BOTANY Higher Secondary Second Year - Textbooks Online

Difference between C 3 and C 4 photosynthetic pathways
C 3 pathway C 4 pathway
1. Photosynthesis occurs in meso - Photosynthesis occurs in meso -
phyll cells. phyll and bundle sheath cells.
2. The CO 2 molecule acceptor is The CO 2 acceptor molecule is
RuBP. phosphoenol pyruvate.
3. The first stable product is a 3C The first stable product is a 4C
compound called 3 – PGA. compound called OAA.
4. Photorespiration rate is high Photorespiration is negligible
and leads to loss of fixed CO2. and it is almost absent. Hence,
It decreases CO 2 fixation rate. it increases CO 2 fixation rate.
5. Optimum temperature is 20 to Optimum temperature is 30 to
25°C. 45°C.
6. Examples of C 3 plants are rice, Examples of C 4 plants are maize,
wheat and potato. sugarcane,Tribulus and
Amaranthus
5.2.7 Photorespiration or C 2 cycle
In animals and bacteria, only one kind of respiration known as dark
respiration occurs. This is not affected by the presence or absence of light.
But in certain green plants, there are two distinct types of respiration –
photorespiration and dark respiration. Respiration that occurs in
photosynthetic tissues in the presence of light and results in increased rate
of carbondioxide evolution is called photorespiration or light respiration.
Photorespiration involves three organelles – chloroplasts, peroxisomes
and mitochondria. Oxidation of RuBP in the presence of high oxygen
is the first reaction of photorespiration. This reaction is catalysed by
Rubisco* enzyme called carboxylase. It leads to the formation of 2C
compound – phosphoglycolic acid and 3C compound PGA. When PGA
is used up in the Calvin cycle, the phosphoglycolic acid is dephosphorylated
to form glycolic acid in the chloroplasts.
From the chloroplast, the glycolic acid diffuses into the peroxisome
where it is oxidised to glyoxalic acid and hydrogen peroxide. In peroxisome
from glyoxalic acid, glycine is formed.
Note : * Rubisco = Ribulose bisphosphate carboxylase
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