1.14. Passive Transport

Transcription

1 Passive Transport 1.14 Simple Diffusion Cell s are selectively permeable only certain substances are able to pass through them. As mentioned in section 1.2, cell s are largely composed of a phospholipid bilayer and proteins. Many small, uncharged molecules, such as water, oxygen, and carbon dioxide, pass through the cell freely (Figure 3). These substances either go through the phospholipid bilayer directly or through channels formed by proteins in the. Since the cell has a hydrophobic middle section, small lipid molecules such as fatty acids are also able to pass through. water oxygen extracellular fluid glucose O cytoplasm nucleic acid carbon dioxide C C fatty acid fatty acid C fatty acid triglyceride selectively permeable allows only certain substances to pass through it Figure 3 Cell s are selectively permeable they let only certain substances through. NEL Cellular Biology 59

2 simple diffusion the movement of particles from an area of higher concentration to an area of lower concentration until particle concentration is equal throughout concentration gradient a difference in concentration between two areas Figure 4 (a) Simple diffusion of airfreshener molecules from an area of higher concentration (in the paste) to an area of lower concentration (in the air surrounding the paste). (b) Carbon dioxide molecules diffuse through the cell from an area of higher concentration in the extracellular fluid to an area of lower concentration in the cytoplasm. owever, ions, small charged molecules, and large molecules such as amino acids, carbohydrates, nucleic acids, and large lipids (triglycerides) cannot pass through easily (Figure 3, on the previous page). Substances that can pass through the do so by a process called simple diffusion. Simple diffusion is the movement of particles from an area where they are more highly concentrated to an area where they are less highly concentrated (Figure 4). A difference in concentration between two areas is called a concentration gradient, and diffusion always occurs down a concentration gradient (from high concentration to low concentration). (a) (b) selectively permeable dynamic equilibrium a state of balance where particles move in all directions at equal rates Diffusion is a natural process that occurs because particles are in constant random motion and have a tendency to spread throughout a given volume. Simple diffusion does not use any of a cell s energy, and ends when the concentration of particles becomes equal everywhere within the given volume. owever, this does not mean that the particles stop moving. In fact, particles continue moving randomly in all directions all the time. During diffusion, molecules move more in one direction than any other. When diffusion ends, we say that a state of dynamic equilibrium has been reached. Dynamic equilibrium is a state of balance, where particles move at equal rates in all directions. When diffusion occurs through a cell, dynamic equilibrium is reached when particles move through the in both directions at equal rates, and the concentration of particles remains the same on both sides of the (Figure 5). 60 Unit 1 NEL

3 Section 1.14 selectively permeable Figure 5 In dynamic equilibrium, particles move through a in both directions at equal rates. The concentration of particles remains constant on both sides of the. The rate (speed) of diffusion depends on temperature and the concentration of solute molecules in solution. Diffusion occurs faster at higher temperatures because molecules move faster. Faster-moving molecules spread out faster and reach dynamic equilibrium more quickly. Dynamic equilibrium will also be reached more quickly if there are a greater number of solute molecules in solution. Facilitated Diffusion Glucose, sodium ions, and chloride ions are three chemicals most cells need to survive. These substances must be able to get across the cell in an efficient manner. owever, large polar molecules such as glucose and large ions such as sodium and chloride cannot go through a by simple diffusion because they cannot easily pass through the hydrophobic middle section of the phospholipid bilayer. To compensate, s have protein molecules that help some substances through. In general, there are three types of proteins (Figure 6). Some proteins go part way through the phospholipid bilayer, some are attached to the outside surface, and others go all the way through. Those that span the bilayer are called trans proteins. Some trans proteins act as carrier proteins that assist certain substances through a. This method of transporting materials across a is called facilitated diffusion, or assisted diffusion. As its name implies, facilitated diffusion is a form of diffusion. This means that particles move down a concentration gradient until dynamic equilibrium is reached. The key difference between simple diffusion and facilitated diffusion is that, in facilitated diffusion, the diffusing particles are assisted through the by trans carrier proteins, whereas in simple diffusion they pass directly through the s phospholipid bilayer or through protein channels. Typically, a given carrier protein transports only one type of substance, or a small group of chemically related substances. An example of carrier protein facilitated diffusion is the movement of glucose into cells of the liver (Figure 7, on the next page). surface protein trans protein channel (simple diffusion) partially embedded protein cell surface trans carrier protein (facilitated diffusion) cytoplasm Figure 6 Membrane showing associated proteins trans protein a protein molecule in a that spans the thickness of the phospholipid bilayer carrier protein a trans protein that facilitates the diffusion of certain substances through a facilitated diffusion the diffusion of solutes through a assisted by proteins NEL Cellular Biology 61

