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Photosynthesis is the fundamental process by which plants, algae and some bacteria convert light energy into chemical energy stored in glucose. This lesson covers the core equation, the site of photosynthesis within plant cells, and the conditions needed for the process to occur. A solid understanding of photosynthesis is essential for the entire Bioenergetics topic in AQA GCSE Biology.
Photosynthesis is an endothermic reaction in which energy from light is used to convert carbon dioxide and water into glucose and oxygen. It is the process by which plants manufacture their own food, making them producers in every food chain.
The word equation for photosynthesis is:
carbon dioxide + water ---light energy---> glucose + oxygen
The balanced symbol equation is:
6CO2 + 6H2O ---light energy---> C6H12O6 + 6O2
| Component | Role in Photosynthesis |
|---|---|
| Carbon dioxide | Raw material absorbed from the air through stomata |
| Water | Raw material absorbed from the soil through root hair cells |
| Light energy | Energy source, usually from the Sun, absorbed by chlorophyll |
| Glucose | Product — an energy-rich sugar used by the plant for growth, respiration and storage |
| Oxygen | By-product — released into the atmosphere through stomata |
Exam Tip: The AQA specification requires you to know both the word equation and the balanced symbol equation for photosynthesis. Practise writing them from memory until you can reproduce them without hesitation.
Photosynthesis takes place primarily in the leaves of a plant. The leaf is highly adapted for this purpose.
Inside leaf cells, photosynthesis occurs within organelles called chloroplasts. Chloroplasts contain a green pigment called chlorophyll which absorbs light energy — particularly from the red and blue parts of the visible light spectrum. Green light is reflected, which is why leaves appear green.
| Leaf Adaptation | How It Helps Photosynthesis |
|---|---|
| Broad, flat shape | Large surface area to absorb maximum light |
| Thin structure | Short diffusion distance for gases |
| Stomata (pores on underside) | Allow CO2 to diffuse in and O2 to diffuse out |
| Guard cells around stomata | Open and close stomata to control gas exchange and water loss |
| Network of veins (xylem and phloem) | Xylem delivers water; phloem transports dissolved sugars away |
| Palisade mesophyll layer near upper surface | Packed with chloroplasts to maximise light absorption |
| Spongy mesophyll with air spaces | Allows efficient gas exchange within the leaf |
| Waxy cuticle on upper surface | Reduces water loss while allowing light to pass through |
graph TD
A[Sunlight hits leaf surface] --> B[Light absorbed by chlorophyll in chloroplasts]
B --> C[Light energy used to split water molecules]
C --> D[Carbon dioxide combined with hydrogen]
D --> E[Glucose produced]
D --> F[Oxygen released as by-product]
E --> G[Glucose used for growth, respiration, storage]
F --> H[Oxygen diffuses out through stomata]
Exam Tip: If asked to describe where photosynthesis occurs, always be specific. Do not just say "in the leaf" — say "in the chloroplasts of leaf cells, particularly the palisade mesophyll cells which contain the most chloroplasts."
Photosynthesis is an endothermic reaction because it takes in energy from the environment (in the form of light). This energy is transferred to the chemical bonds of glucose, storing it as chemical energy.
This is the opposite of an exothermic reaction, which releases energy. Respiration, which you will study later in this topic, is an exothermic reaction.
| Reaction Type | Energy Transfer | Example |
|---|---|---|
| Endothermic | Energy taken in from surroundings | Photosynthesis |
| Exothermic | Energy released to surroundings | Respiration |
Photosynthesis is crucial for life on Earth for several interconnected reasons:
For photosynthesis to occur, three essential conditions must be met:
| Condition | Source | How It Reaches the Leaf |
|---|---|---|
| Light | The Sun (or artificial light) | Passes through transparent cuticle and epidermis to chloroplasts |
| Carbon dioxide | The atmosphere (about 0.04%) | Diffuses through stomata into air spaces, then into mesophyll cells |
| Water | The soil | Absorbed by root hair cells via osmosis, transported up the xylem |
Chlorophyll is also required as it is the pigment that absorbs light energy, but it is not consumed in the reaction — it acts as a catalyst-like molecule.
Exam Tip: When explaining photosynthesis, always link structure to function. For example, "Palisade cells are near the top of the leaf and contain many chloroplasts because this is where the most light is available, so the rate of photosynthesis is maximised."
