The first step of Krebs cycle is the formation of a six-carbon molecule from a two-carbon and a four-carbon molecule. The two-carbon molecule acetyl-CoA combines with the four-carbon molecule oxaloacetate to form a six-carbon molecule, citrate. The citrate molecule undergoes a series of reactions in the Krebs cycle that eventually leads to a five-carbon intermediate and, finally, regeneration of the four-carbon oxaloacetate (to be used for the next cycle). The two-carbon molecule, acetyl-CoA, comes from the pyruvate molecule from glycolysis (recall that pyruvate comes from glucose).

The Krebs cycle starts when oxaloacetate combines with acetyl-CoA in order to create citrate. The process is able to work in a cyclic fashion due to the cycle's ability to remake oxaloacetate at the end, so that it can combine with another acetyl-CoA and start the process again.

The Krebs cycle is useful in not only making ATP molecules, but also for creating high-energy electron carriers, such as NADH and FADH2. As a result, the enzyme isocitrate dehydrogenase will be stimulated when these high energy molecules are depleted in the cell. NAD+, or the oxidized form of NADH, stimulates isocitrate dehydrogenase to work more efficiently. All the other options are high-energy molecules, which would slow down the cycle, as enough energy has already been produced.

Coenzyme Q₁₀ is a fat-soluble molecule that facilitates the transfer of electrons from complex I or II to complex III in the electron transport chain. The mobility of coenzyme Q₁₀ in the membrane allows for this unique function. Each complex in the membrane is then able to use the donated electron to push protons into the intermembrane space, generating the gradient that will eventually be used to synthesize ATP.

Coenzyme Q₁₀ does not directly facilitate the movement of protons. Rather, it aids in the transfer of electrons to initiate the process that allows for proton movement. Coenzyme Q₁₀ is also not involved with the regulation of ATP synthase or with bringing oxygen to the electron transport chain.

NADH is the first electron carrier to be oxidized by the electron transport chain, a process that occurs at complex I. FADH₂ is oxidized further down the chain in complex II, causing it to produce less ATP on average than NADH.

Electron transport chain (ETC) consists of a series of electron carriers on the inner membrane of the mitochondria. The electrons are transferred down these carriers and this movement is used to generate ATP. The question asks for the molecule most abundant surrounding these electron carriers. Since they are found on the inner membrane, the electron carriers in ETC are surrounded by phospholipids (most abundant molecule in a membrane).

The final electron acceptor in electron transport chain is oxygen. The electrons traverse along the electron transport chain and ultimately end up being taken up by an oxygen atom. Upon accepting electrons, the oxygen atom becomes negatively charged and attracts protons (hydrogen atoms). An oxygen atom binds to two hydrogen atoms and forms water.

Energy production in ETC is facilitated by the generation of the proton gradient by the proton pump. This gradient is utilized by ATP synthase to produce ATP. Oxygen does not play a role in this process. Anaerobic respiration is a type of respiration that occurs in the absence of oxygen. Examples of anaerobic respiration include glycolysis and fermentation.

Cellular respiration has three main processes: glycolysis, Krebs cycle, and electron transport chain. Glycolysis is an anaerobic process whereas Krebs cycle and ETC are aerobic processes. A molecule of glucose enters the cell and undergoes glycolysis in the cytosol. Some of the products of glycolysis are transported to mitochondria where they undergo Krebs cycle and then, eventually, the electron transport chain; therefore, anaerobic respiration occurs in the cytosol and aerobic respiration occurs in the mitochondria.

Anaerobic respiration produces very little ATP and the ATP produced is not sufficient to power all of the active processes in a cell. Aerobic respiration, particularly ETC, produces a lot of ATP. This is why mitochondria is called the “powerhouse” of the cell. Note that the products of glycolysis can undergo another type of anaerobic respiration called fermentation. This also produces very little ATP.

Electron transport chain is a series of electron carriers located on the inner membrane of the mitochondria. Electrons traverse across these electron carriers and this motion allows for the proton pump to generate a proton gradient across the inner membrane. This gradient is generated by pumping protons from the inside of the mitochondria to the periplasmic space (space between inner and outer mitochondrial membranes). The excess protons in the periplasmic space are transported back into the mitochondria and this movement facilitates the generation of ATP by the ATP synthase. Note that ATP is generated by ATP synthase, not the proton pump.

 and  are electron carriers that carry electrons from glycolysis and Krebs cycle. These carriers enter the ETC and donate their electrons to the carriers in ETC. This occurs because ETC carriers have higher affinity for electrons. Remember that each subsequent carrier in ETC has a higher affinity for electrons, so that it is able to snatch the electron from the previous carrier. As mentioned, the ATP generation is facilitated by the proton gradient, not the sodium gradient.

The correct answer is proton gradient.  and  donate electrons to electron transport chain complexes and pass them along the membrane, causing protons  to move from the mitochondrial matrix to the intermembrane space. The high proton concentration gradient causes protons to travel through the inner mitochondrial transmembrane ATP synthase to equilibrate the gradient. Passing of protons through ATP synthase promotes synthesis of ATP from ADP from inorganic phosphate.