Limiting Factors

• Limiting factors in photosynthesis

  Environmental factors light, carbon dioxide and temperature that affect the rate of photosynthesis
• If all three limiting factors are present in sufficient quantities, the rate of photosynthesis will be at its optimum
• Limiting factor

  A factor that limits the rate of photosynthesis due to that factor being present at a sub-optimal level
• Light (to provide energy) and carbon dioxide (a raw material) are necessary for photosynthesis, so normally the more of these are present the faster the photosynthesis reaction takes place (up to a maximum)
• Temperature affects the rate of all reactions (e.g. through the speed of molecules diffusing and its effect on enzyme activity), it will affect the rate of the photosynthesis reaction
• Higher tier candidates need to be able to interpret data on limiting factors in graphs or tables
• Figure 2.4 Limiting factors in photosynthesis

  • In this region light is limiting the rate of photosynthesis - if the light intensity is increased, the rate of photosynthesis also increases
  • High carbon dioxide and high temperature. Highest rate due to higher carbon dioxide, higher temperature and high light
  • High carbon dioxide and low temperature; higher rate than A due to extra carbon dioxide
  • Low carbon dioxide and low temperature
• At point *1 both carbon dioxide and temperature levels are limiting the rate of photosynthesis

  At point *2 temperature is limiting the rate
• Hydrogencarbonate (bicarbonate) indicator

  Red in normal atmospheric (0.04%) carbon dioxide levels, turns yellow in increased carbon dioxide levels, turns purple in decreased carbon dioxide levels
• Using hydrogencarbonate indicator to demonstrate photosynthesis and respiration in plants

  1. Bright light

  2. Tube B covered with light-proof foil

  3. Tube C partially shaded with e.g. muslin
• In boiling tube A the rate of photosynthesis exceeded the rate of respiration, resulting in the concentration of carbon dioxide in the tubes decreasing and turning the indicator purple
• In boiling tube B there was only respiration taking place (the foil prevented photosynthesis). The carbon dioxide concentration in the boiling tube increased, turning the indicator yellow
• In boiling tube C both photosynthesis and respiration were taking place but the rate of photosynthesis was much reduced (due to the partial shading) - the rates of photosynthesis and respiration were equal and the amount of carbon dioxide used in photosynthesis was the same as the amount released in respiration. There was no change in carbon dioxide levels and the indicator remained red
• Compensation point

  The point at which the rates of photosynthesis and respiration are equal
• How to use hydrogencarbonate indicator to find the amount of light required to reach the compensation point in pondweed

  1. Add a section of pondweed to hydrogencarbonate indicator in a boiling tube

  2. Add a bung

  3. Leave the pondweed for 20-30 minutes in a particular light intensity and check the colour of the hydrogencarbonate indicator

  4. Increase or decrease the light intensity by moving a lamp or using a dimmer switch and repeat

  5. Repeat until the indicator remains red
• Light intensity affects the rate of photosynthesis - more intense light = faster rate of photosynthesis
• The equation for photosynthesis is carbon dioxide + water -> glucose + oxygen
• Test tube A - normal test tube containing pond weed
• Test Tube B - Covered with light-proof foil containing pond weed
• Test Tube C - Partially shaded with muslin
• Hydrogencarbonate indicator is used to demonstrate photosynthesis and respiration in plants
• Tube 1


  The tinfoil stop's light getting to the plant. This is like night when there is no photosynthesis but respiration still continues so carbon dioxide is released from the plant, turning the hydrogen carbonate indicator
• Tube 1 result


  • yellow
• Tube 2


  There is a lot of light available to the plant. During the day the rate of photosynthesis is greater than the rate of respiration so more carbon dioxide is taken up by the plant than is released in respiration. The low carbon dioxide level makes the hydrogen carbonate indicator purple
• Tube 3


  The muslin reduces the light, this imimics dawn and dusk when the rate of photosynthesis is equal to the rate of respiration All the CO, released in respiration is used in photosynthesis (we say there is no net output of gas). The bicarbonate indicator remains red
• Compensation Point

  when the rate of photosynthesis is equal to the rate of respiration
• Tube 4


  The water beatles are respiring as well as the pondweed and therefore more CO, is produced than can be used in photosynthesis causing the hydrogen carbonate indicator to turn yellow
• THIS IS CALLED THE COMPENSATION POINT
• Respiration


  Glucose + O₁₂-> CO₂+ Water + Energy
• In plants the glucose and oxygen produced in photosynthesis


  Can be used for respiration
• The carbon dioxide and water produced in respiration


  Can be used for photosynthesis
• Sometimes the two processes (photosynthesis and respiration) balance each other out
• Photosynthesis and respiration happen during the day
• Only respiration happens at night
• Hydrogen carbonate indicator


  Can be used to show the relationship between photosynthesis and respiration
• Experiment


  1. Bicarbonate

  2. Tube 1

  3. Tube 2

  4. Tube 3

  5. Tube 4

  6. Indicator

  7. Temperature
• As temperature increases

  The rate of photosynthesis increases, to a point
• At very high temperatures photosynthesis stops as enzymes are destroyed
• As carbon dioxide concentration increases

  The rate of photosynthesis increases, to a point
• After this carbon dioxide concentration has no further affect; either light or temperature becomes a limiting factor