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Biology · Photosynthesis

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Biology · AQA Paper 1

Photosynthesis

The Photosynthetic Reaction

Photosynthesis takes in carbon dioxide and water, and uses light energy to make glucose and oxygen.

  • Photosynthesis The reaction in which plants use light energy to make glucose and oxygen from carbon dioxide and water.
  • Producer An organism that makes its own food, such as a plant.
  • Chlorophyll The green pigment in chloroplasts that absorbs light for photosynthesis.
A leaf with arrows for light, carbon dioxide and water going in and oxygen and glucose coming out, with a chloroplast and the equation for photosynthesis.
Light energy is used to make glucose and oxygen from carbon dioxide and water.

Reactants and products compared

  • What goes in (reactants) Carbon dioxide, from the air through the stomata; Water, from the soil through the roots and xylem; Light energy, absorbed by chlorophyll
  • What comes out (products) Glucose, which is used or stored; Oxygen, which leaves through the stomata; Energy stored in the glucose

Testing a leaf for starch

  • Kill the leaf Dip it in boiling water for about a minute.
  • Remove the chlorophyll Heat the leaf in hot ethanol in a water bath, away from flames.
  • Soften the leaf Rinse it in warm water to make it soft again.
  • Add iodine Spread the leaf on a tile and add iodine solution.

Rate of Photosynthesis and Limiting Factors

The rate of photosynthesis is set by whichever factor is in shortest supply: the limiting factor.

  • Limiting factor A factor that is in short supply and so limits the rate of a reaction.
  • Light intensity How bright the light is.
  • Carbon dioxide concentration The amount of carbon dioxide in the air around a plant.
Three graphs. The rate of photosynthesis rises and then levels off as light intensity or carbon dioxide increases, and rises to a peak and falls with temperature.
Rate rises and then levels off for light and carbon dioxide, but falls at high temperature.
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Biology · AQA Paper 1

Photosynthesis continued

Investigating Light Intensity and Photosynthesis

The bubbles of oxygen released by pondweed show the rate of photosynthesis, and the lamp's distance changes the light intensity.

  • Pondweed A water plant, such as Elodea or Cabomba, used in photosynthesis experiments.
  • Independent variable The variable that the investigator changes.
  • Dependent variable The variable that is measured.
A lamp at a measured distance from a beaker of water containing pondweed, with bubbles of oxygen rising from the cut stem.
The number of bubbles in a set time shows the rate of photosynthesis.

Carrying out the investigation

  • Set up Place the pondweed in the beaker, add sodium hydrogencarbonate…
  • Wait Leave the pondweed for a few minutes to adjust to the light.
  • Count Count the number of bubbles released in one minute, or measure…
  • Repeat Take three readings and calculate the mean.

Uses of Glucose from Photosynthesis

Glucose can be used for energy, or built up into starch, fats and oils, cellulose and proteins.

  • Starch An insoluble carbohydrate that plants use to store glucose.
  • Insoluble Does not dissolve in water.
  • Cellulose A carbohydrate that makes up plant cell walls.
Glucose in the centre with arrows to respiration, starch, fats and oils, cellulose, and amino acids and proteins.
Plants make all of these substances from glucose.

Glucose and starch compared

  • Glucose A small sugar molecule; Soluble in water; Used for respiration and to make other substances
  • Starch A large molecule made of many glucose units; Insoluble in water; Used for storage

Testing for starch, glucose and protein

  • Starch Add iodine solution. It turns from orange-brown to blue-black if…
  • Glucose Add Benedict's reagent and heat in a water bath.
  • Protein Add Biuret reagent. It turns from blue to purple if protein is present.