Bioenergetics
Bioenergetics is the study of energy transformations in a living organism. It encompasses the fundamental energy-harvesting processes of photosynthesis and respiration, as well as other metabolic reactions that require or release energy. Energy in a cell is used for reproduction, synthesis of vital molecules, movement, and other basic functions. Virtually every reaction in the cell either requires or releases energy.
According to the Second Law of Thermodynamics, the entropy of the universe increases with every chemical change. And yet, in the face of this increasing disorder, living organisms carry out exceedingly complex reactions and remain highly organized. How is this possible? This maintenance of order within the living system is accomplished by constantly taking in high-energy, well-ordered materials from the environment, and excreting low-energy, less-ordered waste products. Glucose, for instance, is a high-energy compound used as a food source. Much of its energy is extracted during glycolysis and cell respiration. Its atoms are excreted in the form of carbon dioxide and water. The entropy of the organism therefore remains low, while the entropy of the universe becomes correspondingly higher.
The overall flow of energy through biological systems involves both catabolic and anabolic processes. Together, these two processes constitute metabolism, the sum of the biochemical processes in an organism. Catabolism is the breakdown of complex molecules into simpler constituents, usually with the release of energy. Anabolism is the reverse, involving the synthesis of complex molecules from simpler ones, with the input of energy. Photosynthesis is an anabolic reaction, requiring energy from the sun to build up sugars from carbon dioxide and water. Cell respiration is a catabolic process, returning glucose to carbon dioxide and water, and releasing stored energy.Protein synthesis is an anabolic process, using cellular energy and simple amino acids to build a complex protein.
Another central question addressed by bioenergetics is how reactions are regulated to allow them to occur at rates fast enough to support life. Although glucose and oxygen react spontaneously to liberate energy, they do so exceedingly slowly at room temperature outside of a cell. This is why a bowl of sugar remains essentially unchanged for months or even years, although it is exposed to copious amounts of oxygen during that time. Inside a cell, however, sugar is rapidly reacted.
The key to this difference in reaction rates is the presence of enzymes, proteins that catalyze reactions. A catalyst is a molecule that speeds the rate of a reaction without being used up, and without affecting the net energy change of the reaction.
Like all catalysts, enzymes speed reactions by lowering the activation energy required to begin the reaction. Most reactions initially require that bonds in the reactants be weakened, allowing the reactant atoms to pass into a less stable transition state, and finally to rearrange into their product state. Activation energy is the energy needed to achieve the transition state. Enzymes lower the activation energy by forming temporary weak bonds to the reactants. This stabilizes the transition state, making it less energetic and therefore easier to attain. Since more sets of reactants can achieve the transition state in the presence of the enzyme, the reaction can proceed faster. Regulation of enzymes is a key part of the cellular control of metabolism.
However, not all reactions will proceed faster simply because an enzyme is present. Reactions that release energy (exergonic reactions) will, but those that require energy (endergonic reactions) will not. To speed an endergonic reaction, the cell not only needs an enzyme, but a source of energy to feed into the reaction.
The fundamental way that an endergonic reaction is driven in the cell is by coupling it with an exergonic reaction. In this way, the energy released by the exergonic reaction can be partially harnessed to drive the endergonic one. The energy transfer is not 100% efficient, and the excess energy is released as heat, or thermal motion of the surroundings. In fact, 100% efficiency would generally be counterproductive, since more efficient transfers require longer times, and are more easily reversible. By wasting some of the energy as heat, the cell can insure the reaction happens quickly and irreversibly.
The most common exergonic reaction used for coupling is the hydrolysis of adenosine triphosphate (ATP). ATP reacts with water to cleave off the last of its three phosphate groups, forming adenosine diphosphate (ADP) and inorganic phosphate ion (Pi). This reaction releases approximately 12 kilocalories per mole of ATP consumed, enough to drive most endergonic reactions in the cell. A second phosphate can be hydrolyzed as well, releasing an additional 12 kcal/mole, for reactions requiring a stronger driving force.
Formation of ATP is also accomplished by coupling, in this case to reactions that are even more exergonic, such as the oxidation of glucose. The oxidation of one molecule of glucose via glycolysis and cell respiration releases enough energy to form 38 ATP molecules. Fats and other food molecules can also be oxidized to form ATP. In this way, ATP serves as the "energy currency" of the cell, acting as an intermediate between the energy-releasing reactions of catabolism, and the energy- requiring reactions of anabolism.
Hydrolysis and dehydration are so ubiquitous in cell metabolism that it is worth looking more closely at them. Dehydration--the removal of a water molecule--serves to link small molecules into longer chains. Amino acids are dehydrated to form proteins, nucleotides are dehydrated to form ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) molecules, sugars are dehydrated to form complex carbohydrates, and fatty acids are dehydrated with glycerol to form fats. Dehydration is endergonic not only because the products are less strongly bonded, but because entropy is decreased in the synthesis of the more complex macromolecule. Hydrolysis--the addition of a water to split up a large molecule--serves to return these complex macromolecules to their simpler constituents. The exergonic character of hydrolysis is due to both greater bond strength and greater entropy of the products.
ATP is also used to drive other energy-requiring processes, including the creation of concentration gradients across the plasma membrane of a cell. The transport protein Na+/K+ ATPase uses the energy from ATP hydrolysis to transport sodium ions out of the cell, and bring potassium ions in, creating large differences in the concentration of each ion across the membrane. Such gradients can be used for a variety of purposes. In cells of the intestinal lining, glucose is scavenged from the gut by a cotransporter for glucose and sodium. The high external concentration of sodium creates a strong drive for glucose transport, even though glucose is being moved "uphill" against its concentration gradient. In neurons, the potassium and sodium gradients are used for intracellular transmission of the nerve signal.
The ultimate source of energy for living organisms is the sun. Photosynthesis is the process by which solar energy is captured for the creation of high-energy compounds. Within the chloroplast, light is captured by the molecule chlorophyll, whose electron structure allows the absorption of light in the visible range. Energy is transferred to an electron removed from a water molecule, creating oxygen gas in the process. The energized electron is passed down a series of electron acceptors of increasing electronegativity. At several points along this electron transport chain, the energy released is used to pump H+ ions across the chloroplast membrane, creating a gradient. The energy of this gradient is used to drive the production of ATP by the enzyme ATP synthase, as the H+ ions flow through the enzyme. After being reenergized by another chlorophyll, the electron is combined with other H+ ions and the energy carrier nicotinamide adenine dinucleotide phosphate (NADP+) to form NADPH, a carrier of high-energy hydrogens used in anabolic reactions. The NADPH and ATP are used in the Calvin cycle to drive the creation of a three- carbon sugar, using carbon dioxide as the carbon source.
Energy is harvested from sugars by a three-step process. In glycolysis, the six-carbon glucose is oxidized to form two molecules of the three-carbon organic acid, pyruvate. This forms two ATPs, and two molecules of nicotinamide adenine dinucleotide (NADH), a carrier of high-energy hydrogens used in catabolic reactions. Pyruvate is transported to the mitochondrion, where it undergoes further catabolism, resulting in the production of carbon dioxide and more NADH, as well as another hydrogen carrier, flavin adenine dinucleotide (FADH2), and guanosine triphosphate, similar to ATP. All NADHs and FADH2 are processed in the mitochondria as well. Hydrogens, with their energy-rich electrons, are stripped off. The electrons pass down an electron transport chain similar to the one in the chloroplast, creating an H+ gradient which is used to drive ATP synthesis. The electrons and hydrogens link up with oxygen to form water.
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