Question

Succulent plants have a respiratory quotient of less than one because of
A. Complete oxidation
B. Complete reduction
C. Incomplete oxidation
D. Incomplete reduction

Answer 1

Hint:
The plants have some parts that are more thickened and fleshy than normally mostly for retaining water in arid or soil conditions. Deserts are biomes that receive very low rainfall each year and thus succulent plants grow in such areas. At night, succulent plants do not release carbon dioxide but absorb oxygen for oxidation.

Complete step by step answer:
The respiratory quotient is the ratio of carbon dioxide expelled to the oxygen consumed by an organism. The value of the respiratory quotient is different for each substrate used. In succulent plants, the respiratory substrate used is glucose. Complete oxidation of carbohydrates does not take place in succulent plants, instead, it is partially oxidised to produce organic acids like malic acid and oxalic acids. In some extreme conditions, carbon dioxide is not expelled at all and accumulates in the guard cells. During the incomplete oxidation of carbohydrates in succulent plants, 3 molecules of oxygen are used and no carbon dioxide is released thus, by applying the formula; RQ = No. of molecules of carbon dioxide released / No. of molecules of oxygen used, the respiratory quotient comes out to be zero (0/3). Hence, succulent plants have a respiratory quotient of less than 1 because of incomplete oxidation.
Thus, the correct answer is Incomplete oxidation.

Option ‘C’ is correct

Note:
The stomata of succulent plants are closed during day time and open at night to minimise the loss of water during the dry conditions of the day. The carbon dioxide is not released at night but its uptake takes place at night for further oxidation.