateaus

1. Recall the word equation and identify reactants vs products
2. Locate chloroplasts and chlorophyll and explain why leaves look green
3. Define endothermic and describe where the energy comes from and goes to
4. Introduce limiting factors one at a time and interpret the three rate graphs
5. Apply limiting factors to greenhouse contexts and required practical data
6. Trace the glucose product to its five fates, including nitrate ions for proteins
7. Practise 4-mark mark-scheme phrasing on a worked example

Key terms

photosynthesis

The endothermic reaction in which plants and algae use light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose and oxygen. (Always include 'light energy' and 'chlorophyll' or 'chloroplasts' in the definition to secure the mark.)

endothermic

A reaction that takes in energy from the environment. In photosynthesis, energy is transferred from the environment to the chloroplasts by light. (Examiners want the phrase 'transferred from the environment' — 'takes in energy' on its own often does not score the energy-transfer mark.)

chloroplast

The organelle in plant and algal cells where photosynthesis takes place. Contains chlorophyll.

chlorophyll

The green pigment inside chloroplasts that absorbs light energy (mainly red and blue light) so that it can be used in photosynthesis. (Note 'chlorophyll' is the pigment, 'chloroplast' is the organelle — do not muddle them.)

limiting factor

The factor that is in shortest supply and is therefore limiting the rate of a reaction. For photosynthesis, the limiting factor at any moment may be light intensity, CO₂ concentration, temperature or amount of chlorophyll. (The mark-scheme phrase is 'limits the rate when there is not enough of it'.)

light intensity

A measure of how bright the light is — the amount of light energy hitting a given area per second. Higher light intensity means more energy available to chlorophyll, up to the point where another factor limits the rate.

inverse square law

Light intensity is inversely proportional to the square of the distance from the light source — doubling the distance quarters the intensity. (Useful for explaining the lamp-distance/pondweed required practical.)

denature

When the shape of an enzyme's active site is permanently changed by high temperature (or extreme pH), so the substrate no longer fits and the enzyme can no longer catalyse the reaction. (Use 'active site changes shape', not 'the enzyme dies'.)

starch

An insoluble polymer of glucose that plants use as their main carbohydrate energy store, mainly in leaves, roots and seeds.

cellulose

A strong fibrous polymer of glucose that makes up the plant cell wall and gives plant cells their rigidity.

nitrate ions

Ions absorbed from the soil by the roots and combined with glucose to make amino acids, which are then built into proteins. (If asked how plants make proteins, you MUST mention nitrate ions from the soil for full marks.)

Notes

The reaction

Photosynthesis is the reaction by which plants and algae make their own food. The word equation you must be able to write from memory is:

carbon dioxide + water → glucose + oxygen

The balanced symbol equation (higher tier and triple science) is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Carbon dioxide is taken in from the air through stomata in the leaves. Water is absorbed by the roots from the soil and travels up to the leaves in the xylem. Glucose is the food product — a small soluble sugar. Oxygen is the waste product and it diffuses back out through the stomata, which is where almost all of the oxygen in the atmosphere has come from.

Where it happens — chloroplasts and chlorophyll

Photosynthesis happens inside organelles called chloroplasts, which are packed into the palisade mesophyll cells just under the upper surface of the leaf. Chloroplasts contain the green pigment chlorophyll. Chlorophyll is what absorbs the light energy — it reflects green light (which is why leaves look green) and absorbs the red and blue parts of the visible spectrum. The energy that chlorophyll absorbs is then used to drive the reaction that converts CO₂ and water into glucose. If a plant lacks chlorophyll (for example because it is short of magnesium or because part of the leaf is variegated white), that part of the plant cannot photosynthesise.

