Download Note 5

South Tuen Mun Government Secondary School
Biology Revision Note 5
Mineral requirement in plants
Nitrogen is required to make protein. It is absorbed as nitrate. Protein is used for growth. Deficiency of nitrogen
leads to slow growth and yellow leaves.
Magnesium is required to make chlorophyll. It is absorbed as magnesium ion. Deficiency of magnesium, the
leaf becomes yellow because the leaf cannot make chlorophyll.
Gaseous exchange in green leaf
Respiration : glucose + oxygen  carbon dioxide + water
Photosynthesis : carbon dioxide + water  glucose (carbohydrate) + oxygen
In day time : the rate of photosynthesis is faster than the rate of respiration, therefore, carbon dioxide enters the
green leaf through the stomata and oxygen goes out of the green leaf through the stomata.
At night : the rate of photosynthesis is slower than the rate of respiration (there is no photosynthesis in complete
darkness), therefore carbon dioxide goes out of the green leaf and oxygen goes into the green leaf through the
stomata.
Day
A graph to show the gaseous exchange in green leaves:
Night

When light intensity increases, the rate of photosynthesis increases, but the rate of respiration remains
unchanged.

At low light intensity, the rate of photosynthesis is faster than the rate of respiration. At compensation
point, the rate of photosynthesis equals to the rate of respiration. At high light intensity, the rate of
photosynthesis is faster than the rate of respiration. At very high light intensity, the release / uptake of
carbon dioxide remains unchanged the rate of photosynthesis reaches its maximum.
Calculation of release of oxygen / uptake of carbon dioxide:
Net release of oxygen / net uptake of carbon dioxide (both respiration and photosynthesis) (N)
Release of oxygen / uptake of carbon dioxide by photosynthesis (P)
Release of oxygen / uptake of carbon dioxide by respiration (R)
N = P+ R
R is found by the value when the light intensity is 0. N is usually read from the given curve.