Photorespiration, A competitive of photosynthesis | Is photorespiration a waste process? |

 Photorespiration
A competitive process of Photosynthesis (i.e dark reaction of photosynthesis)

Photorespiration is a competitive process of Photosynthesis (I.e second phase or dark reaction of photosynthesis), but it is said a waste process because the energy in the form of ATP is used and one carbon dioxide molecule is also moved out from the cell however one of the main reason of photorespiration is the shortage of carbon dioxide molecules, and no energy is formed in this process. 

But the question is, why the process of photorespiration is so bad and is it a waste process as it seems to be?

Well, it is not as easy as it seems. Photorespiration is the survival process that helps plants to survive in harsh conditions. Plants release hormones like abscisic acid (also known as Stress hormone) which induces the process of photorespiration in plants by closing their stomatas. It is mostly seen in C3-type plants and rarely seen in C4 and CAM plants.

 

So another question is, why photorespiration is common in C3 plants and rare in C4 and CAM plants?

The simplest answer to this question is that both C4 and CAM plants are mostly found in hot and dry conditions and so they have the mechanisms to deal with the problem of photorespiration. And C3 plants are not as adaptive as C4 and CAM plants.

First, we understand the process of photorespiration, then we will look at the mechanisms or adaptations that C4 and CAM plants have.

• The second phase of photosynthesis starts with the Calvin cycle in which RUBP ( a five-carbon compound) reacts with carbon dioxide that mostly comes from the atmosphere. 
• However, the carbon dioxide released during cellular respiration can also take part in the Calvin cycle but its amount is not enough for a plant’s needs. So mostly plants take atmospheric carbon dioxide through their stomata. 
• When carbon dioxide enters the plant’s cell then it moves towards RUBP or can said rubisco as a short name. 
• The Rubisco can act as carboxylase by binding carbon dioxide with it and oxygenase by binding oxygen with it. 
• When rubisco acts as carboxylase then it runs the Calvin cycle but when it acts as oxygenase then it starts the process of photorespiration.

 

Now, why and when rubisco binds oxygen with it, and when it binds carbon dioxide?

Various factors decide it. But when the concentration of carbon dioxide is high in plant cells then rubisco act as carboxylase and when the concentration of oxygen is high in plant cell then rubisco act as oxygenase. Photorespiration may be due to drought stress also because in drought conditions, plants close their stomata to save water from transpiration and this may lead to the deficiency of carbon dioxide in plants cells.

As we know that oxygen is produced during the process of photosynthesis (i.e. the light reaction of photosynthesis) and the atmosphere is the major source of carbon dioxide for plants.

So the concentration of oxygen is continuously increased in plants (during day time) by the process of light reaction of photosynthesis but what if the plant didn’t allow carbon dioxide to enter into the cell by closing their stomata? Then this may definately leads the process of photorespiration.

Now, why do plants close their stomata and block the way of entrance of carbon dioxide?

• The simplest reason is the external environment. We know that water transpired in the form of vapors through stomata. When a plant is deficient in water or a very hot atmosphere outside (as in the summers), then plants close their stomata to save water. 
• And when the stomata are closed, no carbon dioxide enters into the plant cell or very fewer numbers of molecules of carbon dioxide enter into the cell because the stomata never completely closed themselves, they close partially.

The result of this story highlights that there is not enough carbon dioxide present in the plant’s cell to run the Calvin cycle. 

When the shortage of carbon dioxide is continuous in plants while on the other side, oxygen is continuously produced in the plant’s cell because of photosynthesis (I.e light reaction), then the concentration of oxygen rises in the plant’s cell which leads to the process of photorespiration in plants.

 

We need to understand the carbon-oxygen balance in plants in normal conditions to clarify our concept.

• During the day, two different but very important processes take the plant in plants. One is photosynthesis and the second is respiration. 
• In the process of photosynthesis, carbon dioxide is used to make carbohydrates. Oxygen is released as a by-product of photosynthesis. 
• The carbohydrates (produced during photosynthesis) are stored and used for the sake of energy. This carbohydrates is then utilized in the process of respiration and this process needs oxygen and released carbon dioxide.
• Now, carbon dioxide is used and oxygen is released in photosynthesis while oxygen is used and carbon dioxide is released in respiration. 
• But more carbon dioxide is used in photosynthesis and more oxygen is released as well. 
• While on the other hand, the process of respiration used oxygen that is released in photosynthesis but this oxygen is very excess in quantity. So the remaining or unused molecules of oxygen are released into the atmosphere. 
• That’s why plants give oxygen to the atmosphere that we used to breathe. 
• While a very less quantity of carbon dioxide is released in the respiration process. So plant takes carbon dioxide to full fill their needs. 

