Process and Significance Of Photorespiration Notes

Photorespiration Or C₂-Cycle

Photorespiration is the light-dependent oxidation of intermediates of carbon assimilation which is accompanied by absorption of O₂ and release of CO₂. It was discovered by Dicker and Tio (1959) in a tobacco plant. The. the term ‘photorespiration’ was coined by scientist Gleb Krotkov.

Definition: The oxidation process in plants, that takes place in bright light and high O₂ concentration, producing 2C compound (phosphoglycolic acid) and CO₂ is called photorespiration.

Site of occurrence: This cycle takes place in chloroplasts, peroxisomes and mitochondria.

It is also called the glycolate cycle or C₂ because, the 2C compound, glycolate, is produced as the first intermediate product of this metabolic pathway.

“what is photorespiration “

Mechanism Of Photorespiration

Photorespiration involves the initial fixation of O₂ followed by further O₂ uptakes and CO₂ evolution.

Reactions occurring in chloroplast (first step)

Reactions of photorespiration in chloroplast take place in two steps.

RuBP cleavage, synthesis of phosphoglycolate and its oxidation: The C₂ cycle starts in chloroplasts with phosphoglycolate produced from RuBP due to the oxygenase action of RuBisCO. It converts RuBP to 3-phosphoglycerate and 2-phosphoglycolate.

⇒ \(\text { RuBP }+\mathrm{O}_2 \stackrel{\text { RuBisCO }}{\longrightarrow} \text { 3-PGA + 2-Phosphoglycolate }\)

Conversion of phosphoglycolate to glycolate: 3-phosphoglycerate enters the Calvin cycle. On the other hand, 2-phosphoglycolate undergoes dephosphorylation by phosphoglycolate phosphatase enzyme to form glycolate. This glycolate leaves the chloroplast and enters the peroxisome.

⇒ \(\text { 2-Phosphoglycolate }+\mathrm{H}_2 \mathrm{O} \stackrel{\begin{array}{c} \text { Phosphoglycolate } \\ \text { phosphatase } \end{array}}{\longrightarrow} \text { Glycolate }+\mathrm{Pi}\)

Reactions occurring in peroxisome (second step)

Reactions of photorespiration in peroxisome takes place in three steps. Those steps are discussed below along with their chemical reactions.

Conversion of glycolate to glyoxylate: Glycolate enters peroxisome where it is oxidised to glyoxylate and H202 by enzyme glycolate oxidase and sunlight.

Hydrolysis of H₂O₂: The other product of glycolate oxidation is H₂O₂ which is decomposed to H₂O and O₂. This reaction is catalyzed by the enzyme catalase present in the peroxisome.

⇒ \(\begin{aligned} & \text { Glycolate }+\mathrm{O}_2 \underset{\substack{\text { Glycolate } \\ \text { reductase }}}{\stackrel{\text { Glycolate }}{\rightleftharpoons}} \text { Glyoxylate }+\mathrm{H}_2 \mathrm{O}_2 \\ & 2 \mathrm{H}_2 \mathrm{O}_2 \stackrel{\text { Catalase }}{\longrightarrow} \mathrm{O}_2+\mathrm{H}_2 \mathrm{O} \\ & \end{aligned}\)

It is possible that some of the glyoxylates may return to the chloroplast. There it may get reduced back to glycolate at the expense of photogenerated NADPH by glyoxylate reductase enzyme. Such shuttle reactions, involving glycolate-glyoxylate, are used to dissipate the light-generated reducing power. This is useful in protecting the photosystems when CO₂ supply is limited. Photorespiration is observed in C₃ plants which photosynthetically fix CO₂ exclusively via the Calvin cycle in the mesophyll cells.

Synthesis of glycine: Glyoxylate reacts with glutamate in the presence of glutamate-glyoxylate aminotransferase enzyme to produce glycine (amino acid) and a-ketoglutarate.

⇒ \(\text { Glyoxylate + Glutamate } \stackrel{\begin{array}{c} \text { Glutamate } \\ \text { glyoxylate } \\ \text { aminotransferase } \end{array}}{\longrightarrow} \alpha \text {-Ketoglutarate }\)

Reactions occurring in mitochondria

Glycine enters the mitochondrion from the peroxisome and the cycle moves further.

