BIO NMDCAT

At 4°C, water is densest. Below 4°C, it expands, and ice (0°C) is ~9% less dense, so it floats. This surface ice layer insulates the liquid water below, maintaining a temperature above freezing and allowing aquatic life to survive winter.

Transpiration pull creates negative pressure in xylem. Due to strong cohesion (H-bonds between water molecules), the continuous water column is pulled upwards. Adhesion to xylem walls also assists, but the tensile strength of the water column is a direct result of cohesion.

During the light-dependent reactions, water undergoes photolysis. Water is split (oxidized) by the oxygen-evolving complex, providing replacement electrons to P680 (Photosystem II) and releasing protons (H⁺) and molecular oxygen (O₂) as a byproduct.

When sweat evaporates, the phase change from liquid to gas requires a large amount of heat energy (latent heat of vaporization). This heat is absorbed from the skin's surface, effectively cooling the body. The cooling is due to heat removal, not heat release.

The 5' cap and the 3' poly-A tail are not translated. Their primary roles are to increase the stability of the mRNA by protecting its ends from ribonucleases, and to facilitate the initiation of translation by interacting with translation initiation factors.

tRNA acts as an adaptor, carrying a specific amino acid at its 3' end and recognizing a specific three-nucleotide codon on the mRNA through its complementary anticodon loop. This bridges the genetic code and the amino acid sequence of a protein.

The glycosidic bond in sucrose is formed between the anomeric carbon (C1) of glucose and the anomeric carbon (C2) of fructose. Since neither carbonyl group is free to open into an aldehyde or ketone form, sucrose cannot reduce Cu²⁺ and is thus a non-reducing sugar.

When [S] is saturating (>> Km), all enzyme active sites are occupied, and the reaction rate is solely a function of how fast the enzyme can process substrate (Vmax). Vmax is proportional to the total enzyme concentration, so doubling the enzyme doubles V0 under these conditions.

Metabolism is the sum of all cellular reactions. Catabolism is the breakdown of complex molecules into simpler ones, releasing energy. Anabolism is the synthesis of complex molecules from simpler ones, consuming energy. Their regulation is central to life.

The 2'-OH group in the ribose sugar of RNA is reactive. It can act as a nucleophile and attack the adjacent phosphodiester bond under alkaline conditions, leading to the self-hydrolysis (cleavage) of the RNA strand. DNA, lacking this 2'-OH, is far more chemically stable.

Many enzymes require metal ions (like Mg²⁺, Ca²⁺) as activators or cofactors. EDTA chelates (binds tightly to) these divalent cations, making them unavailable to the enzyme. This effectively inhibits the enzyme's activity. Adding back an excess of the metal ion reverses the inhibition.

Bile salts are amphipathic cholesterol derivatives secreted from the liver. Their hydrophobic side associates with lipid droplets, and their hydrophilic side faces the aqueous intestinal fluid. This coating breaks large globules into smaller ones (micelles), vastly increasing the surface area for lipase action.

With 20 different amino acids as monomers, the number of possible sequences and lengths for a polypeptide is astronomically large. Furthermore, the diverse chemical properties of the 20 R-groups (charged, polar, non-polar, etc.) enable a protein to fold into an immense variety of complex 3D shapes.

The nitrogenous bases in double-stranded DNA are stacked and have a lower absorbance. When the double helix is denatured into two random, single-stranded coils, the bases become unstacked. This unstacking increases their absorbance of UV light at 260 nm, a phenomenon known as the hyperchromic effect.

Specificity is the ability of an enzyme to choose exactly one substrate from a pool of similar molecules. This is due to the exact complementary fit and specific chemical interactions (ionic, H-bonding, hydrophobic) between the substrate and the R-groups lining the active site.

In covalent catalysis, a powerful nucleophilic R-group in the active site (e.g., the -SH of cysteine or -OH of serine) forms a transient covalent bond with the substrate. This acyl-enzyme intermediate is then resolved by another step, releasing the product and regenerating the free enzyme.

Allosteric regulation is mediated by modulator molecules that bind to a site (allosteric site) physically distinct from the active site. This binding causes a conformational change that can either increase (allosteric activator) or decrease (allosteric inhibitor) the activity of the enzyme at its active site.

The definition of competitive inhibition is a "competition" for the active site. At a high enough concentration, the substrate out-competes the inhibitor for the active site, so all enzyme molecules can still bind substrate and reach Vmax. The apparent Km is increased, but Vmax is ultimately unchanged.

The amino-terminal ends of both the light (VL) and heavy (VH) chains form the antigen-binding site. These variable domains have highly diverse amino acid sequences from one antibody clone to another, creating a unique 3D surface that is specific for a single epitope.