Respiration – The Stages of Respiration – Second stage

Respiration – The Stages of Respiration – Second stage

Krebs Cycle

Discovered by Sir Hans Krebs in the 1940s, Krebs Cycle is also known as the citric acid cycle and the tricarboxylic acid cycle.

Acetyl leaves its coA and combines with a 4-carbon oxaloacetate to create a 6-carbon citrate. Over a number of steps the citrate is broken down to reform back into oxaloacetate. During this process CO2 and hydrogen atoms are released, the CO2 diffusing out of the cell and the hydrogen being picked up by either NAD or another hydrogen carrier, FAD. The hydrogen atoms are transported by their carriers to the inner mitochondrial membrane.

The Stages of Respiration

The Link Reaction

The link reaction is the stage between glycolysis and the Krebs Cycle in which pyruvate is converted into the compound acetyl coA. Pyruvate loses CO2 and a hydrogen atom to become acetyl, a 2-carbon compound. It then attaches to coenzyme A, hence the name acetyl coenzyme A or acetyl coA.

CO2 is the waste product and it moves out of the mitochondria and cell membranes via lipid diffusion. From here it travels into the blood through the tissue fluid and is carried to the lungs where it can be expelled. The hydrogen is picked up again by NAD.



Different parts of cellular respiration occur in different areas of the cell which means that each can be easily controlled. However, the majority of it happens in the mitochondria.

Mitochondria are formed of a double membrane. The outer membrane is comprised of porins which allow for the passage of nearly any small molecule. The inner membrane, on the other hand, is impermeable to most molecules. It’s composed of many folds, known as cristae, which increase its surface area. This is where ATP synthase is located. The space after the inner membrane, the matrix, is where Krebs cycle happens. It also contains, DNA, tRNA and ribosomes.