Synthesis of serine, NH3 and CO₂: The glycine then moves to mitochondrion. Two molecules of glycine are converted to one molecule each of serine, CO₂ and NH₃ in a two-step reaction. The reaction requires NAD⁺ as an oxidant and the resultant NADH is reoxidised by the mitochondrial electron transport chain with the generation of ATP. These reactions are catalysed by glycine decarboxylase and serine hydroxymethyl transferase respectively.

” product of photorespiration”

Reactions occurring in peroxisome (last step)

Serine moves back to peroxisome and gets converted to glycerate.

Conversion of serine to hydroxy pyruvate: After being synthesised in mitochondria, serine moves to peroxisome. Here it reacts with or-ketoglutarate to produce glutamate and hydroxypyruvate. This is catalysed by the enzyme glyoxylate aminotransferase.

Reduction of hydroxypyruvate and synthesis of glycerate: Hydroxypyruvate then undergoes reduction to produce glycerate. This reaction is catalysed by the enzyme hydroxy pyruvate reductase.

Reactions occurring in chloroplast

The last step involved moving glycerate into the chloroplast.

Phosphorylation of glycerate: The glycerate then moves into the chloroplast where it is phosphorylated to form 3-phosphoglycerate. This reaction is catalysed by the enzyme glycerate kinase.

“photorespiration pathway “

3-phosphoglycerate now enters the C₃ cycle where it is used for RuBP synthesis. Thus, the photorespiration or C₂ cycle is completed.

Effect of O₂ on photorespiration

When the concentration of O₂ is higher, RuBP carboxylase causes RuBP to bind to O₂ instead of CO₂. This leads to the production of phosphoglycolate, which reduces the rate of carbon assimilation during photosynthesis. The concentration of O₂ in the environment is about 21%, which is maintained by photosynthesis.

But this O₂ content becomes harmful for the C₃ plants, as more and more RuBP will bind O₂, with no RuBP left for binding CO₂. This reduces photosynthesis further. No such carboxylase has been discovered yet, that does not have any affinity to O₂.

Significance Of Photorespiration

1. Photorespiration regenerates CO₂ and PGA which are ultimately used up in the Calvin cycle.
2. It produces amino acids and carbohydrates and maintains CO₂ balance in nature.
3. Photorespiration serves to protect the photochemical apparatus from damage caused by light. This takes place through neutralisation of harmful effects of otherwise damaging products of light reaction. These products tend to accumulate when a low CO₂ concentration limits the progress of the Calvin cycle.
4. Since no ATP is produced in photorespiration, it is not considered true respiration. Instead, ATP is used up during this process along with NADH⁺H⁺.

“significance of photorespiration “

C₃ And C₄ Cycle And Pathway

According to the number of carbon atoms in the intermediate product, the pathway of the Calvin cycle may be of three types—C₃, C₄ and CAM.

C₃ Cycle or Pathway

Definition: The biochemical pathway, within the dark phase, during which carbon is assimilated and phosphoglyceric acid (3C) is produced as the first stable product is called C₃ pathway or Calvin cycle.

Site of occurrence: Stroma of the chloroplast.

Characteristics of C₃ plants:

1. The plants which show the C₃ cycle are called C₃ plants. In most of the plants, it takes place using the RuBisCO enzyme.
2. The first stable compound obtained is the 3-carbon compound phosphoglyceric acid (PGA).
3. RuBP binds CO₂ in the atmosphere and thereby maintains the balance of O₂-CO₂ within the environment.
4. All the compounds produced within the C₃ cycle can be re-synthesised.
5. During hot and dry summers, the stomata of C₃ plants remain closed. Hence, CO₂ cannot enter the plants. So, the C₃ cycle slows down and glucose production is inhibited temporarily. example Paddy, wheat, soybean, etc.

C₄ Cycle Or Pathway

Definition: The pathway of photosynthesis in which carbon assimilation takes place and oxaloacetic acid (4C) is produced as the first stable product is called the C₄ pathway.

Site of occurrence: Cells (mainly chloroplasts) in mesophyll tissue and bundle sheath.

