The primary purpose of fermentation is to regenerate the NAD⁺ that was reduced to NADH during glycolysis. This is essential because the cell will need that NAD⁺ to be able to go through glycolysis. In anaerobic environments, the Krebs cycle and electron transport chain cease to function, leaving glycolysis as the primary metabolic process for the cell to generate ATP. NAD⁺ is an essential reactant for glycolysis, but is rapidly consumed when the electron transport chain is not running. Fermentation takes place in order to replenish this reactant and allow glycolysis to continue.

Ethanol and lactic acid can be products of fermentation, but are not the primary purpose for fermentation. No oxygen or ATP is generated during this process.

During aerobic metabolism, a single glucose molecule produces significantly more ATP molecules than during anaerobic metabolism. The exact number depends on a number of things, including the organism, cell type, and NADH shuttle. Aerobic metabolism of glucose produces more reduced coenzymes, which drop off their electrons at the electron transport chain in the mitochondria. Electron transport provides energy to pump protons from the mitochondrial matrix into the intermembrane space. Both processes produce pyruvate.

Integral proteins are proteins that connect the inside of the cell to the outside by traversing the phospholipid bilayer. Carrier proteins, for example, allow substances to pass the membrane by letting them cross through channels.

The correct answer is nonpolar, small in size, and have a large lipid solubility coefficient. The center of the lipid bilayer is nonpolar and composed of fatty acid chains; therefore, nonpolar molecules diffuse through more easily. Diffusion does not use transport proteins to shuttle molecules through the lipid bilayer, so smaller molecules are able to diffuse more easily between the phospholipids that comprise the bilayer. Finally, a higher lipid solubility coefficient means that the molecule is more soluble in lipids and less soluble in aqueous solutions. A low lipid solubility means that the molecule is more aqueous-soluble than lipid-soluble.  

The correct answer is lipid rafts. Lipid rafts are more dense, less fluid domains of the plasma membrane. In addition to clustering cholesterol and sphingolipids, large transmembrane-domain proteins also localize here.

Integrins are transmembrane receptors, micelles are small lipid spheres, and phosphoinositides are a family of lipid molecules. 

The correct answer is glycoprotein. HIV and many lentiviruses, which are also retroviruses, bind the CD4 glycoprotein. Lipopolysaccharides are found in bacteria, not vertebrates. Oligosaccharides are sugar polymers and generally function in cell-cell signaling. Cholesterol and phosphatidylcholine are components of the plasma membrane, not necessarily extracellularly exposed. The latter two help define the shape and rigidity of the plasma membrane. 

The correct answer is lipid rafts. These are dense portions of the phospholipid bilayer that are slightly thicker due to the increased presence of cholesterols and sphingolipids and are hubs for many processes. Micelles are single layer phospholipids that form spheres. Endosomes are intracellular compartments that are often important for transport of materials to the plasma membrane. 

Cell walls can vary in composition based on what organism is being discussed. In the case of plant cells, cellulose is the primary component. Bacteria use peptidolgycan in their cell walls, and fungi use chitin.

The cell wall is composed of cellulose and encloses a plant cell in a rigid framework. Plant cells have both cell membranes and cell walls. Plant cell walls cannot be digested by humans due to the beta glycosidic bond in its structure. The cell membrane forms the outer boundary of a cell and isolates cell contents from the environment. The cell membrane regulates movement of materials into and out of the cell. The cell membrane is also referred to as the plasma membrane. The nuclear envelope regulates movement into and out of the nucleus. The cytoskeleton gives shape and support to cells and is typically involved in cell movement.

The most abundant molecule in bacterial cell walls is peptidoglycan. Recall that peptidoglycan contains repeating NAG-NAM units, which are made from glucose. In addition, peptidoglycan also has peptides attached to these oligosaccharides. These units form bonds with each other to create a strong and a robust cell wall. Peptidoglycan is synthesized in cytoplasm and on cell membrane before finally being transported to the cell wall.

Recall that phosphate head groups are found on phospholipids, which make up cell membranes (not cell walls).