Photosynthesis in Higher Plants Test - 20

• Peroxisomes lie between chloroplasts and mitochondria in the plant cell and serve to pass the 2-carbon products of oxygenation on for further metabolism.
• In the chloroplast, the phosphoglycolate is dephosphorylated. Glycolate is transported to the peroxisome, where molecular oxygen further oxidizes it into glyoxylate. 
• The product is hydrogen peroxide, which is rapidly broken down by catalase to water and oxygen. The glyoxylate is amidated to the amino acid glycine in the peroxisome.

• The process of photorespiration occurs in plants when the concentration of the oxygen increases and the Ribulose bisphosphate Carboxylase enzyme combines with oxygen, instead of carbon dioxide and functions as an oxygenase enzyme. 
• This process is considered wasteful for plants because it deviates the C$$_{3}$$ cycle. This process needs three organelles, chloroplast, peroxisomes, and mitochondria to be completed. 
• It occurs in the chloroplast where oxygen is utilized by combining with RuBiSCO and glycolate is formed. This glycolate then moves to the peroxisomes and glycolate is converted to glyoxylate using oxygen.
• Glyoxylate then moves to the mitochondria and glycine is formed. From glycine, serine is formed in mitochondria which goes to peroxisomes again where the hydroxypyruvate and glycerate are formed and the glycerate finally arrives back to the chloroplast to take part in glucose formation. So, photorespiration is a deviation of  C$$_{3}$$ cycle. 
• The oxygen-consuming reactions have occurred in the stroma of chloroplast and peroxisomes.

CO$$_2$$ concentration of the atmosphere is 360 ppm. It is a limiting factor for C$$_3$$ as the available CO$$_2$$ concentration is lower than the optimum for photosynthesis. Increase in its concentration upto 0.05% increases the rate of photosynthesis in most C$$_3$$ plants. When CO$$_2$$ concentration is reduced, there comes a point at which illuminated plant parts stop absorbing CO$$_2$$ from their environment. It is called as CO$$_2$$ compensation point or threshold value. The C$$_4$$ plants have this compensation value low as they are more efficient in CO$$_2$$ fixation through PEP- Carboxylase. The optimum CO$$_2$$ concentration for C$$_4$$ plants is 360 ppm. But the atmospheric conc of CO$$_2$$ is increasing. If the trend of CO$$_2$$ increase continues then that would be harmful for C$$_4$$ plants. 

The primary carbon dioxide acceptor in $$C_4$$ plants is phosphoenol pyruvate (PEP), which is a 3 carbon compound. The first stable compound is a 4 carbon compound, oxaloacetic acid.

Hatch and Slack pathway is the other name for $$C_4$$ cycle, named after the discoverers. $$C_4$$ plants have a characteristic feature called as Kranz anatomy in mesophyll. They have small chloroplast in mesophyll, whereas agranal chloroplast in bundle sheath.

C$${_4}$$ is the alternative pathway of Calvin cycle (C$${_3}$$ cycle) taking place during the dark phase of photosynthesis. In the C$${_4}$$ cycle the first stable compound is 4 C compound called as oxaloacetic acid. So, it is called as C$${_4}$$ cycle. The primary CO$${_2}$$acceptor is PEP, a 3 carbon compound. Two CO$${_2}$$ fixation occur. CO$${_2}$$ fixation is fast and more efficient. Fixation of 1 molecule of CO$${_2}$$ requires 5 ATP and 3 NADH. This cycle can operate in low CO$${_2}$$ concentrations. Also, O$${_2}$$ has no inhibitory effect on photosynthesis.

Phosphoenol pyruvate is formed from the decarboxylation of oxloacetate and hydrolysis of one guanosine triphosphate molecule. This reaction is catalyzed by the enzyme phosphoenolpyruvate carboxylase. This reaction is a rate limiting step in gluconeogenesis.

GTP + Oxaloacetate -----> GDP + Phosphoenolpyruvate + $$CO_2$$.

Examples of $$C_3$$ plants: beans, rice, wheat, potatoes, most temperate crops and all woody trees.
Examples for $$C_4$$ plants: corn, sugarcane, amaranth, mostly grasses, but some shrubs too, which are cold tolerant.

$$C_4$$ cycle has two carboxylations in it. In C$$C_4$$ plants, after entering through stomata, CO₂ diffuses into a mesophyll cell. Being close to the leaf surface, these cells are exposed to high levels of O₂, but have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle). Instead, the CO₂ is inserted into a 3-carbon compound (C₃) called phosphoenolpyruvic acid forming the 4-carbon compound oxaloacetic acid. Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is transported into bundle sheath cells. Here the 4-carbon compound is broken down into carbon dioxide, which enters the Calvin cycle to form sugars and starch. Pyruvic acid is transported back to a mesophyll cell where it is converted back into PEP.

The first reaction in photorespiration is oxygenation. In this step, RuBP is converted into one molecule of 3PGA and one molecule of 2PG. This reaction is catalyzed by enzyme RuBisCO. This reaction is affected by environmental factors such as temperature, carbon dioxide, water availability. The next step of photorespiration is processing of 2 phosphoglycolate.