Question

R.Q for malic acid is
A. \[1.3\]
B. \[4\]
C. \[0.7\]
D. \[1\]

Answer 1

Hint: R.Q is the respiratory quotient. And as the name says it is a ratio of some quantities (Think metabolic reactions). The respiratory quotient of a chemical is the ratio of the amount of \[C{O_2}\] expelled to the amount of \[{O_2}\] consumed in what we can essentially call, burning of the chemical. The chemical formula of malic acid is \[{C_4}{H_6}{O_5}\].

Complete answer:
The R.Q for malic acid is \[{\mathbf{1}}.{\mathbf{3}}\]
Respiratory quotient values can be measured by the indirect use of calorimetry. This data can be cross-checked with the theoretical value and it will give us an indication of which macromolecule is getting broken down in different parts of metabolism.
To put this in terms of a formula, $R.Q = \dfrac{{C{O_2}\exp elled}}{{{O_2}consumed}}$
The chemical formula of malic acid is \[{C_4}{H_6}{O_5}\] with two carboxylic acid groups and a hydroxyl group. The reaction with oxygen will follow the subsequent step.
$\dfrac{1}{3}{C_4}{H_6}{O_5} + {O_2} \to \dfrac{4}{3}C{O_2} + {H_2}O$ .
So the corrected equation for one unit of malic acid turns out to be \[{C_4}{H_6}{O_5} + 3{O_2} \to 4C{O_2} + 3{H_2}O\]
Applying the formula for R.Q, we get, $R.Q = \dfrac{{C{O_2}\exp elled}}{{{O_2}consumed}} = \dfrac{4}{3} = 1.3$
The values in other options also correspond to known R.Q values of some other chemicals.
Oxalic acid has an R.Q value of \[4\]. Lipids are generally shown an R.Q value of \[0.7\]. Carbohydrates are the ones that show an R.Q value of \[1.0\]. Some other known values include proteins – \[0.8\], Ketones – \[0.73\], tartaric acid – \[1.6\], and mixed diet usually comes with an R.Q of \[0.8\].

Note:
 The Respiratory quotient is a ratio of amounts of gases in a reaction. So it is a unit less quantity. And since it is a ratio, it is not necessary to correct the equation to one unit of the chemical, as long as the ratio of the values stays the same. It will help to remember the R.Q of basic metabolites for questions like this.