🧫 Cell Membrane (continued) – Selective Permeability

What is selective permeability?

Selective permeability refers to the cell membrane's ability to control what enters and exits the cell.

Which types of molecules can easily pass through the cell membrane via simple diffusion?

Small, nonpolar molecules such as CO2, O2, and small amounts of H_2O can pass through easily.

Which types of molecules require channels or transport proteins to pass through the cell membrane?

Larger or charged molecules, including glucose, ions, and water (via aquaporins), require channels or transport proteins to pass through the membrane (facilitated diffusion or active transport).

Which types of molecules cannot pass through the cell membrane directly?

Very large molecules like starch and proteins cannot directly pass through the cell membrane and need to be broken down first.

Explain the "gatekeeper" analogy for the cell membrane's selective permeability.

The cell membrane acts like a gatekeeper:

• Small gases (CO2, O2) diffuse freely.
• Nutrients like glucose need specific transporters.
• Large molecules must be digested into smaller components before entry.