Glyoxysomes
- Glyoxysomes are found to occur in the cells of yeast,
- Neurospora, and oil rich seeds of many higher plants.
- They resemble with peroxisomes in morphological details, except that, their crystalloid core consists of dense rods of 6.0 mm diameter.
- They have enzymes for fatty acid metabolism and gluconeogenesis, i.e. conversion of stored lipid molecules of spherosomes of germinating seeds into the molecules of carbohydrates.
Functions
Glyoxysomes perform following biochemical activities of plants cells :
(1) Fatty acid metabolism.
- During germination of oily seeds, the stored lipid molecules of spherosomes are hydrolysed by the enzyme lipase (glycerol ester hydrolase) to glycerol and fatty acids.
- The phospholipids molecules are hydrolysed by the enzyme phospholipase.
- The long chain fatty acids which are released by the hydrolysis are then broken down by the successive removal of two carbon or C2 fragments in the process of b-oxidation.
- During b-oxidation process, the fatty acid is first activated by enzyme fatty acid thiokinase to fatty acyl-CoA which is oxidized by a FAD-linked enzyme fatty acyl-CoA dehydrogenase into-2-enoyl-CoA.
- Trans-2-enoyl-CoA is hydrated by an enzyme enoyl hydratase or crotonase to produce the L-3-hydroxyacyl-CoA, which is oxidized a NAD Linked L-3-hydratase or crotonase to produce the L-3-hydroxyacyl-CoA, which is oxidized by a NAD linked L-3-hydroxyacyl-CoA dehydrogenase to produce 3-Ketoacly-CoA.
- The 3-keto acyl-CoA looses a two carbon fragment under the action of the enzyme thiolase to generate an acetyl-CoA and a new fatty acyl-CoA with two less carbon atoms thatn the original.
- This new fatty acyl-CoA is then recycled thought the same series of reactions until the final two molecules of acetyl-CoA are produced.
- In plant seeds b-oxidation occurs in glyoxysomes .
- But in other plant cells b-oxidation occurs in glyxysomes and mitochondria.
- The glyoxysomal b-oxidation requires oxygen for oxidation of reduced flavorprotien produced as a result of the fatty-acyl-CoA dehydrogenase activity.
- In animal cells b-oxidation occurs in mitochondria.
- In plant cells, the acetyl-CoA, the product of b-oxidation chain is not oxidized by Krebs cycle, because it remains spatially separated from the enzymes of Krebs cycle, instead of it, acetyl-CoA undergoes the glyoxylate cycle to be converted into succinate.
(2) Glyoxylate cycle.
- The glyoxylate pathway occurs in glyoxysomes and it involve some of the reactions of the Krebs cycle in which citrate is formed from oxaloacetate and acetyl-CoA under the action of citrate synthetase enzymes.
- The citrate is subsequently converted into isocitrate by aconitase enzyme.
- The cycle then involves the enzymatic conversion of isocitrate to glyoxylate and succinate by isocitratase enzyme :
Isocitratase
Isocitrate 
- The glyoxylate and another mole of acetyl-CoA form a mole of malate by malate synthesis ;
Malate synthetase
Acetyl CoA+Glyoxylate 
- This malate is converted to oxaloacetate by malate dehydrogenase for the cycle to be completed.
- Thus, overall, the glyoxylate pathway involves :
2 Acetyl-CoA+NAD+ 
- Succinate is the end product of the glyoxysomal metabolism of fatty acid and is not further metabolized within this organelle.
- The synthesis of hexose or gluconeogenesis involves the conversion of succinate to oxaloacetate, which presumably takes place in the mitochondria, since the glyoxsomes do not contain the enzymes fumarase and succinic dehydrogenase.
- Two molecules of oxaloacetate are formed from four molecules of acetyl-CoA without carbon loss.
- This oxaloacetate is converted to phosphoenol pyruvate in the phosphoenol pyruvate caboxykinase reaction with the loss of two molecules of CO2 :
2 Oxaloacete + 2ATP 
Q1. Glyoxysomes are primarily found in the cells of:
A) Root hairs
B) Mesophyll cells
C) Yeast and oil-rich seeds
D) Phloem tissues
Answer: C) Yeast and oil-rich seeds
Q2. Morphologically, glyoxysomes are similar to:
A) Mitochondria
B) Ribosomes
C) Peroxisomes
D) Golgi apparatus
Answer: C) Peroxisomes
Q3. The crystalloid core of glyoxysomes contains dense rods of diameter:
A) 2.0 mm
B) 4.0 mm
C) 6.0 mm
D) 8.0 mm
Answer: C) 6.0 mm
Q4. The primary function of glyoxysomes in germinating seeds is:
A) Photosynthesis
B) Nitrogen fixation
C) Lipid to carbohydrate conversion
D) Water absorption
Answer: C) Lipid to carbohydrate conversion
Q5. The first enzyme that acts on stored lipids in spherosomes is:
A) Amylase
B) Lipase
C) Cellulase
D) Protease
Answer: B) Lipase
Q6. Fatty acids are first activated by which enzyme before β-oxidation?
