Water Diffusion Across Membranes

Steps:

• Identify key process: Water moves via osmosis, a type of passive diffusion down its concentration gradient.
• Recall membrane structure: Phospholipid bilayer allows small, uncharged molecules like water to pass directly without proteins.
• Eliminate active mechanisms: No energy required for water entry in animal cells under normal conditions.
• Confirm primary method: Simple diffusion suffices; channels (aquaporins) aid but aren't sole or essential for basic crossing.

Why D is correct:

• Diffusion (option 2) is the direct mechanism per fluid mosaic model, as water's polarity and size permit bilayer solubility without transport proteins.

Why the others are wrong:

• A includes 1 (likely active transport), which requires ATP and opposes gradient—water follows it.
• B includes 1 and 3 (likely channels), but active transport isn't used; channels facilitate, not primary.
• C includes 3 only with 2, but question specifies "cross," emphasizing diffusion over optional channels.

Not enough information on exact statements 1 and 3.

Final answer: D