Respiration is the process by which energy is liberated from organic compounds such as glucose. It is one of the most important characteristics of living organisms. Energy is expended (used) whenever an organism exhibits characteristics of life, such as feeding, excretion and movement. Respiration occurs all the time and if it stops, cellular activities are disrupted due to lack of energy.


This may result in death e.g., if cells in brain lack oxygen that is needed for respiration for a short time, death may occur. This is because living cells need energy in order to perform the numerous activities necessary to maintain life. The energy is used in the cells and much of it is also lost as heat.


In humans it is used to maintain a constant body temperature. Tissue Respiration Respiration takes place inside cells in all tissues. Every living cell requires energy to stay alive. Most organisms require oxygen of the air for respiration and this takes place in the mitochondria.


Mitochondrion Structure and Function


  • Mitochondria are rod-shaped organelles found in the cytoplasm of cells.
  • A mitochondrion has a smooth outer membrane and a folded inner membrane.
  • The folding of the inner membrane is called cristae and the inner compartment is called the matrix.

Adaptations of Mitochondrion to its Function

  • The matrix contains DNA ribosomes for making proteins and has enzymes for the breakdown of pyruvate to carbon (IV) oxide, hydrogen ions and electrons.
  • Cristae increase surface area of mitochondrial inner membranes where attachment of enzymes needed for the transport of hydrogen ions and electrons are found.


There are two types of respiration:

  • Aerobic Respiration
  • Respiration


(1) Aerobic Respiration

This involves breakdown of organic substances in tissue cells in the presence of oxygen . All multicellular organisms and most unicellular organisms e.g. some bactena respire aerobically. In the process, glucose is fully broken down to carbon (IV) oxide and hydrogen which forms water when it combines with the oxygen. Energy produced is used to make an energy rich compound known as adenosine triphosphate (ATP).


It consists of adenine, an organic base, five carbon ribose-sugar and three phosphate groups. ATP is synthesised from adenosine diphosphate (ADP) and inorganic phosphate. The last bond connecting the phosphate group is a high-energy bond. Cellular activities depend directly on ATP as an energy source.


When an ATP molecule is broken down, it yields energy. Process of Respiration The breakdown of glucose takes place in many steps. Each step is catalysed by a specific enzyme. Energy is released in some of these steps and as a result molecules of ATP are synthesised. All the steps can be grouped into three main stages:



The initial steps in the breakdown of glucose are referred to as glycolysis and they take place in the cytoplasm. Glycolysis consists of reactions in which glucose is gradually broken down into molecules of a carbon compound called pyruvic acid or pyruvate. Before glucose can be broken, it is first activated through addition of energy from ATP and phosphate groups.


This is referred to as phosphorylation. The phosphorylated sugar is broken down into two molecules of a 3-carbon sugar (triose sugar) each of which is then converted into pyruvic acid. If oxygen is present, pyruvic acid is converted into a 2-carbon compound called acetyl coenzyme A (acetyl Co A).


Glycolysis results in the net production of two molecules of ATP. The next series of reactions involve decarboxylation i.e. removal of carbon as carbon (IV) oxide and dehydrogenation, removal of hydrogen as hydrogen ions and electrons.


These reactions occur in the mitochondria and constitute the Tri-carboxylic Acid Cycle (T.C.A.) or Kreb’s citric acid cycle. The acetyl Co A combines with 4-carbon compound with oxalo-acetic acid to form citric acid – a 6 carbon compound. The citric acid is incorporated into a cyclical series of reactions that result in removal of carbon (IV) oxide molecules, four pairs of hydrogen, ions and electrons.


Hydrogen ions and electrons are taken to the inner mitochondria membrane where enzymes and electron carriers effect release of a lot of energy. Hydrogen finally combines with oxygen to form water, and 36 molecules of ATP are synthesised.


(2) Anaerobic Respiration

Anaerobic respiration involves breakdown of organic substances in the absence of oxygen. It takes place in some bacteria and some fungi. Organisms which obtain energy by anaerobic respiration are referred to as anaerobes. Obligate anaerobes are those organisms which do not require oxygen at all and may even die if oxygen is present.

Facultative anaerobes are those organisms which survive either in the absence or in the presence of oxygen. Such organisms tend to thrive better when oxygen is present e.g. yeast.


Products of Anaerobic Respiration

The products of anaerobic respiration differ according to whether the process is occurring in plants or animals.


Anaerobic Respiration in Plants

Glucose is broken down to an alcohol, (ethanol) and carbon (IV) oxide. The breakdown is incomplete. Ethanol is an organic compound, which can be broken down further in the presence of oxygen to provide energy, carbon (IV) oxide and water.

C6HI206 _ 2C2H50H + 2C02 + Energy

(Glucose) (Ethanol) (Carbon (IV) oxide)



  • Is the term used to describe formation of ethanol and carbon (IV) oxide from grains.
  • Yeast cells have enzymes that bring about anaerobic respiration.


Lactate Fermentation

  • Is the term given to anaerobic respiration in certain bacteria that results in formation of lactic acid.


Anaerobic Respiration in Animals

Anaerobic respiration in animals produces lactic acid and energy.

C6H1P6 _ 2CH3CHOH.COOH + energy (Glucose) (Lactic acid) + energy

When human muscles are involved in very vigorous activity, oxygen cannot be delivered as rapidly as it is required. The muscle respires anaerobically and lactic acid accumulates. A high level of lactic acid is toxic. During the period of exercise, the body builds up an oxygen debt. After vigorous activity, one has to breathe faster and deeper to take in more oxygen.

Rapid breathing occurs in order to break down lactic acid into carbon (IV) oxide and water and release more energy. Oxygen debt therefore refers to the extra oxygen the body takes in after vigorous exercise.


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