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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:
$$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light energy}} \text{glucose} + \text{oxygen}$$
You must be able to recall the balanced symbol equation:
$$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}6\text{H}{12}\text{O}_6 + 6\text{O}_2$$
| 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