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Transcript
Unit 2: Chapter 5 Biological Membranes Structure of the Cell Membrane 6 main components 1. Phospholipids 2. Proteins 3. Cholesterol 4. Carbohydrates 5. Glycoproteins 6. glycolipids Phospholipids (review) • Membrane is a bilayer – One layer of polar heads – they are hydrophilic because they love water – One layer of nonpolar tails – they are hydrophobic because they hate water Membranes have proteins! • 2 main types 1. Integral proteins • • • • Inserted into the membrane May be unilateral – reach only partway across the membrane Or they may be transmembrane – completely span the membrane Removal disrupts cell membrane 2. Peripheral proteins • • Not embedded in membrane, but attached to the membrane surface by either integral proteins or filaments from the cytoskeleton Removal has little effect on cell membrane 7 functions of membrane proteins 1. 2. 3. 4. 5. Anchoring cell Passive transport Active transport Enzyme activity Signal transduction – transmitting info into cell 6. Cell recognition – like ID tags 7. Junction between cells – cell adhesion Cholesterol • Controls the fluidity of the cell membrane History of the Cell Membrane • 2 models 1. Sandwich Model 2. Fluid Mosaic Model Sandwich Model • Davson and Danielli in 1935 • Perceived the plasma membrane as a sandwich where the proteins were the bread and the phospholipid bilayer was the meat Fluid Mosaic Model • Singer and Nicholson in 1972 • Said the membrane is fluid and must be • If it solidifies, its permeability will change and enzymes will denature • Said the membrane is mosaic – there are proteins embedded in it Nature of protein and lipid mobility • Lateral movement of lipids/proteins is quick • Lipids and proteins rarely flip across the lipid bilayer Basic Terms to Understand • Selectively permeable – prevents passage of most materials through the membrane • Solute – what is dissolving (salt, sugar, etc.) • Solvent – what it is dissolving in (water etc.) • Solution – mixture of solvent and solute 7 ways substances can get into a cell 1. 2. 3. 4. 5. 6. 7. Diffusion Bulk flow Osmosis Facilitated diffusion Active transport Vesicle mediated transport Cell to cell junction Diffusion • Moves materials from a high concentration to a low concentration • Requires no energy – type of passive transport • Easy passage through – oxygen, carbon dioxide, nitrogen, and small polar molecules • Slow passage through – large polar molecules like glucose and charged ions – Proteins allow movement of charged/polar molecules • Particles move until equilibrium Bulk Flow • Molecules move all together in same direction due to hydrostatic pressure Osmosis • ALL things undergo diffusion. • Water also diffuses, however, the water diffusion is not evident unless it crosses a membrane. • Osmosis is the diffusion of water across a membrane from high to a low concentration • No energy is required – type of passive transport • Since cells have membranes, osmosis is important to cells Membranes and Osmosis • Tank w/ semipermeable membrane: water may pass, solute can’t • At first the concentration of solute is very high on the left. • But over time, the water moves across the semipermeable membrane, and dilutes the solute. Water Moves Because It is Polar • Because water is polar, it binds to the solute by hydrogen bonds • The concentration of water is higher on the right • Water will then flow across the membrane, down its concentration gradient, to the left Three osmotic environments • Hypertonic • Hypotonic • Isotonic – Same solute concentration inside and outside the cell – Water flows in and out of the cell equally in both directions – Most cells in our body are isotonic Hypertonic • High concentration of solute outside the cell • Therefore there is more water inside the cell • Water will move out of the cell • If this process continues, the cell collapses and dies – this is called plasmolysis Hypotonic • Low concentration of solute outside the cell • Therefore there is more water outside the cell • Water will move inside the cell • This causes the cell to expand, causing turgor pressure in plant cells Animal cells could burst – this is called cytolysis Plant Cells and Osmotic Pressure • In plants, hypotonic solutions produce osmotic pressure that produces turgor pressure – Turgor means “tight or stiff owing to being very full” – Keeps plant upright; in hypertonic conditions plants wilt Vacuole fills Vacuole shrinks Hypotonic solution Hypertonic Hypertonic Dialysis • Dialysis is the diffusion of solutes across a membrane • The selectively permeable membrane allows small sugar molecules to move across the membrane, but large proteins cannot Facilitated Diffusion • • Transport proteins move materials through membrane 3 kinds of transport proteins 1. Uniport – carries a single molecule across the membrane 2. Symport – moves 2 different molecules at the same time in the same direction 3. Antiport – exhanges 2 molecules in opposite directions Vesicle Mediated Transport • • When vesicles or vacuoles fuse with the membrane to move substances in or out of the cell 2 main types 1. Exocytosis – when vesicle expels contents outside the cell 2. Endocytosis – when vesicles bring substances into the cell • 3 types 3 types of Endocytosis 1. Phagocytosis – solid being taken into the cell 2. Pinocytosis – liquid being taken into the cell 3. Receptor Mediated Endocytosis – substance binds to a specific receptor on the cell before it is brought in Active Transport • Energy is needed • Moves materials against the concentration gradient • Main example: sodium potassium pump Sodium Potassium Pump • Occurs in animal cells • Required ATP (active transport) • Exchanges 3 Na+ ions on inside for 2 K+ ions on outside • This exchange is uneven so an electric potential is generated and so the membrane is now considered to be polarized • Let’s see this in action How do cells stick together and let materials? 1. 2. 3. 4. Plasmodesmata Gap junctions Desmosomes Tight junctions Plasmodesmata • They are channels that allow movement of certain molecules and ions between plant cells Gap Junctions • Cytoplasmic channels between neighboring animal cells • Let adjacent cells communicate • Small dissolved molecules and electrical signals may pass from one cell to the other • Very similar to plasmodesmata Desmosomes • Function as rivets and join animal cells together • They are reinforced by intermediate filaments made of keratin • Still permits materials to move around them in the intercellular space Tight Junctions • Continuous belt around animal cells that fuse membranes of neighboring cells • It is leak proof • Contains no intercellular space Water Potential • Chemical potential of water • Measure of the energy available for reaction or movement • Measures the ability of water to move and water always moves from areas of higher potential to areas of low potential • Has the symbol Ψ (psi) Water Potential Continued… • Is measured in the unit bars • The formula for calculated water potential is osmotic potential (solute) + pressure potential So, the equation is Ψ = Ψs + Ψp Equation Components Ψp = pressure on the system = 0, if the system or container is open ** all of our problems will be open so Ψp will always equal 0. Ψs = change in water potential due to solute molecules The more solute, the lower the water potential Ψs = -iCRT Where i = ionization constant = # of ions in the solute = 1 when there are no ions present C = Molar Concentration usually given in problem equal to Molarity (M), or moles/volume R = pressure constant = 0.0831 Liters X Bars/moles x Kelvin this number never changes T = temperature needs to be in Kelvin Conversion of Celsius to Kelvin is K = 273 + Celsius Suppose we have a beaker of distilled water at room temperature. (0M) What is the water potential? Ψ= Ψs + Ψp Ψ = -iCRT + 0 Ψ = -(1)( 0) (0.0831)(23 + 273) + 0 Ψ=0+0 Ψ=0 Suppose we have an open beaker A that contains Sucrose (2.5M) at 25 C. What is the water potential? Ψ=Ψs+Ψp Ψ = -iCRT + 0 Ψ = -(1)(2.5)(0.0831)(25 + 273) Ψ = -61.91 bars