(Choice A) Simple diffusion refers to the movement of particles along their concentration gradient directly through the cell membrane. Transport of gases (02 and CO2) occurs via simple diffusion, as the membrane is very permeable to small, nonpolar molecules.
(Choice B) Endocytosis allows for cellular uptake through the formation of membrane-bound, typically clathrincoated, vesicles. Uptake of cholesterol by cells occurs by means of receptor-mediated endocytosis (mediated by the LDL receptor).
(Choices D and E) Active transport refers to the movement of a substance against its concentration gradient. This process requires energy as well as transport proteins. In primary active transport, the energy required for transport against the concentration gradient is provided by the hydrolysis of ATP. In secondary active transport, this energy is generated by co-transport of a separate substance down its concentration gradient. Transport of glucose against the concentration gradient occurs via the Na/glucose symporter, which is found in the intestinal and renal tubular epithelium and is used to transfer glucose intracellularly from the lumen.

(Choices A and E) Parathyroid hormone and glucagon act on Gs protein-coupled receptors found on the cell membrane. Binding their respective ligands subsequently activates adenylyl cyclase and increases intracellular cyclic AMP concentration.
(Choice B) Growth hormone receptor is a membrane-bound receptor that works via activating the JAK-STAT pathway.
(Choices C and D) The receptors for insulin-like growth factor-1 (IGF-1) and insulin are structurally similar and located at the cell membrane. The intracellular domains of these receptors have intrinsic tyrosine kinase activity, which is activated on ligand binding. Autophosphorylation of the tyrosine residues on the intracellular part of the receptors then triggers downstream signaling.

(Choice A) Vmax depends on how fast an enzyme can catalyze a reaction when there are enough substrate molecules to fully saturate its active sites. Competitive inhibitors do not affect V max, as higher substrate concentrations are able to overcome the inhibition.
(Choice B) Most competitive inhibitors, such as malate, bind in the substrate-binding pocket. In contrast, most noncompetitive inhibitors bind at allosteric sites, resulting in a conformational change of the enzyme that decreases enzymatic activity and slows the rate of reaction (V max). Noncompetitive inhibition does not change the apparent Km and cannot be overcome with higher substrate concentrations.
(Choice C) Irreversible inhibitors bind to enzymes through strong covalent bonds; this typically renders the enzyme permanently ineffective, decreasing the Vmax.
(Choice D) Competitive inhibitors interfere with substrate binding due to the inhibitor’s own high affinity for the enzyme’s active site. This causes the measured Kn value to increase; however, the actual enzyme affinity for the substrate remains unchanged.