In cellular respiration when is the most energy transferred

The removal of an electron from a molecule via a process called oxidation results in a decrease in the potential energy stored in the oxidized compound.

When oxidation occurs in the cell, the electron sometimes as part of a hydrogen atom does not remain un-bonded in the cytoplasm. Instead, the electron shifts to a second compound, reducing the second compound oxidation of one species always occurs in tandem with reduction of another. 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 via oxidation and reduction is important because most of the energy stored in atoms is in the form of high-energy electrons; it is this energy that is used to fuel cellular functions. 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 as a single, destructive burst.

In living systems, a small class of molecules functions as electron shuttles: they bind and carry high-energy electrons between compounds in cellular pathways. The principal electron carriers we will consider are derived from the vitamin B group, which are derivatives of nucleotides.

These compounds can be easily reduced that is, they accept electrons or oxidized they lose electrons. Nicotinamide adenine dinucleotide NAD is derived from vitamin B3, niacin. When electrons are added to a compound, the compound is reduced. A compound that reduces another is called a reducing agent.

When electrons are removed from a compound, the compound is oxidized. The molecule NADH is critical for cellular respiration and other metabolic pathways. Its reduced form is FADH 2. ATP, produced by glucose catabolized during cellular respiration, serves as the universal energy currency for all living organisms. 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 lead to 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 as needed. Living cells accomplish this by using the compound adenosine triphosphate ATP.

Adenosine triphosphate. 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.

The core 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. 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 a 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.

Once it is reduced, QH 2 , ubiquinone delivers its electrons to the next complex in the electron transport chain. This enzyme and FADH 2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex.

Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH 2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane. The third complex is composed of cytochrome b, another Fe-S protein, Rieske center 2Fe-2S center , and cytochrome c proteins; this complex is also called cytochrome oxidoreductase.

Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex.

Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time. The fourth complex is composed of cytochrome proteins c, a, and a 3.

This complex contains two heme groups one in each of the two cytochromes, a, and a 3 and three copper ions a pair of Cu A and one Cu B in cytochrome a 3.

The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water H 2 O. The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis. In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions protons across the membrane.

If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient.

Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase Figure 8.

This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.

Figure 8. Credit: modification of work by Klaus Hoffmeier. Dinitrophenol DNP is an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until as a weight-loss drug. What effect would you expect DNP to have on the change in pH across the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug? Chemiosmosis Figure 9 is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation.

Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these 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.

At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions protons from the surrounding medium, and water is formed. Figure 9. 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 effect would cyanide have on ATP synthesis?

The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species.

Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria. The NADH generated from glycolysis cannot easily enter mitochondria. Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes.

Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the ideal situations described thus far. For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction.

Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways.

Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose. The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them.

The electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane.

This process contributes to the gradient used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy, High-energy electrons donated to the chain by either NADH or FADH 2 complete the chain, as low-energy electrons reduce oxygen molecules and form water.

The end products of the electron transport chain are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can serve as energy sources for the glucose pathways. Cellular respiration is a collection of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport chain.

Glycolysis is an anaerobic process, while the other two pathways are aerobic. In order to move from glycolysis to the citric acid cycle, pyruvate molecules the output of glycolysis must be oxidized in a process called pyruvate oxidation. Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules. There are two halves of glycolysis, with five steps in each half.

This half splits glucose, and uses up 2 ATP. If the concentration of pyruvate kinase is high enough, the second half of glycolysis can proceed. Some cells e.

However, most cells undergo pyruvate oxidation and continue to the other pathways of cellular respiration. In eukaryotes, pyruvate oxidation takes place in the mitochondria. Pyruvate oxidation can only happen if oxygen is available. In this process, the pyruvate created by glycolysis is oxidized.

In this oxidation process, a carboxyl group is removed from pyruvate, creating acetyl groups, which compound with coenzyme A CoA to form acetyl CoA. This process also releases CO 2. The citric acid cycle also known as the Krebs cycle is the second pathway in cellular respiration, and it also takes place in the mitochondria.

