R.Q. is less than one when aerobic respiration takes place in fats or proteins.

Explanation

### Step-by-Step Solution:

1. **Understanding RQ**: The Respiratory Quotient (RQ) is defined as the ratio of the volume of carbon dioxide (CO2) produced to the volume of oxygen (O2) consumed during respiration. It is mathematically expressed as:
\[ RQ = \frac{\text{Volume of CO2 produced}}{\text{Volume of O2 consumed}} \]

2. **Aerobic Respiration in Fats and Proteins**: When aerobic respiration occurs using fats or proteins as substrates, the biochemical pathways involved lead to different amounts of CO2 being produced relative to O2 consumed.
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