Different organisms oxidise glucose by different methods, some organisms use oxygen (Aerobic respiration) to break down the glucose into carbon-dioxide and water while others do not use oxygen ( Anaerobic respiration)to oxidise glucose
In both the process the first step of break down of glucose (6 carbon molecule) takes place in the cytoplasm of cells of all organisms and gives a three carbon molecule compound called Pyruvate.
Further break down of pyruvate takes place in different manners in different organisms.
(i) Anaerobic respiration: This process takes place in absence of oxygen e.g., in yeast during fermentation. In this case pyruvate is converted into ethanol and carbon dioxide.
(ii) Aerobic respiration: In aerobic respiration break down of pyruvate takes place in presence of oxygen to give rise 3 molecules of carbon dioxide and water. The release of energy in aerobic respiration is much more than anaerobic respiration.
(iii) Lack of oxygen: Sometimes, when there is lack of oxygen, especially during vigorous activity, in our muscles, pyruvate is converted into lactic acid (3 carbon molecule compound). Formation of lactic acid in muscles causes cramp.
Break down of glucose by various pathways during cellular respiration.