Glycolysis occurs in the cytoplasm of a cell and involves the production of 2 molecules of pyruvate which occurs when glucose undergoes phosphorylation, lysis and then oxidation, yielding pyruvate and 2 molecules of ATP along with 2 molecules of reduced NAD. In aerobic respiration only the reduced NAD generated here enters a mitochondrion and goes into the electron transport chair where it is used to generate 6 molecules of ATP. Hence, the net ATP produced from glycolysis are 8 molecules of ATP.
The next stage is krebs cycle which occurs in the matrix of the mitochondria, the resulting product from oxdative decarboxylation of pyruvate is progressively degraded by as series of reactions involving four dehydrogenations, two decarboxylations and one phosphorylation. The krebs cycle liberates 2 ATP molecules. However, most of these are only generated in the electron transport chain in the inner membrane of the mitochondria and the cristae, this is where ATP is generated by transferring electrons from the reduced hydrogen acceptors, generated by glycolysis and the krebs cycle to oxygen, via a series of electron carriers, which undergo oxidation- reduction reactions. The hydrogen ions produced from their removal from the reduced hydrogen acceptors are utilised in the proton pump theory and synthesise 28 ATP molecules. Overall, the whole process of cellular respiration produces 38 ATP molecules (per molecule of glucose) (Glycolysis = 8 ATP, Krebs = 2 ATP, Electron transport chain = 28 ATP).
We have established how ATP is produced in respiration, but the role of ATP is much more widespread. ATP is used in the kidney for example, where the process of active transport is dominant. Active transport is the movement of substances against the concentration gradient using ATP from respiration and carrier proteins in the membrane. The active transport of sodium ions from the ascending limb into the medullary tissue involves ATP, which allows the water to be reabsorbed because the water potential of the tissue surrounding the loop of henle is decreased.
ATP is also used in proteins synthesis, which occurs when the bases are forming bonds by complimentary base pairing e.g. A-T, C-G during semi conservative replication where one half of the original parent DNA is conserved in the daughter DNA, or during transcription, which provides a single stranded mRNA. All these processes use the energy released from ATP and thus go on to synthesise proteins and enzymes; which are themselves essential for biological processes including digestion in the gut, as antigens and plasma membranes.
The role of ATP is also imperative for movement of living organisms, for example, muscle contraction. Here the ATP is utilised in forming cross bridges between Actin and Myosin as well as unbinding the cross bridges. ATP will aid in the movement of tropomyosin to allow the myosin binding sites to become free, allowing cross bridges to form. However, it then binds to the troponin causing the myosin heads to become released and the cross bridge is broken. This ratchet mechanism involves the use of ATP and is essential for muscle contraction. Locomotion is also a type of movement in living organisms such as sperm cells, which have an abundant supply of mitochondria in the middle piece allowing it to swim up the fallopian tube and eventually fuse with the secondary oocyte; it can now clearly be seen that ATP is essential for sexual reproduction in humans, and the existence of humans.
ATP has a role in synaptic transmission, where they release energy for resynthesis of synaptic vesicles, for example in the resynthesis of Acetylcholine, and possibly for the pumping of Ca²+ to restablish the concentration gradient across neuron membrane. ATP is also required constantly in the mammalian eye to maintain active transport of ions and the synthesis of rhodospin from opsin and retinene.
ATP is not only generated by the process of respiration, but autotrophs, which are organisms able to synthesise complex organic compounds from simple inorganic compounds, use an external source of energy such as light, hence photosynthesise produces ATP. The production is associated with the dependant reaction (thylakoids in the chloroplasts) and consumption or role is in the light independent reaction (in the stroma of the chloroplast). The light dependent reaction basically absorbs solar energy, which excites chlorophyll molecules called photosystems; this raises electron energy levels and causes high-energy electrons to be emitted. Energy released from these high electrons are then used in the synthesis of ATP in the process of photophosphorylation (as ATP synthesis now involves light energy).
Photophosphorylation can be cyclic, generating ATP only, or non cyclic which generates ATP and reduced NADP, unlike respiration the reduced hydrogen acceptor does not go on to produce ATP.
The ATP and reduced NADP are then used in the light independent reaction which involves the transformation of carbon dioxide into hexose. This hexose is glucose and is used to produce numerous compounds which are essential to the plants existence, for example, it acts as a respiratory substance, it can be synthesised into cellulose for the cell walls which provide turgidity and prevent osmotic bursting, the glucose can also be used in synthesis of lipids and amino acids, without ATP the hexose would not be formed and none of the compounds would be produced, thus the plant would not exist.
ATP is used in active transport in plants, similar to the kidney in humans. The absorption of minerals, such as nitrates & phosphates require ATP, which are present in root hair cells. These nitrates and phosphates are used in protein and chlorophyll synthesis as well as synthesis of DNA, ATP and NADP, which are essential to the plants growth. However, the mineral salts which are carried in solution by the symplast or apoplast pathway need to cross the endodermal barrier, which is impermeable. They cross the endodermal barrier by active transport and continue their journey in solution as ions in the xylem.
In conclusion, ATP is adapted to its function and is probably one of the most important molecules in biological processes; it is produced in the mitochondria but is used all over the human body and is equally important in plants, where it is produced in the thylakoids. ATP is essential for survival as it allows the growth of autotrophs and thus supports all the food chains in the world, and also is important in maintaining and controlling the human internal environment.
