Which of the following statements about osmosis is correct?

Introduction / Context:
Osmosis is a fundamental process in biology that explains how water moves into and out of cells. It is closely related to diffusion but has specific conditions and directions of movement. Many exam questions test your understanding of whether osmosis is passive or active and how the water moves in response to concentration differences. This question asks you to pick the correct statement that summarises osmosis accurately.

Given Data / Assumptions:

• The term osmosis is being defined and contrasted with general diffusion.
• Options describe its relation to diffusion, direction of movement, and energy requirement.
• We assume basic familiarity with concentration gradients and selectively permeable membranes.

Concept / Approach:
Osmosis is the passive movement of water molecules across a selectively permeable membrane from a region of higher water potential (or lower solute concentration) to a region of lower water potential (or higher solute concentration). It is considered a special form of diffusion because it involves movement down a concentration gradient and does not require energy from ATP. Water does not move “against” its concentration gradient in osmosis, and cells do not have to use energy directly to make it happen.

Step-by-Step Solution:
Step 1: Recall that diffusion is the passive movement of particles from higher to lower concentration, down their concentration gradient. Step 2: Recognise that osmosis is a specific type of diffusion that involves only water molecules moving across a selectively permeable membrane. Step 3: Understand that in osmosis, water moves from where it is more abundant (less solute) to where it is less abundant (more solute), which is still down its own concentration gradient. Step 4: Note that because osmosis is passive, it does not require direct metabolic energy, so any statement claiming it requires energy or moves against the gradient is false.

Verification / Alternative check:
Experiments with potato strips or red blood cells placed in solutions of different concentrations demonstrate osmosis. Potato pieces in concentrated salt solution lose water and shrink because water moves out into the more concentrated solution. Red blood cells swell and may burst in distilled water as water moves in. In both cases, movement is down the water potential gradient and does not require ATP. This experimental evidence supports the correct description of osmosis as passive diffusion of water across a membrane.

Why Other Options Are Wrong:

• Movement against water's concentration gradient: Osmosis involves movement down the concentration gradient; moving against the gradient would be active transport.
• Requires energy: Osmosis is a passive process and does not use cellular energy directly.
• All of the above: Since the previous two statements are incorrect, this combined option must also be incorrect.

Common Pitfalls:
Students sometimes confuse osmosis with active transport because both can involve movement of substances across membranes. The key difference is that osmosis always follows the natural gradient of water concentration and does not require ATP. Another mistake is forgetting the role of a selectively permeable membrane, which allows water but not solute to pass freely. Keeping these key features in mind will help you correctly define osmosis in exam questions.

Final Answer:
The correct statement is that osmosis is a special type of diffusion involving the movement of water across a selectively permeable membrane.