Movement of Molecules + Tonicity - Cell Membrane

Biology 1 8th grade

Diffusion

The passive movement of molecules from high to low concentration (normal concentration gradient)

Why is diffusion passive?

It requires no energy; molecules move naturally down their concentration gradient.

Dynamic Equilibrium?

Molecules continue moving, but there's no net change in concentration across the membrane.

Do substances diffuse together?

No, each substance diffuses independently down its own gradient.

Molecules that diffuse easily into membranes

Small, nonpolar molecules like oxygen (O₂) and carbon dioxide (CO₂).

Osmosis

The diffusion of water across a selectively permeable membrane toward higher solute concentration.

Osmosis is driven by-

Water moves from areas with more free water molecules to areas with fewer (higher solute concentration).

Selectively Permeable Membrane

A membrane that allows certain things like small non-polar molecules( Co₂ & O₂) while blocking large polar molecules (Glucose, Na⁺).

In a 0.5% vs. 2% sucrose solution, which way does water move?

Water moves from the 0.5% side to the 2% side.

What happens to water molecules near solutes?

They cluster around solutes, reducing the number of free water molecules.

Tonicity?

The ability of a solution to cause a cell to gain or lose water.

What happens in an isotonic solution?

Water moves in and out equally; the cell stays the same size.

What happens in a hypotonic solution?

Water enters the cell; animal cells may burst, plant cells become turgid.

What happens in a hypertonic solution?

Water leaves the cell; cells shrink, and plant cells undergo plasmolysis.

Why do plant cells thrive in hypotonic environments?

The cell wall prevents bursting and maintains turgor pressure.