A) Enoyl hydratase
B) Thiolase
C) Fatty acid thiokinase
D) Dehydrogenase
Answer: C) Fatty acid thiokinase
Q7. During β-oxidation, fatty acyl-CoA is first oxidized by:
A) FAD-linked dehydrogenase
B) NAD-linked dehydrogenase
C) Peroxidase
D) Isomerase
Answer: A) FAD-linked dehydrogenase
Q8. Which enzyme hydrates trans-2-enoyl-CoA?
A) Enoyl hydratase
B) Thiolase
C) Aconitase
D) Phospholipase
Answer: A) Enoyl hydratase
Q9. Final product of β-oxidation is:
A) Glycerol
B) Citrate
C) Acetyl-CoA
D) Pyruvate
Answer: C) Acetyl-CoA
Q10. In plants, acetyl-CoA undergoes which cycle instead of the Krebs cycle in glyoxysomes?
A) Calvin cycle
B) Urea cycle
C) Glyoxylate cycle
D) Electron transport chain
Answer: C) Glyoxylate cycle
Q11. The glyoxylate cycle takes place in:
A) Nucleus
B) Glyoxysomes
C) Mitochondria
D) Cytosol
Answer: B) Glyoxysomes
Q12. Which enzyme converts citrate into isocitrate?
A) Citrate synthetase
B) Aconitase
C) Isocitratase
D) Malate dehydrogenase
Answer: B) Aconitase
Q13. The enzyme that splits isocitrate into succinate and glyoxylate is:
A) Isocitratase
B) Aconitase
C) Malate synthetase
D) Enoyl hydratase
Answer: A) Isocitratase
Q14. Malate synthetase catalyzes the reaction between glyoxylate and:
A) Succinate
B) Acetyl-CoA
C) Citrate
D) Oxaloacetate
Answer: B) Acetyl-CoA
Q15. The final product of glyoxysomal metabolism of fatty acids is:
A) Acetyl-CoA
B) Oxaloacetate
C) Succinate
D) Pyruvate
Answer: C) Succinate
Q16. Succinate is converted to oxaloacetate in:
A) Golgi bodies
B) Cytoplasm
C) Glyoxysomes
D) Mitochondria
Answer: D) Mitochondria
Q17. The glyoxysomes lack the enzymes:
A) Isocitratase and malate synthetase
B) Fumarase and succinate dehydrogenase
C) Lipase and phospholipase
D) Aconitase and malate dehydrogenase
Answer: B) Fumarase and succinate dehydrogenase
Q18. Conversion of oxaloacetate to phosphoenol pyruvate involves the enzyme:
A) Pyruvate dehydrogenase
B) Malate synthetase
C) Phosphoenol pyruvate carboxykinase
D) Succinate thiokinase
Answer: C) Phosphoenol pyruvate carboxykinase
Q19. The by-product of the phosphoenol pyruvate carboxykinase reaction is:
A) CO2
B) NADH
C) FADH2
D) GTP
Answer: A) CO2
Q20. How many molecules of oxaloacetate are formed from 4 acetyl-CoA molecules in glyoxylate cycle?
A) 1
B) 2
C) 3
D) 4
Answer: B) 2
Q21. Glyoxysomes are a subtype of:
A) Mitochondria
B) Golgi bodies
C) Peroxisomes
D) Lysosomes
Answer: C) Peroxisomes
Q22. Which of the following enzymes is involved in the hydrolysis of phospholipids?
A) Lipase
B) Thiolase
C) Phospholipase
D) Hydratase
Answer: C) Phospholipase
Q23. Which enzyme catalyzes the conversion of L-3-hydroxyacyl-CoA to 3-ketoacyl-CoA?
A) Enoyl hydratase
B) Thiolase
C) L-3-hydroxyacyl-CoA dehydrogenase
D) Fatty acyl-CoA dehydrogenase
Answer: C) L-3-hydroxyacyl-CoA dehydrogenase
Q24. Which product is generated when 3-ketoacyl-CoA undergoes thiolytic cleavage?
A) NADH
B) Acetyl-CoA
C) Pyruvate
D) Malate
Answer: B) Acetyl-CoA
Q25. In glyoxysomal β-oxidation, the electron acceptor is:
A) NADP+
B) FAD
C) Oxygen
D) Ubiquinone
Answer: C) Oxygen
Q26. Which organelles cooperate with glyoxysomes for gluconeogenesis?
A) Chloroplasts
B) Lysosomes
C) Mitochondria
D) Endoplasmic reticulum
Answer: C) Mitochondria
Q27. Glyoxylate cycle allows bypassing of which decarboxylation step of TCA cycle?
A) Oxidative phosphorylation
B) Decarboxylation of isocitrate
C) ATP synthesis
D) FADH2 generation
Answer: B) Decarboxylation of isocitrate
Q28. What is the role of malate synthetase?
