Problem 115 - Objective Biology for NEET

Question

Photorespiration takes place in (a) Chloroplast, mitochondria (b) Mitochondria, peroxisome (c) Chloroplasts, peroxisome, mitochondria (d) Chloroplasts, cytoplasm, mitochondria

Step-by-Step Solution

Answer

The short answer is (c) Chloroplasts, peroxisome, mitochondria

Step 1: Understanding the question

The first step is to understand the basic concept of photorespiration. It is a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted.

Step 2: Recall the organelles involved

Next, recall which organelles are involved in photorespiration. This process primarily occurs in the chloroplasts, peroxisomes, and mitochondria of plant cells.

Step 3: Match with the correct option

Lastly, match the knowledge from step 2 to the available options. The answer should be the option that lists chloroplasts, peroxisomes, and mitochondria as the sites of photorespiration.

Key Concepts

RuBisCO enzyme

RuBisCO is a crucial enzyme in the process of photosynthesis and photorespiration. It stands for Ribulose-1,5-bisphosphate carboxylase/oxygenase. This enzyme is responsible for fixing carbon dioxide during the Calvin cycle, a part of photosynthesis.
In photorespiration, RuBisCO plays a dual role. Although it primarily facilitates the fixation of carbon dioxide, it can also bind with oxygen, especially under high oxygen or low carbon dioxide conditions. This binding initiates photorespiration.
When RuBisCO reacts with oxygen, it forms a compound called phosphoglycolate, which does not enter the Calvin cycle. This reaction decreases the efficiency of photosynthesis, as it diverts energy and carbon away from sugar production. Understanding RuBisCO's dual functionality helps explain why photorespiration is less efficient for plants.

chloroplast

Chloroplasts are the site of photosynthesis in plant cells. These green organelles contain chlorophyll, which captures sunlight.
In the context of photorespiration, chloroplasts are where the process begins. Chloroplasts are responsible for the initial reaction catalyzed by RuBisCO.
When RuBisCO oxygenates RuBP (ribulose bisphosphate), it produces phosphoglycolate. This compound is not usable in the Calvin cycle and must be processed through other metabolic pathways.
Thus, the chloroplasts initiate the oxygenation step, leading phosphoglycolate to enter the photorespiratory pathway, which involves several organelles.

peroxisome

Peroxisomes are small, membrane-bound organelles that play a role in the photorespiration process. After the formation of phosphoglycolate in the chloroplasts, this compound is converted to glycolate and transferred to peroxisomes.
In peroxisomes, glycolate is converted into glyoxylate and then into glycine. This process involves oxidation reactions, which is a key function of peroxisomes.
The unique environment inside peroxisomes supports these specific chemical reactions. This organelle's role is vital as it continues the conversion pass along photorespiratory intermediates.

• They help recycle glycolate to prevent waste accumulation.
• They provide enzymes necessary for oxidation.

Hence, peroxisomes are essential for effectively managing the byproducts of RuBisCO's oxygenation activity.

mitochondria

Mitochondria are mainly known for their role in energy production through respiration, but they also have a crucial function in photorespiration. Once glycine is produced in the peroxisomes, it is transported into the mitochondria.
Inside the mitochondria, glycine undergoes conversion into serine. This important step releases ammonia and a form of carbon dioxide, which are further used in additional biochemical processes.
Mitochondria contribute significantly to the recycling of compounds derived from the initial oxygenation reaction by RuBisCO. Their involvement ensures that the cycle can continue:

• Provide enzymes for the transformation of glycine into serine.
• Manage metabolic byproducts effectively.

Therefore, mitochondria play a supportive yet essential role in linking the photorespiration process with overall plant metabolism.