Photorespiration: Definition, Diagram, Process, Cycle, Topics, Examples

What is Photorespiration?

Photorespiration is considered to be one of the key metabolic pathways of plants, which links in with the Calvin Cycle of photosynthesis. It is thought to be an energy-wasting route, as it brings down the net efficiency of photosynthesis with the consumption of energy and the evolution of carbon dioxide. Photorespiration is a very important process in understanding how plants adapt to different environmental settings, particularly in terms of carbon fixation.

Conditions Favouring Photorespiration

Some conditions favour photorespiration, such as:

• Hot and dry conditions cause closing of the stomata to prevent the excessive loss of water.

• The concentration of carbon dioxide internally becomes less than the threshold of 5% or 50 ppm.

• The concentration of oxygen rises, RuBisCO starts fixing the oxygen.

• Hence, photorespiration is prominent in tropical consumption, lower photosynthetic yield.

Mechanism of Photorespiration

Photorespiration comprises a few significant steps that add up to the entire process. These are:

Step 1 – Oxygenation (Chloroplast)

At low carbon dioxide, RuBisCO catalyzes the reaction of oxygen with ribulose bisphosphate, RuBP, to produce one molecule of 3-phosphoglycerate, PGA, and one molecule of phosphoglycolate. To prevent the toxic accumulation of phosphoglycolate, it gets converted into glycolic acid by the plant. This occurs rather rapidly to avoid any possible harmful effects.

Step 2 – Conversion In Peroxisomes

Transport of glycolic acid to the peroxisomes for further conversion to glycine, a 2-carbon amino acid. This step is important in detoxifying the plant and making it ready for further metabolism.

Step 3 – Conversion In Mitochondria

Glycine is again transported to mitochondria where it undergoes a conversion into serine, a 3-carbon amino acid. This process is energy-consuming with carbon dioxide being formed as an end-product.

Step 4 – Return To Calvin Cycle (Chloroplast)

The serine converts back to glycerate which re-enters into the Calvin cycle as 3-phosphoglycerate (3-PGA). However, one carbon atom is lost as carbon dioxide during this cycle causing inefficiency.

Impact Of Photorespiration On Plants

Photorespiration has various influences on the metabolism of plants:

Reduced Photosynthetic Efficiency

The energy used to convert the phosphoglycolate to serine is not returned for sugar production, so there is a net loss of carbon. Because of this inefficiency, it may limit the growth and productivity of plants.

Energy Loss

Photorespiration requires ATP and NADPH, the same substrates needed for photosynthesis. This depletes supplies for photosynthesis, and further impacts the general capacity of plants to capture energy efficiently.

Protective Role

Under certain conditions, photorespiration may serve as an energy sink, thus protecting plants from oxidative stress. Under high light intensity or conditions of drought, this protective mechanism may be of value.

Relation Between Photosynthesis And Photorespiration

Photosynthesis and photorespiration can occur in a plant at the same time. During photosynthesis, oxygen is a by-product, but in photorespiration, carbon dioxide is produced. The gases formed in these processes are somewhat interrelated because the oxygen formed during photosynthesis may increase photorespiration when the concentration of carbon dioxide is low.

Adaptations To Reduce Photorespiration

To avoid or reduce the losses through photorespiration, some plants have evolved with alternate mechanisms of carbon fixation :

C4 Pathway

Plants like maize and sugarcane follow the C4 pathway that have evolved a means of concentrating carbon dioxide in the bundle sheath cells. This minimises the chance of photorespiration. They fix carbon dioxide into a 4-carbon compound—first oxaloacetic acid—before it enters the light-independent reactions.

CAM Pathway

CAM plants (Crassulacean Acid Metabolism) such as cacti and succulents assimilate carbon dioxide in the dark. This prevents loss of water during the daytime and reduces the extent of photorespiration.

Photorespiration NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Mechanism of Photorespiration

• Relation between photosynthesis and photorespiration

Practice Questions for NEET

Q1. The substrate for the process of photorespiration is

1. Glycerate

2. Phosphoglycolate

3. Glycine

4. Phosphoglycerate

Correct answer: 2) Phosphoglycolate

Explanation:

Photorespiration is a metabolic process that occurs in plants, particularly under conditions of high oxygen and low carbon dioxide concentrations. The process begins in the chloroplasts, where phosphoglycolate is formed as a byproduct of RuBisCO's oxygenation reaction. Phosphoglycolate is then converted into glycolate, which is transported to the peroxisomes, where it undergoes further metabolism. Finally, the pathway involves the mitochondria, where specific reactions take place to recycle intermediates. Thus, the photorespiration process requires the coordinated activity of chloroplasts, peroxisomes, and mitochondria, highlighting its complexity and energy-consuming nature.

Hence, the correct answer is option 2, Phospoglycolate.

Q2. The sequence of organelles in which photorespiration occurs is

1. Mitochondria - peroxisome - chloroplast

2. Chloroplast - peroxisome - mitochondria

3. Peroxisome - Chloroplast - mitochondria

4. Peroxisome -mitochondria - chloroplast

Correct answer: 2) Chloroplast - peroxisome - mitochondria

Explanation:

Photorespiration is a process that occurs in plants, primarily when the enzyme RuBisCO fixes oxygen instead of carbon dioxide during the Calvin cycle, leading to the formation of phosphoglycolate as a substrate. This process can be wasteful for the plant, as it reduces the efficiency of photosynthesis. Photorespiration involves three key organelles: the chloroplasts, where the initial reaction occurs; the peroxisomes, where part of the process is completed and toxic by-products like hydrogen peroxide are neutralized; and the mitochondria, where some of the carbon from phosphoglycolate is used to regenerate useful compounds. These organelles work together in a complex cycle to reduce the harmful effects of photorespiration, though the process still results in energy loss for the plant.

Hence, the correct answer is option 2) Chloroplast - peroxisome - mitochondria.

Q3. Photorespiration shows formation of

1. Sugar but not ATP

2. ATP but not sugar

3. Both ATP and sugar

4. Neither ATP nor sugar

Correct answer: 4) Neither ATP nor sugar

Explanation:

Photorespiration is the light-dependent process of oxygenation of ribulose biphosphate (RuBP) and the release of carbon dioxide by the photosynthetic organs of a plant. In the light, photosynthetic organs typically absorb CO2 and release O2, the opposite of what happens normally. At high temperatures, RuBP carboxylase acts as an oxygenase and oxidizes ribulose 1, 5-biphosphate to form a 3-carbon phosphoglyceric acid and a 2-carbon phosphoglycolate rather than fixing carbon dioxide (C3 cycle).

Photorespiration doesn't generate power or produce energy. Instead, it uses energy. Additionally, it reverses the effects of photosynthesis. Fixed CO2 is lost by 25%. Photorespiration is therefore a rather wasteful process. This only occurs with C3 plants. Photorespiration has been a concern for C4 plants, but not anymore.

Hence, the correct answer is option 4) Neither ATP nor sugar.

Also Read:

• MCQs on Photorespiration

• Factors Affecting Photosynthesis

• Cyclic Photophosphorylation