This is the process that occurs in plants during daytime in normal conditions. But what happens during the night?

• At night, there is no sunlight and so no photosynthesis. 
• Then just respiration takes place during the night and as we know oxygen is used and carbon dioxide is released in respiration. So, plants take carbon dioxide and release oxygen during the night.

This is the process that occurs under normal conditions. 

Photorespiration is the phenomenon of harsh conditions when there is a shortage of water for plants or high light or temperature in the external environment. 

Now back to our previous question which is why photorespiration is rare in C4 and CAM plants and common in C3 plants?

• In C4 plants, the first stable product that is formed by carbon fixation is malate which is a four-carbon compound and that’s why it is called as C4 cycle. 
• This malate then enters into the bundle sheath cells where C3 or Calvin cycle runs and forms carbohydrates.

Now, why carbon dioxide did not enter directly into Bundle sheath cells to run the Calvin cycle?

The answer is that because of Kranz's anatomy. There is a layer around bundle sheath cells that block carbon dioxide and oxygen to enter the cell. 

• That’s why carbon dioxide is fixed into the mesophyll cells by an enzyme called PEP that fixed carbon dioxide into melic acid and then melate. 
• This malate enters into the bundle sheath cells where it releases one carbon dioxide molecule and moves out from the bundle sheath cell in the form of pyruvic acid or pyruvate.

In this way, photorespiration is controlled or inhibited by a continuous supply of carbon dioxide in the form of malate in C4 plants. 

• While in the case of CAM plants, carbon dioxide is fixed and stored during the nighttime and used in the daytime by opening stomata during the night and closing it during the day. 
• Both rubisco and PEP are present in CAM plants. 
• The most common example of a CAM plant is cactus which is found in deserts.

So photorespiration is a problem faced by C3 plants in the case of water loss or deficiency, high temperature, or sunlight. 

We discussed photorespiration, we discussed why it is a problem, we discuss why and when it occurs, we discussed how plants of deserts (CAM) and plants that grow in hot environments (C4) deal with this problem, we discussed why it is a problem for C3 plants. Now it's important to discuss and understand what is Photorespiration.

 

Photorespiration

The process of photorespiration takes place in three different organelles of cells named: 

1. Chloroplast
2. Peroxisome
3. Mitochondria 

• Photorespiration starts and ends within the same organelle i.e. chloroplast.
• CO2 and O2 compete with each other for reaction with ribulose-1,5-bisphosphate because carboxylation and oxygenation occur within the same active site of the enzyme.
• If equal concentrations of CO2 and O2 are present in a test tube, then rubisco of the angiosperm plant’s cell fixes CO2 about 80 times faster than oxygen. 
• But in unfavorable conditions, oxygen dominates over carbon dioxide.
• A molecule of O2 reacts with a ribulose-1,5 bisphosphate that generates an unstable intermediate that rapidly splits into 2-phosphoglycolate and 3-phosphoglycerate. 
• 3-phosphoglycerate did not participate in photorespiration.
• The 2-phosphoglycolate (that is formed in the chloroplast by oxygenation of ribulose-1,5-bisphosphate) is rapidly hydrolyzed to glycolate by a specific chloroplast’s enzyme phosphatase. 
• Then glycolate enters into the other organelle, the peroxisome.
• Glycolate leaves the chloroplast via a specific transporter protein in the envelope membrane and diffuses to the peroxisome.
• In peroxisome, glycolate is oxidized to glyoxylate and hydrogen peroxide (H2O2) by an enzyme glycolate oxidase
• The vast amounts of hydrogen peroxide released in the peroxisome are destroyed by the action of catalase.
• While the glyoxylate undergoes a transamination reaction.
• The amino donor for this transamination is probably glutamate, and the product is the amino acid glycine.
• Glycine leaves the peroxisome and enters the mitochondria.
• In mitochondria, glycine is converted into serine by losing a carbon dioxide molecule. 
• This conversion takes place with the help of a multienzyme complex. 
• An ammonium ion is also released which then moved from the matrix of mitochondria to chloroplasts, where glutamine synthetase combines it with carbon skeletons to form amino acids.

