BIOLOGICAL MOLECULES

Amphipathic molecules have a dual nature. Phospholipids are a prime example, with a hydrophilic polar "head" (phosphate group) and hydrophobic non-polar "tails" (fatty acid chains). This property is fundamental to the formation of lipid bilayers in water.

Complementarity is the specific pairing dictated by hydrogen bonding potential: adenine pairs only with thymine (or uracil), and guanine pairs only with cytosine. This ensures a purine always pairs with a pyrimidine, maintaining a consistent double helix structure.

Fibrous proteins (e.g., collagen, keratin) have long, chain-like, repetitive secondary structures that form strong, water-insoluble fibers. Their primary role is structural support, contrasting with the soluble, dynamic, roughly spherical nature of globular proteins like enzymes and antibodies.

Lipids contain long hydrocarbon chains rich in C-H bonds, which are in a highly reduced state. Upon oxidation, they yield more energy than the more oxidized C-OH bonds found in carbohydrates. The bulk of energy release comes from the transfer of electrons from these C-H bonds.

Cholesterol is a sterol lipid that intercalates between phospholipids. At high temperatures, it restrains movement, reducing fluidity. At low temperatures, it prevents tight packing, preventing solidification. Thus, it acts as a key fluidity buffer for membrane stability.

The two strands of the DNA double helix are physically linked by hydrogen bonds between complementary base pairs (A-T and G-C). The backbone of each individual strand is held together by covalent phosphodiester bonds.

Linoleic acid is an omega-6 polyunsaturated fatty acid that is essential for mammals, including humans. We lack the desaturase enzymes required to introduce double bonds beyond the ninth carbon, so it must be obtained from the diet.

Feedback inhibition is a regulatory mechanism where the end product of a metabolic pathway acts as an inhibitor of an enzyme earlier in the pathway, usually the first committed step. This prevents the unnecessary accumulation of the product and wasteful use of resources.

While the sugar-phosphate backbone provides structural integrity, genetic information is encoded in the specific linear sequence of the four nitrogenous bases (A, T, G, C). This sequence is the code that dictates protein synthesis and is heritable.

Both glycogen and cellulose are glucose polymers, but glycogen has α-1,4 and α-1,6 glycosidic bonds, allowing it to be a branched, digestible energy source. Cellulose has β-1,4 glycosidic bonds, which create straight chains that form strong structural fibers and are indigestible by most animals.

Condensation (or dehydration synthesis) is the anabolic process where monomers are covalently bonded together with the simultaneous removal of a water molecule. This is the fundamental mechanism for polymer formation. Hydrolysis is the reverse, catabolic process.

Quaternary structure exists only in proteins composed of more than one polypeptide chain (subunit). It describes the specific 3D arrangement and interactions between these individual, folded subunits, as seen in hemoglobin (α2β2).

The test detects the free carbonyl group (C=O) at the anomeric carbon of a reducing sugar. This group can be oxidized, thereby reducing the Cu²⁺ in Benedict's reagent to Cu⁺, forming a colored precipitate. Non-reducing sugars lack this free group.

A non-competitive inhibitor reduces the total amount of functional enzyme, thereby lowering the Vmax. Since the inhibitor does not bind to the active site, increasing the substrate concentration cannot saturate the inhibitor and restore Vmax to its original level.

Phosphorylation is a key reversible covalent modification used to regulate enzyme activity. A kinase adds a phosphate group, causing a shape change that can activate or deactivate the enzyme. A phosphatase removes it, reversing the effect.

Sucrose is dextrorotatory, but upon hydrolysis, the resulting mixture of glucose (dextrorotatory) and fructose (strongly levorotatory) makes the overall solution levorotatory. This change in optical rotation is called inversion, and the product is called invert sugar.

The lock-and-key model proposes a rigid active site that is perfectly complementary only to a specific substrate, ensuring high specificity. The induced fit model expands on this, adding flexibility, but the lock-and-key concept directly explains absolute specificity.

Pepsin is a gastric enzyme that has adapted to function in the highly acidic environment of the stomach, where HCl is present. Therefore, its optimum pH is strongly acidic (around 1.5-2.0), unlike enzymes like trypsin which function in the alkaline small intestine (pH ~8.0).

The covalent bond linking the nitrogenous base (purine or pyrimidine) to the 1' carbon of the pentose sugar (ribose or deoxyribose) is an N-glycosidic bond. The phosphate group is linked to the 5' carbon.