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2.2.7Transport Over Larger Distances

Factors Affecting The Rate Of Photosynthesis

5 min read 9 parts Lesson 17 of 20 1 quiz

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Section 1 of 9Photosynthesis equation

Photosynthesis: light, carbon dioxide and water used to produce glucose and oxygen
Photosynthesis: light, carbon dioxide and water used to produce glucose and oxygen
Word equation
  • Word equation Carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll.
Symbol equation
  • Symbol equation 6CO2 + 6H2O → C6H12O6 + 6O2
Energy change
  • Energy change The reaction is endothermic because it takes in light energy.

Section 2 of 9The three main factors

FactorEffect at low levelsWhy
Light intensityRate rises as light increasesMore energy is available to drive the reaction
Carbon dioxide concentrationRate rises as CO2 increasesMore raw material for glucose production
TemperatureRate rises up to an optimumEnzyme activity and collisions increase
Temperature too highRate drops sharplyEnzymes denature above about 45 °C

Section 3 of 9Limiting factors

1Shortest supply
  • Shortest supply The factor in shortest supply sets the maximum rate.
2Rate rises
  • Rate rises Increasing that factor increases the rate.
3Plateau
  • Plateau A graph levels off when a different factor becomes limiting.
4Change the new limiter
  • Change the new limiter Only increasing the new limiting factor will raise the rate further.

Section 4 of 9Economics for growers

Greenhouse control
Artificial light
  • Artificial light Lamps lengthen the day and raise light intensity.
Heating
  • Heating Keeping the temperature near the optimum speeds growth.
Carbon dioxide enrichment
  • Carbon dioxide enrichment Paraffin heaters or gas supplies increase CO2.
Cost balance
  • Cost balance Increase conditions only while extra yield is worth the cost.

Section 5 of 9Inverse square law (HT only)

1
As a lamp moves away, light intensity falls in proportion to 1 ÷ distance2.
2
Doubling the distance reduces the intensity to one quarter.
3
Because rate tracks light, photosynthesis rate drops in the same pattern when light is limiting.
4
Use this to predict results in lamp-distance experiments.

Section 6 of 9Reading a limiting factor graph

Rising section
  • Rising section On the steep part of the curve the factor being changed is limiting, so increasing it gives a proportional increase in rate.
Plateau
  • Plateau When the line is flat, adding more of the factor makes no difference because another factor,
    • such as carbon dioxide or temperature, is now limiting.
Temperature curve
  • Temperature curve The rate rises to an optimum near 25-35 °C, then falls rapidly as enzymes denature, so the graph is a hump rather than a plateau.

Section 7 of 9Worked example: lamp distance

Lamp distance (cm)Relative intensity (1 ÷ d2)Prediction for rate
101 ÷ 100 = 0.0100Highest rate
201 ÷ 400 = 0.0025One quarter of the intensity at 10 cm
401 ÷ 1600 = 0.000625One sixteenth of the intensity at 10 cm

Section 8 of 9Key terms

TermMeaning
Limiting factorA variable that, in short supply, restricts the rate of a process.
Light intensityHow bright the light is.
Optimum temperatureThe temperature at which the rate is greatest.
DenaturePermanent loss of an enzyme's shape and function.
EndothermicA reaction that takes in energy from the surroundings.

Section 9 of 9Exam technique

1
Saying temperature keeps raising the rate forever; enzymes denature when too hot.
2
Not naming the limiting factor when a graph levels off.
3
Thinking more light helps if CO2 is the limiting factor.
4
Mixing up inverse and direct proportion with distance.
5
Forgetting that chlorophyll must absorb the light for the energy to be used in photosynthesis.
Biology 0610 & 9700 Exam Model Limiting Factors Master

Photosynthesis: Limiting Factors & Graph Plateau

6CO2 + 6H2O → C6H12O6 + 6O2 (Light & Chlorophyll required)

  • Limiting Factor Definition: Something present in the environment in such short supply that it restricts life processes.
  • Graph Plateau: When rate levels off as light intensity increases, light is no longer the limiting factor; CO2 concentration or temperature has become limiting.
  • High Temperature Drop: Beyond optimum temp (~40°C), enzymes (e.g. RuBisCO) denature, permanently decreasing rate.