Photosynthesis — The Basics

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. This is a cornerstone of the AQA GCSE Combined Science Trilogy specification (8464) and underpins the entire Bioenergetics topic. Understanding photosynthesis will help you make sense of food chains, carbon cycling and global energy transfer.

What Is Photosynthesis?

Photosynthesis is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light. Carbon dioxide and water are used as raw materials, and glucose and oxygen are produced.

Word Equation

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

Balanced Symbol Equation

$$ 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light energy}} \text{C}6\text{H}{12}\text{O}_6 + 6\text{O}_2 $$

Exam Tip: The AQA Trilogy specification (8464) requires you to recall both the word equation and the balanced symbol equation. Practise writing them from memory — examiners will penalise missing or incorrect formulae.

Component	Role in Photosynthesis
Carbon dioxide (CO₂)	Reactant — absorbed from the air through stomata
Water (H₂O)	Reactant — absorbed from the soil by root hair cells, transported via xylem
Light energy	Energy source — absorbed by chlorophyll in chloroplasts
Glucose (C₆H₁₂O₆)	Product — energy-rich sugar used for growth, respiration and storage
Oxygen (O₂)	By-product — released through stomata into the atmosphere

Where Does Photosynthesis Happen?

Photosynthesis takes place in the chloroplasts of plant cells. Chloroplasts contain a green pigment called chlorophyll, which absorbs light energy. Chlorophyll absorbs red and blue light most effectively and reflects green light — which is why leaves appear green.

The leaves are the main organs of photosynthesis because they have many adaptations that maximise the rate of the reaction.

Leaf Adaptation	How It Aids Photosynthesis
Broad, flat shape	Large surface area for light absorption
Thin structure	Short diffusion distance for CO₂ and O₂
Palisade mesophyll cells near upper surface	Tightly packed cells containing many chloroplasts
Spongy mesophyll with air spaces	Allows gases to diffuse efficiently throughout the leaf
Stomata on lower surface	Pores that allow CO₂ in and O₂ out by diffusion
Guard cells	Control the opening and closing of stomata
Xylem vessels	Transport water from the roots to the leaf
Phloem vessels	Transport dissolved sugars away from the leaf
Waxy cuticle	Transparent — lets light through while reducing water loss

graph TD
    A["Sunlight strikes the leaf"] --> B["Chlorophyll in chloroplasts absorbs light energy"]
    B --> C["Light energy splits water molecules"]
    C --> D["Hydrogen combines with CO₂ to form glucose"]
    C --> E["Oxygen released as by-product"]
    D --> F["Glucose used for respiration, growth and storage"]
    E --> G["O₂ diffuses out through stomata"]

Exam Tip: When describing where photosynthesis occurs, be precise. Write "in the chloroplasts of leaf cells, particularly the palisade mesophyll cells" rather than simply "in the leaf."

Photosynthesis Is Endothermic

Photosynthesis is classified as an endothermic reaction because energy is taken in from the surroundings (in the form of light). This energy is transferred to the chemical bonds of glucose, storing it as chemical energy.

Reaction Type	Energy Transfer	Example
Endothermic	Energy taken in from surroundings	Photosynthesis
Exothermic	Energy released to surroundings	Respiration

This distinction is important for the Trilogy exam — you must recognise that photosynthesis and respiration are opposite in terms of energy transfer.

Why Photosynthesis Matters

Photosynthesis is essential for life on Earth:

Food production — glucose enters food chains when animals eat plants, making plants the producers at the base of every ecosystem.
Oxygen production — almost all atmospheric oxygen comes from photosynthesis.
Carbon dioxide removal — photosynthesis removes CO₂ from the atmosphere, helping to regulate the greenhouse effect.
Biomass — glucose is converted into cellulose, starch, proteins and lipids, forming plant biomass.

Testing for Photosynthesis

You can test whether a leaf has been photosynthesising by testing it for starch (glucose is quickly converted into starch for storage):

Boil the leaf in water to kill the cells and stop reactions.
Place the leaf in hot ethanol (using a water bath — ethanol is flammable) to remove chlorophyll.
Rinse the decolourised leaf in warm water to soften it.
Add iodine solution to the leaf.
If starch is present, the iodine turns blue-black. If absent, it stays brown/yellow.

By covering parts of a leaf with foil, you can demonstrate that light is essential for photosynthesis.

Common Mistakes

Mistake	Correction
Saying photosynthesis happens "in the leaf"	It happens in the chloroplasts within leaf cells
Saying oxygen is a "product" without qualifying	Oxygen is a by-product — glucose is the main product
Confusing endothermic and exothermic	Photosynthesis takes energy in (endothermic); respiration releases energy out (exothermic)
Writing "CO2" instead of "CO₂" in symbol equations	Always use subscript notation: (\text{CO}_2), (\text{H}_2\text{O}), (\text{C}6\text{H}{12}\text{O}_6), (\text{O}_2)

Summary

Photosynthesis is an endothermic reaction that converts carbon dioxide and water into glucose and oxygen using light energy.
The word equation is: carbon dioxide + water → glucose + oxygen (light energy required).
The balanced symbol equation is: (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).
Photosynthesis occurs in chloroplasts, which contain the green pigment chlorophyll.
Leaves are adapted for photosynthesis with features such as palisade mesophyll cells, stomata, air spaces and veins.
You can test for starch (the storage product of glucose) using the iodine test.
Always link leaf adaptations to how they increase the rate of photosynthesis in your exam answers.

Exam Tip (AQA 8464): A common 6-mark question asks you to describe and explain how a leaf is adapted for efficient photosynthesis. List adaptations, then link each to a specific function — for example, "Palisade mesophyll cells are near the top of the leaf and are packed with chloroplasts, so they absorb the maximum amount of light energy for photosynthesis."