Bio 118 1st Edition Lecture 5 Outline of Last Lecture I Protein Structure and Function a The Nature of Side chains b Important Terminology c Functional Groups Affect Reactivity d Primary Structure e Secondary Structure f Tertiary Structure g R group Interactions That Form Tertiary Structures h Summary of Protein Structure i Protein Function What Do Proteins Do j Folding and Function k Protein Folding Is Often Regulated l Prions and Protein Folding II Nucleic Acids and the RNA World a Summary of DNA s Secondary Structure b How Does DNA Replicate c Is DNA a Catalytic Molecule d RNA Structure and Function e Glycosidic Linkages f Types of Polysaccharides Outline of Current Lecture a Lipids What is a Lipid b The Importance of Cell Membranes c Three Types of Lipids Found in Cells d Fats Oils Triglycerides e Sterols f Phospholipids g Phospholipids Water h Phospholipid Bilayers i Selective Permeability of Lipid Bilayers j Many Factors Affect Membrane Permeability k Bond Saturation and Membrane Permeability Current Lecture These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Lipids What is a Lipid Lipids carbon containing compounds that are found in organisms and are largely nonpolar hydrophobic Unlike carbs proteins nucleic acids they are defined based on their solubility properties o Not by their shared chemical structure Hydrocarbons most commonly known nonpolar lipid molecules that contain only carbon and hydrogen Lipids do not dissolve in water because most have a hydrocarbon chain bonded to a carboxyl COOH group Fatty acids and isoprene are the key building blocks of lipids The Importance of Cell Membranes Plasma Membrane cell membrane separates life from nonlife Made of phospholipid The plasma membrane separates the cell s interior from the external environment Function of membranes o Keep damaging materials out of the cell o Allow entry of materials needed by the cell o Facilitate the chemical reactions necessary for life Three Types of Lipids Found in Cells Fats Oils triacylglycerols triglycerides o Composed of 3 fatty acids linked to glycerol backbone Sterols o Family of lipids with distinctive four ring structure o Cholesterol is an important sterol in mammals o Serves as precursor to important biological molecules including steroid hormones sex hormones vitamin D Phospholipids o Consist of glycerol backbone linked to a phosphate group and to either 2 chains of isoprene or 2 fatty acids o Structure similar to triglycerides Fats Oils Triglycerides otherwise known as fats solid at room temperature and oils liquid at room temperature are comprised of non polar molecules made up of 3 fatty acids linked to a glycerol 3 carbon backbone o Most abundant of lipids found in food body o Saturated mono unsaturated polyunsaturated and trans fats are distinguished from each other based on degree of saturation and the location of the hydrogens Trans fats have hydrogen opposing one another at point of unsaturation resulting in a linear molecule that mimics saturated fats o Also distinguished by long chain 12 carbons or longer medium chain 6 10 carbons short chain less than 6 carbons fatty acids o Fats form due to dehydration reactions o Glycerol is soluble in water due to OH groups o Triglycerides glycerol 3 fatty acids attached to carbon by ester linkages Fatty acids Double O bond at the top Sterols Family of lipids distinguished by bulky four ring structure Differ from one another by the functional groups or side groups attached to different carbons in those hydrophobic rings Structure o Hydrophilic hydroxyl group attached to the top ring and an isoprenoid tail attached at the bottom Cholesterol o An important component of plasma membranes in many organisms o Maintains the fluidity if the cell membrane by regulating the integrity of the cell membrane keeps the cell membrane from becoming too lose or stiff Phospholipids Similar to triglycerides glycerol backbone 2 fatty acids BUTTT instead of 3 rd fatty acid has a phosphorous group that has a charge polar Membrane forming lipids contain both a polar hydrophilic region and a nonpolar hydrophobic region Phospholipids are amphipathic o The head region glycerol phosphate charged group Highly polar covalent bonds o The tail region 2 nonpolar fatty acid or isoprene chains When placed in solution the phospholipid heads interact with water while the tails do not allowing these lipids to form membranes Phospholipids and Water Phospholipids do not dissolve when they are placed in water Water molecules interact with the hydrophilic heads but not with the hydrophobic tails o Drives hydrophobic tails together Upon contact with water phospholipids form either o Micelles Heads face the water and tails face each other o Phospholipid Bilayers lipid bilayers Hydrophobic tails are attracted to one another Very fluid Phospholipid Bilayers Form when 2 sheets of phospholipid molecules align The hydrophilic heads in each layer face a surrounding solution while the hydrophobic tails face one another inside the bilayer Form spontaneously with no outside input of energy required Selective Permeability of Lipid Bilayers The permeability of a structure is its tendency to allow a given substance to pass across it Have selective permeability o Small or nonpolar molecules move across phospholipid bilayers quickly o Charged or large polar substances cross slowly if at all Many Factors Affect Membrane Permeability Many factors influence the behavior of the membrane o Number of double bonds between the carbons in the phospholipid s hydrophobic tail o Length of the tail o Number of cholesterol molecules in the membrane o Temperature Bond Saturation Membrane Permeability Double bonds between carbons in a hydrocarbon chain can cause a kink in the hydrocarbon chain preventing the close packing of hydrocarbon tails and reducing hydrophobic interactions o Unsaturated hydrocarbons have at least one double bond o Called unsaturated because one or more points along hydrocarbon chain where hydrogen is absent thus creating the point of saturation o At this point there will be a double bond between C atoms o Hydrocarbon chains without double bonds are termed saturated saturated referring to how every C is bonded to as many hydrogen as possible hence no double bonds Saturated fats have more chemical energy than unsaturated fats Membranes with unsaturated short phospholipid tails are much
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