Whilst ATP is one of the main products of respiration, it is interesting to note that within its first stage, glycolysis, 2 ATP molecules are actually used initially in order to phosphorylate the glucose molecule, forming phosphorylated glucose. This activates the molecule allowing a series of reactions to occur, first by splitting into 2 glycerate-3-phosphate molecules and then producing 2 NADH molecules and 4 ATP molecules in total (leaving 2 pyruvate molecules for the link reaction), with a net gain of 2 ATP molecules. It is this latter point that is most significant when considering anaerobic respiration for it means that even if the aerobic pathway is blocked that cells will still yield a net gain (albeit not as great) of ATP molecules in order to continue doing cellular work.
ATP is also required in various ways within the nervous system. During the transmission of an impulse across a synapse (where electrical impulses are converted to chemical signals), once acetycholine has bound to Na+ ion recptors on the postsynaptic membrane, it is then hydrolysed by acetycholinesterase into ethanoic acid and choline (to prevent continuous action potentials being generated). Once the acetate group and choline are reabsorbed by the presynaptic membrane, ATP is then hydrolysed and the energy is used alongside choline acetyltransferase to resynthesise acetylcholine, ready to be secreted from their vesicles should another action potential arrive. ATP is also necessary in maintaining the resting potential of cell membranes by its role as the energy source in the primary active transport mechanism of the sodium-potassium pump. ATP is hydrolysed allowing the N+/K+ATPase protein to change shape; in doing so 3 sodium ions are transported out for every 2 potassium ions (whilst the membrane is 100 times more permeable to potassium ions). This results in a potential difference and an overall negative net charge (usually -65mV in humans) within the cell, thereby retaining the negative electrical differences across the membrane. The same protein is also vital in the complete absorption of glucose from the small intestine. This is because it maintains a low concentration of sodium ions within the epithelial cell thereby allowing the sodium-glucose SGLT1 cotransporter to transport glucose (against its concentration gradient) into the epithelial cell (alongside sodium ions, down their concentration gradient); this is secondary active transport, as it utilises the electrochemical potential difference as its source of energy.
The active transport of other mineral ions is also essential for plants because the active transport not only results in uptake of essential nutrient ions essential for growth and function but also because uptake of ions into the xylem during transpiration helps create a concentration gradient thereby forcing water to move into the xylem via osmosis from the surrounding cells (and indirectly, the surrounding soil).
Apart from all the processes which harness ATP as an energy source, we must not forget its role as a multifunctional molecule, most notably as an extracellular signalling molecule. ATP (as well as ADP) is recognised by purinergic receptors (highly abundant in mammalian tissues), which in humans is important in the central and peripheral nervous systems; ATP binds to these receptors and facilitates the modulation of intracellular calcium and cAMP concentrations. Outside of the nervous system, the binding of ATP to purinergic receptors stimulates other responses such as apoptosis (programmed cell death) and cytokine secretion. Whilst still not well understood, increasing research is taking place because of the implications it could have for cancer research and treatment as PCD and cytokines are acknowledged in the scientific community to be important as to the reason why tumours develop.
Furthermore, ATP plays an important role in the synthesis of proteins for it is used in the activation of amino acids. ATP is condensed to AMP and PPi (pyrophosphate) by means of an enzyme call aminocyl-tRNA synthetase which couples the amino acid to the corresponding tRNA molecule’s extending region via an ester bond. The tRNA molecule in its activated state can then be used in the translation stage of protein synthesis.
ATP is also necessary in the regulation of blood glucose levels alongside the hormone adrenaline, where it again (though indirectly) acts as a signalling molecule. When adrenaline binds to its complementary adrenergic receptors, it activates an enzyme inside the cell membrane which converts ATP to cAMP. The cAMP then acts as a signalling molecule (a ‘second messenger’) which activates other enzymes to convert glycogen into glucose, thereby increasing blood glucose concentration of the body which is necessary for the fight or flight response when more metabolic activity will be occurring thereby requiring more respiratory substrate.
In conclusion, whilst ATP serves its function well as an energy input molecule for metabolic reactions, it has also been interesting to note its various other functions not least as a extracellular signalling molecule but also for its input in respiration in order to yield yet more ATP as a result. And considering the vast areas of effect that it has on cellular processes it can be concluded without fear that ATP is one of the most important molecules in plant and animal biology.
