AP BIOLOGY

Detailed explanation-1: -Carrier proteins are responsible for the facilitated diffusion of sugars, amino acids, and nucleosides across the plasma membranes of most cells.

Detailed explanation-2: -Carrier proteins and channel proteins are the two major classes of membrane transport proteins. Carrier proteins (also called carriers, permeases, or transporters) bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane (Figure 11-3).

Detailed explanation-3: -Mammalian plasma membranes are more complex, containing four major phospholipids-phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin-which together constitute 50 to 60% of total membrane lipid.

Detailed explanation-4: -In mammalian cells, secretory proteins are translocated across the Endoplasmic Reticulum (ER) membrane into the lumen co-translationally via the translocon complex. Misfolded proteins in the ER are transported back to the cytosol and degraded by the proteasome, a process termed ER-associated degradation (ERAD).

Detailed explanation-5: -Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly. Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly.