Difference Between Photooxidation And Photorespiration

What Is Photooxidation?

Photooxidation is a process in plants where, due to excessive light intensity, the production of ROS takes place. These ROS are highly damaging to cellular components, including lipids, proteins, chloroplasts, and nucleic acids. The latter damage leads to impairment in photosynthesis and generally in plant health.

Photooxidation typically occurs if the intensity of light is higher than the protective mechanisms developed in the plant and antioxidant defenses can cope with. Excessive light energy overexcites chlorophyll molecules in photosynthetic apparatus and leads to the formation of ROS, which may result in oxidative stress and cellular damage.

Mechanism Of Photooxidation

The mechanism includes the following:

Role Of Reactive Oxygen Species, (ROS)

• Singlet oxygen, superoxide anions, and hydrogen peroxide are examples of ROS formed during photooxidation.

• These ROS are highly reactive and can damage cellular structures and molecules.

Effect Of High Light Intensity

• High light intensity overexcites chlorophyll, and it transfers excess energy to oxygen molecules.

• This excess energy results in the generation of ROS and causes oxidative stress in plant cells.

Chemical Reactions Involved

• Chlorophyll absorbs excessive light energy.

• Transferring this energy to oxygen by excited chlorophyll forms singlet oxygen and other ROS.

• ROS react with lipids, proteins, and nucleic acids, causing cellular damage.

Effects On Plants

• Damage to Chloroplasts and Other Cellular Components:

• ROS can damage thylakoid membranes, hence affecting photosynthetic machinery.

• Peroxidation of lipids by ROS, oxidation of proteins, and DNA damage can take place.

Consequences For Plant Health And Growth

• Reduced photosynthetic efficiency and stunted growth.

• The plants become more susceptible to environmental stresses.

• Probable cell death and a reduced crop yield.

What Is Photorespiration?

Photorespiration is a process in plants whereby, instead of carboxylating RuBP, RuBISCO oxygenises ribulose-1,5-bisphosphate. The result is the formation of phosphoglycolate and eventually the release of CO2. This step consumes energy and reduces photosynthetic efficiency by competition with the Calvin cycle.

Photorespiration generally takes place when there is a high concentration of oxygen and a low concentration of carbon dioxide in the leaf, for instance, when stomata are closed to avoid loss of water. It is more pronounced in plants of the C3 category, whereby RuBisCO has a higher affinity for oxygen.

Mechanism Of Photorespiration

The mechanism of photorespiration is described below:

Role Of The Enzyme RuBisCO

• The enzyme RuBisCO catalyses the oxygenation of RuBP to give phosphoglycolate and 3-phosphoglycerate (3-PGA).

Steps Involved In Photorespiration

• RuBP is oxygenated by RuBisCo to yield phosphoglycolate.

• Phosphoglycolate is converted to glycolate and is transported to peroxisomes.

• Glycolate is converted to glyoxylate and then to glycine.

• Glycine is transported into the mitochondria and there converted to serine, releasing CO2 and NH3.

• The serine in turn goes back to the chloroplast and is converted back to 3-PGA.

Effects On Plants

The details are given below:

Loss Of Energy And Diminished Efficiency Of Photosynthesis

• The photorespiratory pathway uses up both ATP and NADPH without producing sugars.

• This process diminishes the net efficiency of photosynthesis.

Impact On Crop Yield And Productivity Of Plants

• Low net photosynthetic output.

• Lower crop yields and agricultural productivity.

Difference Between Photooxidation And Photorespiration

The difference between photooxidation and photorespiration:

Importance Of Understanding These Processes

Knowing photooxidation and photorespiration is quite important in improving the health of plants and crop yields and in finding ways of minimising their harmful effects.

Agricultural Applications

This includes:

Crop Management And Production Practices

• The understanding of these processes contributes to selecting and breeding crop lines that resist oxidative stress better.

• Optimisation of the light environment in controlled production systems to reduce adverse effects of photooxidation.

Control Measures

• Use of antioxidants to reduce ROS damage.

• Breeding and genetic manipulation to minimise photorespiration.

Ecological Relevance

The impacts on plant ecosystems and biodiversity:

• Photooxidation and photorespiration impact the survival and adaptation of plants.

• Understanding these processes helps in predicting the responses of plants to climate change and other environmental stressors.

Photooxidation vs 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