Peroxisomes

Peroxisomes occur in many animal cells and in a wide range of plants.
They are present in all photosynthetic cells of higher plants in etiolated leaf tissue,
in coleoptiles and hypocotyls, in tobacco stem and callus, in ripening pear fruits and also in Euglenophyta, Protozoa, brown algae, fungi liverworts, mosses and ferns.
Peroxisomes are variable in size and shape, but usually appear circular in cross section having diameter between 0.2 and 1.5mm (0.2 and 0.25 mm diameter in most mammalian tissues : 0.5mm diameter in rat liver cells).
They have a single limiting unit membrane of lipid and protein molecules, which encloses their granular matrix. In some cases (e.g., in the festuciod grasses) the matrix contains numerous threads or fibrils, while in others they are observed to contain either an amorphous nucleoid or a dense inner core which in many species shows a regular crystalloid structure (e.g., tobacco leaf cell).
Little is known about the function of the core, except that it is the site of the enzyme urate oxidase in rat liver peroxisomes and much of the catalase in some plants .
Recently, a possible relationship has been stressed between peroxides and free radicals (such as superoxide anion -O₂^–) with the process of aging.
These radicals may act on DNA molecule to produce mutations altering the transcription into mRNA and the translation into proteins.
In addition, free radicals and peroxides can affect the membranes by causing peroxidation of lipids and proteins.
For these reasons reducing compounds such as vitamin E or enzymes such as superoxide dismutase could play a role in keeping the healthy state of a cell.

Glycolate cycle-

Peroxisomes of plant leaves contain catalaze together with the enzymes of glycolate pathway, as glycolate oxidase, glutamate glyoxylate, serine-glyoxylate and aspirate-a-ketoglutarate aminotransferases, hydroxyl pyruvate reductase and malic dehydrogenase.
They also contain FAD, NAD and NADP coenzymes.
The glycolate cycle is thought to bring about the formation of the amino acids-glycine and serine-from the non-phosphorylated intermediates of photosynthetic carbon reduction cycle, i.e., glycerate to serine, or glycolate to glycine and serine in a sequence of reactions which involve chloroplasts, peroxisomes, mitochondria and cytosol.
The glycolate pathway also generates C₁ compounds and serves as the generator of precursors for nucleic acid biosynthesis.

Photorespiration–

In green leaves, there are peroxisomes that carry out a process called photorespiration which is a light – stimulated production of CO₂ that is different from the generation of CO₂ by mitochondria in the dark.
In photorespiration, glycolic acid (glycolate), a two-carbon product of photosynthesis is released from chloroplasts and oxidized into and H₂O₂ by a peroxisomal enzyme called glycolic acid oxidase.
Later on, glyoxylate is oxidized into CO₂ and formate:

CH₂OH.COOH + O₂ ® CHO – COOH + H₂O₂

CHO – COOH + H₂O₂ ® HCOOH + CO₂ + H₂O

Photorepiration is so called because light induces the synthesis of glycolic acid in chloroplasts.
This process involves intervention of two organelles : cloroplasts and peroxisomes.
Lastly, photorespiration is driven by atmospheric conditions in which the O₂ tension is high and CO₂ tension low.
Apparently O₂ competes with CO₂ for the enzyme ribulose diphosphate carboxylase which normally is the key enzyme in CO₂ fixation during photosynthesis.
When O₂ is used by the enzyme, an unstable intermediate is formed which breaks down into 3-phosphoglycerate and phosphoglycolate.
The latter tends to increase the glycolate concentration by removal of its phosphate group and, therefore, more glycolate is available for additional oxidation and CO₂ release
Photorespiration is a wasteful process for the plant cell, since, it significantly reduces the efficiency of the process of photosynthesis (i.e., it returns a portion of fixed CO₂ to the atmosphere).
It is a particular problem in C₃ plants that are more readily affected by low CO₂ tensions ; C₄plants are much more efficient in this regard.

Biogenesis of Peroxisomes

At one time it was thought that the membrane ‘shell’ of the peroxisomes is formed by building of the endoplasmic reticulum (ER), while the ‘content’ or matrix is imported from the cytosol (cytoplasmic matrix).
However, there is now evidence suggesting that new peroxisomes always arise from pre-existing ones, being formed by growth and fission of old organelles similar to mitochondria and chloroplasts.
Thus, peroxisomes are a collection of organelles with a constant membrane and a variable enzymatic content.
All of their proteins (both structural and enzymatic) are encoded by nuclear genes and are synthesized in the cytosol (cytoplasmic matrix) (i.e., on the free ribosomes).
The proteins present in either lumen or membrane of the peroxisome are taken up post-translationally from the cytosol (cytoplasmic matrix) as the haeme-free monomer;
the monomers are imported into the lumen of peroxisomes,
where they assemble into tetramers in the presence of haeme / Catalase and many peroxisomal proteins are found to have a signal sequence (comprising of three amino acids)
which is located near their carboxyl ends and directs them to peroxisome .
Peroxisomes contain receptors exposed on their cytosolic surface to recognize the signal on the imported proteins.
All of the membrane proteins of the peroxisomes.
Including signal receptor proteins, they are imported directly from the cytosol (cytoplasmic matrix).
The lipids required to make new peroxisomal membranes are also imported from the cytosol (cytoplasmic matrix), possibly being carried by phospholipids transfer proteins from sites of their synthesis in the DR membranes.