This multienzyme complex, present in large concentrations in the matrix of plant mitochondria, comprises four proteins, named: 

1. H-protein (a lipoamide-containing polypeptide) 
2. P-protein (a 200 kDa, homodimer, pyridoxal phosphate-containing protein)
3. T-protein (a folate-dependent protein)
4. L-protein (a flavin adenine nucleotide–containing protein)

(The point to be noted here is that 2-phosphoglycolate is a two-carbon-containing molecule and serine is a three-carbon compound. So two molecules of phosphoglycolate (having four total carbons) lose carbon dioxide and form a three-carbon compound serine).

• The newly formed serine leaves the mitochondria and enters the peroxisome, where it is converted first to hydroxy pyruvate and then to glycerate.
• Finally, glycerate reenters the chloroplast, where it is phosphorylated to yield 3-phosphoglycerate.
• In this way, two molecules of 3-phosphoglycerate enter into the Calvin cycle and produced carbohydrates, and regenerate the RUBP also.

Cycles of Photorespiration. 

In photorespiration, various compounds are circulated in concert through two cycles

1. First cycle, carbon exits the chloroplast in two molecules of glycolate (two-carbon compound) and returns in one molecule of glycerate (three-carbon compound). and one carbon is lost in mitochondria during serine formation.
2. In the second cycle, nitrogen exits the chloroplast in one molecule of glutamate and returns in one molecule of ammonia (together with one molecule of á-ketoglutarate)

 

Summary.

• Two molecules of phosphoglycolate (four carbon atoms), lost from the Calvin cycle by the oxygenation of RuBP, are converted into one molecule of 3-phosphoglycerate (three carbon atoms) and one CO2.
• In other words, 75% of the carbon lost by the oxygenation of ribulose-1,5-bisphosphate is recovered by the C2 oxidative photosynthetic carbon cycle (C2 cycle or photorespiration) and returned to the Calvin cycle.
• On the other hand, the total organic nitrogen remains unchanged because the formation of inorganic nitrogen (NH4+) in the mitochondrion is balanced by the synthesis of glutamine in the chloroplast

 

The main reactions of the photorespiratory cycle. Operation of the C2 oxidative photosynthetic cycle involves the cooperative interaction among three organelles: 

• Chloroplast 
• Mitochondria
• Peroxisome 

1.  Chloroplast to Peroxisome: Two molecules of glycolate (four carbons) are transported from the chloroplast into the peroxisome where they are converted to glycine.
2. Peroxisome to Mitochondria: Glycine is then exported to the mitochondrion and transformed into serine (three carbons) with the loss of carbon dioxide (one carbon). 
3. Mitochondria to Peroxisome: Serine is transported to the peroxisome and transformed into glycerate (three carbons). 
4. Peroxisome to Chloroplast: The latter flows to the chloroplast where it is phosphorylated to 3-phosphoglycerate and incorporated into the Calvin cycle.

 Is photorespiration a waste process?

Photorespiration occurs: 

1. When rubisco acts as an oxygenase by binding oxygen with it.
2. Rubisco binds oxygen when its concentration is more than carbon dioxide in the cell.
3. The concentration of oxygen rises due to the closure of the stomata.
4. Stomata close when there is a very hot temperature outside or an excessive rate of transpiration through leaves.

These all factors are related to each other that represent an unfavorable condition for plants.

• In normal conditions (i.e. when rubisco act as carboxylase), rubisco plus carbon dioxide produces two molecules of 3-phosphoglycerate (having 6 carbons) which then regenerates the rubisco and forms a carbohydrate molecule as well. When oxygen binds with rubisco then only one molecule of 3-phosphoglycerate is produced with a molecule of 2-phosphoglycolate. These two molecules can’t regenerate the rubisco and can’t form the carbohydrate as well. So the process of photorespiration occurs to produce a 3-phosphoglycerate molecule which then enters into the Calvin cycle to regenerate the rubisco and form a carbohydrate as well.
• We can’t compare C2 (photorespiration) and C3 (Calvin cycle) cycle because both occur in different conditions and both have their different processes as well.

So the summary of our’s topic is that photorespiration has its own values however it used ATP and loose carbon dioxide molecules but it can’t be said a waste process because it has great importance for plants (especially C3 plants).