Correct Option: B. Osmosis Explanation of the Correct Answer Osmosis is the process that specifically describes the movement of water molecules across a selectively permeable membrane. This movement occurs in response to a concentration gradient, which means that water moves from an area of lower solute concentration (where there are fewer solute particles) to an area of higher solute concentration (where there are more solute particles). Step-by-Step Explanation:

1. Selectively Permeable Membrane: This type of membrane allows certain molecules to pass through while blocking others. In the case of osmosis, the membrane is permeable to water but not to solutes (like salt or sugar).
2. Concentration Gradient: A concentration gradient exists when there is a difference in the concentration of solutes in two different areas. Water will move to balance the concentration of solutes on both sides of the membrane.
3. Movement of Water: Water molecules will naturally move from the side with a lower concentration of solutes (more water molecules) to the side with a higher concentration of solutes (fewer water molecules). This movement continues until equilibrium is reached, meaning the concentration of solutes is equal on both sides of the membrane.
4. Passive Process: Osmosis is a passive process, meaning it does not require energy (ATP) to occur. The movement of water is driven solely by the concentration gradient.

Why Other Options Are Incorrect:

• A. Active Transport: This process involves the movement of molecules against their concentration gradient, which requires energy (ATP). Active transport is not specific to water and typically involves ions or larger molecules. Since osmosis specifically refers to water movement, this option is incorrect.
• C. Diffusion: While diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, it does not specifically refer to water. Diffusion can apply to any type of molecule, not just water. Therefore, while osmosis is a type of diffusion (specifically for water), the term "diffusion" alone does not accurately describe the process in question.
• D. Facilitated Diffusion: This is a type of passive transport that involves the movement of molecules across a membrane via specific transport proteins. While it does not require energy, it is used for larger or polar molecules that cannot easily pass through the lipid bilayer. Water can move through specialized channels called aquaporins, but the term "facilitated diffusion" does not specifically denote the movement of water across a selectively permeable membrane in the context of a concentration gradient.

Summary of Key Points:

• Osmosis is the movement of water across a selectively permeable membrane due to a concentration gradient.
• Water moves from areas of low solute concentration to areas of high solute concentration.
• Osmosis is a passive process and does not require energy.
• Other processes like active transport, diffusion, and facilitated diffusion do not specifically describe the movement of water in this context.

This understanding of osmosis is crucial in biology, especially in topics related to cell biology, plant physiology, and homeostasis.