Simple diffusion is a process where molecules move from an area of higher concentration to an area of lower concentration across a membrane. This movement occurs without the need for energy or a transport protein. It is a passive process that relies on the natural kinetic energy of molecules.
Here are the options provided:
- Endocytosis during which the transported substance must bind to a specific membrane receptor: This describes receptor-mediated endocytosis, not simple diffusion.
- Transport of a substance across a membrane against a concentration gradient: This describes active transport, which requires energy and often a transport protein.
- Transport protein: This is not a description of simple diffusion, as simple diffusion does not require a transport protein.
Therefore, none of the options provided accurately describe simple diffusion. Simple diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration without the need for energy or a transport protein.