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Published byBarnard McGee Modified over 9 years ago
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extracellular fluid (outside) carbohydrate phospholipid cholesterol binding site phospholipid bilayer recognition protein receptor protein transport protein protein filaments cytoplasm (inside) Membrane Proteins Fig. 3-1
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Phospholipid bilayer blocks passage to most molecules Polar H20 soluble: Salts, AA, sugars Pass freely: Small molecules: H20, uncharged lipid soluble molecules
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3.3 How Does The Plasma Membrane Play Its Gatekeeper Role? Membrane proteins – Transport proteins: allow the movement of water- soluble molecules through the plasma membrane by forming channels or by carrying them across – Receptor proteins: possess a binding site on the outer surface for binding specific chemicals that may alter overall cell function – Recognition proteins: with sugar groups attached to the exterior of the cell, are used by the immune system to identify cells as belonging to “self”
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3.4 What Is Diffusion? Characteristics of molecules in fluids – Concentration: the number of molecules in a given unit of volume – Gradient: a physical difference (concentration, pressure, electrical charge) between two regions of space that causes molecules to move from one region to another
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3.4 What Is Diffusion? Molecules in fluids move in response to gradients. – Diffusion: the movement of molecules from regions of high molecular concentration to low molecular concentration A drop of dye in water illustrates diffusion.
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3.4 What Is Diffusion? Diffusion of a dye in water Fig. 3-4 water molecule drop of dye A drop of dye is placed in water The dye molecules diffuse into the water; the water molecules diffuse into the dye Both dye molecules and water molecules are evenly dispersed 2 3 1
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3.4 What Is Diffusion? Summing up: the principles of diffusion – Diffusion is the movement of molecules down a gradient from high concentration to low concentration. – The greater the concentration gradient, the faster the rate of diffusion. – If no other processes intervene, diffusion will continue until the concentration gradient is eliminated.
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3.5 What Is Osmosis? Osmosis: the diffusion of water molecules from a high water concentration to a low water concentration across a biological membrane Pure water has the highest water concentration. The addition of dissolved solutes to pure water reduces the number of water molecules and thus lowers the water concentration.
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3.5 What Is Osmosis? Osmotic water flow across a membrane takes place across selectively permeable membranes that allow water to pass, but not certain small impermeable molecules, such as sugars.
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selectively permeable membrane pore water sugar Water molecule: can fit through the pore Sugar with water molecules clustered around it: cannot fit through the pore 3.5 What Is Osmosis? A selectively permeable membrane Fig. 3-5
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3.5 What Is Osmosis? Example of osmosis: a water-permeable bag that is impermeable to sugar and has sugar inside of it – Water flows into the bag, down a water concentration gradient. – The bag swells and eventually bursts from the additional water.
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3.5 What Is Osmosis? Osmosis Fig. 3-6 Bag bursts selectively permeable membrane sugar molecule water molecule Water flows in
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3.5 What Is Osmosis? Animation—Osmosis PLAY
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3.5 What Is Osmosis? Summing up: the principles of osmosis – Osmosis is the diffusion of water across a selectively permeable membrane. – Dissolved substances reduce the concentration of water molecules in a solution. – Water moves across a membrane down its concentration gradient from a high concentration of water molecules to a low concentration of water molecules.
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3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? There are two kinds of transport across cell membranes: – Passive transport – Energy-requiring transport
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3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
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Passive transport: movement of molecules across a membrane, down a concentration gradient, without the use of energy – The phospholipid bilayer and transport proteins regulate which molecules can cross the membrane down concentration gradients. – Membranes are selectively permeable, and only allow some molecules to cross and not others.
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3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Examples of passive transport – Simple diffusion: the transfer of gases, water, and lipid-soluble substances—such as ethyl alcohol and vitamin A—across the phospholipid bilayer
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