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The Roles of ATP
ATP, adenosine triphosphate, is an organic compound composed of an adenosine molecule attached to a ribose molecule which then has 3 phosphate groups attached. It is an extremely good energy-carrying molecule and is used universally by plants, animals and even bacteria. This is due to the lack of stability in the phosphate bonds of the molecule, meaning it can be easily hydrolysed to ADP, adenosine diphosphate, and release energy in the process.
The primary way ATP is produced is via aerobic respiration, which occurs in 4 steps, all of which produce varying amounts of ATP. The first step occurs in the cytoplasm of cells and is called glycolysis, which involves converting one molecule of glucose into two of pyruvate, by phosphorylating the glucose, then causing it to undergo lysis and finally oxidising the two products to give the pyruvate. This process has a net production of two ATP molecules. Step two, which occurs within the matrix of a mitochondrion, doesn’t actually produce any ATP but converts the two pyruvate to two acetyl CoA molecules by removing a CO2 molecule and a H2 molecule from each and bonding each with a molecule of co-enzyme A. This process is known as the link reaction. The third step is known as the Kreb’s cycle also occurs within the matrix of a mitochondrion. This takes the acetyl CoA from the link reaction and converts it to a 4-carbon molecule, via a series of steps, producing 2 ATP molecules in the processing. While each of these steps are taking place, other useful molecules are produced aswell, reduced FAD (FAD/H+) and reduced NAD (NAD/H+). All of the first three steps produce reduced NAD, but only the Kreb’s cycle produces reduced FAD (The electron transport chain also produces both, but uses all that is produced). These molecules are what are used in the final step, the electron transport chain, to produce the largest amount of ATP molecules. This final step all happens along the mitochondrial membrane, between the matrix of the mitochondrion and the intermembranal space. Reduced NAD/FAD molecules collide with the membrane, which causes them to release a H+ ion and an electron. The energy released from this happening, aswell as the pull of electrons moving along the phospholipid bilayer, is enough to force the H+ ions through the membrane. The electrons then move out, into the matrix, after using some of their energy to move H+ ions. The H+ ions, which are in a greater concentration within the intermembranal space, then move through protein complexes embedded in the mitochondrial membrane, via simple diffusion, and on their way, produce energy which activates an ATP synthase enzyme binded to the protein complex, which then produces ATP from ADP and inorganic phosphate molecules. After all of this, the H+ ions and electrons that are now in the matrix bond with oxygen molecules (A.K.A. the final electron acceptor) from respiration, to form water. With the reduced NAD and FAD produced from one molecule of glucose, the ETC produces 34 molecules of ATP. Meaning, in turn, one molecule of glucose is enough to produce 38 ATP molecules.
Now, ATP can be used in a large number of ways in each organism. Perhaps the most obvious is the contraction of muscles in animals, which uses the energy from the breakdown of ATP to contract muscle tissue and give movement of limbs. Another Extremely important use, in both animals and plants, is active transport. Active transport is the movement of molecules, against a concentration gradient. This is achieved by protein complexes which vary in shape, depending on what is being actively transported. The proteins have a shape which is complimentary to the required molecule, and when one of said molecules collides with the protein, an ATP molecule binds to the protein and ADP breaks away, leaving only a single phosphate group attached to the protein. This causes the protein to move or change shape and “physically push” said molecule inside of the cell, against the concentration gradient. Then, the phosphate group breaks away and the protein returns to its original shape, ready for use again.
