Photo respiration and its affects

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 Photorespiration and its affects

Photorespiration is defined as the process in which oxygen combineswith ribulose biphosphate (RuBP) in the presence of sunlight and CO, isproduced. The process is called photorespiration because in the presence oflight (photo), oxygen is taken up and CO2 is evolved (respiration).

Photosynthesis needs optimum concentration of the requirementsfor normal functioning. If however one of the requirement is present in lessconcentration than the optimum, the process of photosynthesis is affected and slows down.

In the Dark reaction of photosynthesis, normally CO, combines with

RuBP (carboxylation) forming PGA molecules. The process occurs in thepresence of an enzyme called ribulose biphosphate carboxylase (rubisco).

This enzyme can act both as carboxylase and oxygenase. The reactiondepends on the concentration of CO, and 0, If the concentration of CO, ismore, then rubisco combines with CO, and photosynthesis proceedsnormally. On the other hand if the concentration of O, is more, then rubiscocombines with 0, and photorespiration occurs. (Rubisco can act both ascarboxylase as well as oxygenase).water. In such condition CO, cannot enter the leaf and O, cannot leave it.therefore light reaction occurs with maximum rate which results inand photorespiration proceeds.

Plants have stomata for the exchange of gases. Diffusion of watervapours from leaf to the external environment also occurs through thestomata. In dry and hot weather plants close up stomata so as to conserve. Dry and hot conditions are usually accompanied by intense sunlightmaximum use of Co, Since concentration of Co, lowers down in the leaf. 

The following steps are involved in photorespiration:

Oxygen combines with RuBP (present in stroma of chloroplast) anda compound called Glycolate is produced.Ciscolate

Glycolate is converted into glycine (simplest amino acid) in the

1.O

Chisine

RuBP

2.peroxisome.

Glycolate

3.Glycine is transported to mitochondria where it is converted intoserine and a molecule of CO is produced.

Glisine

Serine + c

Disadvantages of Photorespiration (Consequences)

1. Photorespiration is just the reverse of photosynthesis hampering the fixation of Co,photosynthesis.

2. The process wastes energy and does nothing to serve the needs of the plant.  

Photosynthesis

In normal process of photosynthesis a 3C carbon compound calledPGA is formed as the first detected product of photosynthesis (carbondioxide fixation) and therefore these plants are called C plants. There aresome plants growing in dry and hot conditions produce a 4C carboncompound called oxaloacetate as the first product of carbon dioxide fixationin dark reactions of photosynthesis. These plants are called C. plants andthis type of photosynthesis is called C photosynthesis. C plants use rubiscoto react Co with RuBP. On the other hand C, plants use another enzymecalled pepco (phosphoenolpyruvate carboxylase) to fix Co, to a compoundcalled phosphoenolpyruvate (PEP). This molecule is reduced to anothermolecule called malate. The malate carries Co, to the special type of cellscalled bundle sheath cells where Calvin cycle proceeds.

In C, plants chloroplasts are present only in mesophyll cells of leaf.

However in a C plant chloroplasts are present both in mesophyll cells and in water. In such condition CO, cannot enter the leaf and O, cannot leave it.therefore light reaction occurs with maximum rate which results in and photorespiration proceeds.

Plants have stomata for the exchange of gases. Diffusion of water
vapours from leaf to the external environment also occurs through the
stomata. In dry and hot weather plants close up stomata so as to conserve
Dry and hot conditions are usually accompanied by intense sunlight
maximum use of Co, Since concentration of Co, lowers down in the leaf
The following steps are involved in photorespiration:
Oxygen combines with RuBP (present in stroma of chloroplast) and a compound called Glycolate is produced.
Ciscolate
Glycolate is converted into glycine (simplest amino acid) in the
1.
O
Chisine
RuBP
2.
peroxisome.
Glycolate
3.
Glycine is transported to mitochondria where it is converted into
serine and a molecule of CO is produced.
Glisine
Serine + c
Disadvantages of Photorespiration (Consequences)
1. Photorespiration is just the reverse of photosynthesis hampering the
fixation of Co,photosynthesis.
2. The process wastes energy and does nothing to serve the needs of the
plant.

In C. plants the rate of photosynthesis remains high even when the stomata
are closed and temperature is high. The rate of Co, fixation is also high as
compared to C, plants. C. cycle is basically an adoptability of C. plants to
carry out CO, fixation in dry and hot condition and to reduce the rate of
photorespiration bundle sheath cell. In a C, plant all the mesophyll cells carry out Calvin cycle by fixing Co, and producing glucose. In a C, plant the mesophyll cells only fix Co, by using pepco while the bundle sheath cells carry out Calvin cycle producing glucose. This is a special condition evolved by C. plants so as to prevent photorespiration even in dry and hot environment as pepco does not bind to O, irrespective of the concentration of CO,. Examples of C, plants are sugar cane, maize etc. Vascular bundle (vein) Air space beneath stoma Mesophyll cells Bundle sheath cell Portion of a cross section of a leaf with C. photosynthesis. 

 

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