Photosynthesis

Plants make their own food by photosynthesis. Carbon dioxide and water react together in the presence of light and chlorophyll to make glucose and oxygen. The glucose is converted into starch, fats and oils for storage. It is used to make cellulose for cell walls, and proteins for growth and repair. It is also used by the plant to release energy by respiration.

Photosynthesis and respiration

Photosynthesis

Photosynthesis is a chemical reaction that happens in the chloroplasts of plant cells. It produces glucose for use by the plant, and oxygen as a waste product. Here are the equations for photosynthesis:
Carbon dioxide + water Reaction (light energy, chlorophyll) glucose + oxygen
6CO2 + 6H2O Reaction (light energy, chlorophyll) C6H12O6 + 6O2
Light energy is absorbed by chlorophyll in the chloroplasts for photosynthesis to happen.

Respiration

It is not just animals that respire – plants carry out respiration as well. Plants respire all the time because their cells need energy to stay alive, but plants can only photosynthesise when they are in the light.

 

Time of dayPhotosynthesisRespiration
DayActiveActive
NightNoneActive
The diagrams summarise what this means for the overall release of carbon dioxide or oxygen from plants. Remember that respiration uses oxygen and produces carbon dioxide.
How plants affect the atmosphere: day

Understanding photosynthesis

You should be able to describe how the understanding of the process of photosynthesis has developed.

Ancient Greek scientists

Scientists in ancient Greece believed that plants gained mass only by taking in minerals from the soil. They would not have tested this idea.

Jan Baptist van Helmont (1580-1644)

Van Helmont carried out an experiment to see if the idea from ancient Greece was correct. He grew a willow tree in a weighed amount of soil. After five years, he discovered that the willow tree weighed about 74 kg more than it did at the start. As the weight of the soil had hardly changed, van Helmont concluded that plant growth cannot only be due to minerals from the soil. He thought that the extra plant material had come from the water alone.

Joseph Priestley (1733 - 1804)

Joseph Priestley carried out an experiment that showed that plants produce oxygen. He put a mint plant in a closed container with a burning candle. The candle flame used up the oxygen and went out. After 27 days, Priestley was able to re-light the candle. This showed that plants produce a gas that allows fuels to burn. This gas is oxygen.

Using glucose

The glucose made in photosynthesis is transported around the plant as soluble sugars. Glucose is used in respiration to release energy for use by the plant's cells. However, glucose is converted into insoluble substances for storage. These insoluble storage substances include:
  • Oils
  • Fats
  • Starch
The advantages of using insoluble substances such as starch for storage, rather than soluble substances such as glucose, include:
  • They do not affect the water concentration inside cells
  • They do not move away from the storage areas in the plant
Glucose and starch can be converted into other substances in plants. For example:
  • Cellulose for cell walls
  • Proteins for growth and repair

  • Limiting factors

    Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.
    • rate of photosynthesis plotted against light intensity. the rate begins to slow as the light intensity continues to increase
      Light intensity
      Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.
    • rate of photosynthesis plotted against carbon dioxide concentration. the rate begins to slow as the carbon dioxide concentration continues to increase
      Carbon dioxide concentration
      Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
    • rate of photosynthesis plotted against temperature. the rate begins to slow as the temperature continues to increase
      Temperature
      If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.
    If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.
    In practice, any one of these factors could limit the rate of photosynthesis.

    Read on if you're taking the higher paper.

  • Understanding photosynthesis - Higher tier

    A two-stage process

    Photosynthesis is a process with two main stages:
    1. Light energy is used to split water, releasing oxygen gas and hydrogen ions
    2. Carbon dioxide gas combines with the hydrogen to make glucose
    You do not need to know equations for the two separate stages, only for the overall process.
    6CO2 + 6H2O Reaction (light energy, chlorophyll) C6H12O6 + 6O2

    Evidence for the two stages

    Experiments using isotopes of carbon, hydrogen and oxygen have increased our understanding of photosynthesis. One of these experiments involves an isotope of oxygen, 18O.
    The more common isotope of oxygen in water or carbon dioxide, 16O, can be replaced by 18O. The transfer of these 18O atoms into other substances can be traced. The results were:
    • Plants watered with water containing 18O atoms release oxygen gas containing 18O atoms
    • Plants supplied with carbon dioxide containing 18O atoms do not release oxygen gas containing 18O atoms
    This shows that the oxygen gas produced by photosynthesis comes from water and not carbon dioxide.


For Exercise Please Click Here~!

0 komentar:

Langganan: Posting Komentar (Atom)