Photosynthesis

Photosynthesis is one of the most important biological processes on Earth. It is the process by which plants, algae and some bacteria convert light energy into chemical energy stored in glucose. This lesson covers the equations, conditions and details of photosynthesis as required by the Edexcel GCSE Biology (1BI0) specification, Topic 6: Plant Structures and their Functions.

What is Photosynthesis?

Photosynthesis is the process by which green plants and algae absorb light energy and use it to convert carbon dioxide and water into glucose and oxygen. It is an endothermic reaction because it takes in energy from the surroundings in the form of light.

Photosynthesis takes place in the chloroplasts of plant cells. Chloroplasts contain a green pigment called chlorophyll, which absorbs light energy (particularly red and blue wavelengths) and reflects green light — this is why plants appear green.

Exam Tip: The examiners often ask whether photosynthesis is exothermic or endothermic. Remember: endothermic = energy is taken IN from the surroundings. Light energy is absorbed and converted to chemical energy in glucose.

The Equations for Photosynthesis

Word Equation

carbon dioxide + water → glucose + oxygen

This reaction requires light energy and the presence of chlorophyll.

Symbol Equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

You must learn this symbol equation. Notice that it is balanced:

Element	Reactant side	Product side
Carbon (C)	6	6
Hydrogen (H)	12	12
Oxygen (O)	6 + 6 = 18*	6 + 12 = 18*

*Counting all oxygen atoms across both compounds on each side.

Exam Tip: You may be asked to write either the word equation or the symbol equation. Make sure you can recall both. The symbol equation must be balanced. Do not forget to state that light energy is needed and that chlorophyll acts as the catalyst/absorber of light.

Where Does Photosynthesis Occur?

Photosynthesis occurs primarily in the leaves of a plant, specifically inside the chloroplasts of mesophyll cells.

Structure of a Chloroplast

Chloroplasts are organelles found in plant cells and algal cells. They have a double membrane and contain:

Thylakoid membranes — flattened membrane sacs stacked into structures called grana (singular: granum). This is where the light-dependent reactions occur. Chlorophyll molecules are embedded in these membranes.
Stroma — the fluid-filled matrix surrounding the grana. This is where the light-independent reactions (also called the Calvin cycle at A-level) take place.

At GCSE level, you need to know that chloroplasts contain chlorophyll and that photosynthesis occurs in two broad stages, but the detailed biochemistry of each stage is not required.

Exam Tip: If asked "where in the cell does photosynthesis take place?", the answer is the chloroplast. If asked "where in the plant?", the answer is mainly the leaves (specifically the palisade and spongy mesophyll cells).

The Two Stages of Photosynthesis (Simplified)

Although GCSE does not require detailed knowledge of the two stages, understanding the overview helps you answer higher-tier questions:

Stage 1: Light-Dependent Reactions

Occur in the thylakoid membranes
Light energy is absorbed by chlorophyll
Water molecules are split (photolysis): water → hydrogen ions + electrons + oxygen
Oxygen is released as a by-product
Energy is transferred to the next stage

Stage 2: Light-Independent Reactions

Occur in the stroma
Carbon dioxide is combined with hydrogen (from Stage 1) to produce glucose
This stage does not directly require light, but it depends on the products of Stage 1

Why is Photosynthesis Important?

Photosynthesis is essential because it:

Produces oxygen — almost all the oxygen in Earth's atmosphere comes from photosynthesis
Produces glucose — the starting material for food chains; producers (plants) are the foundation of nearly every ecosystem
Removes carbon dioxide — helping to regulate atmospheric CO₂ levels
Stores energy — converts light energy into chemical energy that can be passed through food chains

Uses of Glucose

Plants do not simply make glucose and leave it as it is. Glucose produced by photosynthesis is used in several important ways:

1. Respiration (Energy Release)

Glucose is broken down during cellular respiration to release energy for life processes. This happens in every living cell of the plant.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)

2. Making Cellulose (Cell Walls)

Glucose molecules are joined together to form cellulose, a structural polysaccharide. Cellulose makes up plant cell walls, providing strength and rigidity.

3. Making Amino Acids and Proteins

Glucose is combined with nitrate ions (NO₃⁻) absorbed from the soil through the roots to produce amino acids. Amino acids are then assembled into proteins needed for growth and repair.

