Cells, however, think of energy as ATP. Cellular respiration is the process of taking the food we eat like sugar and converting it into an energy that can be used by cells - ATP. The breakdown of energy rich molecules like glucose to obtain energy is called cellular respiration.
Cellular respiration occurs in both plants and animals. The energy gained from the breakdown of glucose is stored in cells as ATP. Cellular respiration can be summarized by the following equation:. Glycolysis : Glucose is split into two molecules called pyruvate and 2 ATP molecules are generate per molecule of glucose as well as 2 molecules of NADH.
Glycolysis takes place in the cytoplasm of the cell and does not require oxygen. Krebs Cycle takes place in the mitochondrial matrix and requires oxygen. The ETC takes place in the mitochondrial matrix and requires oxygen.
Glucose : A simple, 6 carbon sugar that serves as the primary energy source in the body. ATP Adenosine triphosphate : The major energy currency of the cell. ATP is a high-energy molecule that stores and transports energy within cells.
Within the cell, where does energy to power such reactions come from? The answer lies with an energy-supplying molecule called adenosine triphosphate, or ATP. ATP is a small, relatively simple molecule, but within its bonds contains the potential for a quick burst of energy that can be harnessed to perform cellular work. This molecule can be thought of as the primary energy currency of cells in the same way that money is the currency that people exchange for things they need.
ATP is used to power the majority of energy-requiring cellular reactions. Excess free energy would result in an increase of heat in the cell, which would denature enzymes and other proteins, and thus destroy the cell.
Rather, a cell must be able to store energy safely and release it for use only as needed. Living cells accomplish this using ATP, which can be used to fill any energy need of the cell. It functions as a rechargeable battery. This energy is used to do work by the cell, usually by the binding of the released phosphate to another molecule, thus activating it. For example, in the mechanical work of muscle contraction, ATP supplies energy to move the contractile muscle proteins. At the heart of ATP is a molecule of adenosine monophosphate AMP , which is composed of an adenine molecule bonded to both a ribose molecule and a single phosphate group Figure 4.
The addition of a second phosphate group to this core molecule results in adenosine di phosphate ADP ; the addition of a third phosphate group forms adenosine tri phosphate ATP. The addition of a phosphate group to a molecule requires a high amount of energy and results in a high-energy bond. The release of one or two phosphate groups from ATP, a process called hydrolysis, releases energy.
You have read that nearly all of the energy used by living things comes to them in the bonds of the sugar, glucose. Glycolysis is the first step in the breakdown of glucose to extract energy for cell metabolism.
Many living organisms carry out glycolysis as part of their metabolism. Glycolysis takes place in the cytoplasm of most prokaryotic and all eukaryotic cells.
Glycolysis begins with the six-carbon, ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. In the first part of the glycolysis pathway, energy is used to make adjustments so that the six-carbon sugar molecule can be split evenly into two three-carbon pyruvate molecules.
If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose. For example, mature mammalian red blood cells are only capable of glycolysis, which is their sole source of ATP. If glycolysis is interrupted, these cells would eventually die. Figure 3: The release of energy from sugar Compare the stepwise oxidation left with the direct burning of sugar right.
Through a series if small steps, free energy is released from sugar and stored in carrier molecules in the cell ATP and NADH, not shown. On the right, the direct burning of sugar requires a larger activation energy. In this reaction, the same total free energy is released as in stepwise oxidation, but none is stored in carrier molecules, so most of it will be lost as heat free energy. This direct burning is therefore very inefficient, as it does not harness energy for later use.
In reality, of course, cells don't work quite like calorimeters. Rather than burning all their energy in one large reaction, cells release the energy stored in their food molecules through a series of oxidation reactions. Oxidation describes a type of chemical reaction in which electrons are transferred from one molecule to another, changing the composition and energy content of both the donor and acceptor molecules.
Food molecules act as electron donors. During each oxidation reaction involved in food breakdown, the product of the reaction has a lower energy content than the donor molecule that preceded it in the pathway. At the same time, electron acceptor molecules capture some of the energy lost from the food molecule during each oxidation reaction and store it for later use. Eventually, when the carbon atoms from a complex organic food molecule are fully oxidized at the end of the reaction chain, they are released as waste in the form of carbon dioxide Figure 3.
Cells do not use the energy from oxidation reactions as soon as it is released. Instead, they convert it into small, energy-rich molecules such as ATP and nicotinamide adenine dinucleotide NADH , which can be used throughout the cell to power metabolism and construct new cellular components. In addition, workhorse proteins called enzymes use this chemical energy to catalyze, or accelerate, chemical reactions within the cell that would otherwise proceed very slowly. Enzymes do not force a reaction to proceed if it wouldn't do so without the catalyst; rather, they simply lower the energy barrier required for the reaction to begin Figure 4.
Figure 4: Enzymes allow activation energies to be lowered. Enzymes lower the activation energy necessary to transform a reactant into a product. On the left is a reaction that is not catalyzed by an enzyme red , and on the right is one that is green. 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.
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.
Figure 6: Metabolism in a eukaryotic cell: Glycolysis, the citric acid cycle, and oxidative phosphorylation Glycolysis takes place in the cytoplasm. 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.
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