The next phase of ATP production is known as the link reaction. It is so-called because it links the aerobic reaction to the anaerobic reaction. In this reaction the two pyruvate molecules are oxidised to reduce another two NAD molecules. The pyruvate molecules form acetate (a two-carbon molecule) and also each gives off a carbon dioxide molecule. Overall two NADH molecules and two carbon dioxide molecules are produced with no ATP usage or production. The acetate molecules go on to react with a further two molecules, known as ‘coenzyme A’. The result is a three-carbon molecule – acetyl coenzyme A.
Acetyl coenzyme A then enters the Krebs Cycle (still in the matrix of the mitochondria). In essence this cycle is a series of oxidation reactions in which there is a net production of 2ATP molecules, four carbon dioxide molecules are released (two have already been released from the link reaction), six NAD molecules are reduced and two FADH2 molecules are produced. FADH2 is very similar to NADH as it is just an electron carrier used in the electron transport chain. It is important to note here that overall twelve hydrogen atoms have been derived for use in reduction. As there are only twelve in hydrogen atoms in glucose, the sheer efficiency of this process can be appreciated as the glucose molecule is oxidised to completion.
From the various reactions already described there are in total ten NADH molecules produced and two FADH2 molecules produced. These molecules diffuse towards the cristae of the mitochondria. The cristae demonstrates the level of adaptation that mitochondria have. A popular theory of the mitochondrial formation is that it first existed as a bacterium that became symbiotic with another cell. This is supported by the fact that there is no sequence in human (or any organism’s) DNA for the expression of mitochondria, the only way they are inherited from parent to offspring is that they exist in the egg and become part of the embryo. The theory of natural selection supports the development of cristae becoming more and more tightly folded because it increased the surface area for enzyme activity in the chain reaction, thus producing ATP at a quicker rate for the host cell.
The cristae consists of phospholipids with a series of proteins specific to mitochondrial membrane. The reaction which occurs here is known as oxidative phosphorylation. FADH2 and NADH, the electron carriers, are reduced causing the first protein in the cristae to become oxidised. This protein then is reduced and the electron is passed along to oxidise the following protein. From this reduction/oxidation reaction energy is derived. This energy is used to phosphorylate ADP to ATP. At optimal conditions each electron from NADH can derive enough energy to phosphorylate three ADP molecules, and each FADH2 can produce two ATP molecules. At the end of this process the electron is accepted by oxygen which combines with hydrogen atoms to form water.
Overall, the reaction is: C6H12O6 + 6O2 → 6CO2 + 6H2O
This reaction occurs in both plants and animals. However, light independent reaction is another source of ATP production exclusive to photosynthetic organisms. It occurs in the thylakoid part of the chloroplast. This houses chlorophyll and it is here that the light dependant reaction takes place. The commencing process is called the photolysis of water. It is the complete breakdown of water using energy from sunlight: H2O → ½O2 + 2H* + 2eˉ
The two electrons gain a lot of energy from the sunlight and are transported to electron carriers. Here, ATP can be produced in much the same way as it is produced in the electron transport chain. A series of oxidation and reduction reactions derive energy and this energy is used to chemically bond inorganic phosphate groups with ADP molecules. At the end of the process the electron can be used to reduce NADP. This molecule is very similar to NAD used in respiration. The only exception is that NADP has a phosphate group (nicotinamide adenine dinucleotide phosphate).
The reaction is: NADP* + 2eˉ + 2H* → NADPH (reduced NADP) + H*
This whole process uses a single molecule of water and produces two ATP molecules and one NADPH molecule. Oxygen is given off as a waste product. This completes the process of non-cyclic photophosphorylation and the NADPH and ATP go on to be used in the light independent stage of photosynthesis. There is, however, a second method in which ATP can be produced in this fashion. Rather than reducing NADP, sometimes the electrons return to the chlorophyll where they are further excited. They go through the electron carrier system again, thus producing more ATP. This process is known as cyclic-photophosphorylation.
