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B8.1 Photosynthesis

GCSE Combined Science · AQA · Chapter B8: Photosynthesis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the photosynthesis equations, why it is endothermic, where it happens in a leaf, and showing that light is needed. 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 and compare the energy in reactants and products
  • Describe how carbon dioxide and water reach the chloroplasts and where the products go
  • Explain how the layers of a leaf help photosynthesis
  • Describe how to show that light and chlorophyll are needed, using the starch test

Key words

  • Producer: An organism that makes its own food. Plants and algae make glucose by photosynthesis.
  • Chloroplast: The organelle where photosynthesis happens. It contains the green pigment chlorophyll.
  • Chlorophyll: The green pigment that absorbs light.
  • Endothermic: A reaction that takes in energy from the surroundings.
  • By-product: A substance made in a reaction that is not the main product. Oxygen is the by-product of photosynthesis.
  • Stomata: Pores, mainly on the lower surface of a leaf, opened and closed by guard cells. Singular: stoma.
  • Translocation: Movement of dissolved sugars around the plant in the phloem.

The photosynthesis equations

Green plants and algae make their own food, so they are producers. Almost every food chain begins with them, and they release the oxygen that animals breathe.

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

Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Worked example: counting atoms

Show that the symbol equation is balanced for hydrogen and carbon.
Hydrogen: left 6H₂O = 6 × 2 = 12; right C₆H₁₂O₆ = 12.
Carbon: left 6CO₂ = 6; right C₆H₁₂O₆ = 6.

Answer: 12 H and 6 C atoms on each side, so it balances

Exam tip: light is not a reactant. Write “light” above the arrow or say “using energy transferred by light”. Oxygen gas is O₂, so the product is 6O₂, not 6O.

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 endothermic. Glucose and oxygen together store more chemical energy than the carbon dioxide and water they were made from. Energy is not created: the extra comes from light.

This is why photosynthesis stops in the dark. Respiration, however, carries on day and night, so oxygen around a plant rises in the light and falls in the dark.

Getting the reactants in and the products out

Carbon dioxide diffuses from the air into the leaf through the stomata, then through the air spaces to the mesophyll cells. Water enters root hair cells by osmosis and travels up to the leaves in the xylem.

Some oxygen is used by the plant’s own cells in respiration; the rest diffuses out through the stomata. Glucose is used in the leaf, or turned into sugar that is carried to the rest of the plant in the phloem (translocation).

  • Broad, flat shape; How it helps photosynthesis Large surface area to absorb light.
  • Thin; How it helps photosynthesis Short diffusion distance for carbon dioxide.
  • Clear upper epidermis; How it helps photosynthesis Lets light through to the cells below.
  • Palisade mesophyll at the top; How it helps photosynthesis Tall cells with the most chloroplasts, where most light falls. Most photosynthesis happens here.
  • Spongy mesophyll with air spaces; How it helps photosynthesis Large internal surface 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.

Showing that light and chlorophyll are needed

Plants store much of their glucose as starch, so starch in a leaf shows that it has photosynthesised. Iodine solution turns from orange-brown to blue-black with starch. First destarch the plant by keeping it in the dark for about two days.

  • 1. Dip the leaf in boiling water for about a minute: this kills the cells and lets the iodine in.
  • 2. Put it in a tube of ethanol standing in a beaker of hot water (no flames) to remove the chlorophyll.
  • 3. Rinse in warm water to soften the leaf, spread it on a white tile and add iodine solution.

Worked example: explaining a result

Ravi destarches a geranium and clips a black card star onto one leaf. After a day in sunlight he tests the leaf with iodine.
The uncovered part turns blue-black: it received light, photosynthesised and stored the glucose as starch.
The star shape stays orange-brown: no light reached it, so there was no photosynthesis there.

Answer: Light is needed for photosynthesis

A variegated leaf (green and white patches) shows that chlorophyll is needed: only the green parts turn blue-black. Safety: ethanol is flammable, so use a water bath, no flames.

Common mistakes

  • Saying plants get food from the soil. They make glucose; soil gives water and mineral ions.
  • Calling photosynthesis exothermic. It takes in energy, so it is endothermic.
  • Saying plants make oxygen “for animals”. It is a by-product.
  • Thinking plants stop respiring in daylight. They respire all the time.
  • Forgetting to destarch the plant, or to remove chlorophyll, before the starch test.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows a cross-section through part of a leaf. Which letter labels the layer where most photosynthesis takes place? (Choose from: A, B, C, D, E.)

Answer: B

Working: B is the palisade mesophyll. Its tall cells are packed with chloroplasts and sit near the upper surface, so they absorb the most light.

Question 2. Name the organelle in a plant cell where photosynthesis takes place.

Answer: chloroplast

Working: Photosynthesis happens in chloroplasts, which contain chlorophyll to absorb light.

Question 3. Photosynthesis is described as an endothermic reaction. What does this tell you? (Choose from: Energy is released to the surroundings as heat, Energy is transferred from the surroundings, by light, into the reaction, The reaction only happens above 25 °C, The reaction needs no energy to start.)

Answer: Energy is transferred from the surroundings, by light, into the reaction

Working: Endothermic reactions take in energy from the surroundings. In photosynthesis the energy is transferred by light and ends up stored in glucose.

Related lessons

Continue in the full course

This lesson is section B8.1 (AGCSC0801) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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