Question

When malic acid is a respiratory substrate the amount of $CO_2$ release is?

Answer 1

Malic acid is an organic compound. It is an intermediate product of the kreb's cycle in metabolic reaction. And the Kreb cycle is the part of cellular respiration. In respiration there are several respiratory substrates which have respiratory quotients. Respiration quotient is the volume of $CO_2$ evolved to the amount of $O_2$ consumed in respiration.

Complete answer:
To answer this question you must know about respiratory quotients.
Respiratory quotient is the ratio of the volume of carbon dioxide evolved to that of oxygen consumed by an organism while food is being metabolized. The substance required for cellular respiration to derive energy through oxidation. Carbohydrates, fats and proteins, and organic acids are examples of respiratory substrates.
When organic acids, such as oxalic acid, malic acid, succinic acid, etc serve as respiratory substrate, then the respiratory quotient is more than 1. It is because organic acids contain more oxygen than carbohydrates. Therefore, a relatively less amount of $O_2$ is required for their oxidation. The respiratory quotient of carbohydrate is 1 and for fats and protein it is less than 1.
As we know respiratory quotient (R.Q.) = volume of $CO_2$ released /volume of $O_2$ consumed. Respiratory quotient depends upon the types of respiratory substrate. If we talk about malic acid the chemical formula is $C_4H_6O$. to find its RQ the reaction is as follow
$C_4H_6O_5 + 3 O_2$ Gives $4CO_2 + 3H_2O$ so $RQ = \dfrac{4CO_2}{O_2} =1.33$.
From the above reaction we can see when a malic acid is a respiratory substrate then the $CO_2$ release is more than oxygen ($O_2$) consumed.

Note:
Respiratory quotients help to determine the respiratory substrate and help in knowing the type of respiration being performed. And most importantly it provides some information about the major transformation of food materials.