5.1 Passive transport  (Page 3/17)

5.1c facilitated transport

In facilitated transport     , also called facilitated diffusion, materials diffuse across the plasma membrane with the help of transport proteins     . A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, if the materials are ions or polar molecules, they will be repelled by the hydrophobic parts of the cell membrane. Facilitated transport proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell.

5.1d osmosis

Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water.

Mechanism

Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high water concentration to one of low water concentration. Imagine a beaker with a semipermeable membrane separating the two sides or halves ( [link] ). On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute     , that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane). If the volume of the solution on both sides of the membrane is the same, but the concentrations of solute are different, then there are also different amounts of water, the solvent, on either side of the membrane.

Figure 6.2

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. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. Osmosis proceeds constantly in living systems.

5.1e tonicity

Tonicity describes how varying the concentration of an extracellular solution can change the volume of a cell by affecting osmosis.

Hypotonic solutions

Three terms—hypotonic, isotonic, and hypertonic—are used to relate the concentration of a cell to the concentration of the extracellular fluid that contains the cells (its surroundings or environment). In a hypotonic     environment, the extracellular fluid has lower solute concentration than the fluid inside the cell. Since osmosis is the movement of water from higher water concentration to lower, water will enters the cell (the cell has higher solute concentration, which means lower water concentration).