Photorespiration

The frequency with which each substrate (CO₂ or O₂) is chosen by RuBisCO during the first step of the Calvin cycle depends on three environmental factors: temperature, moisture content, and the relative concentrations of O₂ and CO₂.

1. Temperature

Increasing temperature also increases oxygenation in two ways. RuBisCO losses affinity for CO₂ as the temperature rises such that it performs oxygenation.

The second possible way temperature affects oxygenation is by its effect on the relative solubilities of O₂ and CO₂. Both the gases must dissolve in the water of the leaf before they can react with RuBisCO. With the increase in temperature, the solubility of any gas in water decreases. There is comparably less O₂ and CO₂ dissolved in cells at a higher temperature. However, the solubility of O₂ decreases less with temperature than CO₂. Thus, as the temperature rises, the CO₂ to O₂ ratio in the cell decreases, which starts favoring the oxygenation of RuBisCO.

2. Moisture Content

As the moisture content in the surroundings decreases, plants close their stomata. Inside the leaf, O₂ is produced by Photosystem II and starts to increase its concentration. At the same time, CO₂ is consumed by carboxylation and thus decreases in concentration. Consequently, CO₂ to O₂ ratio inside the leaf decreases when stomata are closed, which promotes oxygenation of RuBisCO. Thus, hot and dry conditions stimulate RuBisCO to perform photorespiration in plants by increasing its affinity for O₂.

3. Relative Concentrations of O₂ and CO₂

From the above two factors, it is evident that high O₂ to CO₂ concentration favors the oxygenation of RuBisCO.

When does Photorespiration Occur

Photorespiration begins in the chloroplast of a C3 plant when RuBisCO uses O₂ and RuBP as substrates instead of CO₂ producing one molecule of 3PG and one molecule of 2-phosphoglycolate (2PG). The equation is given below:

Equation: RuBP + O₂ → 2-Phosphoglycolate (2PG) + 3-phosphoglycerate (3PG) + 2H⁺

The 2PG is inhibitory to the Calvin cycle enzymes and thus needs to be removed and recycled back to 3PG to prevent depletion of Calvin cycle intermediates. This recycling is complex and involves 3 different organelles – chloroplast, peroxisome, and mitochondria.

Steps of Photorespiration

1. The 2PG formed in the chloroplast by the oxygenase activity of RuBisCO is first dephosphorylated to glycolate.
2. Glycolate is then transported from the chloroplast to the peroxisome. It is then metabolized to glyoxylate by the enzyme glycolate oxidase. This reaction consumes O₂ and generates a toxic molecule, H₂O₂, later detoxified by the enzyme catalase, regenerating O₂.
3. Then, the two glyoxylate molecules are converted to glycine in a transamination reaction with glutamate acting as amino (NH₂) donor for one glycine and serine as an amino donor for the second glycine.
4. Glycine formed is transported to the mitochondrion, where one glycine molecule is catabolized to ammonium (NH₄⁺) and CO₂. This reaction catabolized by the enzyme glycine decarboxylase complex adds the remaining glycine (=CH₂) to a second glycine molecule to form serine.
5. Serine formed in the mitochondria is then transported to the peroxisome. It then undergoes transamination, with the amino portion serving as an amino donor for the second glycine and the remaining carbon forming hydroxypyruvate.
6. Hydroxypyruvate is then reduced to glycerate, exported from the peroxisome to the chloroplast. They are then phosphorylated to form 3-phosphoglycerate, which can reenter the Calvin cycle.

Some of the CO₂ released (4^th step) in the mitochondria enters the chloroplast through the cytosol, where they have access to RuBisCO to participate in the Calvin cycle. The ammonia (as NH₄⁺) concomitantly released with CO₂ in the glycine decarboxylase reaction is then transported from the mitochondrion to the chloroplast, where it is assimilated using the enzyme glutamine synthetase.

This re-assimilation of ammonium consumes both ATP and reduced ferredoxin. When glutamate serves as an amino donor in glycine synthesis, 2-oxoglutarate is regenerated. Glutamate and 2-oxoglutarate are rapidly exchanged between peroxisome and chloroplast to keep the pathway running.

