Human Physiology   7/e   Vander/Sherman/Luciano
Student   Online Learning Center 

Chapter 6: Movement of Molecules Across Cell Membranes


Study Outline

Chapter 6: Movement of Molecules Across Cell Membranes

  1. DIFFUSION

    1. Diffusion is the movement of molecules from one location to another by random thermal motion.

      1. The net flux between two compartments always proceeds from higher to lower concentration of the diffusing substance.
      2. Diffusion equilibrium is reached when the concentrations in the two compartments become equal, resulting in zero net flux.

    2. The magnitude of the net flux F across a plasma membrane is directly proportional to the concentration difference across the membrane C - C, the surface area of the membrane A, and the membrane permeability constant k.
    3. Nonpolar molecules diffuse through the lipid portions of membranes much more rapidly than do polar or ionized molecules because nonpolar molecules can dissolve in the nonpolar lipids in the membrane.
    4. Mineral ions diffuse across membranes by passing through ion channels formed by integral membrane proteins.

      1. The net diffusion of ions across a membrane depends on both the concentration gradient and the membrane potential.
      2. The flux of ions across a membrane can be altered by opening or closing ion channels.

  2. MEDIATED-TRANSPORT SYSTEMS

    1. The mediated transport of molecules or ions across a membrane involves binding of the transported solute to a transporter in the membrane. Changes in the conformation of the transporter move the binding site to the opposite side of the membrane, where the solute dissociates from the protein.

      1. The binding sites on transporters exhibit chemical specificity, affinity and saturation.
      2. The magnitude of the flux through a mediated-transport system depends on the degree of transporter saturation, the number of transporters in the membrane, and the rate at which the conformational change in the transporter occurs.

    2. Facilitated diffusion is a mediated-transport process that moves molecules from higher to lower concentration across a membrane by means of a transporter until the two concentrations become equal. Metabolic energy is not required for this process.
    3. Active transport is a mediated-transport process that moves molecules against an electrochemical difference across a membrane by means of a transporter and requires an input of energy.

      1. Active transport is achieved either by altering the affinity of the binding site so that it differs on the two sides of the membrane or by altering the rate at which the protein changes its conformation from one side of the membrane to the other.
      2. Primary active transport directly uses the phosphorylation of the transporter by ATP to drive the transport process.
      3. Secondary active transport uses the binding of ions (often sodium) to the transporter to drive the transport process.
      4. In secondary active transport the downhill flow of an ion is linked to the uphill movement of a second solute either in the same direction as the ion (cotransport), or in the opposite direction to the ion (countertransport).

  3. OSMOSIS

    1. Water crosses membranes by (1) diffusing through the lipid bilayer, and (2) diffusing through protein channels in the membrane.
    2. Osmosis is the diffusion of water across a membrane from a region of higher water concentration to a region of lower water concentration. The osmolarity 1/m the total solute concentration in a solution1/m determines the water concentration: 1/m. The higher the osmolarity of a solution, the lower the water concentration.
    3. Osmosis across a membrane that is permeable to water but impermeable to solute leads to an increase in the volume of the compartment on the initially high-osmolarity side and a decrease in the volume on the initially low-osmolarity side.
    4. Application to a solution of pressure equal to the osmotic pressure of the solution will prevent the osmotic flow of water into the solution from a compartment of pure water. The greater the osmolarity of a solution, the greater its osmotic pressure. Net water movement occurs from a region of lower osmotic pressure to one of higher osmotic pressure.
    5. The osmolarity of the extracellular fluid is about 300 mOsm. Since water is in diffusion equilibrium across cell membranes, the intracellular fluid also has an osmolarity of 300 mOsm.

      1. Na and Cl ions are the major effectively nonpenetrating solutes in the extracellular fluid, whereas K ions and various organic solutes are the major effectively nonpenetrating solutes in the intracellular fluid.
      2. The terms used to describe the osmolarity of solutions containing different compositions of penetrating and nonpenetrating solutes are (1) isotonic, (2) hypertonic, (3) hypotonic, (4) isoosmotic, (5) hyperosmotic, and (6) hypoosmotic.

  4. ENDOCYTOSIS AND EXOCYTOSIS

    1. During endocytosis, regions of the plasma membrane invaginate and pinch off to form vesicles that enclose a small volume of extracellular material.

      1. The three classes of endocytosis are (1) fluid endocytosis, (2) adsorptive endocytosis, and (3) phagocytosis.
      2. Most endocytotic vesicles fuse with endosomes, which in turn transfer the vesicle contents to Iysosomes where they are digested by Iysosomal enzymes.

    2. Exocytosis, which occurs when intracellular vesicles fuse with the plasma membrane, provides a means of adding components to the plasma membrane and a route by which membrane-impermeable molecules, such as proteins synthesized by cells, can be released into the extracellular fluid.

  5. EPITHELIAL TRANSPORT

    1. Molecules can cross an epithelial layer of cells by two pathways: (1) through the extracellular spaces between the cells (the paracellular pathway), and (2) across both the luminal and basolateral membranes as well as the cell's cytoplasm (the transcellular pathway).
    2. In epithelial cells, the permeability and transport characteristics of the luminal and basolateral plasma membranes differ, resulting in the ability of the cells to actively transport a substance between the fluid on one side of the cell and the fluid on the opposite side of the cell.
    3. The active transport of sodium through an epithelium increases the osmolarity on one side of the cell and decreases it on the other, causing water to move by osmosis in the same direction as the transported sodium.
    4. Glands are composed of epithelial cells that secrete water and solutes in response to stimulation.

      1. There are two categories of glands: (1) exocrine glands, which secrete into ducts, and (2) endocrine glands (ductless glands), which secrete hormones and other substances into the extracellular fluid, from which they diffuse into the blood.
      2. The secretions of glands consist of (1) organic substances that have been synthesized by the gland, and (2) salts and water, which have been transported across the gland cells from the blood

HomeChapter IndexNext

Begin a search: Catalog | Site | Campus Rep

MHHE Home | About MHHE | Help Desk | Legal Policies and Info | Order Info | What's New | Get Involved


Copyright ©1998 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.

Corporate Link