3.1 Movement of Substances Across The Plasma Membrane (continue)

The permeability of the plasma membrane

Keypoints :

• The plasma membrane acts as a partially permeable membrane and selectively controls the movement of substances in and out of a cell.
• Factors that determine whether a molecule can pass through the plasma membrane are the size and polarity of the molecule.

Passive Transport

1. Simple diffusion
2. Facilitated diffusion
3. Osmosis
   

Simple diffusion

• simple diffusion is the random movement of particles (molecules) from a region of high concentration to a region of low concentration.
• this process will continue until a dynamic equilibrium reached.
• example - diffusion of oxygen from the alveoli into the adjacent blood capillaries.
  

The process of diffusion

Simple diffusion through plasma membrane

Facilitated diffusion

• facilitated diffusion is the passive movement of molecules or ions down a concentration gradient.
• the process involves special pore proteins (channel proteins) or specific carrier proteins in the plasma membrane.
• the particular molecules combine with the specific active site of the carrier protein. The carrier protein changes its shape to allow the molecules to pass through to the other side of the plasma membrane. After assisting the movement of molecules to the other side of the plasma membrane, it is free to bind with other molecules.
• example- the reabsorption of glucose and amino acid from the villi into the blood capillaries.
  

Facilitated diffusion through a pore protein in the plasma membrane

Facilitated diffusion through a carrier protein in the plasma membrane

Osmosis

• is the random movement of water molecules from a region of high concentration of water molecules to a region of low concentration of water molecules through a partially permeable membrane.
• Example - the reabsorption of water molecules from the nephrons into the blood capillaries.
  

Important!

Q - What is the difference between simple diffusion & facilitated diffusion?

A - Both describes the tendency of molecules to spread out randomly from the site of high concentration to low concentration (with concentration gradient) except facilitated diffusion requires the help of carrier protein to facilitate the movement of the larger molecules.

Q - What are the similarities between simple diffusion & facilitated diffusion?
A : both are following the concentration gradient and they do not require energy to achieve movement of substances