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.

Interphase in the cell cycle encompasses the G₁, S, and G₂ phases, as it shows the period of growth and DNA replication that a cell must go through to prepare for mitosis. Cell division, which occurs during the M phase, is the only portion of the cell cycle that is not included in interphase.

Because they do not divide, central nervous system nerve cells do not need to experience growth (G₁ and G₂ phases), DNA replication (S phase), or mitosis (M phase). As a result, they spend most of their lives arrested in G₀, a resting phase.

Cancer can often be the result of a problem with the checkpoint at the end of the G₂ phase, as this is the last stop for regulation before the cell undergoes division.

If this checkpoint is not functioning effectively, cells can undergo rapid and unregulated division, resulting in cancer. p53, a cancer suppressor gene, plays a key role in this checkpoint, and is commonly found to be mutated in cancer patients.

The S phase is when the cell replicates its DNA, resulting in chromatid pairs that will split apart during mitosis. The G phases are mainly dedicated to protein synthesis and cell growth. The M phase is the act of mitosis.