Endothermic — what energy phrase to use

Photosynthesis is an endothermic reaction, which means energy is transferred from the environment to the chloroplasts by light. AQA mark schemes are very strict on this phrasing. Do not write 'the reaction gives out energy' — that is exothermic and it is wrong. The mark-scheme-perfect sentence is: 'Light provides/transfers the energy, the energy is transferred to the chloroplasts, and endothermic reactions absorb energy from the environment.' That sentence on its own can score 3 of the 4 marks on the classic 'why is photosynthesis endothermic' question.

Limiting factors — the four to learn

A limiting factor is whatever is in shortest supply and is therefore limiting the rate of the reaction. The four factors AQA wants you to know are:

1. Light intensity — provides the energy. As light intensity rises, the rate rises proportionally until something else becomes the limiting factor. The curve then plateaus.
2. Carbon dioxide concentration — a reactant/raw material. If there is not enough CO₂, the rate is low. As CO₂ rises, the rate rises until light or temperature limits it. The curve plateaus.
3. Temperature — the reactions are controlled by enzymes. Below the optimum (around 25-35 °C for most plants) the rate is low because particles have less kinetic energy. Above about 40-45 °C the enzymes start to denature (their active sites change shape) and the rate falls sharply. So the temperature graph rises, peaks at the optimum, then drops.
4. Amount of chlorophyll — if a plant has less chlorophyll (e.g. because of a magnesium deficiency, disease or variegation), it absorbs less light, so the rate is lower.

In a typical UK greenhouse on a summer day, light intensity is high so the limiting factor is usually CO₂. Commercial growers raise the CO₂ concentration (and heat the greenhouse at night) to push the rate up and increase yield.

Measuring the rate

In the required practical, rate of photosynthesis is usually measured by counting the bubbles of oxygen released by pondweed (e.g. Elodea) in a given time, or by collecting the gas over a measuring cylinder. The lamp is moved to different distances to vary light intensity. Because light intensity ∝ 1 / distance² (the inverse square law), doubling the distance from the lamp reduces the intensity to a quarter.

The five fates of glucose

Glucose is the immediate product of photosynthesis but it is not all used straight away. Plants do five things with it:

• Used in respiration — releases energy for the plant's life processes (active transport, building bigger molecules, etc.). Plants respire 24 hours a day.
• Converted to starch for storage. Starch is insoluble, so it does not affect osmosis, and it can be stockpiled in leaves, roots and stems.
• Used to make cellulose, the strong fibrous material that strengthens the cell wall.
• Converted to lipids (oils/fats) for energy storage, especially in seeds (sunflower seeds, rapeseed).
• Combined with nitrate ions absorbed from the soil to make amino acids, which are then joined into proteins. Without nitrate, plants cannot make protein, so they grow yellow and stunted — this is why farmers add nitrate fertiliser.

Common exam traps

Examiners love to penalise sloppy energy language. 'Photosynthesis makes energy' is wrong — energy cannot be made, only transferred. 'Plants take in oxygen and release CO₂' confuses respiration with photosynthesis — plants do both, but photosynthesis only happens in the light. Finally, on graph questions, name the new limiting factor at the plateau (CO₂ or temperature), do not just say 'something else'.

Exam tips

• Use the phrase 'energy is transferred from the environment to the chloroplasts by light' verbatim — it scores two marks in one sentence.
• On limiting-factor graphs, always NAME the new limiting factor at the plateau (CO₂ or temperature) — vague answers don't score.
• Never write 'photosynthesis makes energy' — energy is transferred, not made. Examiners penalise this every year.
• On uses-of-glucose questions, the protein-synthesis mark requires you to mention NITRATE IONS from the soil — don't forget the nitrate.
• The temperature graph rises, peaks, then drops — be ready to say 'enzymes denature above the optimum' for the drop.
• If asked about CO₂, the magic words are 'CO₂ is a reactant / raw material' — that secures the marking point.
• When sketching graphs, label the axes 'rate of photosynthesis' (y) and the factor (x), and mark the plateau clearly.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in AQA GCSE Biology (8461)

• 4.4.2 Respiration