8.1 Photosynthesis

A free GCSE Biology lesson for AQA (8461) from OBA School: the photosynthesis equations, energy transfer, how a leaf is adapted, and testing a leaf for starch. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Write the word equation and the balanced symbol equation for photosynthesis
• Explain why photosynthesis is an endothermic reaction
• Describe how the structure of a leaf is adapted for photosynthesis
• Describe how to test a leaf for starch and how to show that light, chlorophyll and carbon dioxide are needed

Key words

• Producer: An organism that makes its own food. Green plants and algae make glucose by photosynthesis.
• Chlorophyll: The green pigment in chloroplasts that absorbs light.
• Chloroplast: The organelle where photosynthesis happens. Found in leaf and stem cells, never in roots.
• Endothermic: A reaction that takes in energy from the surroundings. Photosynthesis takes in energy as light.
• Stomata: Tiny pores, mainly on the underside of a leaf. Guard cells open and close them. Singular: stoma.
• Destarch: Keep a plant in the dark for about 48 hours so it uses up its stored starch before an experiment.

The photosynthesis equations

Green plants and algae are producers. They make their own food by photosynthesis, and almost every food chain on Earth starts with them.

Word equation: carbon dioxide + water → glucose + oxygen (light energy absorbed by chlorophyll)

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Carbon dioxide diffuses into the leaf from the air through the stomata. Water is absorbed by the root hair cells and carried up to the leaf in the xylem. Glucose is the useful product. Oxygen is a by-product: some is used for respiration and the rest diffuses out of the leaf.

Worked example: counting atoms

Show that the symbol equation is balanced for oxygen atoms.
Left side: 6CO₂ has 6 × 2 = 12, and 6H₂O has 6 × 1 = 6. Total = 18.
Right side: C₆H₁₂O₆ has 6, and 6O₂ has 6 × 2 = 12. Total = 18.

Answer: 18 oxygen atoms on each side, so it is balanced

An endothermic reaction. Photosynthesis needs a constant supply of energy, which is transferred from the environment by light. Because energy is taken in, photosynthesis is an endothermic reaction. The energy ends up stored in the chemical bonds of glucose.

Light is absorbed by chlorophyll inside the chloroplasts. If a plant is put in the dark, it soon stops making glucose, even though it still has carbon dioxide and water.

Exam tip: “Light” is not a reactant. Do not write it on the left of the equation as a chemical. Write it above the arrow, or say “using energy transferred by light”.

How a leaf is adapted

A leaf is an organ built to collect light and exchange gases. Each layer has a job.

Leaf features and their jobs

• Broad, flat shape; How it helps photosynthesis Large surface area to absorb light.
• Thin; How it helps photosynthesis Short diffusion distance for carbon dioxide to reach the cells.
• Waxy cuticle and transparent upper epidermis; How it helps photosynthesis Cuticle cuts water loss. Light passes straight through the epidermis.
• Palisade mesophyll near the top; How it helps photosynthesis Tall cells packed with chloroplasts, where most light falls.
• Spongy mesophyll with air spaces; How it helps photosynthesis Large internal surface area for gases to diffuse into and out of cells.
• Stomata and guard cells; How it helps photosynthesis Let CO₂ in and O₂ out; guard cells close them to save water.
• Veins (xylem and phloem); How it helps photosynthesis Xylem brings water; phloem carries sugars away.

Testing a leaf for starch

Plants turn much of their glucose into starch for storage, so starch in a leaf shows that photosynthesis has happened. Iodine solution turns from orange-brown to blue-black with starch.

Safety: ethanol is highly flammable. Turn off all Bunsen burners and heat the tube of ethanol in a beaker of hot water, never over a flame.

Showing what photosynthesis needs

Destarch the plant first. Then change one condition and test the leaves:

• Light; Set-up Cover part of a leaf with black card or foil for a day; Result Only the uncovered part turns blue-black
• Chlorophyll; Set-up Use a variegated leaf (green and white patches); Result Only the green parts turn blue-black
• Carbon dioxide; Set-up Seal a leaf or plant in a bag with soda lime, which absorbs CO₂; Result Leaf stays orange-brown

Worked example: explaining a result

Priya destarches a geranium, covers one leaf with a foil strip and leaves it in sunlight for a day. She tests the leaf with iodine.
The uncovered parts turn blue-black because they received light, photosynthesised and made glucose, which was stored as starch.
The covered strip stays orange-brown because no light reached it, so no photosynthesis took place there.

Answer: Light is needed for photosynthesis

Common mistakes

• Saying plants get their food from the soil. Plants make glucose from carbon dioxide and water; the soil only supplies water and mineral ions.
• Writing O for oxygen in the symbol equation. Oxygen gas is O₂, so the product is 6O₂.
• Calling photosynthesis exothermic. It takes in energy, so it is endothermic.
• Forgetting to destarch the plant. Old starch would turn the iodine blue-black even where no photosynthesis happened.
• Testing a leaf with iodine without removing the chlorophyll. The green colour hides the colour change.

Revision questions

Try these, then check your answers below each one.

Question 1. Which is the correct word equation for photosynthesis? (Choose from: carbon dioxide + oxygen → glucose + water, water + oxygen → glucose + carbon dioxide, glucose + oxygen → carbon dioxide + water, carbon dioxide + water → glucose + oxygen.)

Answer: carbon dioxide + water → glucose + oxygen

Working: Carbon dioxide and water react, using energy transferred by light, to make glucose and oxygen.

Question 2. Why is photosynthesis described as an endothermic reaction? (Choose from: It releases energy to the surroundings, It takes in energy from the surroundings, It only happens in warm conditions, It produces heat as a by-product.)

Answer: It takes in energy from the surroundings

Working: Photosynthesis takes in energy, transferred by light, so it is endothermic.

Question 3. Name the green pigment in chloroplasts that absorbs light.

Answer: chlorophyll

Working: Chlorophyll absorbs light energy for photosynthesis.

Related lessons

• Previous lesson: 7.5 Alcohol and Other Carcinogens
• Next lesson: 8.2 The Rate of Photosynthesis
• In this chapter: 8.3 How Plants Use Glucose
• In this chapter: 8.4 Making the Most of Photosynthesis
• In this chapter: 8.5 Required Practicals: Photosynthesis

Continue in the full course

This lesson is section 8.1 (AGCBH0801) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.