Photosynthesis & Limiting Factors

• Photosynthesis produces glucose using light
• Photosynthesis uses energy to change carbon dioxide and water into glucose and oxygen
• Photosynthesis takes place in chloroplasts in green plant cells - they contain pigments like chlorophyll that absorb light
• Photosynthesis is endothermic - this means energy is transferred from the environment in the process
• The word equation for photosynthesis is:

  carbon dioxide + water -(light)-> glucose + oxygen
• The symbol equation for photosynthesis is:

  6CO2 + 6H2O -(light)-> C6H12O6 + 6O2
• Plants use glucose in 4 main ways:

  • For respiration - this transfers energy from glucose which enables the plants to convert the rest of the glucose into various other useful substances
  • Making cellulose - glucose is converted into cellulose for making strong plant cell walls
  • Making amino acids - glucose is combined with nitrate ions (absorbed from the soil) to make amino acids, which are then made into proteins
  • Stored as oils or fats - glucose is turned into lipids for storing in seeds
• Plants also use glucose for:

  • Storing starch - glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't happening, like in the winter. Starch is insoluble, which makes it much better for storing than glucose - a cell with lots of glucose in would draw in loads of water and swell up
• Limiting factors affect the rate of photosynthesis
• The rate of photosynthesis is affected by light intensity, concentration of CO2 and temperature. Any of these 3 factors can become the limiting factor
• A limiting factor in photosynthesis is a factor that's stopping photosynthesis from happening any faster
• Light intensity, concentration of CO2 and temperature have a combined effect on the rate of photosynthesis, but which factor is limiting at a particular time depends on the environmental conditions:

  • At night, the limiting factor is light
  • In winter it's often the temperature
  • If it's warm enough and bright enough, the amount of CO2 is usually limiting
• Chlorophyll can also be a limiting factor of photosynthesis
• The amount of chlorophyll in a plant can be affected by disease (e.g. infection with the tabacco mosaic virus) or environmental stress, such as a lack of nutrients. These factors can cause chloroplasts to become damaged or to not make enough chlorophyll. This means the rate of photosynthesis is reduced because they can't absorb as much light