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This lesson covers photosynthesis — the process by which green plants and algae make their own food using light energy — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to recall the word and symbol equations, explain that the reaction is endothermic, describe where it takes place and give an overview of the light-dependent process.
Photosynthesis is the process by which plants, algae and some bacteria convert light energy into chemical energy stored in glucose. It takes place primarily in the leaves of a plant.
Photosynthesis is an endothermic reaction — it takes in (absorbs) energy from the surroundings in the form of light.
Exam Tip: An endothermic reaction absorbs energy. Photosynthesis absorbs light energy and stores it as chemical energy in glucose. This is a very common mark point.
The word equation for photosynthesis is:
carbon dioxide+waterlight energyglucose+oxygen
You must be able to recall the balanced symbol equation:
6CO2+6H2OlightC6H12O6+6O2
| Reactant | Formula | Number of molecules |
|---|---|---|
| Carbon dioxide | CO₂ | 6 |
| Water | H₂O | 6 |
| Product | Formula | Number of molecules |
| Glucose | C₆H₁₂O₆ | 1 |
| Oxygen | O₂ | 6 |
Exam Tip: Count the atoms on each side to show the equation is balanced — 6 C, 18 O and 12 H on both sides. Examiners love asking you to balance or complete this equation.
Photosynthesis occurs in the chloroplasts of plant cells. Chloroplasts contain a green pigment called chlorophyll.
graph TD
A["Sunlight"] -->|"Light energy absorbed"| B["Chlorophyll in chloroplast"]
B --> C["Light-dependent reactions"]
C --> D["Water split into H⁺ and O₂"]
C --> E["Energy transferred to second stage"]
E --> F["Carbon dioxide + hydrogen → Glucose"]
F --> G["Glucose (C₆H₁₂O₆) — chemical energy store"]
Leaves are highly adapted for efficient photosynthesis:
| Adaptation | How it helps |
|---|---|
| Broad, flat shape | Large surface area to absorb maximum light |
| Thin | Short diffusion distance for gases (CO₂ in, O₂ out) |
| Palisade mesophyll packed with chloroplasts | Maximum light absorption near the top of the leaf |
| Spongy mesophyll with air spaces | Allows CO₂ and O₂ to diffuse between cells |
| Stomata (tiny pores on the lower surface) | Allow CO₂ to enter and O₂ and water vapour to leave |
| Network of veins (xylem and phloem) | Xylem brings water; phloem carries away sugars |
At GCSE level, you need to know the broad outline of how light is used:
Exam Tip: Remember: water is split by light energy (photolysis). Oxygen is a by-product of this process. If asked where the oxygen in photosynthesis comes from, the answer is water, not carbon dioxide.
Plants use the glucose produced in photosynthesis in several ways:
| Use | Details |
|---|---|
| Respiration | Glucose is broken down to release energy for life processes |
| Making cellulose | Glucose is converted into cellulose for cell walls |
| Making amino acids | Glucose combines with nitrate ions (from the soil) to make amino acids, then proteins |
| Making lipids (fats and oils) | Glucose is converted to lipids for energy storage in seeds |
| Stored as starch | Glucose is converted to insoluble starch for storage (starch does not affect osmosis) |
Photosynthesis plays a critical role in the carbon cycle by removing CO₂ from the atmosphere and locking carbon into organic molecules (glucose). This is why plants and trees are described as carbon sinks.
graph LR
A["CO₂ in atmosphere"] -->|"Photosynthesis"| B["Carbon in glucose"]
B -->|"Respiration"| A
B -->|"Eaten by animals"| C["Carbon in animal tissues"]
C -->|"Respiration"| A
B -->|"Decomposition"| A
A student writes the equation as CO₂ + H₂O → C₆H₁₂O₆ + O₂ and asks why it is wrong.
Step 1 — Count atoms on the right: glucose contains 6 C, 12 H and 6 O; O₂ adds 2 more O. Step 2 — Count atoms on the left: only 1 C, 2 H and 3 O are present. Step 3 — Balance by inspection. Six carbons are needed on the left, so place 6 in front of CO₂. Twelve hydrogens are needed, so place 6 in front of H₂O. That gives 18 O on the left; on the right there are 6 O in glucose plus 6 O in 6O₂ = 12 O. Adjust by keeping 6O₂ because the six extra oxygens come from the six water molecules that were split. Final balanced equation: 6CO2+6H2O→C6H12O6+6O2.
