Does osmosis require energy?

No, osmosis is a type of passive transport. Water moves naturally down its concentration gradient, so the cell does not need to spend any ATP (energy) to make it happen.

Is osmosis the same as diffusion?

Osmosis is a specific type of diffusion. While diffusion is the movement of any particles from high to low concentration, osmosis refers specifically to the movement of water molecules across a semipermeable membrane.

Why don't plant cells burst in pure water like animal cells do?

Plant cells have a rigid cell wall outside their cell membrane. When water rushes into a plant cell, the cell swells and pushes against this wall, creating something called turgor pressure. The wall pushes back, preventing the cell from taking in too much water and bursting.

How does osmosis work?

Osmosis is the process where water moves across a semipermeable membrane (like the outer layer of a cell) to balance out the concentration of dissolved particles, called solutes, on both sides. Because the solutes like salt or sugar are too large or charged to cross the membrane themselves, water moves instead. It flows from the side with a lower concentration of solutes to the side with a higher concentration of solutes, effectively diluting the more crowded side.<br><br>Think of it like a party where one room is completely packed with people (solutes) and the other is mostly empty. The door between them has a bouncer who will not let people through, but will let air conditioning (water) flow freely. To make the packed room more comfortable, the cold air rushes in. In biology, water rushes toward the "crowded" side until the concentration is equalized.

Understanding the Semipermeable Membrane

For osmosis to happen, you need a semipermeable membrane. This is a barrier that allows certain small molecules, like water, to pass through easily, but blocks larger molecules like sugars or ions like sodium. In living organisms, the cell membrane acts as this barrier. Because the solutes are trapped on their respective sides, the only way the universe can balance the concentration differences is by moving water.

The Three States of Tonicity

Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. We use three terms to describe this. An isotonic solution has the same solute concentration as the inside of the cell, so water moves in and out at equal rates. A hypertonic solution has a higher solute concentration than the cell, causing water to rush out, making the cell shrivel. A hypotonic solution has a lower solute concentration than the cell, so water rushes in, causing the cell to swell.

Where Students Slip Up

The most common mistake is thinking that the solutes are moving. Remember, in osmosis, the membrane stops the solutes; only the water moves. Another point of confusion is the direction of flow. A helpful phrase to remember is "water follows salt." Water always wants to move toward the side that is "saltier" or more concentrated to try and water it down.

Worked through

A red blood cell, which has an internal salt concentration of about $0.9\%$ , is placed in a beaker of pure distilled water ( $0\%$ salt). What will happen to the cell, and why?

First, identify the concentrations. The inside of the cell is $0.9\%$ salt, and the outside environment is $0\%$ salt. Second, determine the tonicity. Because the pure water outside has less solute than the inside of the cell, the pure water is a hypotonic solution. Finally, apply the rule of osmosis: water moves from the area of lower solute concentration to the area of higher solute concentration (water follows salt). Water will rush across the cell membrane into the red blood cell. As a result, the cell will swell and likely burst (a process called hemolysis).

Questions students ask

Ask about this topic

Explain at

Where this comes from: OpenStax Biology 2e, Chapter 5: Structure and Function of Plasma Membranes · Campbell Biology, Chapter 7: Membrane Structure and Function · Khan Academy, Unit: Cell structure and function (Membranes and transport)