Question

Which of the following is correct in regards to photorespiration?
A) In mitochondria, serine is converted into hydroxypyruvate
B) In mitochondria, two glycine molecules unite to form serine
C) In mitochondria, glycolate is oxidised to form glyoxylate
D) In peroxisomes, three molecules of glycine unite to form serine

Answer 1

Hint: Photorespiration is also known as oxidative photosynthetic carbon cycle. It involves the movement of molecules among chloroplast, peroxisomes and mitochondria in a metabolic pathway that utilises oxygen and releases carbon dioxide at the cost of ATP.

Complete Answer:
- Photorespiration is the process in plant metabolism in which enzyme RuBisCO oxygenates RuBP by utilising the ATP produced during photosynthesis. It involves a complex network of enzyme reactions in which the metabolites are exchanged in between chloroplast, peroxisomes and mitochondria.
- In Chloroplast- molecular oxygen is added to ribulose-1,5-bisphosphate to produce 3-phosphoglycerate and 2-phosphoglycolate. The phosphoglycolate is converted to glycerate by a series of reactions in the peroxisome and mitochondria. Glycerate re-enters the chloroplast.
- In peroxisome- Hydrogen peroxide is produced. It splits into water and oxygen by the enzyme catalase. Glycine is transported out to mitochondria.
- In Mitochondria- two molecules of glycine are converted to serine by enzyme glycine-decarboxylase which releases carbon dioxide. The ammonia released is transported to cytoplasm.

Thus the correct answer is B. In photorespiration two glycine molecules unite to form serine in mitochondria.

Note: Photorespiration is an energy consuming process. It involves uptake of oxygen and release of carbon dioxide in light. It is initiated in chloroplast.