Rate of Respiration

Our Objective

To study the rate of respiration in germinating seeds having different substances such as wheat (carbohydrates), mustard (fats) and bean (proteins).


Respiration is the process during which simple carbohydrates, like glucose, break down into simpler substances and liberate carbon dioxide and energy.  The compound used, or oxidized, during respiration is called a respiratory substrate. Carbohydrates, fats, and proteins are examples of respiratory substrates, and carbohydrates are the preferred respiratory substrate among them. The rate of respiration can be measured in terms of gas exchange, that is, consumption of the respiratory substrate oxygen, or evolution of carbon dioxide.

What is Respiratory Quotient?

As we know, during aerobic respiration, oxygen is consumed and carbon dioxide is released.  The respiratory quotient (RQ) is the ratio of CO2 produced to that of the O2 consumed while food is being metabolized.

Let’s see how respiratory quotient depends upon different respiratory substances.

The respiratory quotient depends upon the type of respiratory substrate used during respiration. Different respiratory substrates have different numbers of carbon and oxygen atoms in their molecules. So, during respiration the amount of carbon dioxide evolved from per gram weight of the substrate also differs. The carbohydrates have equal numbers of carbon and oxygen in their molecules. When carbohydrates are used as substrate, then the RQ will be 1, because equal amounts of carbon dioxide and oxygen are evolved and consumed.

Fats and proteins contain a smaller number of oxygen atoms than carbon atoms in their molecules. When fats are used as a substrate in respiration, the RQ is less than 1 because the amount of oxygen utilised is always higher than the amount of carbon dioxide released.


Thus, we can study the rate of respiration for different respiratory substrates by calculating the amount of carbon dioxide evolved per gm weight of the substrate.

Let’s see the factors affecting the rate of respiration.

Here are some of the few factors that affect the rate of respiration.

  • Temperature: At a very high temperature, the rate of respiration decreases with time, and at very low temperature, the respiration rate is insignificant. Optimum temperature for respiration is 20 - 30oC.
  • Carbon dioxide concentration: The higher the carbon dioxide concentration, the lower the rate of respiration. Increase in carbon dioxide concentration and absence of oxygen adversely affects the rate of aerobic respiration.
  • Water: The respiratory rate increases with the increase in water content of the respiring organism.
  • Light: Light controls respiration by raising the temperature of an organism.

Respiration in plants can be studied in moist germinating seeds that release carbon dioxide (CO2) during respiration. The seeds are kept in an air tight conical flask. A small test tube containing potassium hydroxide (KOH) solution is placed in the flask. Potassium hydroxide absorbs carbon dioxide released by the seeds and a partial vacuum is created in the flask as a result. This causes the water level in the delivery tube to rise. Thus the rise in water level at the end of the delivery tube dipping in the beaker proves that germinating seeds release carbon dioxide during respiration. In the case of mustard and bean seeds the rise in water level is relatively less because these seeds use fat and proteins as respiratory substrate and release very small amount of carbon dioxide. But in the case wheat grains the rise in water level is more because it uses carbohydrate as respiratory substrate.

Learning Outcomes:

  • Students understand the terms respiration and respiratory quotient.
  • Students understand the dependency of respiratory quotient on different respiratory substances.
  • Students understand the factors affecting the rate of respiration.
  • Students do the experiment better in the real lab having gone through the animation and simulation.