Understanding the Electron Transport Chain - AP Biology

The direct purpose of moving electrons down the electron transport chain is to pump protons (hydrogen ions) into the intermembrane space. This creates a chemiosmotic gradient that the cell uses to generate ATP by selectively allowing hydrogen ions to move back into the mitochondrial matrix.

Energy is not directly captured as electrons travel down the electron transport chain to synthesize ATP. GTP is a product of the Krebs cycle and can be used to generate cellular energy, but is not involved in synthesizing ATP or the electron transport chain. Other metabolic processes are often used to regulate glucose concentrations in the blood, indirectly influencing the rate of glycolysis and cellular respiration, but these processes do not directly provide energy for the electron transport chain.

Oxygen is the final electron acceptor in the electron transport chain, showing the need for aerobic conditions to undergo such a process. ATP is produced as a product of the electron transport chain, while glucose and CO2 play a role in earlier processes of cellular respiration.

In cellular respiration, oxygen is the final electron acceptor. Oxygen accepts the electrons after they have passed through the electron transport chain and ATPase, the enzyme responsible for creating high-energy ATP molecules. Just remember cellular **respiration—**respiration means breathing, and you cannot breathe without oxygen.

Glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate, and one NADH is also produced during that step. NADH enters the electron transport chain, and is therefore worth ATP. Normally, an NADH is worth about 2.5 ATP; however, an NADH produced in glycolysis is only worth 1.5 ATP because it costs 1 ATP to move that NADH from the cytoplasm into the mitochondria. So, in this first step, we have a total of 1.5 ATP.

As the molecule continues on its path to become pyruvate, it will also produce two ATP directly; therefore, we have a net total of 3.5 potential ATP.

Cellular respiration typically follows three steps, under aerobic conditions. Glycolysis generates NADH and converts glucose to pyruvate, while producing small amounts of ATP through substrate-level phosphorylation. The citric acids cycle, or Krebs cycle, uses pyruvate to generate more NADH and FADH2. These NADH and FADH2 molecules donate electrons to the electron transport chain, which are used to pump protons into the intermembrane space of the mitochondrion. The protons in the intermembrane space then flow through ATP synthase to generate large amounts of ATP via oxidative phosphorylation.

Oxygen serves as the terminal electron acceptor for the electron transport chain. Electrons are donated by NADH molecules and passed through several different proteins to generate the proton gradient in the intermembrane space. Upon reaching the final protein, the electron is bonded to an oxygen molecule to create water. Without oxygen, there would be nowhere for the electrons to go after being pumped through the electron transport chain, and aerobic cellular respiration would be impossible.

Oxygen is the final electron acceptor in the electron transport chain, which allows for oxidative phosphorylation. Without oxygen, the electrons will be backed up, eventually causing the electron transport chain to halt. This will cause the products of glycolysis to go through fermentation instead of going to the citric acid cycle. Without oxygen, oxidative phosphorylation (the electron transport chain) is impossible, but substrate-level phosphorylation (glycolysis) continues.

Cellular respiration is only about 38% efficient, with the rest of the energy in glucose lost as heat.

Water and carbon dioxide are not used to store energy. Fats can be synthesized from acetyl CoA and glycerol, but are not generally created in large quantities during cellular respiration. Starches are generally used for energy storage in plants, but can be synthesized from glucose; however, starches are not a standard product of cellular respiration.

Most of the reactions in cellular respiration are exothermic, in order to support spontaneous reaction. The result is release of heat energy with most steps.

Oxygen acts as the terminal electron acceptor in the electron transport chain (ETC). This accounts for the reason as to why, when cells are starved of oxygen, the ETC "backs up" and the cell will divert to using anaerobic respiration, such as fermentation. At the end of the electron transport chain, the electron and a proton are passed to an oxygen molecule to produce water.