Common Mistake: Students sometimes forget to balance both reactants and products, leaving the equation with uneven oxygen counts. Always check all three elements (C, H, O) separately.
Leaves are thin and flat specifically to maximise surface area to volume ratio so that gases can diffuse rapidly in and out of every mesophyll cell.
Consider a leaf modelled as a flat rectangular block of dimensions 5 cm x 3 cm x 0.02 cm (a typical leaf thickness of 0.2 mm).
Compare this to a cubic leaf of volume 0.3 cm³ (side length 0.67 cm): surface area = 6 x 0.67² = 2.69 cm², ratio = 9 : 1. The flat leaf has a surface area to volume ratio roughly 11 times larger, making gas diffusion dramatically faster. This same principle — high surface area to volume ratio — underpins alveoli, villi and gills.
| Condition | Rate compared to optimum | Why |
|---|---|---|
| Bright sunlight, 25 °C, plentiful CO₂ | Maximum | All factors at or near ideal values |
| Bright sunlight, 5 °C | Low | Enzymes have low kinetic energy; reactions slow |
| Dim light, 25 °C | Low | Limiting factor is light — cannot split water fast enough |
| Bright sunlight, 45 °C | Very low / zero | Enzymes begin to denature |
| Bright sunlight, 25 °C, drought | Low | Stomata close to conserve water → CO₂ cannot enter |
Exam Tip: Always identify which factor is limiting when explaining why a rate is low. Generic answers like "not enough light" lose marks — examiners want the correct technical term "limiting factor".
The carbon cycle links photosynthesis, respiration, decomposition and combustion. Photosynthesis is the only major biological process that removes CO₂ from the atmosphere.
graph TD
A["Atmospheric CO₂"] -->|"Photosynthesis"| B["Glucose in plants"]
B -->|"Respiration by plants"| A
B -->|"Consumed by herbivores"| C["Carbon in animal tissues"]
C -->|"Respiration"| A
C -->|"Death and decay by decomposers"| D["Decomposer respiration"]
D --> A
B -->|"Death and burial over millions of years"| E["Fossil fuels"]
E -->|"Combustion"| A
A -->|"Dissolves in oceans"| F["Dissolved CO₂"]
F -->|"Photosynthesis by algae"| G["Marine food chains"]
Deforestation and fossil fuel burning both increase atmospheric CO₂ because they release carbon that was locked away and reduce the rate at which CO₂ is removed by photosynthesis.
Grade 3–4 answer: "Photosynthesis happens in the leaves. Plants take in carbon dioxide and water and make glucose and oxygen. It needs light."
Grade 5–6 answer: "Photosynthesis is an endothermic reaction that absorbs light energy. It takes place in the chloroplasts of palisade mesophyll cells, which contain chlorophyll. The balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The glucose is used for respiration or converted to starch for storage."
Grade 7–9 answer: "Photosynthesis is an endothermic, light-driven series of enzyme-controlled reactions in which light energy absorbed by chlorophyll is used to carry out photolysis — the splitting of water into hydrogen ions and oxygen. The hydrogen is combined with carbon dioxide to produce glucose (C₆H₁₂O₆), while oxygen is released as a by-product. Leaves are adapted with a high surface area to volume ratio, stomata for gas exchange, palisade cells packed with chloroplasts and a vascular network of xylem and phloem that delivers water and removes sugars. The rate depends on light intensity, CO₂ concentration and temperature, with any of these acting as a limiting factor. Glucose is stored as insoluble starch (which does not affect water potential) or used to synthesise cellulose, amino acids and lipids."
The step up from grade 5 to grade 9 is precise vocabulary — photolysis, limiting factor, surface area to volume ratio, gas exchange — and showing that photosynthesis is an enzyme-controlled sequence rather than a single reaction.
Photosynthesis connects to almost every other biology topic on the Edexcel specification:
Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Biology Topic 4 Natural selection / Topic 5 Health, plant structures / Topic 8 Exchange and transport — specifically CB5 Photosynthesis. Assessed on Biology Paper 2.