A disadvantage of photorespiration is that it is bad for plants. Photorespiration is thus a wasteful pathway because it significantly lowers the efficiency of photosynthesis and thus decreases the productivity of plants like wheat and soybean. It does so by decreasing the efficiency of photosynthesis by preventing them from using ATP and NADPH during the carbohydrate synthesis. It also results in a loss of 3 fixed carbon atoms.

Many theories are proposed to explain the significance and importance of photorespiration in plants. One possibility is that when plants first evolved, primitive earth had little oxygen. Thus, the inability to distinguish between O₂ and CO₂ was natural. As the oxygen level in the atmosphere gradually increased, the formation of glycolate began to occur, thus causing photorespiration.

An alternative theory about why plants need photorespiration is that it has some benefits in plants. It shields plants from the harmful effects of very high internal oxygen concentrations or high-energy phosphate molecules like ATP. The high concentrations of these substances occur when plants perform photosynthesis at a higher rate than usual. It is assumed that photorespiration consumes those oxygen and ATP molecules to a level that does not harm the plants.  

Certain plants species and algae can escape the worst effects of photorespiration. Accordingly, there is two such adaptation arising by natural selection – C4 and CAM pathways. These mechanisms reduce uptake of molecular oxygen by RuBisCO and are called Carbon Concentration Mechanisms.

How Do C4 Plants Minimize Photorespiration

In C4-plants, the light-dependent reaction and the Calvin cycle are physically separated. While the light-dependent reaction occurs in the mesophyll cells (spongy tissue in the middle of the leaf), the Calvin cycle occurs in the bundle sheath cells (special cells around the leaf veins).

The initial step of the light-dependent reaction involves the fixation of carbon as CO₂ in the mesophyll cells to form a 4-carbon organic acid (oxaloacetate). This step is catalyzed by PEP carboxylase, a non-rubisco enzyme. Oxaloacetate is converted to malate, which is then transported to the bundle sheath cells. Inside the bundle sheath cells, malate breaks down by releasing a molecule of CO₂. Finally, the CO₂ is fixed by RuBisCO and made into sugar by the Calvin cycle, similar to C3 photosynthesis.

This pathway requires ATP expenditure. However, since the mesophyll cells constantly pump CO₂ into bundle sheath cells in the form of malate, there is always a high concentration of CO₂ than O₂ around RuBisCO, thus suppressing photorespiration.

How Do CAM Plants Minimize Photorespiration

Plants adapted to dry environments such as cacti and pineapples minimize photorespiration by this pathway. The name ‘CAM’ was derived from the family of plants – the Crassulaceae, in which scientists first discovered the pathway.

Here, instead of physically separating the light-dependent reaction and the Calvin cycle, CAM plants separate the two processes on time. During the night, plants open their stomata, allowing CO₂ to diffuse into the leaves. The CO₂ is fixed into oxaloacetate by the enzyme PEP carboxylase and then to malate or another type of organic acid. The organic acid formed is stored inside vacuoles until the next day. During the daytime, the CAM plants do not open their stomata but perform photosynthesis. The organic acids are exported out of the vacuole. They are subsequently broken down to release CO₂, entering the Calvin cycle pathway. This controlled release of CO₂ maintains a high concentration of CO₂ around RuBisCO, thus helping to suppress photorespiration. This pathway, similar to C4-plants, requires ATP at multiple steps and thus cannot work without energy.

Differences

Photorespiration and photosynthesis are two processes that occur in plants. Photorespiration is a side reaction of the Calvin cycle. The enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted. In contrast, photosynthesis is the process by which autotrophs produce carbohydrates with the help of sunlight, CO₂, and H₂O.

The key differences between the two processes are given below:

Similarities

• Occurs in plants during the production of glucose
• Uses the enzyme RuBisCO
• Occurs within cellular organelles
• Requires enzymes
• Undergo light reaction

The key differences between photorespiration and dark respiration are given below: