When glucose is in adequate supply, such as shortly after consumption of a meal, the hormone insulin from the pancreas increases glycogen formation glycogenesis in the liver.
When glucose levels drop between meals, the hormone glucagon is released from the pancreas and stimulates the conversion of glycogen into glucose by the process of glycogenolysis. If all glycogen supplies are depleted, then other substances in the body are converted into glucose or intermediate products that can enter the above-outlined cellular respiration pathway.
The conversion of fatty acids from lipids or amino acids from proteins into glucose or intermediate products is called gluconeogenesis p. Fats lipids are stored in adipose tissue. These stored fat molecules are synthesized in the body from the breakdown products of fat digestion glycerol and fatty acids , in a process known as lipogenesis p.
When needed as an energy source, the fat reserves are mobilized, moved out of adipose tissue, and broken down into glycerol and fatty acids in the liver by the process of lipolysis. The energy of the electrons is harvested and used to generate a electrochemical gradient across the inner mitochondrial membrane. The potential energy of this gradient is used to generate ATP. The entirety of this process is called oxidative phosphorylation.
The electron transport chain Figure 4. Oxygen continuously diffuses into plants for this purpose. In animals, oxygen enters the body through the respiratory system.
Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where oxygen is the final electron acceptor and water is produced. There are four complexes composed of proteins, labeled I through IV in Figure 4.
The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of prokaryotes. In each transfer of an electron through the electron transport chain, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions across the inner mitochondrial membrane into the intermembrane space, creating an electrochemical gradient.
Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? What affect would cyanide have on ATP synthesis? As they are passed from one complex to another there are a total of four , the electrons lose energy, and some of that energy is used to pump hydrogen ions from the mitochondrial matrix into the intermembrane space.
In the fourth protein complex, the electrons are accepted by oxygen, the terminal acceptor. The oxygen with its extra electrons then combines with two hydrogen ions, further enhancing the electrochemical gradient, to form water. If there were no oxygen present in the mitochondrion, the electrons could not be removed from the system, and the entire electron transport chain would back up and stop. The mitochondria would be unable to generate new ATP in this way, and the cell would ultimately die from lack of energy.
This is the reason we must breathe to draw in new oxygen. In the electron transport chain, the free energy from the series of reactions just described is used to pump hydrogen ions across the membrane. Hydrogen ions diffuse through the inner membrane through an integral membrane protein called ATP synthase Figure 4. This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient from the intermembrane space, where there are many mutually repelling hydrogen ions to the matrix, where there are few.
This flow of hydrogen ions across the membrane through ATP synthase is called chemiosmosis. Chemiosmosis Figure 4. The result of the reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule.
But how do cells get this energy in the first place? And how do they use it in the most efficient manner possible? Figure 1: For photosynthetic cells, the main energy source is the sun. For photosynthetic cells, the main energy source is the sun.
Cells, like humans, cannot generate energy without locating a source in their environment. However, whereas humans search for substances like fossil fuels to power their homes and businesses, cells seek their energy in the form of food molecules or sunlight.
In fact, the Sun is the ultimate source of energy for almost all cells, because photosynthetic prokaryotes, algae, and plant cells harness solar energy and use it to make the complex organic food molecules that other cells rely on for the energy required to sustain growth, metabolism, and reproduction Figure 1.
Cellular nutrients come in many forms, including sugars and fats. In order to provide a cell with energy, these molecules have to pass across the cell membrane, which functions as a barrier — but not an impassable one. Like the exterior walls of a house, the plasma membrane is semi-permeable. In much the same way that doors and windows allow necessities to enter the house, various proteins that span the cell membrane permit specific molecules into the cell, although they may require some energy input to accomplish this task Figure 2.
Figure 2: Cells can incorporate nutrients by phagocytosis. This amoeba, a single-celled organism, acquires energy by engulfing nutrients in the form of a yeast cell red. Through a process called phagocytosis, the amoeba encloses the yeast cell with its membrane and draws it inside. Specialized plasma membrane proteins in the amoeba in green are involved in this act of phagocytosis, and they are later recycled back into the amoeba after the nutrients are engulfed.
Figure Detail. Complex organic food molecules such as sugars, fats, and proteins are rich sources of energy for cells because much of the energy used to form these molecules is literally stored within the chemical bonds that hold them together.
Scientists can measure the amount of energy stored in foods using a device called a bomb calorimeter. With this technique, food is placed inside the calorimeter and heated until it burns. The excess heat released by the reaction is directly proportional to the amount of energy contained in the food. 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.
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