Reactions in plants (BIOLOGY)
PHOTOSYNTHESIS:
Limiting Factor:
 A variable that prevents the rate (speed) of a process from becoming any
faster.
 With less light, or a shortage of a raw material, photosynthesis is slower
Factors affecting photosynthesis
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 rate of photosynthesis.
Carbon dioxide concentration
Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient
carbon dioxide.
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 shown above.
 In practice, any one of these factors could limit the rate of photosynthesis.
Aerobic Respiration
 Plant cells need a supply of glucose for energy to help the plant grow and
make new substances.
 The chemical energy stored in the glucose is released by the reaction called
aerobic respiration because oxygen from the air is needed for it to work.
 The word equation and symbol equation for aerobic respiration:
 glucose + oxygen → carbon dioxide + water (+energy)
C6H12O6 + 6O2 → 6CO2 + 6H2O (+energy)
 The energy released from glucose is transferred to molecules of ATP, which
store energy for a cell until it is needed.
Photosynthesis and respiration
 Respiration is the opposite of photosynthesis.
 Photosynthesis only happens when there is light but
respiration happens all the time.
 During the day, a plant produces more oxygen from
photosynthesis than it needs for respiration and so
oxygen is given off.
 Glucose is carried to all parts of the plant in the form of
sugars dissolved in water.
 Phloem vessels carry the sugar solution. This is how the
roots get glucose for respiration.
 Roots need oxygen for respiration which they get from
the soil.