This is a part of the online lecture for the undergraduate students of Botany, Government First Grade College, Yelahanka, Bangalore

1. PHOTORESPIRATION
BY
MALLIKHARJUNA PB
GOVERNMENT FIRST GRADE COLLEGE YELAHANKA BANGALORE
16-04-2020

2. 1. Photorespiration is a special physiological phenomenon that
occurs only in green plants.
2. It is a process wherein the uptake of O2 and release of CO2 occurs
in the presence of light in the chlorenchyma / mesophyll cell.
3. This phenomenon was first observed by Krotkov (1963).
4. It is also known as C2 - cycle or Glycolate pathway.
5. It is a bypass or the unnecessary process of the C3 cycle or Calvin
cycle.

3. A) At higher CO2 concentration, it produces two molecules of
3-Phosphoglycerate (3-PGA) which in turn yields a Glucose molecule.
Ribulose 1,5 Biphosphate + CO2
𝑹𝒖𝑩𝑰𝑺𝑪𝑶
𝑪𝒂𝒓𝒃𝒐𝒙𝒚𝒍𝒂𝒔𝒆
> 2 (3-Phosphoglycerate)
B) At higher O2 concentration, it produces one molecule of
3-Phosphoglycerate (3-PGA) and one molecule of 2-Phosphoglycolate,
the starting molecule of Photorespiration.
Ribulose 1,5 Biphosphate + O2
𝑹𝒖𝑩𝑰𝑺𝑪𝑶
𝒐𝒙𝒚𝒈𝒆𝒏𝒂𝒔𝒆
> 3-Phosphoglycerate + 2-Phosphoglycolate

4. C3 PLANTSDUAL BEHAVIOUR OF RUBISCO

5. 1. Photorespiration occurs only in Chlorenchyma during daytime or
in the light.
2. It is very common in C3 plants, but rare or absent in C4 and CAM
plants.
3. It occurs due to the O2: CO2 ratio in the tissue atmosphere
4. ( <50ppm of CO2).
5. This process requires the co-ordination of 3-cell organelles viz.,
Chloroplast, Peroxisome and Mitochondrion.
6. In this process, the uptake of 3-O2 molecules and releasing of one
CO2 takes place.

6. 6. This process releases no energy unlike Aerobic respiration. However,
utilizes 2 molecules each of ATP and NADH2.
7. It is a multistep biochemical process (12 reactions) mediated by a
series enzymes led by the RuBISCO oxygenase (11).
8. The starting molecule of Photorespiration is the 2-Phosphoglycolate,
which is a two carbon containing molecule, hence it is called C2 cycle.
9. The end molecule of photorespiration is the 3-Phosphoglycerate, which is
synthesized from the 2-Phosphoglycolate through a series of events,
hence termed as Glycolate pathway.

8. Role of Photorespiration : Although it is energy consuming
unnecessary process. It has playing some important role in
• The removal of the Toxic Metabolic Intermediates
• Protecting the RuBISCO from the Photoinhibition
• Supports the Plant Defense mechanism and
• It is Intimately Integrated Into Primary Metabolism