When oxygen isn’t available, cells can’t fully oxidize pyruvate in the mitochondria, so they rely on fermentation to recycle NAD+ and keep glycolysis going. Pyruvate from glycolysis is converted into either ethanol and CO2 (in yeast and many microbes) or into lactate (in many bacteria and in muscle cells). In both cases, NADH is oxidized back to NAD+, allowing glycolysis to continue and a small amount of ATP to be produced. That’s why the typical fermentation end products are ethyl alcohol (ethanol) or lactic acid (lactate), depending on the organism. The other options don’t fit as end products of fermentation under anaerobic conditions: complete oxidation yields CO2 and water with oxygen; fermentation primarily regenerates NAD+ and produces alcohol or lactate rather than ATP and NADH; and oxygen is not produced in fermentation.

A peptide bond covalently links amino acids in proteins. It forms by a condensation reaction between the carboxyl group of one amino acid and the amino group of the next, releasing a molecule of water. This creates a -CO-NH- linkage along the backbone, giving the polypeptide its directional sequence from N-terminus to C-terminus. The peptide bond has partial double-bond character from resonance, making it planar and limiting rotation, which helps determine the protein’s overall shape. Other types of bonds—like hydrogen bonds, ionic interactions, or glycosidic linkages—do not serve as the main linkage between amino acids in the polypeptide chain.

Teichoic acids are characteristic components of Gram-positive bacterial cell walls. They come in two forms: wall teichoic acids, which are covalently linked to peptidoglycan, and lipoteichoic acids, which are anchored in the cytoplasmic membrane. Their presence contributes to the thick, teeming peptidoglycan layer and gives the cell surface a negative charge that helps with ion binding. Gram-negative bacteria, in contrast, have a thin peptidoglycan layer and an outer membrane, and they do not contain teichoic acids. This structural distinction makes Gram-positive organisms the ones that contain teichoic acids.

Active humoral immunity happens when B cells directly recognize a foreign antigen and then produce antibodies. After B cells bind the antigen and receive help from helper T cells, they proliferate and differentiate into plasma cells that secrete specific antibodies, as well as memory B cells for quicker recall later. These antibodies circulate to neutralize pathogens, tag them for destruction, and can activate complement—if needed—but the defining feature is the body's own production of antibodies in response to the antigen. The other options describe different scenarios: receiving antibodies from another person is passive immunity, cytotoxic T cells mediate cell-mediated immunity, and while antibodies can engage complement, active humoral immunity centers on B cells generating antibodies themselves.