B4.3 How do living organisms obtain energy?

B4.3 How do living organisms obtain energy?

All living organisms require energy released by respiration for some chemical reaction in cells – the energy is used for:

  • Movement
  • Synthesising (making) larger molecules
  • Active transport

Large molecules, such as starch and cellulose are synthesised from smaller molecules such as glucose in plant cells. This involves joining the glucose molecules (monomers) together to form a polymer (made of many units)






Amino acids are synthesised from glucose and nitrates. Proteins are made in plant, animal and bacterial cells from strings of amino acids joined together


AEROBIC RESPIRATION releases energy through the breakdown of glucose molecules, by combining them with oxygen inside living cells. The majority of animal and plant cells  and some microorganisms respire aerobically.


ANAEROBIC RESPIRATION takes place in conditions of low oxygen or absence of oxygen to include:

  • Animals cells (e.g. in humans during vigorous exercise)
  • Plant cells (e.g. in plant roots in waterlogged soil)
  • Microbial cells (bacteria in puncture wounds)

The equation for anaerobic respiration in animal cells and some bacteria is:

The equation for anaerobic respiration in plant cells and some microorganism (including yeast, which is used in brewing and making bread) is:

Aerobic respiration releases more energy per glucose molecule than anaerobic respiration – a maximum of 18 times as much. In humans anaerobic respiration can only occur for a short period.

Animal cell:

  • Cell membrane – allows gases and water to enter and leave the cell freely while acting as a barrier to other, larger chemicals
  • Nucleus – contains the DNA that carries the genetic code for making proteins, including the enzymes needed in respiration
  • Cytoplasm – where proteins, including enzymes used in anerobic respiration are made
  • Mitochondria – where aerobic repiration occurs.

Microbial cells have similar structures, but with some important differences:

BACTERIA – an important feature of bacteria is that they do not have any membrane-bound organelles. Therefore they do not have a nucleus or mitochondria




YEAST – yeast is a type of fungus – it is used to make bread and alcohol. Unlike bacteria yeast does have membrane-bound organelles.



Applications of Anaerobic respiration – Biotechnology has enabled us to use the products of anaerobic respiration

Making bread – Yeast is added to a dough, made from flour, salt, water and other ingredients. The dough is effectively a source of glucose that is needed for anaerobic respiration

Brewing alcohol – Brewing involves a fermentation process:

Aerobic fermentation – the yeast is exposed to air and grows rapidly on the sugar provided. Some alcohol is produced but the majority of energy is used to produce more yeast cells.

Anaerobic respiration – this takes place in the absence of oxygen. The yeast respires anaerobically and produces alcohol and carbon dioxide instead of multiplying

Biogas – It is now possible to introduce bacteria to biodegradable substances such as manure, sewage and household waste in landfill sites. The anaerobic digestion leads to the production of methane (an explosive gas) and carbon dioxide. The methane can be used as a low-cost fuel.