Chapter#6 - Lipid Structure and Function PDF

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Lipid Structure and Function...

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6.1 Lipid Structure and Function ● Lipids: ○ Carbon-containing compounds insoluble in water ● Hydrocarbons ○ nonpolar molecules that contain only carbon and hydrogen ■ Hydrophobic ● Lipids, Membranes, and the First Cells ○ Plasma Membrane, or cell membrane, separates life from nonlife: ■ Serves as selective barrier ■ Facilitates chemical reactions necessary for life ● How Does Bond Saturation Affect Hydrocarbon Structure? ○ Fatty acids—simple lipid made of hydrocarbon chain bonded to polar carboxyl functional group ○ Saturated—hydrocarbon chains consist of only single bonds between carbons: ■ Has maximum number of hydrogen atoms ○ Unsaturated hydrocarbon chains have one or more double bonds in hydrocarbon chains: ■ Hydrogen atoms are removed to make double bond ■ Forms “kink” in chain ■ Polyunsaturated chains have many double bonds ○ Fig 6.9 Lipid Bilayers show selective permeability ● Fats are: ○ Composed of three fatty acids linked to glycerol ○ Also called triacylglycerols or triglycerides ○ The primary role of fats is energy storage ● Phospholipids ○ Consist of glycerol linked to phosphate group and two hydrocarbon chains ○ Primary role of phospholipids is to form cell membranes ● How Membrane Lipids Interact with Water ○ Lipids have wide array of functions: ■ Store chemical energy

■ Act as pigments that capture/respond to sunlight ■ Serve as signals between cells ■ Form waterproof coatings on skin and cells ■ Act as vitamins in cellular processes ○ Phospholipids are amphipathic ■ Amphipathic nature responsible for plasma membrane—life’s defining barrier 6.2 Phospholipid Bilayers ● Amphipathic lipids do not dissolve in water: ○ Hydrophilic heads interact with water ○ Hydrophobic tails do not ● Amphipathic lipids assume one of the two types of structure ○ Micelles ■ Tiny spherical aggregates ■ Form free fatty acids ○ Lipidbilayers ■ Created when lipid molecules align in paired sheets ● Artificial Membranes as an Experimental System ○ Vesicles ■ Small bubble-like structures surrounded by phospholipids ■ Artificial membrane-bound vesicles are called liposomes 6.3 How Substances Move Across Lipid Bilayers: Diffusion and Osmosis ● Dissolved solutes are in constant random motion due to their thermal energy ● Diffusion—Spontaneous movement of molecule and ions ○ Concentration Gradient created by difference in solute concentrations ○ Net movement from high-concentration regions to low-concentration regions ● Diffusion along concentration gradient increases entropy ● Diffusion ○ Equilibrium occurs when molecules or ions are randomly distributed throughout solution

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■ Molecules are still moving randomly ■ But there is no more net movement ○ Passive Transport occurs when substances diffuse across membrane in absence of an outside energy source Osmosis ○ Water moves from regions of low solute concentration to regions of high solute concentration ■ Dilutes higher concentration of solute ■ Equalizes concentration on both sides of bilayer ○ Concentration of solution outside cell may differ from concentration inside cell ○ Hypertonic: Outside solution with higher concentration than inside of cell ○ Hypotonic: Solution with lower concentration than inside of cell ○ If solute concentrations are equal on outside and inside of cell, solutions are isotonic to each other Membranes and Chemical Evolution ○ First lipid bilayers probably provided container for replicating first “living” molecule ○ Negatively charged ribonucleotides can get across lipid bilayers and inside lipid-bound vesicles ○ Protocells ■ Simple vesicle-like structures that harbor nucleic acids ■ Possible intermediates in evolution of cell Systems for Studying Membrane Proteins (Fig 6.20) ○ Membrane proteins may be separated from membrane with detergents ■ Small amphipathic molecules that can form micelles ■ Unlike amphipathic lipids, detergents are water soluble ○ Allowed identification of membrane proteins that affect permeability ■ Channels ■ Carriers ■ Pumps Protein Structure Determines Channel Selectivity

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○ Cells have different types of pore-like channel proteins in their membranes ■ Channel proteins are selective ○ Aquaporins (“water-pores”) ■ Permit water to cross plasma membrane Movement through Many Membrane Channels Is Regulated ○ Gated channels ■ Open or close in response to signal ○ Flow of ions and small molecules through membrane channels carefully controlled ○ Movement of substances through channel proteins is passive ○ Facilitated Diffusion: ■ Transmembrane proteins assist passive transport Carrier Proteins Facilitate Diffusion (Fig 6.25) ○ Carrier proteins selectively pick up solute on one side of membrane and drop it on the other side ○ Best studied carrier protein is glucose Pumps Perform Active Transport ○ Active Transport: ■ Moves substances against their gradient ■ Requires input of energy ○ ATP often provides energy in cells: ■ Phosphate group transferred via active transport protein (“pump”) The Sodium–Potassium Pump ○ Uses ATP ○ Transports Na+ andK+ ions against their concentration gradient Secondary Active Transport (co-transport) ○ Gradients are crucial to function of cell ■ Make it possible for cells to engage in secondary active transport ○ By moving materials against their concentration gradients, pumps set up electrochemical gradients

■ Gradients represent potential energy ● Electrochemical gradients power movement of another molecule against its gradient ● Plasma Membranes Define the Intracellular Environment ○ Biological membranes allow for cell’s internal environment to differ from its outside environment ○ Evolution of membrane proteins enable early cells ability to create internal environment conduce to life ○ Cells with efficient and selective membranes favored by natural selection

Diffusion

Facilitate Diffusion

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Active Transport -Energy...