Next Video 8. This process requires oxygen in humans and most other organisms and produces carbon dioxide, water, heat, and usable energy in the form of ATP. While many different organic molecules, sugars, amino acids, and lipids, can be used in cellular respiration, glucose is used as the prototype. Thus the equation for cellular respiration is C6 H12 O6 plus six O2, leads to six CO2 plus six H20 plus energy, the reverse of photosynthesis.
This reaction actually occurs in multiple steps. Glycolysis is in the cytoplasm, pyruvate oxidation and the citric acid cycle occur in the mitochondria, and oxidative phosphorylation takes place over the inner mitochondrial membrane. Together these processes power cellular activities from flagellar movement, muscle contraction, through the breakdown of organic molecules to produce ATP.
Organisms harvest energy from food, but this energy cannot be directly used by cells. Cells convert the energy stored in nutrients into a more usable form: adenosine triphosphate ATP.
ATP stores energy in chemical bonds that can be quickly released when needed. Cells produce energy in the form of ATP through the process of cellular respiration. Although much of the energy from cellular respiration is released as heat, some of it is used to make ATP. During cellular respiration, several oxidation-reduction redox reactions transfer electrons from organic molecules to other molecules. Here, oxidation refers to electron loss and reduction to electron gain. Some prokaryotes use anaerobic respiration, which does not require oxygen.
Most organisms use aerobic oxygen-requiring respiration, which produces much more ATP. Aerobic respiration generates ATP by breaking down glucose and oxygen into carbon dioxide and water.
Both aerobic and anaerobic respiration begin with glycolysis, which does not require oxygen. Glycolysis breaks down glucose into pyruvate, yielding ATP. Importantly, several types of yeast use alcoholic fermentation. Human muscle cells can use lactic acid fermentation when oxygen is depleted. Anaerobic respiration ends with fermentation.
Aerobic respiration, however, continues with pyruvate oxidation. Pyruvate oxidation generates acetyl-CoA, which enters the citric acid cycle. The final stage of cellular respiration, oxidative phosphorylation, generates most of the ATP. The electron transport chain releases energy that is used to expel protons, creating a proton gradient that enables ATP synthesis.
Lane, N. Martin, W. The Origin of Mitochondria. Nature Education 3 9 To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove.
Your access has now expired. Provide feedback to your librarian. If you have any questions, please do not hesitate to reach out to our customer success team. As scientists discovered more about the underlying mechanisms, however, it became clear that in oxidation, an element was losing one or more electrons to oxygen, and in reduction, an element was gaining electrons.
The ATP produced in cellular respiration is a chemical fuel that powers every reaction in the cell, either directly or indirectly. Respiration happens in every cell in the human body, as well as the cells of almost every eukaryote. The fact that our cells depend on this reaction is the reason that humans breathe in oxygen and breathe out carbon dioxide. The process of cellular respiration involves two main steps. In the first step, which scientists call glycolysis, glucose breaks down.
In the second, aerobic respiration breaks the remains of the glucose down further. During aerobic respiration, oxygen is reduced, donating an electron to hydrogen to form water. The entire process of cellular respiration oxidizes glucose. This produces the majority of the energy released in cellular respiration. Thus, relative to its state before the reaction, carbon has lost electron density because oxygen is now hogging its electrons , while oxygen has gained electron density because it can now hog electrons shared with other elements.
Hydrogen arguably loses a little electron density too, though its electrons were being hogged to some degree in either case. Biologists often refer to whole molecules, rather than individual atoms, as being reduced or oxidized; thus, we can say that butane—the source of the carbons—is oxidized, while molecular oxygen—the source of the oxygen atoms—is reduced. In the context of biology, however, you may find it helpful to use the gain or loss of H and O atoms as a proxy for the transfer of electrons.
In glucose, carbon is associated with H atoms, while in carbon dioxide, no Hs are present. Thus, we would predict that glucose is oxidized in this reaction.
Figure 3. Click on the image for a larger view. Image based on similar diagram by Ryan Gutierrez. Like other chemical reactions, redox reactions involve a free energy change.
Reactions that move the system from a higher to a lower energy state are spontaneous and release energy, while those that do the opposite require an input of energy. In redox reactions, energy is released when an electron loses potential energy as a result of the transfer. Electrons have more potential energy when they are associated with less electronegative atoms such as C or H , and less potential energy when they are associated with a more electronegative atom such as O.
Thus, a redox reaction that moves electrons or electron density from a less to a more electronegative atom will be spontaneous and release energy. For instance, the combustion of butane above releases energy because there is a net shift of electron density away from carbon and hydrogen and onto oxygen. Instead, cells harvest energy from glucose in a controlled fashion, capturing as much of it as possible in the form of ATP. This is accomplished by oxidizing glucose in a gradual, rather than an explosive, sort of way.
There are two important ways in which this oxidation is gradual:. The removal of an electron from a molecule, oxidizing it, results in a decrease in potential energy in the oxidized compound. The electron sometimes as part of a hydrogen atom , does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound.
The shift of an electron from one compound to another removes some potential energy from the first compound the oxidized compound and increases the potential energy of the second compound the reduced compound. The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons.
The transfer of energy in the form of electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst. This module focuses on the extraction of energy from food; you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.
Electron carriers, sometimes called electron shuttles, are small organic molecules that readily cycle between oxidized and reduced forms and are used to transport electrons during metabolic reactions.
Figure 4. FAD is a similar type of molecule, although its functional groups are different. They deposit their electrons at or near the beginning of the transport chain, and the electrons are then passed along from one protein or organic molecule to the next in a predictable series of steps. In redox terms, this means that each member of the electron transport chain is more electronegative electron-hungry that the one before it, and less electronegative than the one after [2].
A living cell cannot store significant amounts of free energy. 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 5. 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 5.
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.
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