Explain the difference between simple diffusion and facilitated diffusion.

Simple diffusion is the passive movement of substances from high to low concentration, while facilitated diffusion requires carrier proteins.

Simple diffusion and facilitated diffusion are both types of passive transport, meaning they do not require energy to occur. They are both driven by the concentration gradient, moving substances from an area of high concentration to an area of low concentration. However, the key difference between the two lies in the way the substances are transported across the cell membrane.

Simple diffusion is a process where small, non-polar molecules, such as oxygen and carbon dioxide, move directly through the phospholipid bilayer of the cell membrane. This process is driven by the concentration gradient, with molecules moving from an area of high concentration to an area of low concentration until equilibrium is reached. Simple diffusion does not require any assistance from proteins, hence the term 'simple'.

On the other hand, facilitated diffusion involves larger, polar molecules or ions, such as glucose and sodium ions, which cannot easily pass through the lipid bilayer. These substances require the assistance of specific carrier proteins or channel proteins embedded in the cell membrane to facilitate their movement across. These proteins provide a hydrophilic pathway for the substances to pass through, effectively 'facilitating' their diffusion. Like simple diffusion, facilitated diffusion is also driven by the concentration gradient and does not require energy.

IB Biology Tutor Summary: In summary, simple diffusion and facilitated diffusion both move substances from a high to low concentration without using energy. Simple diffusion allows small, non-polar molecules to pass through the cell membrane on their own. Facilitated diffusion, however, needs special proteins to help larger, polar molecules or ions cross the membrane. Both processes are driven by the concentration gradient.