Passive Movement

Passive movement of particles or ions across the cell mem­brane is directly influenced by chemical or electrical gra­dients and does not require an expenditure of energy. A difference in the number of particles on either side of the membrane creates a chemical gradient, and a difference in charged particle or ions creates an electrical gradient. Chem­ical and electrical gradients are often linked and are called electrochemical gradients.

 

Diffusion. Diffusion refers to the process by which mole­cules and other particles in a solution become widely dis­persed and reach a uniform concentration because of energy created by their spontaneous kinetic movements (Fig. 4-16). Electrolytes and other substances move from an area of higher to an area of lower concentration. With

 

ions, diffusion is affected by energy supplied by their elec­trical charge. Lipid-soluble molecules, such as oxygen, car­bon dioxide, alcohol, and fatty acids, become dissolved in the lipid matrix of the cell membrane and diffuse through the membrane in the same manner that diffusion occurs in water. Other substances diffuse through minute pores of the cell membrane. The rate of movement depends on how many particles are available for diffusion and the ve­locity of the kinetic movement of the particles. The num­ber of openings in the cell membrane through which the particles can move also determines transfer rates. Temper­ature changes the motion of the particles; the greater the temperature, the greater is the thermal motion of the mol­ecules. Thus, diffusion increases in proportion to the in­creased temperature.

Osmosis. Most cell membranes are semipermeable in that they are permeable to water but not all solute particles. Water moves through a semipermeable membrane along a concentration gradient, moving from an area of higher to one of lower concentration (see Fig. 4-16). This process is called osmosis, and the pressure that water generates as it moves through the membrane is called osmotic pressure. Osmosis is regulated by the concentration of nondif-fusible particles on either side of a semipermeable mem­brane. When there is a difference in the concentration of particles, water moves from the side with the lower con­centration of particles and higher concentration of water to the side with the higher concentration of particles and lower concentration of water. The movement of water con­tinues until the concentration of particles on both sides of the membrane is equally diluted or until the hydrostatic (osmotic) pressure created by the movement of water opposes its flow.

Facilitated Diffusion. Facilitated diffusion occurs through a transport protein that is not linked to metabolic energy (see Fig. 4-16). Some substances, such as glucose, cannot pass unassisted through the cell membrane because they are not lipid soluble or are too large to pass through the membrane's pores. These substances, combined with spe­cial transport proteins at the membrane's outer surface, are carried across the membrane attached to the transporter and then released. In facilitated diffusion, a substance can move only from an area of higher concentration to one of lower concentration. The rate at which a substance moves across the membrane because of facilitated diffusion de­pends on the difference in concentration between the two sides of the membrane. Also important are the availability of transport proteins and the rapidity with which they can bind and release the substance being transported. It is thought that insulin, which facilitates the movement of glucose into cells, acts by increasing the availability of glu­cose transporters in the cell membrane.