Photosynthesis

Plants make their own food. In their leaves they use energy from light to turn carbon dioxide and water into glucose, giving out oxygen. Almost every food chain starts here, and the oxygen you breathe was made this way. How fast it happens depends on light, carbon dioxide and temperature.

Key facts

  • Word equation: carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll.
  • Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Photosynthesis is endothermic: energy is taken in from light.
  • It happens in the chloroplasts, mostly in the palisade cells near the top of the leaf. Carbon dioxide gets in through stomata; water comes up from the roots in the xylem.
  • Plants use the glucose for respiration, store it as starch, and make cellulose (for cell walls), proteins (with nitrate ions from the soil) and fats and oils.
  • Test a leaf for starch: boil it in water, then in ethanol to remove the chlorophyll, rinse, and add iodine. Starch turns the iodine from orange-brown to blue-black.
  • Limiting factors: light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply limits the rate. Too hot, and the enzymes are denatured.
  • Light intensity is inversely proportional to the square of the distance from the lamp (light intensity ∝ 1 ÷ d²): double the distance and the light is a quarter as strong.

Worked examples

Example 1

Write the word equation for photosynthesis and say where the energy comes from.

  1. Light shines on the leaf; chlorophyll in the chloroplasts absorbs its energy.
  2. Carbon dioxide comes in from the air, through the stomata, and water comes up from the roots.
  3. Glucose is made and oxygen goes out into the air. carbon dioxide + water → glucose + oxygen

Answer: carbon dioxide + water → glucose + oxygen; the energy comes from light

Example 2

A piece of pondweed gives off 48 bubbles of oxygen in 4 minutes. What is the rate of photosynthesis in bubbles per minute?

  1. The bubbles are oxygen, a product of photosynthesis, so counting them measures the rate.
  2. Divide the number of bubbles by the time. 48 ÷ 4 = 12
  3. The rate is 12 bubbles per minute.

Answer: 12 bubbles per minute

Example 3

A lamp is moved from 10 cm to 20 cm away from some pondweed. How does the light intensity change?

  1. Light intensity ∝ 1 ÷ d². The distance doubles: × 2.
  2. So the intensity changes by 1 ÷ 2². 1 ÷ 2^2 = 1/4
  3. The light intensity is a quarter of what it was.

Answer: it becomes a quarter (÷ 4)

Example 4

The graph shows the rate of photosynthesis as light intensity increases. Why does the rate stop rising?

  1. At first, more light gives a faster rate: light is the limiting factor.
  2. Then the line levels off: more light makes no difference.
  3. So something else is now in short supply: the carbon dioxide concentration or the temperature.

Answer: something else (carbon dioxide or temperature) has become the limiting factor