NEET Biology Notes Concept of Photorespiration (C2-Cycle)
It is a special type of respiration shown by many green plants, when they are exposed to light. It was discovered by Dicker and Tio (1959) in tobacco.
The site for photorespiration is chloroplast. Peroxisome is required to complete the process. RuBP carboxylase is changed to RuBP-oxygenase at high temperature and high oxygen concentration. At high temperature, RuBP carboxylase functions as oxygenase and instead of fixing carbon dioxide, oxidises ribulose 1, 5-bisphosphate to produce phosphoglyceric acid and phosphoglycolate.
Phosphoglycolate is hydrolysed to form glycolate. Glycolate usually passes into peroxisomes of the mesophyll cells and forms glyoxalate. Glyoxalate is now converted into an amino acid glycine. This is a transamination reaction. The glycine formed in peroxisomes migrates into mitochondria where 2 molecules of glycine reacts to form one molecule of another amino acid serine with the liberation of C02 (post-illumination burst of C02 and photorespiration) and also NH2. This reaction is catalysed by the enzyme serine hydroxymethyl transferase. Serine can further be deaminated to form PGA. The later passes into chloroplast for the synthesis of photosynthetic products as well as photorespiration.
Since, photorespiration involves the synthesis of two carbon compounds, it is also called as C2 -cycle (glyoxylate and glycine are 2-C compounds). C2 cycle is also known as glycolate metabolism
CAM (Crassulacean Acid Metabolism) is an alternative ofC3 and C4-pathway ofC02 fixation. This pathway was first of all reported in Bryophyllum, a member of family-Crassulaceae and hence, is called crassulacean acid metabolism.
In CAM plants, stomata open’ during night and close during daytime. There is no Kranz anatomy but dark acidification takes place, i.e. during night malic acid is formed. This malic acid breaks up into C02 and pyruvic acid in daytime and C02 released is utilised in C 3-cycle.
Factors Affecting Photosynthesis
Law of limiting factors was proposed by FF Blackman in 1905. According to this law, ‘when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor’ (i.e. factor present in minimum amount).
Many factors affect the rate of photosynthesis some are as follows:
- Atmospheric C02 Concentration
Atmospheric concentration of C02 is 0.03% (300 ppm). Increase in this concentration up to 0.1%, increases the rate of photosynthesis in plants. Compensation point is reached at 50-100 ppm in C3 -plants and 1-10 ppm in C 4-plants.
Maximum photosynthesis occurs in blue and red light, while minimum photosynthesis takes place in green light. Red light is more efficient in photosynthesis as compared to blue light. However, maximum photosynthesis rate has been observed in full sunlight.
Very high light intensity decreases the rate of photosynthesis and this phenomenon is called as solarisation. It may be due to the photoinhibition and photoxidation. In case of C4-plants, saturation point is not reached even in full sunlight.
It affects only enzyme controlled dark reaction. The optimum temperature is 10-35°C for C3 -plants and 30-45°C for C 4-plants.
The maximum temperature, at which photosynthesis can occur, is 55^ in desert plants and 75°C for some algae found in hot springs and in some conifers, it occurs even at -35°C. When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10°C rise in temperature.
- Rate of Respiration
Usually, in the mornings and evenings, the rate of respiration is approximately equal to rate of photosynthesis, there shall not be any apparent gaseous exchange.
The 02 evolved during photosynthesis will be utilised in respiration and C02 evolved during respiration will be used in photosynthesis. This stage is theoretically called compensation point.
In C 3-plants, optimum oxygen for photosynthesis is 2.5%. Rate of photosynthesis in C3-plants is reduced at normal atmospheric concentration of oxygen. No such effect is found in C4-plant. Above 21%, there is reduction in photosynthesis. Thi affect is called as Warburg effect.
This is due to 02 which is a strong quencher of excited state of chlorophyll and high concentration of 02 that converts RuBP carboxylase to RuBP oxygenase.
- Organic and Inorganic Nutrients
Sulphur dioxide, ozone, chlorofluorocarbon, other atmospheric pollutants and deficiency of minerals such as Mg, Fe, Cu, Zn, Mn, N decrease the rate of photosynthesis. Accumulation of food in the chloroplast also reduces the rate of photosynthesis.