Final answer:
Cell membranes are most permeable to water (H2O) because it can easily pass through aquaporins, whereas ions like Ca2+, Cl−, and K+ require specific ion channels to be transported across the membrane.
Step-by-step explanation:
Cell membranes are selectively permeable structures that regulate the passage of substances into and out of cells. While membranes are impermeable to electrically charged ions such as sodium (Na+), chloride (Cl−), and calcium (Ca2+), they are most permeable to water (H2O), which can easily pass through due to its small size and ability to move through aquaporins, specialized channels for water. Ions like Ca2+, Cl−, and K+ require specific ion channel proteins to facilitate their transport across the hydrophobic interior of the membrane due to their polar nature.