Physics Made Easy


Respiration can be divided into two parts:

  1. Cellular/internal/tissue respiration. This describes the metabolic processes within cells that release energy from glucose.
  2. Gaseous exchange/external respiration. This is the process of gaining oxygen for cellular respiration and removing waste CO2.

Overall equation of respiration

Respiratory substrate (energy source) + oxygen carbon dioxide + water + energy

In reality respiration is more complex than this simple equation (scroll down for the biochemistry of respiration). The point of respiration is to provide energy for living creatures (this energy is then used in metabolism etc).

Note: the above equation describes aerobic respiration, which occurs when oxygen is present. If oxygen is absent, an organism can also undergo anaerobic respiration (scroll down for more information on this).

Respiratory substrate: Usually a mixture of carbohydrate and fat is respired. Carbohydrates are broken down into simple sugars like glucose and fructose by digestion before they are respired. Protein is rarely respired unless the body is malnourished or starved.

Respiratory Quotient (RQ): this is a measure of the ratio of CO2 evolved to O2 consumed in unit time during respiration.

RQ = CO2 evolved / O2 consumed

For different respiratory substrates the RQ varies. Examples are:

  1. Glucose and other hexose sugars

C6H12O6 + 6O2 6CO2 + 6H2O +energy

This equation is the one you’ll be expected to remember.

  1. For fats, RQ=0.7 (typically), i.e. fats require more oxygen than carbohydrates for complete oxidation (this makes sense, as fats contain proportionally less oxygen than carbohydrates).
  2. For proteins, RQ≈0.9 (due to the wide variation in protein structures and exact figure cannot be given for proteins in general)

The mixture of glucose and fats typically respired by living creatures gives an average RQ of 0.8-9.
Note: substances are rarely completely oxidised during respiration, so the theoretical RQ and actual RQ for a particular situation may not be the same.

Respirometers are experimental tools used to measure the aerobic respiration rate of small life-forms. They are used as follows:

  1. Mark initial position of meniscus of water
  2. The organisms respire, taking in O2 and evolving CO2, which is absorbed by the KOH.
  3. Measure the movement of the meniscus of the water towards the organism in unit time to find the volume of O2 consumed in that time.
  4. Repeat the experiment using water in place of KOH (CO2 is no longer absorbed, but the total volume of apparatus inside the tube is kept constant). Record the distance moved by the meniscus of the water towards the organisms in unit time. This distance equals O2 consumed – CO2 evolved
  5. Repeat as necessary to get a decent sample of results. The average amounts of O2 consumed and CO2 evolved in unit time can also be used to calculate the RQ of the organisms.

Limitations of the respirometer:

  1. Does not take into account changes in gas volume due to temperature or pressure changes during the experiment.
  2. When KOH or NaOH absorbs CO2, it alters the composition of gases in the respirometer, possibly altering the organisms’ breathing rate.

ATP (adenosine triphosphate) is a short-term energy store which is easily transported around the body. Energy is released by the hydrolysis of ATP by the enzyme ATPase- the following reactions occur:

ATP ADP + Pi + energy
ADP AMP + Pi + energy
AMP A + Pi + some energy

(ADP= adenosine diphosphate; AMP= adenosine monophosphate; A= adenosine; Pi = phosphate group).

ATP transfers energy from energy-rich compounds to cellular reactions and provides energy for anabolic processes (building up large molecules), movement (muscular contraction), active transport, cell division, secretion (vesicle formation) and activation of chemicals.

Structure of ATP: [to be uploaded]

Phosphorylation: the addition of phosphate groups, e.g. to ATP. If the process requires oxygen, then it is called oxidative phosphorylation.

Biochemistry of respiration

  1. Glycolysis takes place in the cytoplasm and is the same in both aerobic and anaerobic respiration


  1. The link reaction takes place in the mitochondria

pyruvic acid + fatty acids +amino acids acetyl coenzyme A + 3 ATP + CO2

Acetyl coenzyme A is a 2 carbon compound.
Remember, each molecule of glucose produces 2 molecules of pyruvic acid, so 6 ATP molecules are produced in the link reaction per molecule of glucose respired.

  1. Kreb’s Cycle/tricarboxylic acid cycle/citric acid cycle breaks down macromolecules to CO2 and water. It takes place in the mitochondria.


The intermediate compounds in Kreb’s cycle can be used to manufacture other substances, e.g. fatty acids, amino acids and glycerol.

Note: Kreb’s cycle produces 12 molecules of ATP per molecule of acetyl coenzyme A, which means that 24 ATP molecules are produced per molecule of glucose.

  1. Electron/hydrogen transport system/chain / mitochondrial shunt is how H+ ions are used to make ATP. H+ ions and electrons are passed down a chain of carriers on the crista membranes of the mitochondria. As they are passed from one carrier to the next, the energy released is used to produce ATP (this process is called oxidative phosphorylation).
    At the end of the chain, H+ ions and electrons recombine to form hydrogen atoms. Oxygen acts as the final hydrogen acceptor by reacting with it to form water (this is known as metabolic water).
    Water formation is catalysed by cytochrome oxidase, which is inhibited by cyanide. This causes the accumulation of hydrogen atoms, and the cessation of aerobic respiration, which is why cyanide is such an effective poison.

Energy produced by respiration
Per molecule of glucose:
2 ATP molecules required
40 ATP molecules produced: 4 from glycolysis, 16 from the link reaction and 24 from Kreb’s cycle
Therefore, overall aerobic respiration produces 38 ATP molecules per molecule of glucose.

Mitochondrion structure

[to be uploaded]

Anaerobic respiration only produces 2 ATP molecules per molecule of glucose. Oxygen is not present, so oxidative phosphorylation (i.e. Kreb’s cycle and the hydrogen transport chain cannot take place. Instead, H+ ions combine with pyruvic acid to form lactic acid (animals) or ethanol (plants). Glucose is not completely broken down and NAD is regenerated.

Organisms which can respire anaerobically: micro-organisms can respire anaerobically. Animals only respire anaerobically during vigorous activity, when there may not be enough oxygen available to provide energy. Plants may respire anaerobically in waterlogged soils (roots cannot obtain enough oxygen).
There are two types of anaerobically respiring organism:

  1. Obligate anaerobes live permanently in areas of low, if any, oxygen and always respire anaerobically. Oxygen is poisonous to some obligate anaerobes.

Facultative anaerobes only respire anaerobically when oxygen is in short supply. Most anaerobes are facultative.

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