Photosynthesis: The Basics

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.

What Is Photosynthesis?

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.

Where Does Photosynthesis Happen?

Photosynthesis takes place primarily in the leaves of a plant. The leaf is highly adapted for this purpose.

The Role of Chloroplasts

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 as an Endothermic Reaction

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

The Importance of Photosynthesis

Photosynthesis is crucial for life on Earth for several interconnected reasons:

Food production — plants produce glucose which enters food chains when animals eat plants. Without photosynthesis, there would be no food for any organism.
Oxygen production — the oxygen we breathe is a by-product of photosynthesis. Almost all atmospheric oxygen has been produced by photosynthetic organisms.
Carbon dioxide removal — photosynthesis removes CO2 from the atmosphere, helping to regulate the greenhouse effect and global temperatures.
Biomass production — the glucose produced is used to build plant biomass (cellulose, starch, proteins, lipids), which forms the base of ecosystems.
Fossil fuels — coal, oil and natural gas were formed from ancient photosynthetic organisms, storing solar energy captured millions of years ago.

Conditions Needed for Photosynthesis

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."

Investigating Photosynthesis

A classic experiment to demonstrate that photosynthesis produces a sugar involves testing a leaf for starch using iodine solution:

Boil the leaf in water to kill the cells and stop reactions.
Place the leaf in hot ethanol (in a water bath, not over a flame — ethanol is flammable) to remove the green chlorophyll.
Rinse the leaf in warm water to soften it.
Add iodine solution to the leaf.
If starch is present, the iodine turns blue-black. If no starch, it remains brown/yellow.

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.

Adaptations of Leaf Cells for Photosynthesis

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:

Tall and column-shaped to pack tightly together
Located near the upper surface to receive maximum light
Packed with chloroplasts (they may contain 50 or more per cell)

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.

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 (with light energy)
The balanced symbol equation is: 6CO2 + 6H2O ---> C6H12O6 + 6O2
Photosynthesis occurs in chloroplasts, which contain the green pigment chlorophyll
The palisade mesophyll cells contain the most chloroplasts and are the main site of photosynthesis
Leaves are highly adapted for photosynthesis with features such as a large surface area, stomata, veins and air spaces
Photosynthesis is essential for producing food, oxygen and biomass on Earth
The iodine test for starch can be used to demonstrate that photosynthesis has occurred

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.