Passive transport
Osmosis and diffusion are processes that explain how water, lipids, lipid-soluble molecules, and gases permeate membranes. These particles either dissolve in the lipid bilayer or are small enough to pass between the lipid molecules. Passage across the membrane is due to their random motion from a region of greater to lesser concentration. The cell plays no active role in moving the particles. This type of movement is called passive transport because the cell uses no energy for osmosis or diffusion to occur. Many particles, such as certain ions, sugars, and amino acids are in greater concentration outside the membrane than inside. The cell needs such particles but they cannot dissolve in or pass between the lipids in the bilayer. How do such important particles pass across the membrane? Recall that many of the proteins of the membrane play a role in passage of particles. Use of proteins to aid the passage of particles from a region of greater to lesser concentration is called facilitated diffusion. Facilitated diffusion is a special form of passive transport. In general, proteins involved in facilitated diffusion are known as permeases. A single permease is composed of two or more proteins. One of the simplest types of permeases is the protein pore. These pores extend through the bilayer and form a channel that allows certain ions to pass through. Different ions pass through different pores. Which ion passes through a given pore depends upon the size and charge of the ion and the inside diameter of the pore. Some permeases are more complex. They aid in the passage of two particles at once. Glucose molecules and sodium ions are passed across a membrane in this way. A sodium ion outside the cell combines with a protein in the permease and is thought to change the permease’s shape. This change in shape allows the permease to pick up a glucose molecule. Once the glucose is picked up, the shape of the permease changes again. It then becomes closed to the outside and open to the inside. The permease, in this shape, “loses its hold” on glucose and sodium, and the two particles then enter the cell. Once the particles have entered the cell, the permease resumes its original shape. It is once again open to the outside. Whether passive transport occurs by osmosis, simple diffusion, or facilitated diffusion, it always involves a net movement of particles from a region of greater to lesser concentration. The particles pass across the membrane as a result of their own energy of motion.
áËÅ觢éÍÁÙÅ: BIOLOGY living system. Page 70.
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