Membrane Structure and Function

I.         Fluid Mosaic Model (Fig. 8.2)

            A.        The phospholipid bilayer is in a fluid (i.e.,flowing) state. Proteins in the membrane are able to drift sideways through the membrane.

            B.        The fluidity of the membrane can be altered by changing the amount of unsaturated phospholipids. (Fig. 8.3). By adding more double bonds, organisms can prevent membranes from freezing at lower temperatures.

            C.        Cholesterol inserted in the membrane plays a role in membrane fluidity in animal cells.

                        1.         At warm temperatures, cholesterol makes the membrane less fluid by restricting the movement of phospholipids.

                        2.         Cholesterol prevents close packing of the phospholipids so it also increases membrane fluidity at lower temperatures.

            D.        Membrane Proteins (Fig. 8.5)

                        1.         Integral proteins - span the membrane so that the hydrophobic regions are in the hydrocarbon tails of the lipids while the hydrophilic portions are exposed to water.

                        2.         Peripheral proteins - are attached to the surface of the membrane.

                        3.         Other proteins perform a variety of other functions as well. (Fig. 8.6)

            E.        Cell-Cell Recognition

                        1.         carbohydrates attached to the membrane help cells recognize one another.

                        2.         Glycoprotein - membrane proteins with an attached oligosaccharide.

                        3.         Glycolipid - membrane lipids with an attached oligosaccharide.

                        4.         These oligosaccharides vary from species to species, from individual to individual within a species, and from cell type to cell type in an individual.

II.        Permeability - the plasma membrane is a selectively permeable membrane.

            A.        What can pass

                        1.         Small, hydrophobic molecules like hydrocarbons and oxygen can dissolve in the lipid bilayer and cross.

                        2.         Some small, polar (but uncharged) molecules can cross as well. Carbon dioxide and, surprisingly, water are small enough to squeeze between the lipids.

            B.        What can’t pass

                        1.         Large, polar molecules like glucose are unable to cross the membrane.

                        2.         All ions, even small ones.

            C.        Transport proteins - provide channels through which hydrophilic substances can pass by keeping them away from the hydrophobic tails of the phospholipids. Transport proteins are specific for the substance they transport.

III.       Passive Transport

            A.        Diffusion (Fig. 8.8)

                        1.         the tendency for molecules to move from an area where it is more concentrated to an area where it is less concentrated. In other words, they will move down their concentration gradient.

                        2.         The movement is caused by the random vibrational motion of all particles called Brownian motion.

                        3.         This process increases entropy so it is a spontaneous process requiring no work to make it happen.

                        4.         Most of the movement across cell membranes is by diffusion. It is called passive transport because the cell does not have to spend energy to make it happen.

                        5.         Factors that affect the rate of diffusion

                                    a.         Temperature - T ∝ rate of diffusion. As temperature increases, the movement of particles increases and so the rate of diffusion increases.

                                    b.         Concentration - [particle] ∝ rate of diffusion. As concentration increases, particles collide more often and so diffuse faster.

                                    c.         Size - particle size 1/∝ rate of diffusion. Larger particles require more force to overcome their inertia and so diffuse more slowly.

                        6.         Facilitated diffusion (Fig. 8.12)

                                    a.         Many substances have specific transport proteins that speed their transfer across the cell membrane.

                                    b.         Like enzymes, these proteins are specific, can be saturated, and are subject to inhibition by molecules that resemble the one they are designed to transport.

                                    c.         Also a passive process that moves solutes down the concentration gradient.

            B.        Osmosis - the diffusion of water through a selectively permeable membrane. In general, water will move through a selectively permeable membrane from the solution with a lower [solute] (i.e., higher [water]) to the solution with the higher [solute] (i.e., lower [water]). Remember that we are talking here about a solute which is too large to pass through the pores in the membrane. (Fig. 8.9)

                        1.         Hypertonic - of two solutions, the one that has a higher concentration of solutes. A cell in a hypertonic solution will tend to lose water to that solution. (Fig. 8.10)

                        2.         Hypotonic - of two solutions, the one that has a lower concentration of solutes. A cell in a hypotonic solution will tend to gain water from that solution. (Fig. 8.10)

                        3.         Isotonic - solutions having equal concentrations of solutes. Water flows equally in both directions but there is no net movement.

                        4.         Water balance is of great significance to all cells. Many cells live in environments that are isotonic.

IV.      Active Transport

            A.        The pumping of solutes against the concentration gradient.

            B.        Transport proteins move solutes from the side of the membrane where they are les concentrated to the side on which they are more concentrated.

            C.        This requires energy in the form of ATP.

            D.        Membrane potential (Fig. 8.13)

                        1.         By selectively pumping ions across the membrane, a cell can maintain a voltage difference between the cytosol and the extracellular fluid.

                        2.         This separation of charge can be used by the cell to do work. Because the inside of the cell is negative with respect to the outside, cations tend to move into the cell and anions tend to move out.

                        3.         This electrical force (electrical gradient) combined with the chemical force (concentration gradient) form what we call the electrochemical gradient. This means that ions don’t diffuse down a simple concentration gradient, but rather an electrochemical gradient.

                        4.         Sodium-potassium pump is the protein responsible for maintaining a membrane potential in animal cells.

            E.        Cotransport (Fig. 8.16)

                        1.         A special case of active transport where the active transport of one solute is used to drive the active transport of a different solute.

                        2.         The first solute is pumped across the membrane and as it diffuses back the other direction, does the work of bringing a different solute with it.

            F.        Movement of larger molecules (Fig. 8.17)

                        1.         Exocytosis - the secretion of macromolecules enclosed in a Golgi vesicle fusing with the plasma membrane.

                        2.         Endocytosis - the uptake of macromolecules by enclosing them in a vesicle pinched from the plasma membrane. Three categories of endocytosis:

                                    a.         Phagocytosis - a cell engulfing a particle

                                    b.         Pinocytosis - a cell engulfing a droplet of extracellular fluid to obtain whatever solutes might be dissolved in it.

                                    c.         Receptor-mediated endocytosis - specific receptors embedded in the plasma membrane bind to specific molecules in the extracellular fluid.

V.        Signal Transduction (Fig. 8.18)

            A.        Proteins built into the membrane are able to transmit chemical signals from the extracellular environment to the inside of the cell.

            B.        The process involves proteins in a series of events ending int eh triggering of some metabolic response.

            C.        A receptor on the plasma membrane binds a messenger molecule of some kind. This binding triggers the activation of a protein inside the cell which in turn activates an effector protein. The effector protein makes something happen inside the cell in response to the original messenger molecule outside the cell.