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General Biology 121- Chapter 7: Membrane Structure and Function

by: Audrey Notetaker

General Biology 121- Chapter 7: Membrane Structure and Function Bio 121

Marketplace > Syracuse University > Biological Sciences > Bio 121 > General Biology 121 Chapter 7 Membrane Structure and Function
Audrey Notetaker
Intro to biological sciences

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Intro to biological sciences
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This 5 page Class Notes was uploaded by Audrey Notetaker on Sunday October 4, 2015. The Class Notes belongs to Bio 121 at Syracuse University taught by Wiles in Fall 2015. Since its upload, it has received 20 views. For similar materials see Intro to biological sciences in Biological Sciences at Syracuse University.

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Date Created: 10/04/15
Chapter 7 Membrane Structure and Function 71 Cellular membranes are uid mosaics of lipids and proteins The most abundant lipids in most membranes are The ability of phospholipids to form membranes is inherent in their molecular structure A phospholipid is an amphipathic molecule meaning it has both a hydrophilic region and hydrophobic region A phospholipid bilayer can exist as a stable boundary between 2 aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water In the the membrane is a mosaic of protein molecules bobbing in a uid bilayer of phospholipids The Fluidity of Membranes A membrane is held together primarily by hydrophobic interactions Most of the lipids and some of the proteins can shift laterally The lateral movement of phospholipids within the membrane is rapid Proteins are much larger than lipids and move more slowly but some membrane proteins do drift Membranes must be uid to work properly the uidity of a membrane affects both its permeability and the ability of membrane proteins to move to where their function is needed When a membrane solidi es its permeability changes and enzymatic proteins in the membrane may become inactive if their activity requires movement within the membrane Membranes that are too uid can t support protein function either Membrane Proteins and Their Functions A membrane is a collage of different proteins often clustered together in groups embedded in the uid matrix of the lipid bilayer Phospholipids form the main fabric of the membrane but proteins determine most of the membrane s functions Different types of cells contain differences sets of membrane proteins and the various membranes within a cell each have a unique collection of proteins There are 2 major populations of membrane proteins Integral proteins penetrate the hydrophobic interior of the lipid bilayer The majority are which are a type of membrane protein spanning the entirety of the biological membrane to which it is permanently attached They span from one side of a membrane through to the other side of the membrane are a proteins loosely bound to the surface of a membrane or to part of an exposed integral protein and not embedded in the lipid bilayer 72 Membrane structure results in selective permeability Permeability of the Lipid Bilayer Nonpolar molecules are hydrophobic They can therefore dissolve in the lipid bilayer of the membrane and cross it easily without the aid of membrane proteins But the hydrophobic interior of the membrane impedes direct passage through the membrane of ions and polar molecules which are hydrophilic Polar molecules pass only slowly through a lipid bilayer and even water doesn t cross rapidly The lipid bilayer is responsible for a cell s selective permeability Selective permeable membranes allow passage of some substances but not others They control volume and internal composition of cell s ions and molecules Transport Proteins 73 are transmembrane proteins that help a certain substances or class of closely related substances to cross the membrane A transport protein is speci c for the substance it translocates allowing only a certain substance to cross the membrane The selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the speci c transport proteins built into the membrane Passive transport is diffusion of a substance across a membrane with no energy investment 0 is the movement of particles of any substance so that they spread out into the available space 0 Each molecule moves randomly yet diffusion of a population of molecules may be directional o A is the region along which the density of a chemical substance increases or decreases Diffusion is a spontaneous process needing no energy 0 The diffusion of a substance across a biological membrane is called because the cell doesn t have to expand energy to make it happen 0 The concentration gradient itself represents potential energy and drives diffusion Effects of Osmosis on Water Balance 0 is the diffusion of free water across a selectively permeable membrane from higher free water concentration to lower water concentration 0 The movement of water across cell membranes and the balance of water between the cell and its environment Water Balance of Cells Without Cell Walls 0 is the ability of a surrounding solution to cause a cell to gain or lose water 0 The tonicity of a solution depends in part on its concentration of solutes that can t cross the membrane relative to that inside the cell 0 If there s a higher concentration of nonpenetrating solutes in the surrounding solution water will tend to leave the cell and so on o refers to a solution that when surrounding a cell causes no net movement of water into or out of the cell Water diffuses across the membrane but at the same rate in both directions 0 refers to a solution that when surrounding a cell causes the cell to lose water Once this happens the cell will shrivel and probably die 0 to a solution that when surrounding a cell causes the cell to take up water This will cause the cell to swell and burst like an over lled water balloon is the regulation of solute concentrations and water balance by a cell or organism Cells regulate internal osmotic pressure to prevent shrinking or bursting Water Balance of Cells With Cell Walls 0 Plant cells are surrounded by cell walls These cell walls help maintain the cell s water balance when immersed in a hypotonic solution o means swollen or distended regarding plant cells in a hypotonic solution 0 means limp regarding plant cells in an isotonic solution 0 is a phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall this occurs when the cell loses water to a hypertonic environment The plant will then wilt and can lead to death Facilitated Diffusion Passive Transport Aided by Proteins is the passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of speci c transmembrane transport proteins requiring no energy expenditure 0 The 2 types of transport proteins are which provide corridors that allow speci c molecules or ones to cross the membrane and which undergo a subtle change in shape that translocates the solutebinding site across the membrane This change in shape may be triggered by the binding and release of the transported molecule Like ion channels carrier proteins involved in facilitated diffusion result in the net movement of a substance down its concentration gradient No energy is required 0 Channel proteins that transport ions are called 0 Many ion channels function as which open or close in response to a stimulus 74 Active transport uses energy to move solutes against their gradients Facilitated diffusion is considered passive transport because the solute is moving down its concentration gradient a process that requires no energy Facilitated diffusion speeds transport of a solute by providing the efficient passage through the membrane but it doesn t alter the direction of transport The Need for Energy in Active Transport 0 To pump a solute across a membrane against its gradient requires work the cell must expand energy Therefore this type of membrane traffic is called This is mediated by speci c transport proteins and requiring an expenditure of energy Active transport enables a cell to maintain internal concentration of small solutes that differ from concentrations in its environment For example a uses ATP to pump sodium ions out and potassium ions in Cotransport Couples Transport by a Membrane Protein 0 is the coupling of the downhill diffusion of one substance to the uphill transport of another against its own concentration gradient 75 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Exocytosis is the cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane Materials exit the cell and the membrane surface area increases 0 Many secretory cells use exocytosis to export products 0 is the cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane Materials enter the cell and the membrane surface area decreases The 3 types of endocytosis are Phagocytosis is when large particles enter the cell quotCellular eatingquot 0 Pinocytosis is when dissolved materials enter the cell quotCellular drinkingquot 0 Human ces use receptormediated endocytosis to take in cholesterol for membrane synthesis and the synthesis of other steroids


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