Practice Questions

Enantiomers are a pair of molecules that are non-superimposable mirror images of each other. D-glucose and L-glucose are mirror images at all four chiral centers (C2, C3, C4, and C5), making them enantiomers. Diastereomers differ at one or more, but not all, chiral centers.

A reducing sugar has a free anomeric carbon whose carbonyl group can be oxidized, thereby reducing another agent like Cu²⁺ in Benedict's reagent. The free aldehyde or α-hydroxyketone group is essential for this property. Non-reducing sugars lack this free group.

The nucleophilic addition of an alcohol (hydroxyl group) to a carbonyl group forms a hemiacetal (from an aldehyde) or a hemiketal (from a ketone). This intramolecular reaction converts the linear monosaccharide into its cyclic form, creating a new chiral center (the anomeric carbon).

Monosaccharides are polyhydroxy carbonyl compounds. If the carbonyl group is at the end of the carbon chain (C1), it is an aldehyde and the sugar is an aldose (e.g., glucose). If the carbonyl group is on an internal carbon (C2), it is a ketone and the sugar is a ketose (e.g., fructose).

Antifreeze proteins adsorb to the surface of nascent ice nuclei. Their large, hydrophilic structure sterically hinders and geometrically mismatches the approach and orderly crystallization of further water molecules, effectively stopping ice crystal growth.

Heat energy disrupts the internal hydrogen bonds of the starch granule. Water molecules then penetrate and form new H-bonds with the exposed -OH groups of the starch polymers (imbibition). This swelling, called gelatinization, eventually ruptures the granules, thickening the mixture.

Aquaporins are integral membrane proteins that form water-specific channels. While water can slowly diffuse through the lipid bilayer, aquaporins allow for a much faster, regulated flux of water in tissues like kidney tubules, red blood cells, and plant roots where rapid osmosis is required.

Water buffers temperature because much of the thermal energy added to a cell is used not to increase molecular kinetic energy (and thus temperature) but to disrupt the extensive hydrogen-bonded network. This high heat capacity is a direct function of hydrogen bonding.

Excess water intake lowers plasma osmolarity, making it hypotonic to the intracellular fluid. Water moves by osmosis into the cells. In the brain, this can lead to cerebral edema (swelling) within the rigid skull, causing increased intracranial pressure, which is a life-threatening condition.

Historically and by definition, one calorie is the amount of heat energy needed to raise the temperature of exactly one gram of pure water by exactly one degree Celsius. This makes water the standard reference for calorimetry and the definition of heat units.