ATP

Energy is a tricky concept when it comes to biology. Because biology is often considered a very a applied science, it is not always possible to easily describe biological processes in terms of fundamental 'pure' processes like energy transfer. Science knows a lot about energy transfer in simple, idealised situations which we can use the laws of thermodynamics for, but when we consider how complex biology is, it doesn't really make sense to talk about energy in these terms. Fortunately, that gives biologists the licence to talk slightly loosely about energy. Nevertheless, there are still some important points about energy which must be maintained. For example:

• Energy cannot be created or destroyed, only transferred between forms.
• You cannot say something has used or created energy as this contravenes the first law of thermodynamics.
• It is however, acceptable to say things like 'energy is transferred and utilised' by something.

Everything in the universe has a tendency to want to be in it's lowest energy state possible and shed its energy. This is the second law of thermodynamics and paints a scary picture for the future of the universe (but not for a few years yet). Adenosine triphosphate (ATP) - that glorious molecule - is no exception to this rule, and biology has learned to use this property to its advantage. Pretty much every active (energy-dependent) process in biology depends on ATP to function. These include:

• Photosynthesis
• Active transport
• DNA synthesis and replication
• Protein synthesis
• Cell division
• Muscle contraction
• Homeostasis

You might notice that a lot of these involve movement. Movement of course requires energy which is served up by the almighty ATP.

ATP has its origins in two antagonistic reactions: photosynthesis and respiration. These have opposing equations which can be seen in the diagram to the right. With the addition of energy, glucose can be synthesised and used as a store of energy. This is a very important feature of glucose as opposed to ATP as ATP is a very unstable molecule and cannot be stored, so must be used almost as soon as it is produced.

In order to fully understand ATP, it is important to understand its structure. In the diagram to the left, you can see that it contains a double-ring nitrogenous base (adenine) which bonds to the pentose sugar ribose. Furthermore, the ribose is bonded to three inorganic phosphate groups in a chain. Each time one of these is hydrolytically removed, there is a net exothermic reaction. So ATP-->ADP-->AMP.

When ATP is produced, it is sent (somehow) to wherever it needs to be and it will undergo a hydrolysis reaction in the presence of the catabolic enzyme ATPase. This breaks ATP down into ADP and an inorganic phosphate group. In doing so, the net enthalpy change of the reaction is negative (so it is exothermic). The energy released is then transferred to the part of the cell which needs it (e.g. a proton pump) which allows it to do useful work. 

However, if the reaction was only conducted in one direction, there would be a huge demand for adenine and a large waste of ADP and inorganic phosphate. But biology is seldom as wasteful as this. ADP and inorganic phosphate can be reformed into ATP using the enzyme ATP synthase during respiration in oxidative phosphorylation. ATP can also be reformed in cyclic and non-cyclic photophosphorylation during the light dependent stage of photosynthesis. Please note that even though ATP is produced during the LDR of photosynthesis, the same amount is used up in the Calvin cycle, so the net production of ATP in photosynthesis is 0.

You might now be wondering why ATP has been elevated to the status of 'God molecule' in biology. After al, it doesn't look particularly amazing, and surely a similar job could be carried out by other molecules. Correct, it probably could. But all known organisms have evolved around this molecule, suggesting that it arose in the simplest organisms at the dawn of life, and reap its benefits. There are some key features of ATP which make it such a good energy currency:

• It is small and soluble so can be moved easily around the cell.
• It is easily hydrolysed, so energy can be quickly released.
• It can transfer energy to other molecules.
• ATP releases small amounts of energy, so the energy can be precisely controlled.
• ATP won't leak from cells as it needs a protein pump which isn't present on cell surface membranes to pass through the non-polar hydrophobic core. 

And that mostly sums up ATP, or [(2''R'',3''S'',4''R'',5''R'')-5-(6-aminopurin-9-yl)-3,4-dihydroxyoxolan-2-yl]methyl(hydroxyphosphonooxyphosphoryl)hydrogen phosphate as UIPAC have catchily named it using systematic nomenclature. I think I'll stick to ATP though.