produced by glycolysis and the citric acid cycle and  produced by the citric acid cycle as electron carries. These molecules "drop off" electrons to the complexes in the electron transport chain.  is also an electron carrier, however is main role is during the Calvin cycle of photosynthesis. 

The electron transport chain (ETC) is responsible for oxidative phosphorylation, resulting in the production of ATP. The ATP is produced by a proton gradient made as electrons are transported throughout the chain. Inhibition of this process by "Substance Y" will block the formation of a proton gradient. Blocking the proton gradient will not allow for oxidative phosphorylation to occur, thus greatly decreasing the amount of ATP produced.

The correct answer to this question is oxygen.

Oxygen is the final electron acceptor of electrons as they are passed down the electron chain. The electrons move and combine with oxygen to produce . Water and hydrogen are just byproducts of the acceptance of the electron, not the acceptor. The electrons are actually brought to the electron transport chain by carries like  and .

This is because glucose can net 36 ATP, NADH actually nets 3, FADH2 can net 2, and pyruvate can net 15. This answer involves a careful examination of respiration processes.

FADH2 is directly attached onto the second protein of the electron transport chain and therefore the electrons of FADH2 (electron carrier) are dropped off at the second protein not the first. As a result, the electrons from FADH2 do not pump as much electrons across the membrane as NADH. This results in a lower proton gradient created from FADH2 then NADH and therefore less ATP production from FADH2. 

Remember that NADH and FADH2 are electron carriers and do not directly create any energy. The movement of the electrons through the electron transport chain also does not create energy directly, but does create a proton gradient that is later used to create energy. The movement of protons down its proton gradient through ATP synthase does, however, generate energy. It actually generates around 30 ATP molecules per one glucose. 

In both cellular respiration and photosynthesis, chemiosmosis occurs. Chemiosmosis is the process in which the creation of a proton gradient leads to the transport of proton down its concentration gradient to produce ATP. This occurs in the electron transport chain in both mitochondrias and chloroplast. In the photosynthesis it occurs when the electron is transported from photosystem II to photosystem I. 

Apoptosis is programmed cell death, and it usually occurs when the DNA of the cell is damaged beyond repair.

Photosynthesis and glycolysis are normal metabolic processes of the cell, and would not result from irreversible damage. Endocytosis and exocytosis are also normal cell processes or taking up substances into the cell (endocytosis) or expelling them (exocytosis) in the form of vesicles.

The M phase is also known as mitosis, and is the time where the cell is ready to divide. At this time, the cell has synthesized enough proteins and has successfully replicated its DNA, so growth and synthesis are not priorities.

Remember that G1, S, and G2 are all divisions of interphase. In interphase, the cell is preparing to divide by synthesizing proteins and replicating DNA, so these three phases place a heavy emphasis on growth and protein synthesis.

During G₁, the cell undergoes growth as it increases in size and produces organelles. This is followed by DNA replication is S phase, further growth in G₂, and mitosis in M phase.