In plants, an example of active transport being used is in the root hair cells of the plant, to move much-needed ions from the soil into the root of the plant. This also causes the water potential within the plant to decrease/become more negative, so more water moves in by osmosis, which is obviously beneficial to the plant. Animals also use active transport in a similar way, to move glucose and other important molecules, present from digested food, from the lumen of the small intestine, into the cells lining it, to be used for respiration, protein production and other processes. Another use of ATP in animals, that plants do not share, is within neurones. Nerve cells/neurones require ATP to provide energy to sodium-potassium pumps, which are embedded in the membrane of the cell body and within the nodes of Ranvier of the axons of (myelinated) neurones. These sodium-potassium pumps are required to achieve and maintain a resting potential before and after action potentials have been generated along the neurones. These pumps are also proteins and work in the same way as those within root hair cells and the small intestine. Three sodium plus ions from within the cell enter the pump (due to its complimentary shape), and an ATP molecule binds with the protein, and ADP molecule breaks away and a phosphate group is left attached to the protein, the energy this produces causes the protein to change shape and release the sodium plus ions into the tissue fluid surrounding the neurone. Then, two potassium plus ions move into the pump (once again, due to its shape), the phosphate group breaks away, the protein changes and in doing so releases the two potassium ions into the cytoplasm/axoplasm of the neurone. Also, potassium ion channels in the membrane of the neurone are somewhat “faulty” and allow potassium ions to move back out of the cytoplasm due to being partially open, along the concentration gradient, into the surrounding tissue fluid. These factors combined mean there is a much larger positive charge outside of the neurone which induces a potential difference. This results in the neurone, when measured using a voltmeter, having a voltage of -40mV (0.04V). This is important as the neurone must become depolarised to carry an action potential, meaning it must be negative to begin with. As well as using ATP for resting potential within neurones, it is also used at the end of neurones, the synapse. When an action potential reaches the end of a sensory or intermediate neurone, it arrives at a point known as the synaptic knob. The arrival of the action potential cause voltage-gated calcium ion channels to open and calcium 2+ ions to rush in. These calcium ions cause neurotransmitter-containing vesicles to bind with the membrane of the presynaptic neurone and release their contents, neurotransmitters such as acetylcholine, into the synaptic cleft. These can cause an action potential in the postsynaptic neurone. After the release of the neurotransmitters, one of two things may happen. All of the neurotransmitter may be taken up by the postsynaptic neurone or some may remain in the synaptic cleft. Any that remains in the synaptic cleft is then broken by enzymes, e.g. cholinesterase which breaks down specifically acetylcholine. Its constituent parts are then absorbed by the presynaptic neurone and enzymes within the synaptic knob use ATP, from mitochondria also with the knob, to reproduce acetylcholine for future use.
Returning to the use of ATP by plants; plants produce and use ATP, via respiration and photosynthesis, in an extremely clever and efficient manner. As we know, respiration produces ATP and plants use respiration in the exact same way as animals, as do bacteria. However, plants also undergo photosynthesis, which occurs in two stages. The light dependent reaction and the light independent reaction. The first produces more ATP, while the latter uses it to produce glucose for later use in respiration (When light is not available for the light dependent reaction). The light dependent reaction is rather similar to the ETC in how it produces ATP. Different to the ETC, this takes place along a thylakoid membrane. It starts when photons collide with chlorophyll-containing proteins within the membrane. These photons use some of their energy to release electrons from the chlorophyll which then move along the membrane via protein electron carriers (These electrons are being constantly replaced by electrons produced from the photolysis of water). The electrons pull H+ ions through the membrane, into the thylakoid lumen, during their movement, they then leave the membrane. The H+ ions then move through a protein complex identical to the one in the ETC and the energy provided by this movement actives the ATP synthase enzyme and ATP is produced from ADP + inorganic phosphates. H+ ions are also produced in the thylakoid lumen by the photolysis of water. All H+ ions and electrons that move out of the thylakoid membrane are used to bond with NADP and produce reduced NADP. The light independent, also known as the Kalvin cycle, then uses the reduced NADP and ATP from the light dependent reaction to convert CO2 to triose phosphate. This is done by using the enzyme rubisco (ribulose bisphosphate carboxylase) to bind carbon dioxide and RuBP (ribulose bisphosphate), which gives a 6 carbon intermediate which gives 2 GP molecules (glycerate-3-phosphate, which is then converted, using ATP and reduced NADP, to TP (triose phosphate) molecules which can later be used to produce glucose, cellulose, proteins and many other organic molecules. So, plants basically continuously recycle ATP to produce their own food and building materials for growth and energy to allow this growth.
In conclusion, ATP plays an absolutely massively major role in all living organisms. Even viruses, which have no metabolic system at all, so do not require ATP directly, would be unable to reproduce at all without the ATP the host cells they infect use to create more viruses. If ATP were not present in living organisms, there would most definitely have to be a very good substitute in its place and with how well, ATP does its job as an energy-carrier, it is highly unlikely anything could replace it and work as well within organisms, which could produce all manner of unknown affects.