100+ Free Valencian Community PAU Biology Practice Questions

Explanation: Water possesses a high specific heat capacity (4.184 J/g·°C) due to extensive hydrogen bonding, allowing organisms to absorb significant heat with minimal temperature change. Additionally, its high heat of vaporization requires substantial energy to break hydrogen bonds during evaporation, enabling effective cooling through perspiration or transpiration.

Explanation: A 0.3 M NaCl solution dissociates into ~0.6 Osm/L of solute particles, making it hypertonic relative to the cell cytosol (~0.3 Osm/L). Water moves by osmosis down its concentration gradient from the intracellular environment to the hypertonic extracellular environment, causing the red blood cell to shrink and undergo crenation.

Explanation: D-Glucose is an aldohexose (a six-carbon sugar containing an aldehyde group at C1). In aqueous solution, intramolecular cyclization between the C1 aldehyde and C5 hydroxyl forms a six-membered pyranose ring ($\beta$-D-glucopyranose), which predominantly adopts a stable chair conformation.

Explanation: Maltose is composed of two D-glucose monomers linked via an $\alpha(1 \rightarrow 4)$ glycosidic bond. Because one of the hemiacetal anomeric carbons (at C1 of the second glucose unit) remains free, maltose can undergo mutarotation and act as a reducing sugar.

Explanation: Cellulose consists of linear, unbranched polymer chains of D-glucose linked by $\beta(1 \rightarrow 4)$ glycosidic bonds. This configuration allows long chains to lie parallel and form extensive inter-chain hydrogen bonds, creating rigid microfibrils with high tensile strength. Starch and glycogen contain $\alpha(1 \rightarrow 4)$ bonds that yield helical structures easily hydrolyzed by enzymes.

Explanation: Glycerophospholipids (e.g., phosphatidylcholine) consist of a glycerol backbone esterified at C1 and C2 to hydrophobic fatty acid tails, and at C3 to a hydrophilic phosphate group, which is further linked to a polar head group such as choline, ethanolamine, or serine.

Explanation: Cis-double bonds create permanent kinks in fatty acid hydrocarbon tails. These kinks prevent adjacent lipids from packing tightly into a rigid crystalline lattice, weakening intermolecular van der Waals forces and lowering the phase transition temperature, thereby increasing membrane fluidity.

Explanation: Cholesterol is an amphipathic steroid. Its polar region consists of a single hydroxyl group (-OH) at position C3, which resides near the membrane surface interacting with water and polar head groups of phospholipids. The non-polar planar steroid ring and hydrocarbon tail insert deeply into the hydrophobic lipid core.

Explanation: At physiological pH 7.4, which is well above the $pK_a$ of the $\alpha$-carboxyl group (~2.2) and below the $pK_a$ of the $\alpha$-amino group (~9.4), the carboxyl group loses its proton ($-\text{COO}^-$) and the amino group gains a proton ($-\text{NH}_3^+$). This dipolar ion with a net charge of zero is termed a zwitterion.

Explanation: The peptide bond ($-\text{CO}-\text{NH}-$) exhibits resonance delocalization of electrons between the carbonyl oxygen and amide nitrogen. This confers ~40% double-bond character to the C-N bond, locking the six participating atoms into a rigid planar conformation (usually trans) and restricting rotation.