Crossing the Plasma Membrane
The plasma membrane keeps a cell intact. It allows only certain molecules and ions to enter and exit the cytoplasm freely; therefore, the plasma membrane is said to be selectively permeable. Both passive and active methods are used to cross the plasma membrane (see Table 3.2).
Diffusion is the random movement of molecules from the area of higher concentration to the area of lower concentration until they are equally distributed. To illustrate diffusion, imagine putting a tablet of dye into water. The water eventually takes on the color of the dye as the dye molecules diffuse. The chemical and physical properties of the plasma membrane allow only a few types of molecules to enter and exit a cell simply by diffusion. Lipid-soluble molecules such as alcohols can diffuse through the membrane because lipids are the membrane’s main structural components. Gases can also diffuse through the lipid bilayer; this is the mechanism by which oxygen enters cells and carbon dioxide exits cells. As an example, consider the movement of oxygen from the alveoli (air sacs) of the lungs to the blood in the lung capillaries. After inhalation (breathing in), the concentration of oxygen in the alveoli is higher than that in the blood; therefore, oxygen diffuses into the blood. When molecules simply diffuse from higher to lower concentration across plasma membranes, no cellular energy is involved.
Osmosis is the diffusion of water across a plasma membrane. It occurs whenever an unequal concentration of water exists on either side of a selectively permeable membrane. Normally, body fluids are isotonic to cells (Fig. 3.8a)-that is, there is an equal concentration of solutes (substances) and solvent (water) on both sides of the plasma membrane, and cells maintain their usual size and shape.
Figure 3.8 Tonicity. The arrows indicate the movement of water.
Intravenous solutions medically administered usually have this tonicity. Tonicity is the degree to which a solution’s concentration of solute versus water causes water to move into or out of cells. Solutions (solute plus solvent) that cause cells to swell or even to burst due to an intake of water are said to be hypotonic solutions. If red blood cells are placed in a hypotonic solution, which has a higher concentration of water (lower concentration of solute) than do the cells, water enters the cells and they swell to bursting (Fig. 3.8b). The term lysis refers to disrupted cells; hemolysis, then, is disrupted red blood cells. Solutions that cause cells to shrink or to shrivel due to a loss of water are said to be hypertonic solutions. If red blood cells are placed in a hypertonic solution, which has a lower concentration of water (higher concentration of solute) than do the cells, water leaves the cells and they shrink (Fig. 3.8c). The term crenation refers to red blood cells in this condition. These changes have occurred due to osmotic pressure. Osmotic pressure is the force exerted on a selectively permeable membrane because water has moved from the area of higher concentration of water to the area of lower concentration (higher concentration of solute).
Active transport requires a protein carrier and the use of cellular energy obtained from the breakdown of ATP. When ATP is broken down, energy is released, and in this case the energy is used by a carrier to carry out active transport. Therefore, it is not surprising that cells involved in active transport have a large number of mitochondria near the plasma membrane at which active transport is occurring. Proteins involved in active transport often are called pumps because just as a water pump uses energy to move water against the force of gravity, proteins use energy to move substances against their concentration gradients. One type of pump that is active in all cells but is especially associated with nerve and muscle cells moves sodium ions (Na) to the outside of the cell and potassium ions (K) to the inside of the cell. The passage of salt (NaCl) across a plasma membrane is of primary importance in cells. First, sodium ions are pumped across a membrane; then, chloride ions simply diffuse through channels that allow their passage. Chloride ion channels malfunction in persons with cystic fibrosis, and this leads to the symptoms of this inherited (genetic) disorder.
Endocytosis and Exocytosis
During endocytosis, commonly called phagocytosis, a portion of the plasma membrane invaginates to envelop a substance, and then the membrane pinches off to form an intracellular vesicle (see Fig. 3.1, top). Digestion may be required before molecules can cross a vesicle membrane to enter the cytoplasm. During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs (see Fig. 3.1, bottom). This is the way insulin leaves insulin-secreting cells, for instance. Table 3.2 summarizes the various ways molecules cross the plasma membrane.
Because capillary walls are only one cell thick, small molecules (e.g., water or small solutes) tend to passively diffuse across these walls, from areas of higher concentration to those of lower concentration. However, blood pressure aids matters by pushing water and dissolved solutes out of the capillary. This process is called filtration. Filtration is easily observed in the laboratory when a solution is poured past filter paper into a flask. Large substances stay behind, but small molecules and water pass through. Filtration of water and substances in the region of capillaries is largely responsible for the formation of tissue fluid, the fluid that surrounds the cells. Filtration is also at work in the kidneys when water and small molecules move from the blood to the inside of the kidney tubules.
Transport by Carriers
Most solutes do not simply diffuse across a plasma membrane; rather, they are transported by means of protein carriers within the membrane. During facilitated transport, a molecule (e.g., an amino acid or glucose) is transported across the plasma membrane from the side of higher concentration to the side of lower concentration. The cell does not need to expend energy for this type of transport because the molecules are moving down their concentration gradient. During active transport, a molecule is moving contrary to the normal direction-that is, from lower to higher concentration (Fig. 3.9). For example, iodine collects in the cells of the thyroid gland; sugar is completely absorbed from the gut by cells that line the digestive tract; and sodium (Na) is sometimes almost completely withdrawn from urine by cells lining kidney tubules.
Figure 3.9 Active transport through a plasma membrane. Active transport allows a molecule to cross the membrane from lower concentration to higher concentration.1 Molecule enters carrier. 2 Breakdown of ATP induces a change in shape that 3 drives the molecule across the membrane.