In this system, the solute cannot pass through the selectively permeable membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it
- Only some molecules move from a higher concentration to a lower concentration.
Selective permeability of the membrane means that membrane regulates which molecules (ions, substances) and how much of them can pass through it. Permeabilty of the membrane is determinated by its structure (hydrophobic heads and hydrophilic tails, cholesterol), temperature
The simplest form of transport through membrane is diffusion, passive transport of the molecule down its concentration gradient 9from higher to lower concentration). Some molecules can move down their concentration gradients directly, some have to pass through membrane proteins in a process called facilitated diffusion while others require energy and use active transport.