1. Peroxisomes are surrounded by:

A) Double membrane
B) No membrane
C) Single membrane
D) Triple membrane
✅ Answer: C) Single membrane

2. The matrix of peroxisomes can contain:

A) DNA strands
B) Cristae
C) Threads, fibrils, or dense cores
D) Ribosomes
✅ Answer: C) Threads, fibrils, or dense cores

3. The enzyme urate oxidase in rat liver peroxisomes is localized in:

A) Nucleus
B) Cytoplasm
C) Inner core
D) Ribosomes
✅ Answer: C) Inner core

4. Peroxisomes in tobacco leaves may show:

A) Lysosomal enzymes
B) Crystalloid structure
C) Mitochondrial DNA
D) Lipid bodies
✅ Answer: B) Crystalloid structure

5. Peroxisomes are absent in:

A) Mosses
B) Euglenophyta
C) Brown algae
D) None of these
✅ Answer: D) None of these

6. Peroxisomes are commonly found in:

A) Mature RBCs
B) Bacteria
C) Fungi
D) Cyanobacteria
✅ Answer: C) Fungi

7. Peroxisomes play a key role in:

A) Protein synthesis
B) Lipid peroxidation
C) Cell wall formation
D) DNA replication
✅ Answer: B) Lipid peroxidation

8. Free radicals like superoxide anion are associated with:

A) Chloroplasts
B) DNA transcription
C) Aging
D) Photosynthesis
✅ Answer: C) Aging

9. Superoxide dismutase plays a role in neutralizing:

A) Proteins
B) Carbohydrates
C) Free radicals
D) Nucleic acids
✅ Answer: C) Free radicals

10. The antioxidant that protects membrane lipids from peroxidation is:

A) Vitamin D
B) Vitamin E
C) Vitamin C
D) Vitamin B12
✅ Answer: B) Vitamin E

11. Peroxisomes contain enzymes of the:

A) Krebs cycle
B) Glycolysis
C) Glycolate cycle
D) Photosystem II
✅ Answer: C) Glycolate cycle

12. Glycolate oxidase is located in:

A) Mitochondria
B) Chloroplast
C) Peroxisomes
D) Cytosol
✅ Answer: C) Peroxisomes

13. The glycolate cycle leads to the production of:

A) Glucose
B) Oxygen
C) Glycine and Serine
D) Alanine and Valine
✅ Answer: C) Glycine and Serine

14. Glycolate is converted to glycine in a process involving:

A) Golgi body
B) ER
C) Mitochondria
D) Chloroplast
✅ Answer: C) Mitochondria

15. Which of these is NOT involved in the glycolate cycle?

A) Cytosol
B) Ribosomes
C) Mitochondria
D) Peroxisomes
✅ Answer: B) Ribosomes

16. Glycolate cycle coenzymes include all EXCEPT:

A) NAD
B) FAD
C) NADP
D) CoA
✅ Answer: D) CoA

17. Which compound is a non-phosphorylated intermediate of the glycolate pathway?

A) Glucose-6-phosphate
B) Phosphoenolpyruvate
C) Glycerate
D) ATP
✅ Answer: C) Glycerate

18. Photorespiration occurs in:

A) Darkness
B) Presence of light
C) Absence of oxygen
D) High CO₂ tension
✅ Answer: B) Presence of light

19. The enzyme involved in converting glycolate to glyoxylate is:

A) Catalase
B) Glycolate oxidase
C) Malate dehydrogenase
D) RuBisCO
✅ Answer: B) Glycolate oxidase

20. In photorespiration, which gas is released?

A) O₂
B) CO₂
C) NH₃
D) H₂S
✅ Answer: B) CO₂

21. Photorespiration is more common in:

A) C4 plants
B) C3 plants
C) CAM plants
D) Algae
✅ Answer: B) C3 plants

22. Phosphoglycolate is a product of:

A) Light reaction
B) RuBisCO oxygenase activity
C) Fermentation
D) Krebs cycle
✅ Answer: B) RuBisCO oxygenase activity

23. The unstable product of RuBisCO reacting with O₂ breaks down into:

A) Glucose and fructose
B) 3-PGA and phosphoglycolate
C) Acetyl-CoA and CO₂
D) NADH and CO₂
✅ Answer: B) 3-PGA and phosphoglycolate

