Photosynthesis

• Photosynthesis

  An endothermic reaction in which energy is transferred from the environment to the chloroplasts by light.

  Takes place in the specialised mesophyll tissues in leaves which are packed with chloroplasts containing chlorophyll to absorb as much light as possible.
• Photosynthetic reaction

  Carbon dioxide + Water → Glucose + Oxygen.

  6CO2 + 6H20 → C6H1206 + 602.
• How do plants gain the reactants for photosynthesis?

  Carbon dioxide - diffuses into the leaf through the stomata.

  Water - taken up by the roots and transported through the xylem to the leaves.
• What are the four factors that affect the rate of photosynthesis?

  Temperature.

  Light intensity.

  Carbon dioxide concentration.

  Amount of chlorophyll.
• Effect of temperature on photosynthesis

  Increasing temperature increases kinetic energy of particles, increasing likelihood of collisions between reactants and enzymes which results in the formation of products.

  At higher temperatures however, enzymes controlling photosynthesis can denature causing a fall in rate of photosynthesis.

  
• Effect of light intensity on photosynthesis

  The more light a plant receives, the faster the rate of photosynthesis up to a certain point.

  At this point, increasing light intensity stops increasing the rate.

  Rate becomes constant because another factor is limiting the rate.

  
• Effect of carbon dioxide concentration on photosynthesis

  The more carbon dioxide that is present, the faster photosynthesis will occur up to a certain point.

  At this point, increasing carbon dioxide concentration stops increasing the rate.

  Rate becomes constant because another factor is limiting the rate.

  
• Effect of chlorophyll on photosynthesis

  Chloroplasts contain the green pigment chlorophyll which absorbs the light energy used for photosynthesis.

  Therefore the more chloroplasts a plant has, the faster the rate of photosynthesis.

  Amount of chlorophyll is affected by diseases, lack of nutrients and loss of leaves.
• Interactions between two Limiting Factors

  At the start of the graph, rate of photosynthesis is limited by light intensity so both lines show the same rate.

  As light intensity increases rate of photosynthesis at 15 degrees is lower than at 25 degrees.

  Both lines level off, this shows that light intensity is no longer the limiting factor.

  
• Interactions between three Limiting Factors

  All three experiments level off when light intensity is no longer the limiting factor.

  The top line has the highest temperature and carbon dioxide concentration so rate of photosynthesis is much higher.

  In experiment 2, the limiting factor is the carbon dioxide concentration.

  In the graph, rate of photosynthesis is controlled by carbon dioxide levels.

  
• Calculating light intensity

  Light intensity and distance are inversely proportional to each other.

  Light intensity = 1/Distance^2.

  Light intensity is measured in au (arbitrary units).
• Growing in a Greenhouse

  Commercial horticulturists will grow their plants in a greenhouse because they can control as many of the limiting factors of photosynthesis as possible.

  This allows them to gain the maximum rate of photosynthesis while maintaining profit.

  Running a greenhouse is costly, but the increased yield of the crop and the fact that the crop can be harvested more frequently means the farmer will make more money.
• How Limiting Factors are controlled in a Greenhouse

  The levels of heat, light, water, carbon dioxide and nutrients are carefully controlled so only the smallest amounts needed are used so that farmers are not wasting money.

  E.g. spending money on increasing the concentration of carbon dioxide beyond a point when some other factor limits the rate of photosynthesis is a waste.

  
• Rate of Photosynthesis PRACTICAL


  Place a piece of pondweed into a beaker of water and place the light source a set distance away from the beaker.

  Record the number of bubbles observed in three minutes.

  Repeat steps at different distances.

  IMPROVEMENTS: Use a gas syringe to collect exact volume of oxygen gas produced. Repeat experiment multiple times for each distance and calculate mean number of bubbles. Place a glass tank between light and plant to prevent heating the plant.

  
• Uses of glucose produced in photosynthesis

  Used for aerobic and anaerobic respiration to release energy.

  Converted into insoluble starch for storage in the stems, leaves and roots.

  Used to produce fat or oil for storage in seeds.

  Used to produce cellulose which strengthens cell walls.

  Combined with nitrate ions absorbed from the soil to produce amino acids for protein synthesis.