Humans generally store enough fat to supply their cells with several weeks' worth of energy Figure 7. Figure 7: Examples of energy storage within cells. A In this cross section of a rat kidney cell, the cytoplasm is filled with glycogen granules, shown here labeled with a black dye, and spread throughout the cell G , surrounding the nucleus N.
B In this cross-section of a plant cell, starch granules st are present inside a chloroplast, near the thylakoid membranes striped pattern. C In this amoeba, a single celled organism, there is both starch storage compartments S , lipid storage L inside the cell, near the nucleus N.
Qian H. Letcher P. A Bamri-Ezzine, S. All rights reserved. This page appears in the following eBook. Aa Aa Aa. Cell Energy and Cell Functions. Figure 3: The release of energy from sugar. Compare the stepwise oxidation left with the direct burning of sugar right. Figure 5: An ATP molecule. ATP consists of an adenosine base blue , a ribose sugar pink and a phosphate chain.
Figure 6: Metabolism in a eukaryotic cell: Glycolysis, the citric acid cycle, and oxidative phosphorylation. Glycolysis takes place in the cytoplasm. Cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, eukaryotic cells make energy-rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation.
Any excess energy is then stored in larger, energy-rich molecules such as polysaccharides starch and glycogen and lipids.
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You have authorized LearnCasting of your reading list in Scitable. Do you want to LearnCast this session? Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate ATP.
It functions similarly to a rechargeable battery. When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The energy is used to do work by the cell, usually by the released phosphate binding to another molecule, activating it.
For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients. Figure 1.
ATP adenosine triphosphate has three phosphate groups that can be removed by hydrolysis to form ADP adenosine diphosphate or AMP adenosine monophosphate. The negative charges on the phosphate group naturally repel each other, requiring energy to bond them together and releasing energy when these bonds are broken.
At the heart of ATP is a molecule of adenosine monophosphate AMP , which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group Figure 1. The addition of a second phosphate group to this core molecule results in the formation of adenosine diphosphate ADP ; the addition of a third phosphate group forms adenosine triphosphate ATP.
The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. The release of one or two phosphate groups from ATP, a process called dephosphorylation , releases energy.
Hydrolysis is the process of breaking complex macromolecules apart. Water, which was broken down into its hydrogen atom and hydroxyl group during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP. Obviously, energy must be infused into the system to regenerate ATP.
Where does this energy come from? In nearly every living thing on earth, the energy comes from the metabolism of glucose. Do you want to LearnCast this session? This article has been posted to your Facebook page via Scitable LearnCast. Change LearnCast Settings. Scitable Chat. Register Sign In.
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