A classic experiment to demonstrate that photosynthesis produces a sugar involves testing a leaf for starch using iodine solution:
By covering parts of a leaf with foil (blocking light) or using variegated leaves (parts without chlorophyll), you can show that both light and chlorophyll are required for photosynthesis to produce starch.
graph LR
A[Leaf Structure] --> B[Upper epidermis]
A --> C[Palisade mesophyll]
A --> D[Spongy mesophyll]
A --> E[Lower epidermis]
B --> B1[Transparent — lets light through]
C --> C1[Tall, tightly packed cells with many chloroplasts]
D --> D1[Air spaces for gas exchange]
E --> E1[Contains stomata for gas exchange]
Palisade mesophyll cells are the main site of photosynthesis. They are:
Spongy mesophyll cells also photosynthesise but have fewer chloroplasts. Their main role is to provide large air spaces for the efficient diffusion of CO2 and O2.
Exam Tip: A common 6-mark question asks you to explain how a leaf is adapted for photosynthesis. Structure your answer by listing adaptations and linking each one to how it increases the rate of photosynthesis. Always use the key terms: chloroplast, chlorophyll, stomata, palisade mesophyll, diffusion.
Exam questions sometimes give you an unbalanced equation and ask you to balance it. Consider this version:
CO2 + H2O ---> C6H12O6 + O2
Count the atoms on each side:
To balance:
Common mistake: Students often write the word equation using an equals sign (=) or forget to include "light energy" above the arrow. Always use an arrow (--->) and write "light energy" (or "light") above it. The word equation is not balanced by atoms; only the symbol equation needs to be balanced.
Common mistake: Confusing glucose (C6H12O6) with starch when describing the products of photosynthesis. The direct product of photosynthesis is glucose. Starch is produced later by joining many glucose molecules together for storage. If a question asks "What is the product of photosynthesis?" the answer is glucose, not starch.
The distinction between endothermic and exothermic reactions is fundamental. During photosynthesis, the energy from sunlight is absorbed and locked into the chemical bonds of glucose. The total energy of the products (glucose + oxygen) is greater than the total energy of the reactants (carbon dioxide + water). The difference has come from the sunlight.
When a plant (or animal) later respires, these bonds are broken and the stored energy is released — that is why respiration is exothermic.
| Property | Endothermic (Photosynthesis) | Exothermic (Respiration) |
|---|---|---|
| Direction of energy flow | Energy taken in from surroundings | Energy released to surroundings |
| Effect on surroundings | Surroundings lose energy (cool slightly) | Surroundings gain energy (warm slightly) |
| Products vs reactants | Products contain more stored energy | Products contain less stored energy |
| Example | Light energy stored in glucose | Glucose energy released for life processes |
Exam-style question: A plant produces glucose and oxygen when placed in sunlight. Explain how the plant produces these substances, and why this reaction is described as endothermic. (4 marks)
Grade 4-5 answer: The plant photosynthesises. It uses carbon dioxide and water and light to make glucose and oxygen. The equation is carbon dioxide + water ---> glucose + oxygen. It is endothermic because it uses energy.
Grade 8-9 answer: Photosynthesis occurs in the chloroplasts of the palisade mesophyll cells of the leaf. Light energy is absorbed by chlorophyll and used to drive the reaction between carbon dioxide (absorbed through the stomata) and water (absorbed from the soil by root hair cells). The word equation is: carbon dioxide + water ---(light energy)---> glucose + oxygen, and the balanced symbol equation is 6CO2 + 6H2O ---> C6H12O6 + 6O2. The reaction is described as an endothermic reaction because energy is transferred from the surroundings (the Sun) to the reacting chemicals — the products (glucose and oxygen) therefore contain more stored chemical energy than the reactants. This is the reverse of aerobic respiration, which is exothermic. A common misconception is that plants only photosynthesise during the day and do not respire; in fact, plants respire continuously, and photosynthesis only exceeds respiration in net terms when light intensity is above the compensation point.
AQA alignment: This content is aligned with AQA GCSE Biology (8461) specification section 4.4 Bioenergetics — specifically 4.4.1.1 Photosynthesis reaction. Assessed on Paper 1.