Types of chloroplast involved in the C₄ cycle: Chloroplasts are of two types—

Mesophyll chloroplast (MC): Chloroplast present in the mesophyll cells.

Bundle sheath chloroplast (BSC): Chloroplast present in the bundle sheath cells.

Characteristics of C₄ plants:

1. The plants which show C₄ cycle are called C₄ plants. Most of the C₄ species are monocots, especially grasses, although more than 300 are dicots.
2. They are generally found in tropical and subtropical regions.
3. They have more bundle sheath cells.
4. Only spongy parenchyma is present in mesophyll cells.
5. The initial products of C₂ fixations are the 4-carbon dicarboxylic acids—oxalate, malate and aspartate. Hence the pathway is known as C₄ pathway. The first stable compound formed in this pathway is a 4-carbon compound, oxaloacetic acid (OAA).
6. The rate of transpiration is more than C₃ plants.
7. Their ability to photosynthesise is high, as compared to C₃ plants.
8. Generally, photorespiration is absent in these plants.
9. Photosynthesis continues even in bright sunlight, water stress and high temperature.
10. The presence of a prominent layer of bundle sheath cells containing chloroplasts, around the vascular tissue of the leaf, is the feature of C₄ plants. This feature is called Kranz anatomy.
11. The stroma in the chloroplasts within bundle sheath cells is more organised than the grana.
12. The stroma of chloroplast in mesophyll cells is less organised than the grana. Moreover, there are differences in the ultrastructures of chloroplasts between mesophyll cells and bundle sheath cells. example Sugarcane, maize, jowar, bajra, etc.

“photorespiration diagram “

C₄ plants and photorespiration

C₄ cycle effectively pumps CO₂ from the atmosphere into the bundle sheath cells. This transport process generates a much higher concentration of CO₂ in the bundle sheath cells than would occur in equilibrium with the external atmosphere. This elevated concentration of CO₂ at the site of carboxylation of RuBP, results in suppression of the oxygenation of RuBP. Hence, photorespiration is prevented.

Description of C₄ Cycle: C₄ cycle consists of four stages—

1. Fixation of CO₂ by the carboxylation of phosphoenolpyruvate (PEP) in the mesophyll cells to form 4-carbon acid.
2. Transport of the 4C acid to the bundle sheath cells.
3. Decarboxylation of the 4C acid within the bundle sheath cells and generation of CO₂, which is then reduced to carbohydrates via the Calvin cycle.
4. Transport of the 3C acid (pyruvate or alanine). that is formed by the decarboxylation, back to the mesophyll cells for regeneration of CO₂ acceptor, PEP.

Reactions in mesophyll cells

1. Carbon dioxide present in the air enters the mesophyll cells and reacts with water to form carbonic acid. This reaction is catalysed by the enzyme carbonic anhydrase.

⇒ \(\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \stackrel{\text { Carbonic anhydrase }}{\longrightarrow} \mathrm{H}^{+}+\mathrm{HCO}_3^{-}\)

The primary carboxylation of the C₄ cycle is done by phosphoenol pyruvate carboxylase (PEP carboxylase) using HCO_3^– as the substrate to yield oxaloacetate (OAA). PEP carboxylase is found in the cytosol of mesophyll cells. It is activated by Mg2+ and inhibited by malate and aspartate feedback control. OAA (4C) is the first stable compound formed in C₄ cycle.

“photorespiration definition “

⇒ \(\text { PEP } \underset{\text { dehydrogenase }}{\stackrel{\text { Malate }}{\longrightarrow}} \mathrm{OAA}+\mathrm{Pi}\)

OAA formed now enters the mesophyll cells. It is reduced to malate in the chloroplast at the expense of NADPH by the enzyme malate dehydrogenase. In the case of some C₄ plants, aspartate is generated from OAA.

⇒ \(\text { OAA } \underset{\text { dehydrogenase }}{\stackrel{\text { Malate }}{\longrightarrow} \text { Malate + NADP }}{ }^{+}\)

Malate so formed is then exported to the bundle sheath cell chloroplast.