The citric acid cycle depends on oxygen in an indirect sense. The main purpose of the cycle is to produce electron donors for the electron transport chain. If the chain is not functional (due to lack of oxygen), the citric acid cycle also stops functioning. Glycolysis is not dependent on oxygen, and can function in anaerobic environments.

In ATP synthesis, the proton gradient is an interconvertible form of energy in electron transport. 2,4-dinitrophenol is an inhibitor of ATP production in cells with mitochondria. Its mechanism of action involves carrying protons across the mitochondrial membrane, which leads to the consumption of energy without ATP production.

The other answer choices are not directly related to the generation of the proton gradient.

The events of the electron transport chain take place on the inner membrane of the mitochondria. The transmembrane proteins used to shuttle electrons through the electron transport chain are embedded on the inner membrane. Electrons are donated to these proteins and used to transfer protons into the intermembrane space from the matrix. After reaching the final inner membrane protein in the chain, the electron is transferred to oxygen to form water.

The mitochondrial matrix is where the ATP eventually is eventually synthesized, as well as the site of the citric acid cycle. The cytoplasm is the site of glycolysis. The outer mitochondrial membrane is not directly involved in cellular respiration.

Oxidative phosphorylation is composed of electron transport and chemiosmosis. Chemiosmosis occurs when ions cross a selectively permeable membrane down their concentration gradient. In cellular respiration, hydrogen ions (protons) move down their concentration gradient through a membrane protein to produce ATP. The gradient of protons is established by the electron transport portion of oxidative phosphorylation, which is used to transfer protons into the intermembrane space. Protein complexes I, II, III, and IV help protons to cross the membrane.

Substrate-level phosphorylation occurs during glycolysis, and does not utilize chemiosmosis.

Both NADH and FADH2 donate two electrons to the electron transport chain, so theoretically they should make the same amount of ATP. However, NADH donates its electrons to complex I while FADH2 donates its electrons further "downstream" at complex II. Because complex I is a site for pumping protons into the intermembrane space, FADH2's electrons will not pump as many protons as those from NADH. This results in more ATP being generated from a single molecule of NADH than a single molecule of FADH2.

The Krebs cycle creates a proton gradient between the intermembrane space and the inner mitochondrial membrane. This proton gradient provides the energy necessary to drive the proton through the ATP synthase. As the protons are passively diffusing through the ATP synthase, the energy is coupled to phosphorylate ADP to ATP. If a base were injected into this space, then it would would consume these protons due to its electronegativity and decrease ATP synthase’s ability to transform ADP to ATP.

The enzyme ATP synthase uses the electromotive force generated by the unequal concentrations of protons across both sides of the membrane to attach a phosphate group to ADP, generating ATP. The passing of a proton from a high concentration to low concentration permits the formation of the ATP molecule. Cytochrome c is an enzyme embedded in the inner mitochondrial membrane, but is not directly associated with ATP synthesis. Succinate dehydrogenase and aldolase are enzymes involved in the Krebs cycle.

The most acidic environment, or the lowest pH, would be found in the intermembrane space. This is because as and pass their electrons to the enzymes in the electron transport chain, protons are pumped into the intermembrane space. This is where a high concentration protons is generated, which is considered acidic. The low concentration of protons is generated in the mitochondrial matrix rendering it basic.

The final electron acceptor in the electron transport chain is oxygen. It gets reduced by accepting two electrons and two protons from the ATP synthase to form water via the following equation:

Each produces 3 molecules in the electron transport chain while each produces 2 molecules. Each glucose molecule results in the formation of 10 molecules, which go on to produce 30 . Each glucose molecule results in the formation of 2 molecules, which go on to produce 4 . Note that some references may indicate that each produces 2.5 , while each produces 1.5 . These are theoretical maximums and depend on the organism, cell type, and cellular environment.

A total of 38 ATP molecules are produced from one molecule of glucose. 2 ATP from glycolysis, 2 ATP from the Krebs cycle, and about 34 ATP from the electron transport chain. Note that this is a theoretical maximum and is rarely seen in nature.