Respiration is the process of releasing energy from the breakdown of glucose. Respiration takes place in every living cell, all of the time and all cells need to respire in order to produce the energy that they require.

What is the energy used for?

The energy produced during respiration is used in many different ways, some examples of what it is used for are:

  • Working your muscles
  • Growth and repair of cells
  • Building larger molecules from smaller ones i.e. proteins from amino acids
  • Allowing chemical reactions to take place
  • Absorbing molecules in active transport
  • Keeping your body temperature constant
  • Sending messages along nerves

Types of Respiration

There are two main types of respiration, aerobic and anaerobic we will look at each one of these in detail now.

1. Aerobic Respiration

Aerobic means “with air”. This type of respiration needs oxygen for it to occur so it is called aerobic respiration. The word equation for aerobic respiration is:

Glucose + Oxygen Carbon dioxide + Water + Energy

The chemical equation is:

C6H12O6 + 6O2       6CO2 + 6H2O + 2900 kJ/mol

It is important that you learn both the word and chemical equation.

In the above equations we see that glucose is broken down by oxygen to release energy with carbon dioxide and water being produced as by-products of the reaction. Approximately 2900 kJ of energy is released when one mole of glucose is broken down.  The released energy is used to make a special energy molecule called Adenosine triphosphate (ATP).  ATP is where the energy is stored for use later on by the body.

Aerobic respiration occurs in plants as well as animals. Oxygen enters plant cells through the stomata. Plants produce their food via photosynthesis and release energy from it through the process of respiration. Below is a reminder of what the equation for photosynthesis is:

(Energy via sunlight)
Carbon dioxide + Water   Glucose + Oxygen

Comparing the two equations we can see that aerobic respiration works in the opposite way to photosynthesis. During the day both photosynthesis and respiration are taking place at the same time, though photosynthesis is occurring at a faster rate. At night when there is no light only respiration takes place.



2. Anaerobic Respiration

Anaerobic means without air (“an” means without).  Sometimes there is not enough oxygen around for animals and plants to respire, but they still need energy to survive.  Instead they carry out respiration in the absence of oxygen to produce the energy they require this is called anaerobic respiration.

a) In animals

Our muscles need oxygen and glucose to respire aerobically and produce the energy they require, these are carried to the muscle via the blood.  However if we were to carry out vigorous exercise our heart and lungs would not be able to get sufficient oxygen to our muscles in order for them to respire. In this case muscles carry out anaerobic respiration. The word and chemical equation for anaerobic respiration in is:

Glucose       Lactic acid + Energy

C6H12O6      2C3H6O3 + 120 kJ/mol

As you can see anaerobic respiration is not as efficient as aerobic and only a small amount of energy is released.  This is because glucose can only be partially broken down.  As well as this inefficiency a poisonous chemical, lactic acid is also produced, if this builds up in the body it stops the muscles from working and causes a cramp.  To rid the body of lactic acid oxygen is needed, the amount of oxygen required to break down the lactic acid is referred to as the oxygen debt.

b) In Plants

The oxygen supply to plants can also run out, this happens for example if the soil gets waterlogged. In this case they have to obtain their energy via anaerobic respiration. Below is the word and chemical equation for anaerobic respiration in plants:

Glucose       Ethanol + Carbon dioxide + Energy

C6H12O6      2C2H5OH + 2CO2 + Energy

When the above reaction occurs in yeast cells it is referred to as fermentation. Fermentation is the process used for baking bread and brewing alcohol.