Final answer:
Water (H₂O) can diffuse across the phospholipid bilayer due to its very small and polar nature. Calcium ions (Ca₂⁺) and glucose require specific transport mechanisms to cross the bilayer as they are either charged or too large and polar.
Step-by-step explanation:
The question asks which compounds can diffuse across a phospholipid bilayer: 1) Ca₂⁺ 2) H₂O 3) glucose 4) All of these. To answer this question, we need to consider the properties of the phospholipid bilayer and the characteristics of each compound.
Lipid bilayers are selectively permeable, meaning they allow certain substances to pass through while blocking others. Large polar or ionic molecules, which are hydrophilic, cannot easily cross the bilayer without assistance. Very small polar molecules, such as water (H₂O), can cross via simple diffusion. However, charged ions like calcium (Ca₂⁺), are repelled by the bilayer's hydrophobic interior and cannot pass through via simple diffusion. Glucose is a large polar molecule and also requires assistance from transport proteins to move across the bilayer via facilitated diffusion.
Hence, the compound from the given options that can diffuse across the phospholipid bilayer is water (H₂O) because of its small size and despite being polar. Calcium ions (Ca₂⁺) and glucose cannot diffuse across the bilayer without specific transport mechanisms.