Peroxisomes: Structure and Cellular Roles | UPSC Mains BOTANY-PAPER-II 2022

Structure and functions of peroxisomes

How to Approach

This question requires a detailed understanding of peroxisomes, covering both their structural components and the diverse functions they perform within the cell. The answer should begin with a clear definition of peroxisomes and their discovery. Then, it should delve into their structural features – membrane, matrix, enzymes – and subsequently elaborate on their functions like fatty acid oxidation, detoxification, and photorespiration. A concise and organized presentation, utilizing subheadings, will enhance clarity and ensure a comprehensive response.

Structure of Peroxisomes

Peroxisomes are generally spherical or oval-shaped organelles, ranging in diameter from 0.1 to 1 μm. Their structure can be broadly divided into three main components:

Membrane: A single phospholipid bilayer enclosing the peroxisomal matrix. It contains transport proteins (peroxins) that regulate the movement of molecules into and out of the organelle.
Matrix: The space enclosed by the membrane, containing a dense arrangement of enzymes, including oxidases, catalases, and other metabolic enzymes.
Crystalline Core: Often observed in mammalian peroxisomes, this core is composed of urate oxidase, an enzyme involved in purine catabolism.

Functions of Peroxisomes

1. Fatty Acid Oxidation

Peroxisomes play a critical role in the β-oxidation of very long-chain fatty acids (VLCFAs). Unlike mitochondria, which primarily oxidize fatty acids with even numbers of carbon atoms, peroxisomes can oxidize VLCFAs and branched-chain fatty acids. This process shortens the fatty acid chain, which is then transported to mitochondria for complete oxidation. Defects in peroxisomal β-oxidation lead to accumulation of VLCFAs, causing disorders like Zellweger syndrome.

2. Detoxification of Harmful Compounds

Peroxisomes contain catalase, an enzyme that decomposes hydrogen peroxide (H₂O₂) into water and oxygen. H₂O₂ is a toxic byproduct of many metabolic reactions and is produced during fatty acid oxidation. Catalase prevents the accumulation of H₂O₂, protecting the cell from oxidative damage. They also detoxify other harmful compounds like methanol and ethanol.

3. Photorespiration in Plants

In plant cells, peroxisomes are essential for photorespiration, a metabolic pathway that occurs in chloroplasts, peroxisomes, and mitochondria. Photorespiration involves the oxidation of glycolate, produced during the oxygenase activity of RuBisCO. Peroxisomes convert glycolate to glyoxylate, which is then further processed in other organelles. This process helps to recover some of the carbon lost during photorespiration.

4. Synthesis of Plasmalogens

Peroxisomes are the site of synthesis for plasmalogens, a class of phospholipids abundant in the brain and heart. Plasmalogens are crucial for nerve impulse transmission and membrane structure. Their synthesis requires the transport of specific precursors across the peroxisomal membrane.

5. Cholesterol and Dolichol Metabolism

Peroxisomes participate in the initial steps of cholesterol synthesis and the metabolism of dolichol, a lipid involved in protein glycosylation. These processes are essential for maintaining cellular membrane integrity and protein function.

Function	Description	Significance
Fatty Acid Oxidation	β-oxidation of VLCFAs and branched-chain fatty acids	Energy production, prevents VLCFA accumulation
Detoxification	Decomposition of H₂O₂ and other toxins	Protects cells from oxidative damage
Photorespiration	Metabolism of glycolate in plants	Recovers carbon lost during photosynthesis

Additional Resources

Key Definitions

Peroxins

Peroxins are a family of proteins involved in the biogenesis and maintenance of peroxisomes. They function as import receptors, chaperones, and scaffolding proteins, facilitating the transport of proteins and lipids into and out of the organelle.

Plasmalogens

Plasmalogens are a class of phospholipids containing a vinyl ether linkage instead of the typical ester linkage at the sn-1 position. They are particularly abundant in the brain and heart and play a role in membrane structure and nerve impulse transmission.

Key Statistics

Zellweger syndrome, a genetic disorder caused by defects in peroxisomal biogenesis, affects approximately 1 in 50,000 newborns.

Source: National Organization for Rare Disorders (NORD) - as of 2023

Approximately 70% of the enzymes in the peroxisomal matrix are involved in fatty acid metabolism.

Source: Lodish et al., Molecular Cell Biology, 4th edition (Knowledge cutoff 2023)

Examples

Adrenoleukodystrophy (ALD)

ALD is a genetic disorder caused by the accumulation of VLCFAs due to a defect in peroxisomal β-oxidation. It primarily affects the nervous system and adrenal glands, leading to neurological deterioration and adrenal insufficiency.

Frequently Asked Questions

▶What happens if peroxisomes are absent?

The absence of functional peroxisomes, as seen in Zellweger syndrome, leads to a severe accumulation of VLCFAs, impaired detoxification, and defects in plasmalogen synthesis. This results in a range of neurological and developmental abnormalities, often leading to early mortality.