Diffusion, osmosis and active transport

diffusion

The movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. This process does not require energy (passive transport).

diffusion example

Oxygen moving from the lungs into the bloodstream, or carbon dioxide moving from cells into the bloodstream.

concentration gradient

The difference in the concentration of particles between two areas. The steeper the gradient, the faster the diffusion.

concentration gradient example

When a drop of dye is placed in water, the concentration of dye is higher near the drop and lower farther away. Over time, the dye will diffuse through the water, reducing the concentration gradient.

passive transport

The movement of substances across a cell membrane without the use of energy. Diffusion is a type of passive transport.

passive transport example

Oxygen diffusing into cells from the bloodstream, or glucose moving into a cell from an area of high concentration to low concentration.

osmosis

The diffusion of water molecules through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.

selective permeability

Only water molecules can move through the membrane, not solutes.

water potential

Water moves towards the side with lower water potential (higher solute concentration).

osmosis example

Water moving into plant roots from the soil or into red blood cells placed in pure water.

water potential example

In plant cells, water moves from the soil (higher water potential) into the roots (lower water potential) through osmosis.

active transport

The movement of substances across a cell membrane against their concentration gradient, from an area of lower concentration to higher concentration. Active transport requires energy, usually in the form of ATP.

active transport example

The sodium-potassium pump in nerve cells, where sodium ions are pumped out and potassium ions are pumped into the cell, both against their concentration gradients.

transport proteins

Proteins embedded in the cell membrane that facilitate the movement of substances across the membrane. Some transport proteins work in passive transport (e.g., facilitated diffusion), while others are involved in active transport.

transport proteins example

Ion channels (for passive transport) and pumps (for active transport, like the sodium-potassium pump).