24. Photorespiration is considered a ______ process.

A) Productive
B) Beneficial
C) Wasteful
D) Synthetic
✅ Answer: C) Wasteful

25. Biogenesis of peroxisomes occurs through:

A) Golgi bodies
B) ER budding
C) Growth and division of existing peroxisomes
D) Mitochondrial fusion
✅ Answer: C) Growth and division of existing peroxisomes

26. Peroxisomal proteins are encoded by:

A) Mitochondrial DNA
B) Chloroplast DNA
C) Nuclear DNA
D) Plasmids
✅ Answer: C) Nuclear DNA

27. Peroxisomal proteins are synthesized:

A) Inside peroxisomes
B) In nucleus
C) On free ribosomes in cytosol
D) On rough ER
✅ Answer: C) On free ribosomes in cytosol

28. Protein import into peroxisomes is:

A) Co-translational
B) Post-translational
C) Through lysosomes
D) DNA-mediated
✅ Answer: B) Post-translational

29. Catalase is assembled as a tetramer in:

A) Nucleus
B) Cytosol
C) Peroxisomal lumen
D) ER
✅ Answer: C) Peroxisomal lumen

30. The peroxisomal targeting signal (PTS) consists of how many amino acids?

A) 1
B) 2
C) 3
D) 5
✅ Answer: C) 3

31. PTS is found at which end of the protein?

A) N-terminal
B) Middle
C) Carboxyl-terminal (C-end)
D) Anywhere
✅ Answer: C) Carboxyl-terminal (C-end)

32. Peroxisomes import:

A) Proteins only
B) Lipids only
C) Both proteins and lipids
D) Only RNA
✅ Answer: C) Both proteins and lipids

33. Peroxisomal membrane proteins are imported from:

A) Mitochondria
B) Golgi bodies
C) Cytosol
D) Nucleus
✅ Answer: C) Cytosol

34. Lipids are transported to peroxisomal membranes by:

A) Enzymes
B) DNA
C) Phospholipid transfer proteins
D) Vesicles
✅ Answer: C) Phospholipid transfer proteins

35. Which of the following enzymes is present in peroxisomes but not lysosomes?

A) Hydrolase
B) Catalase
C) Nuclease
D) Phosphatase
✅ Answer: B) Catalase

36. Peroxisomes contribute to metabolism by:

A) Synthesizing starch
B) Storing fats
C) Neutralizing H₂O₂
D) Synthesizing proteins
✅ Answer: C) Neutralizing H₂O₂

37. FAD is a coenzyme in:

A) Lipid synthesis
B) Glycolysis
C) Glycolate cycle
D) Fermentation
✅ Answer: C) Glycolate cycle

38. Photorespiration reduces:

A) CO₂ intake
B) ATP production
C) Photosynthetic efficiency
D) Light absorption
✅ Answer: C) Photosynthetic efficiency

39. In photorespiration, H₂O₂ is:

A) Stored
B) Excreted
C) Broken down by catalase
D) Used for ATP production
✅ Answer: C) Broken down by catalase

40. The byproduct of photorespiration includes:

A) Glucose
B) Urea
C) Formate
D) NADPH
✅ Answer: C) Formate

41. Peroxisomes import enzymes as:

A) Dimers
B) Monomers
C) DNA
D) Polypeptides with PTS
✅ Answer: B) Monomers

42. Peroxisomal growth and division is similar to:

A) Nucleus
B) Lysosomes
C) Mitochondria
D) ER
✅ Answer: C) Mitochondria

43. Function of glutamate-glyoxylate aminotransferase is found in:

A) Mitochondria
B) Golgi
C) Peroxisomes
D) Lysosomes
✅ Answer: C) Peroxisomes

44. Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) is responsible for:

A) Lipid synthesis
B) Glycolate oxidation
C) CO₂ fixation or O₂ competition
D) Protein folding
✅ Answer: C) CO₂ fixation or O₂ competition

45. The problem of photorespiration is reduced in:

A) C3 plants
B) C4 plants
C) CAM plants
D) Aquatic plants
✅ Answer: B) C4 plants

46. The unstable intermediate from RuBisCO + O₂ breaks into:

A) Glycerol and glucose
B) 3-PGA and phosphoglycolate
C) Fructose and glucose
D) Pyruvate and lactate
✅ Answer: B) 3-PGA and phosphoglycolate

47. The oxidizing agent in glycolate to glyoxylate reaction is:

A) H₂O
B) NAD+
C) O₂
D) CO₂
✅ Answer: C) O₂

48. Phosphoglycolate is converted into:

A) Acetyl CoA
B) Glycine
C) Glycolate
D) Serine
✅ Answer: C) Glycolate

49. Catalase in peroxisomes helps in:

A) Synthesizing RNA
B) Breaking H₂O₂
C) DNA replication
D) Peptide bonding
✅ Answer: B) Breaking H₂O₂

50. Which process is most wasteful to plants?

A) Respiration
B) Fermentation
C) Photorespiration
D) Glycolysis
✅ Answer: C) Photorespiration