Question

Give an example of a substance that can diffuse across a membrane. Give an example of a substance that must cross a membrane by mediated transport.

Short answer

The substance that can diffuse across a membrane is carbon dioxide and oxygen. However, the substances that cross a membrane by mediated transport are drugs.

Step-by-step solution

Diffusion

Diffusion is a common way for chemicals to move through cells and a way for vital tiny molecules to get across the cell membrane. The only simple molecules that can permeate the cell membrane through diffusion are water, carbon dioxide, and oxygen (or a type of diffusion known as osmosis).

Substance that diffuses across membrane

Because non-polar and tiny polar molecules may pass through the cell membrane, concentration gradients cause them to diffuse across it. Carbon dioxide and oxygen are two chemicals that pass through the membrane in this manner.

Substance that moves across membrane by mediated transport

Membrane transport proteins are required for the transfer of large polar compounds. Thus, they are called mediated transport. Mediated transport can be passive and active.

The movement down the concentration gradient is passive transport, while the transport against the concentration gradient is active transportation that necessitates the use of energy. Drugs move through mediated transport.