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Photosynthesis in Higher Plants - Photorespiration

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Photorespiration, also known as the C2C_2 cycle, is a wasteful process occurring in C3C_3 plants where O2O_2 is consumed and CO2CO_2 is released without the production of ATPATP or NADPHNADPH.

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The enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) exhibits a dual nature. It acts as an oxygenase when the concentration of O2O_2 is high and CO2CO_2 is low, particularly at higher temperatures.

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The process takes place across three organelles in a specific sequence: Chloroplast, Peroxisome, and Mitochondrion.

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In the chloroplast, Ribulose-1,5-bisphosphate (RuBPRuBP) reacts with O2O_2 to form one molecule of 3-phosphoglyceric acid (3−PGA3-PGA) and one molecule of 2-phosphoglycolate (2C2C compound).

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The 2-phosphoglycolate is converted to glycolate and transported to the peroxisome, where it eventually forms the amino acid glycine.

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In the mitochondria, two molecules of glycine (2×2C2 \times 2C) are converted into one molecule of serine (3C3C), resulting in the release of one molecule of CO2CO_2 and NH3NH_3.

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Photorespiration is absent in C4C_4 plants because they possess a mechanism (Kranz anatomy) that increases the CO2CO_2 concentration at the RuBisCO site, ensuring the enzyme functions only as a carboxylase.

📐Formulae

RuBP+O2→RuBisCO3-PGA (3C)+2-Phosphoglycolate (2C)RuBP + O_2 \xrightarrow{RuBisCO} \text{3-PGA (3C)} + \text{2-Phosphoglycolate (2C)}

2×Glycine (2C)→MitochondriaSerine (3C)+CO2↑+NH32 \times \text{Glycine (2C)} \xrightarrow{\text{Mitochondria}} \text{Serine (3C)} + CO_2 \uparrow + NH_3

Net Carbon Loss=14 of fixed Carbon in the C2 pathway\text{Net Carbon Loss} = \frac{1}{4} \text{ of fixed Carbon in the } C_2 \text{ pathway}

💡Examples

Problem 1:

Why is photorespiration considered a 'wasteful' process in C3C_3 plants?

Solution:

It results in the loss of fixed carbon and consumes energy.

Explanation:

Unlike the Calvin cycle, photorespiration does not synthesize sugars, ATPATP, or NADPHNADPH. Instead, it utilizes ATPATP and releases CO2CO_2, essentially reversing the work done by photosynthesis. About 25%25\% of the carbon fixed by the Calvin cycle can be lost through this process.

Problem 2:

Calculate the number of carbon atoms lost as CO2CO_2 if 4 molecules of 2-phosphoglycolate enter the photorespiratory pathway.

Solution:

2 molecules of CO22 \text{ molecules of } CO_2

Explanation:

In the photorespiratory pathway, for every 2 molecules of 2-phosphoglycolate (2×2C=4C2 \times 2C = 4C), they eventually form 2 molecules of Glycine (2×2C2 \times 2C). In the mitochondria, 2 molecules of Glycine react to form 1 molecule of Serine (3C3C) and release 1 molecule of CO2CO_2 (1C1C). Therefore, if 4 molecules of 2-phosphoglycolate enter, they produce 4 Glycine molecules, which yield 4÷2=24 \div 2 = 2 molecules of CO2CO_2.