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More Than Matter?: Is There More to Life Than Molecules?
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In the enzyme-catalyzed reaction, an enzyme will bind to a reactant and facilitate its transformation into a product. Consequently, an enzyme-catalyzed reaction pathway has a smaller energy barrier activation energy to overcome before the reaction can proceed. The high-energy phosphate bond in this phosphate chain is the key to ATP's energy storage potential. Figure Detail The particular energy pathway that a cell employs depends in large part on whether that cell is a eukaryote or a prokaryote.
Eukaryotic cells use three major processes to transform the energy held in the chemical bonds of food molecules into more readily usable forms — often energy-rich carrier molecules. Adenosine 5'-triphosphate, or ATP, is the most abundant energy carrier molecule in cells. This molecule is made of a nitrogen base adenine , a ribose sugar, and three phosphate groups. The word adenosine refers to the adenine plus the ribose sugar. The bond between the second and third phosphates is a high-energy bond Figure 5. The first process in the eukaryotic energy pathway is glycolysis , which literally means "sugar splitting.
Glycolysis is actually a series of ten chemical reactions that requires the input of two ATP molecules. Two NADH molecules are also produced; these molecules serve as electron carriers for other biochemical reactions in the cell. Glycolysis is an ancient, major ATP-producing pathway that occurs in almost all cells, eukaryotes and prokaryotes alike.
This process, which is also known as fermentation , takes place in the cytoplasm and does not require oxygen. However, the fate of the pyruvate produced during glycolysis depends upon whether oxygen is present.
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In the absence of oxygen, the pyruvate cannot be completely oxidized to carbon dioxide, so various intermediate products result. For example, when oxygen levels are low, skeletal muscle cells rely on glycolysis to meet their intense energy requirements. This reliance on glycolysis results in the buildup of an intermediate known as lactic acid, which can cause a person's muscles to feel as if they are "on fire. In contrast, when oxygen is available, the pyruvates produced by glycolysis become the input for the next portion of the eukaryotic energy pathway.
During this stage, each pyruvate molecule in the cytoplasm enters the mitochondrion, where it is converted into acetyl CoA , a two-carbon energy carrier, and its third carbon combines with oxygen and is released as carbon dioxide. At the same time, an NADH carrier is also generated.
Acetyl CoA then enters a pathway called the citric acid cycle , which is the second major energy process used by cells. Within the mitochondrion, the citric acid cycle occurs in the mitochondrial matrix, and oxidative metabolism occurs at the internal folded mitochondrial membranes cristae. The third major process in the eukaryotic energy pathway involves an electron transport chain , catalyzed by several protein complexes located in the mitochondrional inner membrane. This process, called oxidative phosphorylation, transfers electrons from NADH and FADH 2 through the membrane protein complexes, and ultimately to oxygen, where they combine to form water.
As electrons travel through the protein complexes in the chain, a gradient of hydrogen ions, or protons, forms across the mitochondrial membrane. Cells harness the energy of this proton gradient to create three additional ATP molecules for every electron that travels along the chain. Overall, the combination of the citric acid cycle and oxidative phosphorylation yields much more energy than fermentation - 15 times as much energy per glucose molecule! Together, these processes that occur inside the mitochondion, the citric acid cycle and oxidative phosphorylation, are referred to as respiration , a term used for processes that couple the uptake of oxygen and the production of carbon dioxide Figure 6.
The electron transport chain in the mitochondrial membrane is not the only one that generates energy in living cells. In plant and other photosynthetic cells, chloroplasts also have an electron transport chain that harvests solar energy. Even though they do not contain mithcondria or chloroplatss, prokaryotes have other kinds of energy-yielding electron transport chains within their plasma membranes that also generate energy. When energy is abundant, eukaryotic cells make larger, energy-rich molecules to store their excess energy.
The resulting sugars and fats — in other words, polysaccharides and lipids — are then held in reservoirs within the cells, some of which are large enough to be visible in electron micrographs.
Animal cells can also synthesize branched polymers of glucose known as glycogen , which in turn aggregate into particles that are observable via electron microscopy. A cell can rapidly mobilize these particles whenever it needs quick energy. Athletes who "carbo-load" by eating pasta the night before a competition are trying to increase their glycogen reserves.
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Under normal circumstances, though, humans store just enough glycogen to provide a day's worth of energy. Plant cells don't produce glycogen but instead make different glucose polymers known as starches , which they store in granules. In addition, both plant and animal cells store energy by shunting glucose into fat synthesis pathways. One gram of fat contains nearly six times the energy of the same amount of glycogen, but the energy from fat is less readily available than that from glycogen. Log In. Don't have an account? Sign Up. Update your profile Let us wish you a happy birthday!
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