Respiration is a set of metabolic reactions and processes that take place with in the cells of organisms. It stores biochemical energy within adenosine triphosphate (ATP) molecules. There are two types of respiration, aerobic and anaerobic. Respiration is also the process of making ATP rather that breaking it down.

ATP (Adenosine triphosphate)

  • ATP is the main energy storage and transfer molecule in the cell. It’s a nucleic acid similar to RNA.
  • It’s formed from ADP (Adenosine diphosphate) + Pι and consists of a ribose sugar, a nitrogenous base (adenine) and three phosphate groups each carrying a negative charge.
  • li>ATP + H2O = ADP + Pι + H+ = 30.6Kj

  • This reaction is catalysed by the enzyme ATPase


The diagram above shows the usual mitochondria which would be found in a eukaryotic cell. As seen, it contains an outer and inner membrane along with folds in the inner membrane called cristea. The center of the mitochondria is called the matrix. This is the site of action where the Krebs cycle and the ETC (electron transport chain) occur.

1. Glycolysis

The conversion to phosphorylated is simply to make it more reactive and the conversion to sugar phosphate easier.
  • This is the first stage of three in respiration and takes place within the cytoplasm of the cell.
  • Glycolysis is the same process for aerobic and anaerobic respiration.
  • In this process the cell gains a net of 2 ATP molecules.

2. The Link Reaction

Pyruvate or Pyruvic acid then diffuses from the cytoplasm into the matrix of the mitochondria. Here it gets converted into acetyl coenzyme A, which is a two carbon structure. A carbon was lost through the release of CO2.

3. The Krebs Cycle

  • The function of the Krebs Cycle is a way to liberate energy from carbon bonds to provide ATP and reduced NAD with the release of carbon dioxide.
  • NADH2 and FADH2 produced by the Krebs cycle are used under aerobic conditions to supply electrons for the electron transport chain but also to supply protons for the proton pumps, across the inner membrane.
  • At the end of this process you are left with 6 CO2 compounds, 4 ATP molecules, 10 NADH2 and 2 FADH2.
  • The Krebs cycle happens twice as much as glycolysis because at the end of glycolysis two, three carbon pyruvates are formed and only one is used during each turn of the krebs cycle.

3. Electron Transport Chain

  • The third stage consists mainly of the movement of H+, carried by NADH2, from the matrix into the intermembrane space via proton pumps in the mitochondria. Then once the H+ molecules are in the intermembrane space they can then pass back through the ATPase into the matrix forming ATP. As you can see from the diagram below ADP is used in this reaction along with Pι (organic phosphate molecule) to form ATP.
  • There is a higher H+ concentration in the inter-membrane space which is why the proton pumps are required.
  • The NADH2 from the Krebs cycle when beginning the electron transport cycle get converted into two electrons (e) and two hydrogen’s (H+).
  • Once the electrons lose their energy they can be used in the formation of H2O

Anaerobic Respiration

  • The process of anaerobic respiration is similar to the aerobic respiration. The first stage, glycolysis, is the same because it doesn’t require oxygen but it does require NAD+.

  • In aerobic respiration the electron transport chain turns NADH back into NAD with the aid of oxygen and thus recycles the NAD. With anaerobic respiration the shortage of oxygen in the cells means that they must find another way to convert NADH back into NAD, this process is called fermentation.
  • Lactate fermentation occurs in mammals when there is a deficiency of oxygen. It has many advantages including strenuous exercise and oxygen demand under water. It works by each pyruvate molecule produced taking up two hydrogen molecules (from glycolysis) to form lactate. This then leaves the NAD+ to be recycled.
  • The lactate produced can cause problems in itself, it’s a toxic chemical and can form cramp in muscles. This lactate can be taken away from the cells by the blood to the liver to be converted into glycogen.

  • In plants fermentation occurs but not in the form of lactate but instead producing ethanol and CO2.

The Respiring of Fats and Proteins

How Fats and Proteins get respired without oxygen.

  • Fats and proteins can also be used to respire. When fats are about to be respired they are broken down into fatty acids and glycerol. The glycerol is converted into triose phosphate and enters the glycolysis stage. The fatty acids are broken down into two carbon fragments and entered into the Krebs cycle via acetyl co-enzyme A.
  • Proteins cannot be stored by mammals so have to either be used or excreted.
  • The respiration of proteins only occurs when there is a excess of them in the diet. To begin their process the amino group is removed (called deamination). The amino group then combines with CO2 forming urea, later to be excreted. The remains of the amino acid, now without an amino group, are called organic acids and are fed back into the Krebs cycle.

Respiratory Quotient

Respiratory Quotient, or RQ for short, is a number used in estimating carbon dioxide production. The equation to find the RQ is:

RQ = CO2 Released ÷ O2 Used

For example a question might ask you: The equation to calculate the oxidation of a lipid is

C57H104O6 + 80O2 = 57CO2 + 52H2O + energy

Use the equation to calculate the respiratory quotient of this lipid. Show your workings.

And the answer? Using the RQ equation, we need to know the amount of CO2 Released and the amount of O2 Used. This will be 57 for CO2 and 80 for O2 and therefore 57 ÷ 80 = 0.7125

An easier question would be: what’s the RQ for this equation:

C6H12O6 + 6O2 = 6CO2 + 6H2

Try it out for yourself!

6 responses to “Respiration

  1. 1. In the TCA cycle what is the process by which NAD+ IS CONVERTED TO NADH?
    2.Excess glucose after a meal will first form glycogen in a process called:
    Glycolysis Glycogenesis,Glycogenolysis and Gluconeogenesis

  2. Can i ask question about marathon runner and a sprinter? A sprinter would depend more on anaerobic respiration is that correct and the marathon runner aerobic and anaerobic? Why? Also atp would that Be stores in the muscles?

    1. Hey Nikole,
      A sprinter would be using anaerobic respiration because they are working at such a high intensity that there is not a supply of oxygen readily available to them. The oxygen in their blood wouldn’t get to the muscles in time to replenish the need, so anaerobic respiration takes place.

      A marathon runner would primarily use the aerobic system while using the ATP system at the very beginning of the race and the lactic acid system will become predominant when the lactate threshold is reached. The body only stores a small amount of ATP in the muscle cells so that there is a readily available source of energy which will last around 2-3 seconds, this is why the body has to resynthesise ATP constantly.

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