Reactions in bundle sheath cells

1. CO₂ removal and decarboxylation:

1. Malate dehydrogenase present in bundle sheath cells, acts upon malate to produce pyruvate and release CO₂.
2. Pyruvate so formed is transported to mesophyll cells,
3. The released CO₂ reacts with RuBP to form PGA, which enters the C₃ cycle. This is catalysed by the enzyme RuBP carboxylase. Instead of malate, if aspartate is generated, it is catalysed by PEP carboxykinase.

⇒ \(Malic acid +\mathrm{NADPH}+\mathrm{H}^{+} \underset{\text { dehydrogenase }}{\longrightarrow} Pyruvic acid + NADP\)

⇒ \(\begin{aligned} & \text { Aspartate }+\alpha \text {-ketoglutaric acid } \frac{\text { Iransaminase }}{\text { OAA }+ \text { Glutamic acid }} \\ & \mathrm{OAA}+\mathrm{ATP} \stackrel{\text { Carboxykinase }}{\longrightarrow} \mathrm{PEP}+\mathrm{CO}_2+\mathrm{ADP} \\ & \text { PEP } \stackrel{\begin{array}{c} \text { Pyruvate } \\ \text { kinase } \end{array}}{\longrightarrow} \text { Pyruvate } \stackrel{\begin{array}{c} \text { Alanine } \\ \text { transferase } \end{array}}{\longrightarrow} \text { Alanine } \\ & \end{aligned}\)

Types of C₄ plants

There are three subgroups according to the different mechanisms by which decarboxylation takes place in bundle sheath cells.

1. NADP*-malic enzyme (NAD⁺-ME) type: In this type, malic acid is formed from oxaloacetic acid in the presence of NADPH⁺H⁺. This is catalysed by the enzyme malate dehydrogenase. Malate is then transported to bundle sheath cells from the mesophyll cell. Since NADP-dependent malate dehydrogenase is the main enzyme involved, hence the name NADP⁺-ME type.
2. PEP-carboxykinase (PEP-CK or PCK) type: In this type, OAA is directly decarboxylated by the enzyme PEP-carboxykinase in bundle sheath cell chloroplast. This provides for assimilation through a cycle. The formed PEP is converted by pyruvate kinase to pyruvate, which is then converted to alanine by alanine aminotransferase. This alanine is returned to the mesophyll cell. Since PEP-carboxykinase is the main enzyme involved, hence the name PEP-carboxykinase type.
3. NAD+-malic enzyme (NAD⁺+-ME) type: In this type, transfer of aspartate and return of alanine takes place. In this case, malate is directly decarboxylated to form by NAD-dependent malic enzyme, hence the name NAD⁺+-ME-type. Pyruvate moving from mitochondria is then converted to alanine in cytoplasm by alanine aminotransferase.

Re-formation of PEP: The pyruvate sent to the chloroplast of mesophyll cells, is converted to PEP at the expense of ATP by a unique enzyme named pyruvate orthophosphate dikinase. Alanine that enters the mesophyll cells also gets converted to PEP.

This is the final step of the C₄ cycle.

Significance of C₄ cycle:

1. The rate of photosynthesis is higher than C₃ plants. Hence, they can produce more glucose than C₃ plants.
2. C₄ plants are partially adapted to drought conditions, where a high rate of CO₂ fixation is maintained even with almost closed stomata. Hence, they can grow and produce more seeds than C₃ plants.
3. The C4 photosynthesis is more efficient at high temperatures. Such high-temperature tolerance of C₄ plants is due to the stability of some enzymes like PEP-carboxylase.
4. Oxygen has no inhibitory effect on C₄ photosynthesis because PEPcase is insensitive to oxygen and photorespiration is absent.
5. C₄ plants have low CO₂ compensation points. They can rapidly take up CO₂ even at reduced C02 levels, with almost closed stomata and thus they can conserve water.
6. The C₄ pathway is commonly found in tropical plants, which are normally exposed to abundant sunlight. This pathway supports a higher rate of photosynthesis and growth in these plants.
7. Photorespiration is not observed in C₄ plants.
8. An adequate amount of nitrogen assimilation enzymes and an efficient capacity to use nitrogen for biomass production are additional features associated with C₄ pathway.