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Transcript
Biology 102 Lectures 6 & 7: Biological Membranes Lecture outline 1. Relationship of membrane structure and function 2. Movement of substances across membranes 3. Functions Structure: The fluid-mosaic model of membranes Principles of Diffusion Passive and active transport of solutes Osmosis Endocytosis and exocytosis Specialization of cell surfaces 1. Membrane structure and function Biological membranes Thin barrier separating the inside of the cell (or structure) from the outside environment Functions (focus on plasma membrane) Selectively isolate the cell’s contents from the external environment Regulate the exchange of substances between the inside and outside of the cell Communicate with other cells Fluid-mosaic model of membrane structure The phospholipid bilayer is the fluid portion of the membrane Double layer Polar head group: hydrophilic exterior Non-polar hydrocarbon tails: hydrophobic interior Unsaturated hydrocarbon chains: maintains fluidity Phospholipid bilayer as a barrier Hydrophilic molecules cannot pass freely through the membrane’s hydrophobic interior amino acids, charged ions (i.e. Na+ and Cl-) are some examples Though polar, H20 is so small it does pass through. Sugars, Many hydrophobic molecules can pass freely through the membrane’s hydrophobic interior Steroid hormones and other lipids are some examples Cholesterol molecules are part of the lipid bilayer Adds strength Adds flexibility Affects fluidity Decreases fluidity at “moderate” temperatures Harder for phospholipids to move Prevents solidification at low temperatures Keeps phospholipids from binding to each other A mosaic of proteins is embeded in the membrane Glycoproteins: proteins with attached carbohydrates Types of membrane proteins Transport proteins For passage of materials through the plasma membrane Channel vs. carrier proteins Receptor proteins Bind molecules and trigger cellular responses Example: hormones Recognition proteins Self vs. non-self (glycoprotein-based) recognition Markers during development 2. Movement of substances across membranes Definitions Concentration Number of molecules in a given volume Gradient Differences in concentration between two regions of space. This causes molecules to move from one region to the other (if no barrier to movement) Diffusion Net movement of molecules from regions of high concentration to regions of low concentration Considered as movement “down” its concentration gradient Diffusion of Dye in Water Dispersing Random Dispersal Time 0 Time 1 Time 2 Steep Concentration Gradient Reduced Concentration Gradient No Concentration Gradient Passive vs. active transport Passive transport Movement of molecules down their concentration gradients Requires no net energy expenditure The gradients themselves provide energy Active transport Movement of molecules against their concentration gradients Requires energy! Focus: Passive transport 1. Simple diffusion 2. Facilitated diffusion 3. Osmosis Remember that no energy is required, and molecules move down their concentration gradients Focus: Passive transport 1. Simple diffusion Molecules simply cross cell membrane on their own, down their concentration gradients Possible only for molecules that can cross the lipid bilayer on their own Lipid-soluble molecules Very small molecules Examples: ethyl alcohol, vitamin A, steroid hormones Examples: water, carbon dioxide Rate depends upon Concentration gradient Size Lipid solubility Focus: Passive transport (cont.) 2. Facilitated diffusion Molecules move down their concentration gradients (as for simple diffusion), but… Transport proteins assist these molecules in crossing the membrane No net energy expenditure! (This is a type of diffusion…) Focus: Passive transport (cont.): Facilitated diffusion via a channel Focus: Passive transport (cont.): Facilitated diffusion via a carrier protein Diffusion Channel Protein Molecule in Transit Diffusion Gradient (Outside Cell) Carrier protein has binding site for molecule Molecule enters binding site (Inside Cell) Carrier protein changes shape, transporting molecule across membrane Carrier protein resumes original shape Focus: Passive transport (cont.) 3. Osmosis Movement of water from a high [water] to an area of low [water] concentration across a semipermeable membrane Note here that water can pass through, but glucose cannot Think about which way water will move (blackboard demo) The effects of osmosis Compare solute and water concentrations outside vs. inside the cell (sketches) Focus: Active Transport 1. Movement via active transport proteins 2. Endocytosis 3. Exocytosis Remember that energy is required, and molecules are moved against their concentration gradients Focus: Active transport 1. Movement via active transport proteins ATP required (has own binding site) Note movement of particles (Ca++) against their concentration gradient Focus: Active transport 2. Endocytosis Three types of endocytosis Pinocytosis “cell drinking” Extracellular fluid taken in Receptor-mediated endocytosis Specific for particular molecules Molecules bind to receptors. Receptor-molecule complex taken in Phagocytosis Large particles engulfed Focus: Active transport 3. Exocytosis 3. Specialization of cell surfaces Connections between cells Desmosome: Membranes of adjacent cells glued together by proteins and carbohydrates Tight junction: Cells sealed together with proteins 3. Specialization of cell surfaces (cont.) Communication between cells Gap junctions: Channels connect adjacent cells Plasmodesmata: Continuous cytoplasm bridges between two cells (plants) Note also cell walls. Only certain cell types have cell walls!