Question

 During non-cyclic photophosphorylation, when electrons are lost from the reaction centre at PS-II, what is the source which replaces these electrons? [NEET (Oct.) 2020]

(a) Oxygen

(b) Water

(c) Carbon dioxide

(d) Light

Answer/Explanation

Ans. (b)

During non-cyclic photophosphorylation, electrons expelled by the excited PS-II photocentre does not return to it. Therefore, it requires an external electron donor and that purpose is
served by water. $\mathrm{H}_2 \mathrm{O}$ undergo photolysis and the electrons thus released are picked up by PS-1I $\left(\mathrm{P}_{600}\right)$ and handed over to PS-I or $P_{700}$.

Question

 The process which makes major difference between $\mathrm{C}_3$ and $\mathrm{C}_4$-plants is [NEET 2016, Phase II]

(a) glycolysis

(b) Calvin cycle

(c) photorespiration

(d) respiration

Answer/Explanation

Ans. (c)

Photorespiration is the process which makes a difference between the $\mathrm{C}_3$ and $\mathrm{C}_4$-plants. In this process, there is a continuous loss of carbon fixed in the form of $\mathrm{CO}_2$.
It occurs due to the high $\mathrm{O}_2$ content, high temperature conditions in which RuBP carboxylase starts working as RuBP oxygenate and normal photosynthesis does not occur.

Question

 The correct sequence of cell organelles during photorespiration is [CBSE AIPMT 2012

(a) chloroplast-Golgi bodies-mitochondria

(b) chloroplast-rough endoplasmic reticulum-dictyosomes

(c) chloroplast-mitochondria -peroxisome

(d) chloroplast-vacuole-peroxisome

Answer/Explanation

Ans. (c)

None of the option is correct. Photorespiration required three cell organelles in sequence of chloroplast, peroxisome and mitochondria. Option (c) may be correct if be read as said sequence.

Question

 During photorespiration, the oxygen consuming reaction(s) occur in [CBSE AIPMT 2006 .

(a) stroma of chloroplasts and peroxisomes

(b) grana of chloroplasts and peroxisomes

(c) stroma of chloroplasts

(d) stroma of chloroplasts and mitochondria

Answer/Explanation

Ans. (a)

The first reaction of photorespiration occurs in stroma of chloroplast. In this reaction the RuBP(Ribulose 1-5 biphosphate) consumes one oxygen molecule in presence of enzyme RuBisCO.
In peroxisome the glycolate transferred from chloroplast takes $\mathrm{up}_2$ and formed the glyoxylate whereas, the $\mathrm{H}_2 \mathrm{O}_2$ released as byproduct.

Question

 Which one of the following is wrong in relation to photorespiration? [CBSE AIPMT 2003

(a) It is a characteristic of $\mathrm{C}_3$-plants

(b) It occurs in chloroplasts

(c) It occurs in day time only

(d) It is a characteristic of $\mathrm{C}_4$-plants

Answer/Explanation

Ans. (d)
Dicker and Tio (1959) discovered photorespiration in tobacco plant. It is light dependent process of oxygenatior of Ribulose Bisphosphate(RuBP). During this process $\mathrm{CO}_2$ is liberated ano $\mathrm{O}_2$ is consumed. $\mathrm{C}_4$-plants avoid photorespiration by following Hatch Slack pathway.

Question

 Which enzyme is most abundantly found on earth? [CBSE AIPMT 1999]

(a) Catalase

(b) RuBisCO

(c) Nitrogenase

(d) Invertase

Answer/Explanation

Ans. (b)

RuBisCO(RuBP carboxylase) is the most abundant protein on this planet. RuBisC0 constitutes $16 \%$ of chloroplast protein. It is required for $\mathrm{CO}_2$ fixation with RuBP(Ribulose Biphosphate) in Calvin cycle.

Question

 Photorespiration is favoured by [CBSE AIPMT 1996]

(a) high $\mathrm{O}_2$ and low $\mathrm{CO}_2$

(b) low light and high $\mathrm{O}_2$

(c) low temperature and high $\mathrm{O}_2$

(d) low $\mathrm{O}_2$ and high $\mathrm{CO}_2$

Answer/Explanation

Ans. (a)

Photorespiration is light induced oxidation of photosynthetic intermediates with the help of oxygen. It is stimulated by high $\mathrm{O}_2$ concentration or low $\mathrm{CO}_2$, high light intensity, high temperature and ageing of leaf.

Question

 The substrate for photorespiration is [CBSE AIPMT 1989]

(a) ribulose bis-phosphate

(b) glycolate

(c) serine

(d) glycine

Answer/Explanation

Ans. (b)
Photorespiration is the oxidation of photosynthetic intermediate without production of $\mathrm{CO}_2, \mathrm{ATP}$ and $\mathrm{NADH}_2$. The substrate for photorespiration is a 2-carbon compound glycolic acid (glycolate).

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