Introducing ATP

by L. R. B. Mann
Chemistry Dept., Victoria University of Wellington

Many scientists today are noticing and rejoicing that the ‘separate disciplines’ of science are tending to merge or at least to overlap. This very desirable movement is clearly seen (and is extremely important) in the relationship between Chemistry and Biology.

Only a few decades ago, chemists mostly regarded living things as bewildering sources of an inexhaustible fund of strange compounds, and as possible cheats in the game of physical chemistry. Biologists were still largely occupied with classification, and of the few who asked the question ‘What chemical changes are going on in these living cells?’, most despaired of there being any answer obtainable by scientific investigation.

Today things are different. The catalogue of chemicals which occur in cells seems largely complete, and good progress has been made towards following the processes whereby food is built up into an organism's own substance, or burnt as fuel.

One outstanding discovery has been the ubiquitous biochemical known familiarly as ATP. This compound seems to be of immense importance in the workings of organisms. It is in fact a ‘portable power-pack’, assembled at a few special factories in a cell and carried about all over the other parts for use as a source of energy in mechanical or chemical operations.

‘ATP’ stands for ‘adenosine triphosphate’. Its molecular structure is:

The non-chemist need not take fright and flee from either the name or the formula! For our present purposes, we can represent the molecule quite simply with this equivalent picture:

A — (P) — (P) — (P)

The complexities of the ‘A’ part (adenosine) need not concern us: attention should be focussed on the string of three phosphate units, each drawn as (P), which give rise to the epithet ‘triphosphate’.

The splitting of ADP, leaving the monophosphate AMP, is also very exergonic.

This energy is used to drive end-ergonic reactions, i.e. those which must absorb energy if they are to proceed. Now cells abound with endergonic reactions — for making proteins, starch, nucleic acids and many other essential substances. ATP is used as a source of energy to drive along these reactions.

Some people get the idea that the way this works involves merely ‘letting off an ATP squib’ in the vicinity of some recalcitrant compound which is thereby somehow hustled along and undergoes the desired endergonic reaction in double-quick time. This is quite wrong. To give an example of how ATP is in fact used to make such reactions go, let us take the joining of two simple sugars, glucose and fructose, to form the ‘double sugar’ sucrose. What happens is actually a ‘coupling’, or gearing together of the two reactions

H₂O + ATP = ADP + (P) (1)

glucose + fructose = sucrose + H₂O (2)

These reactions (1) and (2) are not simply conducted in the same vicinity. The actual reactions which occur are:

glucose + ATP = glucose—(P) + ADP (3)

and then glucose (P) + fructose = sucrose + (P) (4)

Although (1) and (2) add up to the same nett reaction as (3) and (4), they do not in fact represent the true process.

Many different compounds needed by a living cell are built up in a comparable way.

ATP is thought also to provide the energy for muscular work, for bioluminescence, for absorption and secretion and for generation of high voltages. It has been known for about 30 years that ATP is used in the action of muscle; but just how its stored energy is converted to mechanical work is not understood, though dozens of theories have been suggested.

How does a cell assemble this energy bundle? Exactly as much energy as is given off when ATP is split must be supplied to form ATP from ADP and free phosphate. A cell's energy comes from oxidising food, usually to carbon dioxide; and green plants have the additional resource of trapping sunlight and using its energy for their own chemical needs. Both these processes, respiration and photosynthesis, are so arranged in the cell^* that much of the chemical energy made available is stored in ATP. The ATP formed can then be sent off to where it is needed for driving endergonic reactions, making muscles do work, etc.

Here we have seen one example of what can be gained by applying knowledge and methods from one ‘;separate discipline’ of science to problems which had been considered to belong in another sphere. Biologists need not think that the movement of overlapping is one-way. At least some chemists are predicting that biologists will help tackle chemical problems a good deal more in the future than they do now; and of course their present aid is quite considerable. A good deal of specialisation in our studies is no doubt necessary, but it should not be allowed to put blinkers on our scientific outlook.