4 extracellular fluid glucose cytoplasm carrier protein phospholipid bilayer Figure 7 Facilitated diffusion of glucose. The solute (glucose) attaches to a binding site on a carrier protein at one side of the. The attachment causes the carrier to undergo a series of structural changes that have the effect of carrying the solute to the other side of the. The carrier then releases the solute and, through another structural change, transforms itself to its original state, ready to accept another glucose molecule. osmosis the net movement of water across a selectively permeable from an area of high concentration to an area of low concentration Figure 8 Osmosis. Two containers of equal volume are separated by a selectively permeable that allows free passage of water but totally restricts the passage of large solute molecules such as proteins. Since side B has a lower solute concentration (higher concentration of water) than side A, a net amount of water will move (by osmosis) from side B into side A. Osmosis Water molecules move freely through cell s. Under normal conditions, large quantities of water molecules move into and out of a cell by simple diffusion. The cell remains the same size because equal amounts of water go into and out of the cell. There are, however, many cases in which a net amount of water flows into or out of a cell. This means that more water may enter the cell than leaves the cell, so that the cell gains water, or more water may leave the cell than enters it, so that the cell loses water. In such situations, water still moves through the cell by simple diffusion, but the process is important enough to warrant a special name osmosis. Osmosis is the net movement of water across a selectively permeable from the side where water is more concentrated to the side where it is less concentrated. Note that solutions have a high concentration of water when they have a low concentration of solute, and vice versa. Osmosis occurs because more water molecules strike the on the side with a higher concentration of water molecules (i.e., a lower solute concentration) than on the side with a lower concentration of water molecules (i.e., a higher solute concentration). More strikes result in more water molecules passing through the and a net diffusion of water from one side to the other. The key point to remember about osmosis is that water moves through a from the side with a lower solute concentration to the side with a higher solute concentration. Dynamic equilibrium is reached when sufficient water has moved to equalize the solute concentrations on both sides of the, and at that point, net movement of water (osmosis) ceases (Figure 8). Membrane is permeable to water but not to protein. water protein molecules side A side B selectively permeable side A side B osmosis equal concentrations of protein and water in side A and side B 62 Unit 1 NEL

5 Section 1.14 Osmosis occurs whenever there is a difference in solute concentration across a selectively permeable. A number of special terms are commonly used to describe differences in solute concentration (Figure 9). Isotonic solutions are solutions that have equal solute concentrations. When a selectively permeable separates isotonic solutions, osmosis does not occur. A hypertonic solution is one with a higher concentration of solutes than another solution. The solution with the lower concentration of solutes is called a hypotonic solution. isotonic solution a solution of equal solute concentrations hypertonic solution a solution that has a higher solute concentration than some other solution hypotonic solution a solution that has a lower solute concentration than some other solution NEL solution A solution B solution C We may now understand why it is important for patients to receive a solution of a particular concentration in an IV drip. Blood serum (the liquid part of blood) is normally isotonic with respect to red blood cell cytoplasm. Under normal conditions, osmosis does not occur into or out of red blood cells. The cells maintain their normal size and shape (Figure 10(a)). When a patient receives an IV drip, the IV solution mixes directly with blood serum. If the solution contains a lower solute concentration than blood serum (i.e., a hypotonic solution), it may dilute the blood serum until it is hypotonic to blood cell cytoplasm. If so, osmosis will occur into the red blood cells, causing the cells to swell, and maybe burst (Figure 10(b)). This condition is called hemolysis, and may be fatal because the blood cells will be unable to transport oxygen to body tissues efficiently. If a patient receives an IV solution that is hypertonic to blood serum, it may concentrate blood serum until it is hypertonic to blood cell cytoplasm. Osmosis will occur out of the blood cells. The cells will lose water and become small and scallop-shaped (Figure 10(c)). Scallop-shaped cells have a tendency to stick to one another and clog small veins and arteries, preventing oxygen from reaching body tissues. This condition, called crenation, may also be fatal. (a) isotonic (b) hypotonic (c) hypertonic Figure 9 Solutions A and B are isotonic; solution C is hypotonic to solutions A and B; and solutions A and B are hypertonic to solution C. hemolysis swelling and bursting of red blood cells placed in a hypotonic solution crenation clumping of cells (usually red blood cells) that have become scallop-shaped when placed in a hypertonic solution Figure 10 (a) Red blood cells in an isotonic solution. The cells are normal in shape and size. (b) Red blood cells in a hypotonic solution take on water by osmosis and burst. The empty cells that remain are called redblood-cell ghosts. (c) Red blood cells in a hypertonic solution. The cells lose water by osmosis and are scallop-shaped and smaller than normal. Cellular Biology 63

6 Any solution injected directly into veins and arteries must be isotonic with blood serum. Typical isotonic IV solutions include 5% glucose (also called 5% dextrose, or D5W) and 0.9% sodium chloride (also called isotonic saline). 64 Unit 1 NEL