4. Making Starch (Storage)

Glucose is converted into starch for storage. Starch is insoluble in water, which is important because:

It does not affect the water potential of cells (does not affect osmosis)
It stays where it is made rather than dissolving and moving away
It can be converted back to glucose when energy is needed

5. Making Lipids (Fats and Oils — Storage)

Glucose can be converted into lipids (fats and oils) for long-term energy storage. Seeds, for example, often contain high levels of lipids.

6. Making Other Molecules

Glucose is also used to produce sucrose (for transport in phloem), nucleic acids (DNA and RNA) and other organic molecules.

Use of glucose	Product	Purpose
Respiration	CO₂ + H₂O + energy	Energy for life processes
Cellulose synthesis	Cellulose	Structural — cell walls
Amino acid synthesis	Amino acids → proteins	Growth, repair, enzymes
Starch synthesis	Starch	Energy storage (insoluble)
Lipid synthesis	Fats and oils	Long-term energy storage

Exam Tip: A common question is "Why do plants store glucose as starch rather than as glucose?" The answer is that starch is insoluble, so it does not dissolve in cell sap and therefore does not affect osmosis (water movement) in the cell. Glucose is soluble and would affect the water potential of the cell.

Testing a Leaf for Starch

To show that photosynthesis has occurred, you can test a leaf for starch:

Boil the leaf in water for 1–2 minutes — this kills the cells and stops further reactions
Place the leaf in hot ethanol (in a water bath, not over a naked flame — ethanol is flammable) — this removes the green chlorophyll, making colour changes easier to see
Dip the leaf in hot water briefly — this softens the leaf (ethanol makes it brittle)
Spread the leaf on a white tile and add a few drops of iodine solution
Observe the colour change: a colour change from brown/orange to blue-black indicates that starch is present

Exam Tip: If a question asks you to explain the purpose of each step, make sure you know: boiling water = kill cells; ethanol = remove chlorophyll; iodine = test for starch (blue-black = positive).

Key Conditions for Photosynthesis

For photosynthesis to take place, the plant needs:

Light energy — usually from the Sun (or artificial light)
Carbon dioxide — enters through stomata in the leaves
Water — absorbed from the soil through root hair cells and transported up through xylem
Chlorophyll — the green pigment in chloroplasts that absorbs light

If any one of these is missing, photosynthesis cannot occur.

Photosynthesis and Respiration: The Link

Photosynthesis and respiration are closely linked processes that are often compared in exams:

Feature	Photosynthesis	Aerobic Respiration
Equation	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Energy	Endothermic (takes in light energy)	Exothermic (releases energy)
Where	Chloroplasts (in cells with chlorophyll)	Mitochondria (in all living cells)
When	Only in the light	All the time (day and night)
Gas taken in	CO₂	O₂
Gas released	O₂	CO₂

Notice that the equations are essentially the reverse of each other. The glucose and oxygen produced by photosynthesis are the reactants for respiration, and vice versa.

During the daytime, a plant carries out both photosynthesis AND respiration. If photosynthesis is faster than respiration, there is a net uptake of CO₂ and a net release of O₂. At night, only respiration occurs, so the plant takes in O₂ and releases CO₂.

The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point, there is no net gas exchange.

Exam Tip: Be clear that plants respire ALL the time (24 hours a day), not just at night. Photosynthesis only occurs in the light, but respiration is continuous. A very common misconception is that "plants photosynthesise during the day and respire at night" — in reality, they respire constantly.

The Role of Minerals

Plants need more than just glucose to grow. They require mineral ions absorbed from the soil:

Nitrate ions (NO₃⁻) — needed to combine with glucose to make amino acids and then proteins. A deficiency causes stunted growth and yellow older leaves (chlorosis)
Magnesium ions (Mg²⁺) — needed to make chlorophyll. A deficiency causes yellow leaves because chlorophyll cannot be produced

These minerals are absorbed by the roots through active transport and carried upward in the xylem.

Summary

Photosynthesis is an endothermic reaction that converts light energy into chemical energy in glucose
It occurs in chloroplasts, which contain chlorophyll
Word equation: carbon dioxide + water → glucose + oxygen
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Glucose is used for respiration, making cellulose, amino acids, starch and lipids
Starch is the storage form of glucose because it is insoluble and does not affect osmosis
You can test for starch using the iodine test (blue-black = positive)
Photosynthesis and respiration are reverse reactions; plants respire all the time but only photosynthesise in the light
Nitrate ions are needed for amino acids/proteins; magnesium ions are needed for chlorophyll

Exam Tip: Be prepared to link photosynthesis to other topics such as respiration, food chains and the carbon cycle. Examiners love cross-topic questions.