The rate of the cycle is controlled by ATP concentration. This pathway is a closed loop: the final step produces the compound needed for the first step. The citric acid cycle is considered an aerobic pathway because the NADH and FADH 2 it produces act as temporary electron storage compounds, transferring their electrons to the next pathway electron transport chain , which uses atmospheric oxygen. Most ATP from glucose is generated in the electron transport chain.

It is the only part of cellular respiration that directly consumes oxygen; however, in some prokaryotes, this is an anaerobic pathway. In eukaryotes, this pathway takes place in the inner mitochondrial membrane. In prokaryotes it occurs in the plasma membrane. The electron transport chain is made up of 4 proteins along the membrane and a proton pump.

A cofactor shuttles electrons between proteins I—III. Click here for a text-only version of the activity. Answer the question s below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section.

Skip to main content. Module 6: Metabolic Pathways. Search for:. Learning Objectives Describe the process of glycolysis and identify its reactants and products Describe the process of pyruvate oxidation and identify its reactants and products Describe the process of the citric acid cycle Krebs cycle and identify its reactants and products Describe the respiratory chain electron transport chain and its role in cellular respiration.

After citric acid forms, it goes through a series of reactions that release energy. This energy is captured in molecules of ATP and electron carriers. Carbon dioxide is also released as a waste product of these reactions.

This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvate molecules when it splits glucose. After the second turn through the Citric Acid Cycle, the original glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules.

These molecules are:. Oxidative phosphorylation is the final stage of aerobic cellular respiration. There are two substages of oxidative phosphorylation, Electron transport chain and Chemiosmosis. During this stage, high-energy electrons are released from NADH and FADH 2 , and they move along electron-transport chains found in the inner membrane of the mitochondrion.

An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by chemical reactions. This ion transfer creates an electrochemical gradient that drives the synthesis of ATP. The electrons from the final protein of the ETC are gained by the oxygen molecule, and it is reduced to water in the matrix of the mitochondrion.

The pumping of hydrogen ions across the inner membrane creates a greater concentration of these ions in the intermembrane space than in the matrix — producing an electrochemical gradient. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower.

The ATP synthase acts as a channel protein, helping the hydrogen ions across the membrane. The flow of protons through ATP synthase is considered chemiosmosis. After passing through the electron-transport chain, the low-energy electrons combine with oxygen to form water.

You have seen how the three stages of aerobic respiration use the energy in glucose to make ATP. How much ATP is produced in all three stages combined? Glycolysis produces 2 ATP molecules, and the Krebs cycle produces 2 more. Therefore, a total of up to 36 molecules of ATP can be made from just one molecule of glucose in the process of cellular respiration.

Bring on the S'mores! Where do organisms get energy from? What is ATP? When the covalent bond between the terminal phosphate group and the middle phosphate group breaks, energy is released which is used by the cells to do work.

What Is Cellular Respiration? The process begins with Glycolysis. In this first step, a molecule of glucose, which has six carbon atoms, is split into two three-carbon molecules. The three-carbon molecule is called pyruvate. Pyruvate is oxidized and converted into Acetyl CoA. These two steps occur in the cytoplasm of the cell. Acetyl CoA enters into the matrix of mitochondria, where it is fully oxidized into Carbon Dioxide via the Krebs cycle.

Finally, During the process of oxidative phosphorylation, the electrons extracted from food move down the electron transport chain in the inner membrane of the mitochondrion. As the electrons move down the ETC and finally to oxygen, they lose energy. Glycolysis The first stage of cellular respiration is glycolysis. Results of Glycolysis Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules.

Transformation of Pyruvate into Acetyl-CoA In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are sites of cellular respiration. Citric Acid Cycle Before you read about the last two stages of cellular respiration, you need to review the structure of the mitochondrion, where these two stages take place.

The space inside the inner membrane is full of fluid, enzymes, ribosomes, and mitochondrial DNA. This space is called a matrix. The inner membrane has a larger surface area as compared to the outer membrane. Therefore, it creases.