Leading on from the light dependent stage of the reaction is the first use of ATP: the light independent reaction. This process happens in the stroma of the chloroplast. It uses reduced NADP and ATP in the production of starch, a polymer of hexose sugar. Starch is stored by plants rather in this form rather than as glucose or other non-polymers because starch is osmotically inert. Therefore it does not disrupt the metabolism or overall chemistry of a cell. Starch can be used when required for the production of amino acids, lipids or other forms of carbohydrate.
This reaction is activated as carbon dioxide diffuses through the stomata into the cell and then diffuses through the palisade layer of the leaf up to the chloroplasts, and then it diffuses through the cell membrane and cytoplasm into the stroma. A five-carbon compound, ribulose bisphosphate, then combines with one molecule of carbon dioxide to form an unstable six-carbon compound. This breaks down into a pair of three-carbon molecules: glycerate-3-phosphate. The use of one mole of ATP is now required to lower the activation energy for the formation of one mole of triose phosphate from one mole of glycerate-3-phosphate. The reduced NADP is used here also, it is oxidised and this allows the glycerate-3-phosphate to be reduced to triose phosphate. The NADP* molecule now returns to the light dependent stage to be reduced once more. Sometimes a pair of the triose phosphate molecules will combine to form an intermediate molecule called hexose sugar. This sugar is polymerised to for starch. On other occasions five molecules of triose phosphate can be converted to form three molecules of the five-molecule compound ribulose bisphosphate. These molecules are now ready to react with carbon dioxide once again. The formation of these molecules is however energy requiring. The remaining ATP molecule produced from the light dependent stage is used in this reaction.
An important role for ATP in animal organisms is through the nervous system. This system is responsible for everything from emotion, to homeostasis, to blood pressure to skeletal movement. ATP is used at the synapse of a nerve, be it a nerve-nerve junction of a neuromuscular junction. A stimulus causes an action potential to carry across the axon of the nerve cell. When it reaches the synaptic knob the potential difference causes calcium channels to open which causes calcium ions to flow into the knob. If the action potential is above threshold then enough calcium channels will open causing more vesicles containing acetyl choline to move towards the presynaptic membrane. The vesicles fuse with this membrane releasing the acetyl choline into the synapse. These molecules diffuse across and bind with receptor proteins on the postsynaptic membrane. A change in the permeability of the membrane is the result, causing sodium ions to pass into the cell, causing the development of the excitatory postsynaptic potential. Provided that this potential difference reaches threshold, an action potential is fired allowing the nerve impulse to carry on past the first neurone. Once this action potential has been fired the acetyl choline is released and acetyl choline esterase breaks down the molecule. The products are reabsorbed back into the synaptic knob where ATP is used to reproduce the acetyl choline molecules. This is important in order to maintain the correct diffusion gradients.
Another use for ATP is in muscle contraction. Because mitochondria provide the site for aerobic respiration and ATP production, there are substantial numbers of them in muscle tissue. A single muscle fibre is constructed by rows of sarcomeres. These comprise of a band of myosin with bands of actin on either side. When muscle contraction occurs, the actin bands move in towards the centre of the myosin bands, thus making the ‘H-band’ smaller. With the majority of skeletal muscles, there is always a second antagonistic muscle. This counteracts its opposite muscle to return the limb to the original position. On each myosin molecule there is a head containing ADP and an inorganic phosphate group. This head can attach to an actin group, and upon bonding the tertiary structure of the myosin head changes, causing the rest of the myosin to move along to accommodate the change in structure. The reaction between the actin and myosin now causes the head to release the ADP and Pi which is taken up by a mitochondrion. In the processes outlined above, this is converted to ATP via respiration, and ATP is released back into the muscle tissue. This ATP is needed by the actin-myosin bridge to be released. ATP diffuses into the myosin head, and the donation of a phosphate group to the bond lowers the activation energy, allowing the head to be released. It resumes its original tertiary structure and is available to bridge with another myosin further down the fibre. This process occurs over and over again, making the overall contraction a ratchet motion.
