Cellular respiration

Glucose is an energy-rich molecule which can be used as an energy source to fuel many activities. Glucose can either be broken down in the presence of oxygen (aerobic respiration) or in the absence of oxygen (anaerobic respiration) both are discussed below.

Aerobic respiration


The first stage of glycolysis involves the break down  of glucose that consists of 10 different steps (you do not need to learn these!). It occurs in the cytosol of cells. One molecule of glucose is broken down into 2 three carbon molecules known as pyruvate. Two net ATP molecules are produced and hydrogen atoms removed form the glucose throughout the process are captured by the acceptor molecule NAD that forms NADH (two of which are produced).

Krebs cycle (TCA cycle or Citric acid cycle)

The Kreb’s cycle consist of a series of  reactions that occur in the matrix of the mitochondria. As the cycle progresses more H+ atoms are removed and accepted by acceptor molecules NAD and FAD to produce NADH and FADH2. At the completion of the cycle 6 carbon dioxide molecules, 2 ATP ,8 NADH and 2 FADH2 are produced.

Electron transport

Occurs in the inner membranes of the mitochondria (cisternae). The acceptor molecules NADH and FADH2 are brought to the complexes that make up the electron transport chain that donate their electrons to be transferred from one complex to another, that occurs in conjunction with the pumping of Hydrogen ions into the intermembrane space. The electrons are finally get accepted by oxygen. The hydrogen ions get transferred back to the other side of the membrane which results in the production of ATP and they combine with the oxygen to produce water. As a result the electron transport chain produces 6 water and 30-32 ATP.

Note the amount of ATP produced depends on the type of cell for example muscle cells may have higher energy needs.

Anaerobic respiration

Anaerobic respiration goes through glycolysis but fails to enter the Kreb’s cycle and then the electron transport chain. This produces only 2 ATP. It is a fast way to obtain a bit of energy quickly but cannot be sustained. Pyruvate gets further converted after glycolysis into lactic acid in animals (which builds up in the muscles to cause fatigue) and ethanol in yeast and plants. This allows for the regeneration of acceptor molecules.

See also