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Transcript
BIOL 106 General Biology I Chapter 3: A Closer Look at Cell Membranes Impacts, Issues: One Bad Transporter and Cystic Fibrosis Fig 5.1 Transporter proteins regulate the movement of substances in and out of cells; failure of one of these proteins causes cystic fibrosis I. Organization of Cell Membranes 5.1 The basic structure of all cell membranes is the lipid bilayer with many embedded proteins. A membrane is a continuous, selectively permeable barrier. A. Revisiting the Lipid Bilayer Fig 5.2 Phospholipid molecules -phosphate-containing “heads” are ____________ - fatty acid “tails” are ____________ Form bilayer B. The Fluid Mosaic Model A cell membrane is a mixture of phospholipids, steroids, proteins and other molecules that drift and move Fig 5.3 C. Variations 1. Differences in Membrane Composition •Different kinds and numbers of carbohydrates are attached to membrane proteins and lipids; face outside of cell •Different kinds of phospholipids 2. Differences in Fluidity •Some proteins are attached to the cytoskeleton; others drift around Fig 5.4 •Cell membranes of Archaea do NOT contain fatty acids and are more rigid than the cell membranes of Bacteria or eukaryotes II. Membrane Proteins 5.2 Cell membrane function begins with the many proteins associated with the lipid bilayer. ____________________________________- permanently embedded in a membrane ____________________________________- temporarily attached to one of a membrane’s surfaces 1 Fig 5.5 III. Diffusion, Membranes, and Metabolism 5.3 Ions and molecules tend to move from one region to another, in response to gradients. A. Membrane Permeability ________________________- the ability of a cell membrane to control which substances and how much of them enter or leave the cell Fig 5.6 - allows the cell to maintain a difference between its internal environment and extracellular fluid - supplies the cell with nutrients, removes wastes, and maintains volume and pH 2 B. Concentration Gradients ____________- the number of molecules (or ions) of substance per unit volume of fluid ________________________- a difference in concentration between two adjacent regions Molecules move down their concentration gradient, from a region of higher concentration to one of lower concentration. ____________- the net movement of molecules down a concentration gradient Fig 5.7 - a substance tends to diffuse in a direction set by its own concentration gradient, not by the gradients of other molecules around it - moves substances into, through, and out of cells C. The Rate of Diffusion The rate of diffusion depends on five factors: 1. Size 2. Temperature 3. Steepness of the concentration gradient 4. Charge 5. Pressure D. How Substances Cross Membranes Gases and nonpolar molecules diffuse freely across a lipid bilayer Ions and large, polar molecules require other mechanisms to cross the cell membrane Fig 5.8 A, B and C ________________________________________________- allows a specific solute to follow its gradient across a membrane ________________________- pumps a specific solute across a membrane against its gradient using energy 3 Fig 5.8 D and E ________________________ - endocytosis - exocytosis IV. Passive and Active Transport 5.4 Many types of molecules and ions diffuse across a lipid bilayer only with the help of specific transport proteins. A. Passive Transport Requires no energy input Some passive transporters are open channels Other passive transporters are gated and change shape when a specific molecule binds to them or in response to a change in charge e.g. glucose transporter Fig 5.9 B. Active Transport Requires energy input (usually from ATP) e.g. calcium pumps Fig 5.10 ____________- an active transport protein that moves two substances across a membrane at the same time e.g. sodium-potassium pump moves Na+ out of the cell and K+ into the cell Fig 5.11 V. Membrane Trafficking 5.5 By processes of endocytosis and exocytosis, vesicles help cells take in and expel particles that are too big for transport proteins, as well as substances in bulk. ________________________- the formation and movement of vesicles formed from membranes, involving motor proteins and ATP A. Endocytosis and Exocytosis Fig 5.12 ________________________- fusion of a vesicle with the plasma membrane, releasing its contents to the surroundings ________________________- formation of a vesicle from the plasma membrane, enclosing materials near the cell surface and bringing them into the cell ____________________________________- specific molecules bind to receptors on the plasma membrane, which are then enclosed in an endocytic vesicle Fig 5.13 4 ________________________- “cell eating”; pseudopods engulf target particle and merge as a vesicle which fuses with a lysosome in the cell Fig 5.14 ____________________________________- extracellular fluid is captured in a vesicle and brought into the cell; not as selective B. Membrane Cycling Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane New membrane proteins and lipids are made in the ER and modified in Golgi bodies before forming vesicles that fuse with the plasma membrane Fig 5.15 VI. Which Way Will Water Move? 5.6 Water diffuses across cell membranes by osmosis. Osmosis is driven by tonicity, and is countered by turgor. A. Osmosis ____________- the movement of water down its concentration gradient from a region of lower solute concentration to a region of higher solute concentration Fig 5.16 B. Tonicity ____________- the relative concentrations of solutes in two fluids that are separated by a selectively permeable membrane ____________- solution with a lower concentration of solutes ____________- solution with a higher concentration of solutes ____________- solutions with the same concentration of solutes Water diffuses from a hypotonic solution to a hypertonic solution until the solutions are isotonic. Fig 5.16 When a cell is placed in an isotonic solution, its volume does not change. When a cell is placed in a hypertonic solution, water will diffuse out of the cell and the cell will shrink. When a cell is placed in a hypotonic solution, water will diffuse into the cell and the cell will swell (and may burst). 5 C. Effects of Fluid Pressure ____________________________________- the pressure exerted by a volume of fluid against a surrounding structure (tube, plasma membrane or cell wall) which resists changes in volume ________________________- the amount of hydrostatic pressure that can stop water from diffusing into cytoplasmic fluid or other hypertonic solutions Fig 5.18 6