Photosynthesis

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.

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What Is Photosynthesis?

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.

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Word Equation

The word equation for photosynthesis is:

$$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light energy}} \text{glucose} + \text{oxygen}$$

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Symbol Equation

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.

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Where Does Photosynthesis Take Place?

Photosynthesis occurs in the chloroplasts of plant cells. Chloroplasts contain a green pigment called chlorophyll.

The Role of Chlorophyll

• Chlorophyll absorbs light energy (mainly red and blue wavelengths) and reflects green light, which is why plants look green.
• The absorbed light energy drives the photosynthesis reaction.
• Chloroplasts are most abundant in the palisade mesophyll cells of leaves because these cells are near the upper surface and receive the most light.

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"]

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Leaf Adaptations for Photosynthesis

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

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Overview of the Light-Dependent Stage

At GCSE level, you need to know the broad outline of how light is used:

1. Light energy is absorbed by chlorophyll in the chloroplasts.
2. This energy is used to split water molecules into hydrogen and oxygen — a process called photolysis ("photo" = light, "lysis" = splitting).
3. The oxygen is released as a by-product (it diffuses out through the stomata).
4. The hydrogen is combined with carbon dioxide in a series of enzyme-controlled reactions to produce glucose.

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.

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What Happens to the Glucose?

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)

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Photosynthesis and the Carbon Cycle

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

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Summary

• Photosynthesis is an endothermic reaction that converts light energy into chemical energy in glucose.
• Word equation: carbon dioxide + water → glucose + oxygen (in the presence of light energy).
• Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• The reaction takes place in chloroplasts, which contain chlorophyll.
• Chlorophyll absorbs light energy, which is used to split water (photolysis).
• Leaves are adapted with a large surface area, stomata, palisade cells and air spaces.
• Glucose is used for respiration, making cellulose, amino acids, lipids or stored as starch.