Diffusion

• Passive energy comes from the natural, inbuilt motion of particles rather than external sources of energy (e.g. ATP).
• All particles are constantly in random motion due to the kinetic energy that they possess. The particles are constantly bouncing off each other as well as off other objects (e.g. the walls of the vessel they are contained in).
• Diffusion can be defined as the net movement of molecules or ions from a region of higher concentration to a region of lower concentration until they are evenly distributed.
• Charged ions and polar molecules do not easily diffuse across the phospholipid bilayer, so this movement is facilitated by transmembrane channels and carriers that span the membrane. This is known as facilitated diffusion.
• Facilitated diffusion is a passive process. It relies solely on the kinetic energy of the diffusing molecules, and there is no external input of ATP from respiration.
• Facilitated diffusion also occurs down a concentration gradient, but this gradient only occurs at specific points on the plasma membrane where the protein channels and carrier proteins are located.
• Protein channels are water-filled hydrophilic channels across the membrane, and allow specific water-soluble ions to pass through.
• Protein channels are selective, each opening in the presence of a specific ion. If the specific ion is not present, then the channel will remain closed. This gives a degree of control over the entry and exit of ions.
• In protein channels, the ions bind with the protein, causing it to change shape in a way that closes it to one side of the membrane and opens it to another side.
• Carrier proteins permit an alternative form of facilitated diffusion, whereby a specific molecule such as glucose can bind to a specific protein, causing it to change shape in such a way that the molecule is released to the inside of the membrane.
• Carrier proteins also do not depend on ATP energy (their function is passive).