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   Chapter: Respiration


 A set of chemical reactions that break down nutrient molecules in living cells to release energy is called as respiration.

What is respiration ?

Respiration is a process in which oxidation of food molecules occurs resulting in the release of energy. The food we take basically contains carbon, hydrogen and oxygen. The complete oxidation which occurs in the process of respiration converts them into carbon dioxide and water. Complete oxidation occurs only in the presence of oxygen. Let us take a look at the chemical reaction.

      C6H12O6 + 6O2        →           energy + 6CO2 + H2O

So much of energy is released during this process. The energy is released in small packets which are then used to carry out different reactions to keep the cell alive. These energy packets act as short term stores of energy. Adenosine triphosphate (ATP) is the most commonly used form of energy by the cells. The energy that is released during the process of respiration is used to carry out various activities in the body in order to keep the organisms alive. Let us look into detail of various activities that are driven by energy released during respiration.

1. Growth:

Energy is utilized for the synthesis of biomolecules like proteins from amino acids. Proteins play a very important role in the growth of an organism.

2. Maintenance of body temperature:

All the chemical reactions that take place in the body, especially the liver releases some heat which is used to maintain normal body temperature.

3. Active transport:

Energy is required for the movement of molecules against a concentration gradient. For example uptake of ions from the soil by the roots utilizes energy in the form of ATP.

4. Cell division:

A lot of energy is required for the replication and separation of chromosomes. Billions of cells are replaced every day in a human being.

5. Movement:

Contraction of muscles causes movement and the contraction of muscles is an energy requiring process.

6. The passage of nerve impulses:

The resting nerve cell has to maintain electrical balance and the cell has to do work to maintain the ion concentration. ATP, molecules are used to provide the required amount of energy for maintaining ion concentration.

Aerobic respiration:

The process of respiration which releases energy in the presence of oxygen is called aerobic respiration. Aerobic respiration occurs in mitochondria of the cell. Some amount of energy released during aerobic respiration is released as heat which maintains the normal body temperature of an organism.

Glucose + oxygen        →          energy + carbon dioxide + water

Anaerobic respiration:

The process of respiration which releases energy in the absence of oxygen is called as anaerobic respiration. For example: Muscles can release energy from food without using oxygen. During heavy work outs glucose is converted into lactic acid in the absence of oxygen. Let us look at this word equation to understand.

 Glucose       →       energy + lactic acid

In case of microorganisms like yeast, anaerobic respiration results in the production of alcohol and carbon dioxide.

 Glucose         →         alcohol + carbon dioxide

The two main draw backs of anaerobic respiration are:

  1. Very less amount of energy is produced compared to the energy produced during aerobic respiration.
  2. Lactic acid which is produced in muscles during anaerobic respiration in muscles becomes poisonous if it gets built up in the cell. Excess amount of lactic acid inhibits muscular contraction which leads to fatigue and thereby death.

Anaerobic respiration in yeast during bread making and brewing:

Yeasts are generally used for the baking purpose and for the production of alcohol. Anaerobic respiration which takes places in yeast results in the production of alcohol and carbon dioxide. These products are useful in the baking and brewing industry. The alcohol that is produced makes beer and wine. Carbon dioxide makes the bread or pastry rise (fermentation) and the alcohol evaporates during baking process.

Difference between aerobic respiration and anaerobic respiration



          Aerobic respiration

      Anaerobic respiration


Occurs in the presence of oxygen

Occurs in the absence of oxygen


Does not produce ethanol or lactic acid

Produces ethanol or lactic acid


Large amount of energy is released

Small amount of energy is released


Mitochondria involved


Mitochondria not involved


Carbon dioxide is always produced


Carbon dioxide is produced only sometimes.


Gas Exchange

Respiration is a process that uses oxygen to burn food and release energy. Living organisms obtain oxygen from the air and release carbon dioxide into the air. Swapping of oxygen and carbon dioxide in this way is called gas exchange or gaseous exchange. Gas exchange in living organisms takes place through a surface called as gas exchange surface or otherwise called as respiratory surface. The properties of an ideal gas exchange surface are given below.

Features of gas exchange surface:

  1. Gas exchange surface is thin, about one cell thick. This property of the surface allows dissolved gases to pass through easily.
  2. The respiratory surface/gas exchange surface has a large surface area which allows the diffusion of many number of gas molecules at the same time.
  3. The surface is always moist to keep the cells alive.
  4. The surface is well ventilated so that concentration gradients for oxygen and carbon dioxide are kept up by regular fresh supply of air.
  5. The respiratory surface is close to blood supply so that gases can be carried to and from the cells that need or produce them.

Gas exchange in humans

  1. In human beings, air enters the body through nostrils. The air passing through nostrils is filtered by fine hairs lining that passage.
  2. From nostrils air passes through the throat and then to the lungs. Rings of cartilage present in the throat ensure that the air passage does not collapse.
  3. Inside the lungs the passage divides into smaller tubes which terminate into balloon like structures called alveoli which serves as a surface for the exchange of gases. Alveoli contain an extensive network of blood vessels.
  4. When we breathe in air we lift our ribs and flatten our diaphragm as a result of which the chest cavity becomes larger.
  5. As the chest cavity enlarges the air is sucked into the lungs and fills the expanded alveoli.
  6. The blood brings carbon dioxide from the rest of the body and releases into the alveoli and the oxygen in the alveolar air is taken up by blood in the alveolar blood vessels to be transported to all the cells in the body.
  7. Lungs always contain residual volume of air during breathing cycle so that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released.
  8. In animals with large size, the diffusion pressure alone cannot deliver oxygen to all the body parts instead respiratory pigments help in delivering oxygen. In humans the respiratory pigment haemoglobin has high affinity for oxygen and this pigment is present in red blood corpuscles.

Differences in composition between inspired and expired air

Component of air

Inspired air (in %)

Expired air (in %)





Oxygen has diffused from alveoli to blood

Carbon dioxide



Carbon dioxide has diffused from blood into the alveoli




Nitrogen gas is not used by the body

Water vapour

Very variable


Water evaporates from the surface in the alveoli


Very variable


Heat is lost to the air from the lung surfaces.


Lime water test

Lime water test is done to investigate the differences in the composition of inspired air and expired air. Lime water is used because it changes its colour to a milky white colour when reacts with carbon dioxide. Since there is more amount of carbon dioxide in the expired air, it makes lime water change its colour more quickly than the inspired air.

Effects of physical activity on rate and depth of breathing

Physical activity is any movement that uses more amount of energy. Exercise is a type of physical activity which consumes a lot of energy. Physical exercise is required by the body to keep it fit and healthy.  During exercise muscles work hard and therefore they need to release more energy through respiration. As they need to release more amount of energy more amount of oxygen is required. Therefore a greater volume of air is breathed in and out by increasing the breathing rate and the tidal volume. Increase in the breathing rate means more breaths per minute and increase in tidal volume means more air per breath. These two changes can increase the volume of the air passing in and out of the lungs from 8 dm3 at rest to 50 – 60 dm3 per minute during a heavy work out.