The production and use of ATP in living organisms

Adenosine Triphosphate, also known as ATP, is the molecule responsible for the energy that we, and all other organisms, need to survive. It is produced primarily in the processes of aerobic and anaerobic respiration by oxidative and substrate phosphorylation. 4 molecules of ATP are produced from 4 ADP and 4 inorganic phosphates in glycolysis in the cytoplasm of every cell, by the oxidation of a triose phosphate into two molecules of pyruvate.

In anaerobic respiration these are the only 4 ATP molecules produced per molecule of glucose as there is no oxygen available for the link reaction or electron transport chain to occur in the cytoplasm, instead the pyruvate molecules are reduced into either lactate in muscles or ethanol and CO2 in yeast. However 2 ATP molecules are used in the phosphorylation of glucose at the start of glycolysis so the net product of anaerobic respiration is just 2 ATP. In aerobic respiration the pyruvate molecules move into the mitochondrial matrix where they undergo the link reaction, releasing one CO2 molecule and one NADH each.

This leaves two acetyl co-enzyme A molecules which enter the Krebs cycle to release another 2 CO2 molecules, 1 ATP, 3 NADH and 1 FADH each. So far we have a net production of 4 ATP (subtracting the 2 used in glycolysis). The electron transport chain is where the majority of ATP is produced. 10 NADH and 2 FADH (produced from glycolysis, link reaction and the Krebs cycle) are oxidised to NAD and FAD, releasing 12 hydrogens. These hydrogens are split into protons and electrons.

The electrons are passed from carrier to carrier in the bilayer of the inner membrane of the mitochondrial cristae, releasing energy at each one. This energy is used to pump the protons through the carriers into the intermembrane space, creating a gradient. Due to this gradient, the protons flow from the intermembrane space back into the matrix by ATP Synthase in the inner membrane. This movement of protons allows 28 ADP and 28 inorganic phosphates to form 28 ATP molecules, while the protons and electrons are left to react with oxygen to form H2O.

Overall, respiration produces 32 ATP molecules per glucose molecule, making it a very efficient source of energy. A small amount of ATP is also produced in photosynthesis, specifically in the light dependent reactions of photosynthesis in the thylakoids of chloroplasts. Once photoexcitation has taken place, the two electrons released from a chlorophyll molecule move along the electron transport chain, losing energy at each carrier. This energy allows ADP and inorganic phosphate to form ATP in the same way as the electron transport chain in aerobic respiration.

As you can see the production of ATP is not simple, but it is necessary due to its large number of uses in living organisms. I have already mentioned the use of ATP in glycolysis in the phosphorylation of glucose, but ATP is also required in the light independent reactions of photosynthesis in the stroma. RuBP is converted into 2 GP molecules by the fixing of CO2. These GP molecules are then reduced to two GALP by the oxidation of NADPH to NADP and the energy released by the breaking down of an ATP molecule into an ADP and an inorganic phosphate.

Some of this GALP is used in the making of glucose, while most of it is recycled back into RuBP again by the energy released from the breaking of a single bond in an ATP molecule to produce ADP and inorganic phosphate, thus allowing the cycle to continue. An ATP molecule is able to provide energy due to the fact that breaking bonds releases energy. But for bonds to be made, energy is required. This is a key use of ATP in living organisms as it is essential that we can synthesise certain molecules in our bodies for growth, repair and energy stores.

These synthetic reactions can also be called condensation reactions, in which two small molecules are bonded to form one larger molecule and water, for example amino acids to proteins, glycerol and fatty acids to lipids, nucleic acids to DNA etc. Another more obvious use of ATP is in muscle contraction in animals to allow movement. The enzyme ATPase is released due to the calcium ions released in skeletal muscle tissues when an electrical impulse is received by the central nervous system.

This breaks down ATP into ADP and inorganic phosphate, releasing the energy required to pull the filaments of muscle tissues and therefore for the muscles to contract. ATP is also largely used in active transport of substances against a concentration gradient. ATP binds to a carrier protein bonded to a molecule or ion in low concentration on one side of a membrane, causing it to split into ADP and inorganic phosphate and causing the protein to change shape. This change in shape opens the protein to the other side of the membrane, releasing the molecule or ion into the higher concentration on the other side.

The phosphate is released from the protein, allowing it to return to its original shape and for ATP to again form from ADP and phosphate. An example of this in plants would be the active transport of mineral ions into the xylem from the endodermal cells in roots, creating a lower water potential in the xylem so water can move from the endodermal cells into the xylem to the be used in cells for processes such as photosynthesis. An example of active transport in animals is the absorption of glucose in the small intestine.

A sodium potassium pump requires ATP to pump sodium out of the epithelial cells of the intestine and into the blood stream, against a concentration gradient. This creates a concentration gradient of sodium from the ileum to the epithelial cells, causing sodium ions to move into the epithelial cells by facilitated diffusion by a sodium glucose co-transport protein, bringing with it any glucose molecules in the intestine. These are not the only examples of ways in which ATP is used but they are the most common and most important ones and highlight how hugely important ATP is for all living organisms.

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