DNA base pairing uses complementary rules: adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds. This A–T and G–C pattern follows Chargaff’s rules (A = T, G = C). Uracil is used in RNA instead of thymine and pairs with adenine. The difference in bond numbers also affects stability, with GC pairs being more thermally stable than AT pairs. So the arrangement where A pairs with T via two hydrogen bonds and G pairs with C via three hydrogen bonds is correct for DNA.

The key idea is the difference in energy yield and oxygen use between aerobic respiration and fermentation. In aerobic respiration, glucose is fully oxidized through glycolysis, the pyruvate step, the citric acid cycle, and oxidative phosphorylation. Oxygen acts as the final electron acceptor in the electron transport chain, allowing a large amount of ATP to be produced by phosph from the proton gradient. That full process typically yields about 30–32 ATP per glucose, though exact numbers can vary by organism and shuttle systems. In fermentation, there is no functional electron transport chain operating to extract that much energy; electrons from NADH are dumped back into pyruvate or a derivative to regenerate NAD+, enabling glycolysis to continue, but only a small amount of ATP is produced—about 2 ATP per glucose. Fermentation also generates the fermentation products like lactate or ethanol, but the crucial point for the comparison is the much lower ATP yield and the lack of reliance on oxygen. So the statement that aerobic respiration yields roughly 30–32 ATP per glucose while fermentation yields about 2 ATP captures the big difference in energy efficiency and explains why aerobically respiring cells produce far more ATP than those relying on fermentation. The other points are less precise descriptions of the systems: fermentation doesn’t use oxygen as the final electron acceptor, aerobic respiration isn’t defined by yielding less ATP, and while glycolysis is shared, saying aerobic respiration requires glycolysis doesn’t compare the two processes as clearly.

PLC signaling generates two second messengers, IP3 and DAG, which coordinate calcium release and PKC activation. When PLC cleaves PIP2 in the inner plasma membrane, it produces IP3, a soluble molecule that diffuses through the cytosol to bind receptors on the endoplasmic reticulum and open Ca2+ channels. This raises cytosolic Ca2+, enabling many Ca2+-dependent processes. The other product, DAG, remains in the membrane and, together with the Ca2+ rise, activates protein kinase C (PKC) by helping recruit PKC to the membrane and promoting its catalytic activity. Thus, IP3’s role is to release Ca2+, while DAG helps activate PKC; both arise from PLC-mediated cleavage of PIP2.

The nucleus acts as the cell’s control center: it houses the genetic material (DNA) and directs gene expression and DNA replication to coordinate all cellular activities. It’s enclosed by a double membrane with nuclear pores that regulate traffic in and out, and it contains the nucleolus where ribosomal RNA is made, with ribosomes then functioning in the cytoplasm or on the rough endoplasmic reticulum. Energy production happens in mitochondria, not in the nucleus, and calcium storage is mainly in the endoplasmic reticulum, not in the nucleus.