How is the body’s energy actually created? Where does this energy come from? Well Energy is always derived from food, mainly carbohydrates; these carbohydrates then go through a series of chemical processes, leading to oxidative phosphorylation in the mitochondria. Oxidative phosphorylation is when these reactions come to fruition by combining chemical reaction to dynamic physiology creating ATP - the bodys fundamental energy unit.


There are four classes of biomolecules, these are, proteins, nucleic acids, lipids and carbohydrates. Carbohydrates make up the majority of organic matter on earth due to their multifunctionality; this includes their role as an energy source.

The central carbohydrate as far as energy is concerned is glucose. Glucose is a simple sugar or monosaccharide; this means it is a single sugar unit. Table sugar (sucrose) is a disaccharide meaning it is made up of two monosaccharides; one glucose and one fructose molecule are joined by a glycosidic bond to form sucrose. Disaccharides are broken down in the body to form monosaccharides using various enzymes e.g. sucrase.

Various other more complex carbohydrates from different food stuffs e.g. bread, pasta or rice can be broken down in to simple sugars in a similar way. These sugars can then be passed through a biochemical process called glycolysis (the name is derived from the Greek prefix Glyk-, meaning sweet and lysis meaning dissolution – i.e. breakdown of sugar).


Glycolysis is a very important step; the main purpose is it converts sugars in to pyruvate. A side gain is a small amount of energy (2 ATP molecules) produced from the bond energy broken in the process. The production of pyruvate from sugars is almost identical in all organisms.

Pyruvate can be converted in to one of three things depending on organism and physiological condition.

  • Firstly in yeast and several bacteria, it is converted in to ethanol as a bi-product of respiration.
  • Secondly pyruvate can be converted to lactate. This happens in a variety of microorganisms, it also happens in higher organisms (such as you) when exercising hard. Since energy demands outstrip what can be supplied by normal aerobic respiration (requiring oxygen), anaerobic respiration takes place. This form of respiration can only take place for a short period and is responsible for lactic acid (a toxin) production leading to muscle cramp.
  • Thirdly it can be converted in to acetyl CoA and enters the Kreb’s or Citric Acid cycle.

Citric Acid Cycle

The Kreb's cycle is a very important pathway that links the metabolism of all biomolecules; and is the main set of reactions that take place in higher organisms responsible for the production of energy compound intermediates. I wont talk too much about glycolysis or the Kreb’s cycle as there is a lot of excellent information available in other nodes on E2.

The products of the citric acid cycle are converted in to ATP by the electron transport chain; this transmits an electronic charge by a process of oxidation. The electrons are taken from NADH (Kreb product) and transferred to oxygen. To cut a long story a little bit shorter, this reaction releases a lot of energy, which is harnessed to add a phosphate to a compound called ADP (adenosine diphosphate) to form ATP. When energy is used, ATP is broken down to ADP, the bond broken releases a lot of pent up energy and hence an energy cycle is created.

So how do these chemical processes lead to phosphorylation of ATP?
This is the cleaver bit! If everything else has washed over you so far, now is the time to put on your thinking cap and visualise.

A good metaphor to think about is a water pump, pumping water up-hill. When the water then flows down again it drives a water turbine and creates energy.

So here we go:
All these processes happen in the mitochondria (the powerhouse of the cell). The mitochondria (see fig.1) are composed of an outer membrane, an intermembrane space, an inner membrane and the matrix (the centre of the mitochondria). The electron flow created by oxidation within the electron transport chain, is used by proton pumps to pump the hydrogen ions from the matrix, through the inner membrane and in to the intermembrane space (uphill bit). This leads to a high H+ concentration in this intermembrane space creating a concentration gradient across the inner membrane, the protons then ‘flow’ (downhill bit)down the concentration gradient through the most accessible route back in to the matrix. The process by which this happens is called facilitative diffusion. The protein through which this diffusion occurs is via a very specialised pore in the inner membrane, which is called ATPsynthase.

fig.1 Mitochondria (the ATP generator)
 /                Intermembrane Space                \
 |         ________________________________          |
 |        |                                |         |
 |        |                                |         |
 |        |             MATRIX             |         |
 |        |                                |         |
 |        |________________________________|         |
 |                                                   |

F0/F1 ATPase (ATPsynthase)

ATPsynthase is made up of two parts, F0 and F1. F0 is the pore-forming unit, which is a hydrophobic segment that spans the inner mitochondrial membrane. This pore (F0) is what the protons will flow through and is made up of 4 types of polypeptide chains each of a different length. The F1 section has a diameter of 85 angstroms and is the catalytic section of the complex and is made up of 6 different types of polypeptides. If the F1 is removed from F0, it is capable of hydrolysing ATP to ADP, since ATP phosphorylation is a reversible process. ADP can only be phosphorylated to ATP by the F1 unit, when F0 is coupled to F1. The driving force of the ATPsynthase is known as the proton motive force.

So the final stage of metabolism requires oxidation to create an electron flow, which leads to phosphorylation of ADP to ATP (the body’s energy). Collectively, this is known as Oxidative Phosphorylation.

Thanks for feedback on this node, soon to be linked due to popular demand is how ATP is used
Stryer, L. (1995). Biochemistry - 4th Edition. W.H. Freeman and Company.

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