Photorespiration is the light-dependent process of oxygenation of ribulose biphosphate and release of carbon dioxide by the photosynthetic organs of a plant.
It occurs in the chloroplast, Peroxisome and mitochondria are required for completing the process. This process creates a significant difference between C3and C4 plants as it is absent in C4 plants.
In photorespiration, the rate of which increases under the influence of light, and during which CO2 is released, and O2 is used but not ATP is formed. It involves three organelles chloroplast, mitochondria and peroxisomes.
O2 is utilized as ribulose biphosphate is oxidized and a molecule of phosphoglycolic acid and a molecule of 3-phosphoglyceric acid are formed. This oxidation takes place under the influence of high light intensity. Ribulose diphosphate carboxylase, an enzyme, facilitates this reaction as it behaves as an oxygenase. only one molecule of oxygen is fixed here.
In Kranz anatomy the mesophyll is undifferentiated, and its cells occur in concentric layers around vascular bundles. Vascular bundles are surrounded by largely sized bundle sheath cells which are arranged in a wreath-like manner in one to several layers. C4 plants, both monocots, and dicots, such as sugarcane, maize, sorghum have Kranz anatomy leaf.
Crassulacean acid metabolism
CAM is the photosynthesis by the C4 pathway in which CO2 is taken up by plants during the night time. In night the plant’s stomata are open and fixed into malic acid.
During the day, when the stomata are closed carbon dioxide is released from malic acid for use in the Calvin cycle.
This is necessary for plants that live in arid conditions as it enables them to keep their stomata closed during the day time to reduce water loss from evaporation. Crassulacean acid metabolism is common in succulent plants of desert regions, including cacti and spurges, and in certain ferns.
The C4 plants are adapted to dry tropical regions and have a greater productivity of biomass. C4 plants have a different type of leaf anatomy known as Kranz anatomy.
In this type of anatomy the bundle sheath cells from several layers around the vascular bundles; they are characterized by having a large number of chloroplasts, which walls impervious to gaseous exchange and no intercellular spaces.
C4 plants are photosynthetically more efficient than C3 plants because the C4 plant contains two types of the chloroplast, i.e., bundle sheath chloroplast and mesophyll chloroplast. So such plants operate a dicarboxylic acid cycle in addition to Calvin cycle. CO2 acceptor molecule (PEP) is present in large bundle sheath cell which has higher efficiency in picking up CO2. Thus, photosynthesis continues even at low CO2 concentration, and the rate of photorespiration is also negligible.
CAM plants fix CO2 ar night, form malate which is stored in large vacuoles in mesophyll cells until next day. CAM plants use the enzyme pepco (PEP carboxylase). The malate formed at night release CO2 during the day to Calvin cycle within the same cell,.The cell now has NADPH and ATP available for light-dependent reactions.CAM represents Crassulacean acid metabolism.
The Crassulaceae is the family of flowering succulents. In CAM plants stomata open during the night only and is an adaption fo xerophytic succulents. Photosynthesis in such cases is usually not as efficient as in C3 or C4 plants. but however, it allows the CAM plants to live under stressful environment.
Cacti plants like Byrophyllum and some members of family Euphorbiaceae show CAM mechanism. These have basically the C4 pathway of C fixation. Such plants contain a large amount of organic acids like malic acids and oxalic acid. They fix carbon in the form of 4-C organic acids like C4 plants. But they fix CO2 at night and reduce CO2 via the Calvin cycle bu using NADPH2 form during the day. They keep their stomata closed during the daytime and open during the night for inflow of CO2. In this way, they reduce the rate of transpiration.Such plants have separated the process of CO2 fixation and electron transfer for reduction of organic acids. Thus, they can carry on one step during the day other during the night.
Energetic of the C4 and C3 Plants
In C4 plants the cost of concentrating CO2 within the bundle sheath cells is 2ATP per CO2.
Energetics of the C4 photosynthetic carbon cycle:-
PEP + H2O + NADPH → Malate + NADP+ + iP + CO2
Malate + NADP+ + iP → Pyruvate + NADPH + CO2
Pyruvate + iP + ATP → PEP + AMP + PiPP
iPP + H2O → 2iP
AMP + ATP → 2ADP
Net : CO2 (mesophyll) → CO2 (bundlesheath) + ATP + 2H2O + 2ADP + 2iP
In C4 plants, 12 ATP molecules are required for producing one hexose sugar. In the bundle sheath cells, C3 cycle operates which requires 18 ATP and 12 NADPH2 molecules are required in C4 cycle.
Whereas in C3 cycle 18 ATP nad 12 NADPH2 molecules are required.