A) Converts malate to fumarate
B) Converts acetyl-CoA and glyoxylate to malate
C) Converts succinate to oxaloacetate
D) Hydrolyzes citrate
Answer: B) Converts acetyl-CoA and glyoxylate to malate
Q29. Which of the following is not a product of glyoxylate cycle?
A) Succinate
B) Glyoxylate
C) Oxaloacetate
D) Glucose
Answer: D) Glucose
Q30. Which enzyme starts the glyoxylate cycle by forming citrate?
A) Isocitratase
B) Aconitase
C) Citrate synthetase
D) Malate dehydrogenase
Answer: C) Citrate synthetase
Q31. What is the energy molecule consumed in the conversion of oxaloacetate to phosphoenol pyruvate?
A) GTP
B) ATP
C) NADPH
D) AMP
Answer: B) ATP
Q32. The process of gluconeogenesis is completed in which cellular location?
A) Nucleus
B) Glyoxysome
C) Cytosol and mitochondria
D) Golgi body
Answer: C) Cytosol and mitochondria
Q33. Which compound is the direct precursor of glucose in gluconeogenesis?
A) Malate
B) Oxaloacetate
C) Phosphoenol pyruvate
D) Pyruvate
Answer: C) Phosphoenol pyruvate
Q34. Glyoxylate pathway helps in:
A) CO₂ fixation
B) Glucose oxidation
C) Lipid breakdown to carbohydrate
D) ATP synthesis
Answer: C) Lipid breakdown to carbohydrate
Q35. In yeast and Neurospora, glyoxysomes function during:
A) Anaerobic respiration
B) Sporulation
C) Fat metabolism
D) Alcohol fermentation
Answer: C) Fat metabolism
Q36. Which enzyme is absent in glyoxysomes?
A) Isocitratase
B) Malate synthetase
C) Succinic dehydrogenase
D) Citrate synthetase
Answer: C) Succinic dehydrogenase
Q37. Glyoxysomes are commonly present in which part of the seed?
A) Cotyledon
B) Plumule
C) Endosperm
D) Seed coat
Answer: A) Cotyledon
Q38. Glyoxylate cycle produces succinate which enters:
A) Calvin cycle
B) Glycolysis
C) TCA cycle
D) Pentose phosphate pathway
Answer: C) TCA cycle
Q39. Which of the following is not involved in β-oxidation?
A) Fatty acid thiokinase
B) Enoyl hydratase
C) Pyruvate kinase
D) Thiolase
Answer: C) Pyruvate kinase
Q40. The number of carbon atoms removed per cycle in β-oxidation is:
A) 1
B) 2
C) 3
D) 4
Answer: B) 2
Q41. What is the fate of glycerol released during lipid hydrolysis?
A) Enters the TCA cycle directly
B) Converted to glucose via gluconeogenesis
C) Excreted as waste
D) Used in β-oxidation
Answer: B) Converted to glucose via gluconeogenesis
Q42. Which process generates NADH in the glyoxysome?
A) Lipid hydrolysis
B) Fatty acid activation
C) Oxidation of L-3-hydroxyacyl-CoA
D) Conversion of succinate to fumarate
Answer: C) Oxidation of L-3-hydroxyacyl-CoA
Q43. Which of the following best describes glyoxysomal metabolism?
A) Anaerobic
B) Photosynthetic
C) Oxygen-dependent
D) Nucleus-dependent
Answer: C) Oxygen-dependent
Q44. The enzyme that regenerates oxaloacetate from malate is:
A) Malate synthetase
B) Malate dehydrogenase
C) Succinate dehydrogenase
D) Enoyl hydratase
Answer: B) Malate dehydrogenase
Q45. Which enzyme converts isocitrate to glyoxylate?
A) Isocitratase
B) Citrate synthase
C) Aconitase
D) Thiolase
Answer: A) Isocitratase
Q46. How many ATP molecules are used in the conversion of oxaloacetate to phosphoenol pyruvate (2 molecules)?
A) 1
B) 2
C) 3
D) 4
Answer: B) 2
Q47. What is the main reason for glyoxylate cycle to bypass CO₂-producing steps?
A) To conserve energy
B) To fix oxygen
C) To preserve carbon skeletons
D) To form lipids
Answer: C) To preserve carbon skeletons
Q48. Which of the following enzymes is common to both glyoxylate cycle and TCA cycle?
A) Succinate dehydrogenase
B) Isocitratase
C) Aconitase
D) Malate synthetase
Answer: C) Aconitase
Q49. Glyoxysomes are especially important in seeds that are rich in:
A) Protein
B) Starch
C) Oil
D) DNA
Answer: C) Oil
Q50. What is the correct sequence of the glyoxylate cycle?
A) Citrate → Isocitrate → Glyoxylate → Malate → Oxaloacetate
B) Pyruvate → Citrate → Malate → Glyoxylate
C) Acetyl-CoA → Glyoxylate → Pyruvate
D) Oxaloacetate → Pyruvate → Citrate
Answer: A) Citrate → Isocitrate → Glyoxylate → Malate → Oxaloacetate