State that water diffuses through partially permeable membranes by osmosis.

What is Osmosis?

Osmosis is the movement of water molecules through a partially permeable membrane from an area of low solute concentration to an area of high solute concentration until equilibrium is reached. Think of it like a crowd moving from a less crowded room to a more crowded room until the number of people in each room is balanced.

In scientific notation: water diffuses from a region of lower solute concentration $C_1$ to a region of higher solute concentration $C_2$ across a membrane that allows only water to pass.

Key Concepts

• Water moves from low to high solute concentration.
• The membrane is partially permeable – it lets water through but blocks most solutes.
• Osmosis continues until the solute concentrations on both sides are equal (equilibrium).
• Osmotic pressure ($\Pi$) is the pressure needed to stop water flow and can be calculated using $ \Pi = iMRT $ (ideal solution).

Analogy: The Water Filter

Imagine a water filter that only lets water molecules pass but blocks sugar crystals. If you pour a sugary drink on one side of the filter and plain water on the other, the water will slowly move through the filter to dilute the sugary side, just like osmosis.

💡 Remember: Osmosis is not the same as diffusion – it specifically involves water and a semi‑permeable membrane.

Examples in Biology

1. Root cells absorb water from soil through osmosis.
2. Plant cells maintain turgor pressure, keeping stems rigid.
3. Red blood cells in a hypotonic solution swell and may burst (hemolysis).

Exam Tips

When answering exam questions about osmosis:

• Define osmosis clearly and mention the role of a partially permeable membrane.
• Use the correct direction: water moves from low to high solute concentration.
• Explain the concept of osmotic pressure if asked.
• Include a simple diagram (drawn by hand) showing the membrane and concentration gradient.
• Use terms like hypotonic, hypertonic, and isotonic where relevant.

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