Chapter 7 Respiration

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  • For an organism to maintain life, energy is
  • required.
  • Energy is required for cells (the basic unit of organisms) to perform their vital activities.
  • Energy is released by the metabolic breakdown of organic compounds mainly carbohydrates.
  • The reactions constitute what is known as cell tissue or internal respiration.
  • Oxygen is normally required and carbon dioxide and water are produced.
  • Cell respiration generally involves the oxidation of sugar to release energy for the cell.
  • C6H12O6 + 602 à 6H2O + 6CO2 + Energy
  • Respiration can be divided into two types depending on the usage of oxygen in the process.
(a) Aerobic respiration is the oxidation of glucose using oxygen to produce energy, carbon dioxide and water. This process involves a series of reactions which occurs in stages. Aerobic respiration can be represented by the following equation.

C6H12O6 + 602 à 6C02 + 6H20 + Energy)
(b) Anaerobic respiration is a process where by sugar is broken down to release energy in the absence of oxygen. Certain organisms derive energy from breaking down sugar in the complete absence of oxygen. Yeast can respire anaerobically, as can various fungi and bacteria.


  • Anaerobic respiration in plants (including yeast and other fungi) yields ethanol, carbon dioxide and energy. This is shown in the following equation.
  • C6H12O6 à 2CH3CH20H + 2CO2 + 210 kJ.
  • Anaerobic respiration in animals yields energy and lactic acid as in the following equation.
  • C6H1206 à 2CH3CH(OH)COOH +150 kJ
  • As the breakdown of the sugar is incomplete in anaerobic respiration, less energy is released than when sugar is broken down in aerobic conditions. The energy released in aerobic condition is 2 880 kJ
  • The respiratory surface is the surface where, gaseous exchange takes place between the organism and the environment. The respiratory surface in the amoeba is the cell membrane or plasma membrane, whereas in the humans the lungs with its numerous alveoli form the respiratory surface.
  • Unicellular organisms such as amoeba and paramecium do not require any special respiratory system because they possess a big total surface area to volume ratio ( TSA/Vol ). Gaseous exchange occurs through the whole surface area by simple diffusion.
  • Multicellular organisms need specific respiratory organs like the tracheal system (Insects), gills (fish) and lungs (mammals).
  • Even though there are physical differences in the various types of respiratory organs all of them share certain characteristics like having
(a) a thin membrane to shorten the distance of movement by the gaseous molecules.
(b) a moist respiratory surface so that the gases can dissolve in the water present.
(c) a big surface area.
(d) a transport system (capillary system for example) to transport the gases.
  • In the lungs the partial pressure of oxygen is higher in the alveolar air compared with the blood capillaries. The partial pressure of carbon dioxide in the alveolar air is lower compared with the blood capillaries. This results in the diffusion of oxygen from the alveolar air into the blood capillary vessels via the alveolar and blood capillaries' membranous layers. The reverse occurs in the diffusion of carbon dioxide from the blood capillaries into the alveolus.
  • In the capillary vessels the oxygen diffuses into the erythrocyte and combines with the hemoglobin to form oxyhemoglobin. The carbon dioxide is transported in 3 ways
(a) As hydrogen carbonate ion
(b) As carbonic acid
(c) As carbaminohemoglobin.
  • The rate of respiration is regulated by the respiratory centre located in the medulla oblongata. An increase in the blood carbon dioxide concentration causes the blood to become acidic. The respiratory centre which is sensitive to blood pH will cause the respiratory rate to increase so that the excess carbon dioxide can be eliminated from the blood.
  • Chemoreceptors located on the aorta and carotid artery are sensitive to the partial pressure of oxygen in the blood. When the partial pressure of blood decreases the chemoreceptors will stimulate the respirator) centre to increase the ventilation of the lungs.
  • Gaseous exchange between plant cells and the atmosphere takes place through
(a) the stomata in the leaves
(b) the lenticels in the woody stems and old roots.
(c) the root hairs in the roots.
  • With increase in the light intensity, the rate of photosynthesis increases and is higher than the rate of respiration. The carbon dioxide released from respiration is absorbed and used in photosynthesis.
  • In the dark, plant cells carry out only respiration. The oxygen required is absorbed from the atmosphere and carbon dioxide is released.
  • The compensation point is reached when the light intensity increases to a certain level when the rate of photosynthesis and the rate of respiration are the same.
  • Some plants are able to carry out anaerobic respiration in the absence of oxygen. For example the rice seedlings growing in waterlogged padi fields respire anaerobically.

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