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BSC 114 Chapter 7

by: Lauren Dutch

BSC 114 Chapter 7 BSC 114

Lauren Dutch
GPA 4.0

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lecture notes and book information combined
The Principles of Biology 1
Dr. Stephenson
Class Notes
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This 4 page Class Notes was uploaded by Lauren Dutch on Saturday September 17, 2016. The Class Notes belongs to BSC 114 at University of Alabama - Tuscaloosa taught by Dr. Stephenson in Fall 2016. Since its upload, it has received 6 views. For similar materials see The Principles of Biology 1 in Biology at University of Alabama - Tuscaloosa.


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Date Created: 09/17/16
Chapter 7 Membrane Structure and Function I. Cellular membranes are fluid mosaics of lipids and proteins A. Membranes separate external from internal and subdivide cytoplasm into organelles 1. Cell contents and the external environment are aqueous so membranes are hydrophobic in order to separate the hydrophilic areas B. Fluid mosaic model 1. Membranes are composed of lipids and proteins 2. Lipid bilayer constitutes the “fluid”  Phospholipid structure  Fatty acid tails are hydrophobic  Polar head is hydrophilic  The phospholipid is amphipathic, meaning it has both hydrophobic and hydrophilic regions  Double layer of lipids  The hydrophilic heads are out towards the aqueous environment - Polar head groups on the periphery hydrogen bond with water and dissolved polar molecules  Hydrophobic core is made of nonpolar fatty acid tails 3. Proteins constitute the “mosaic”  Integral membrane proteins float like icebergs in the sea of lipids  Transmembrane proteins extend all the way through the lipid bilayer with parts exposed on each side  Other integral membrane proteins only go part way through the membrane  Most proteins span the membrane many times - Membrane embedded parts are usually alpha helices with hydrophobic amino acids - Loops of protein exposed on in the cytoplasm and outside the cell are hydrophilic  Proteins can move around the lipid bilayer; they are not locked in place  Peripheral membrane proteins are not embedded in the membrane but are instead attached by covalent or noncovalent bonds to the lipid head groups, integral proteins, or other peripheral proteins 4. Membrane fluidity  Degree to which lipids and proteins move in the plane of the membrane  Varies with temperature; as temperature increases, fluidity increases  Varies with lipid composition  Many unsaturated phospholipids leads to high fluidity because the kinks in the tails prevent packing  Many saturated phospholipids lead to low fluidity/viscous because the tails can pack together tightly  Cholesterol in between the tails hinders movement; decreases fluidity but also prevents phospholipid packing II. Membrane Function A. Permeability of the lipid bilayer 1. Movement of molecule across the lipid bilayer WITHOUT proteins:  Nonpolar molecules cross easily  Small polar molecules cross slowly  Large polar molecules or charged molecules of any size cannot pass the hydrophobic core B. Diffusion is the spreading of a molecule in the available space 1. Molecules move spontaneously from high to low concentration; move down the concentration gradient 2. Equilibrium is reached when there is no net movement and the concentration is equal  Example: If you put sugar in water, the sugar concentration will be high at the bottom of the cup and lower at the top while the water concentration will be low at the bottom of the cup and high at the top. However, eventually the concentration will become uniform. C. Osmosis is a special type of diffusion in which water is diffused across a membrane 1. Water can cross lipid bilayers but dissolved chemicals cannot 2. Osmoregulation  The plasma membranes of cells are semipermeable, allowing water to pass but preventing the passage of most solutes  Solute is the molecules dissolved in the solvent; water is the solvent in most biological processes  Hypotonic solutions are solutions where the solute concentration outside the cell is lower than that inside the cell; external water concentration is higher  Animal cells quickly absorb water and may swell or explode  Plants/fungi/bacteria have rigid cell walls that help prevent expansion  Protists continually pump water from cytoplasm using a contractile vacuole  Hypertonic solutions are solutions where the solute concentration outside the cell is higher than that inside the cell; external water concentration is lower than internal  Animal and plant cells lose water and shrivel up  Isotonic solutions are solutions where the solute concentration is the same inside and outside the cell  Animals live in isotonic situations  Plants rely on internal cell pressure so they do not live in isotonic situations; they need hypotonic situations III. Membrane Transport  Cells move molecules across membranes using proteins A. Passive transport is diffusion of a substance across a membrane with no energy investment 1. Diffusion without assistance 2. Down the concentration gradient 3. Facilitated diffusion is when larger polar or charged molecules cannot cross the lipid bilayer, e.g. ions, sugars, amino acids  Require transporter proteins  Channel proteins are hydrophilic channels on inside of proteins  Carrier proteins contain a hydrophilic space that changes shape, alternately opened to the inside and the outside  Always gated; pores can be opened and closed according to when the cell wants it to happen 4. Water can cross by diffusion and facilitated diffusion  Through diffusion, the water leaks through the lipid bilayer slowly because of its polarity  Through facilitated diffusion, the water moves through channels called aquaporins (keep turgor in plants; reclaim water from urine in kidneys) B. Active transport uses energy to move solutes against their gradients 1. The transport proteins used in active transport are all carrier proteins 2. ATP driven  ATP transfers its terminal phosphate group directly to the carrier protein  Example: Sodium potassium pump in which energy is obtained by breaking ATP bonds through ATP hydrolysis to pump sodium ions out of cell and potassium ions into cell, both going against their gradients 3. Cotransport is the movement of one solute down its concentration gradient to provide energy to move another up its concentration gradient  The molecules must move in the same direction, accompanying each other  Example: the sucrose-hydrogen ion transporter moves hydrogen ions down its concentration gradient while it moves sucrose against its concentration gradient C. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis 1. Exocytosis occurs when small membrane bound vesicles fuse with the plasma membrane to deliver contents of the vesicles outside of the cell  Last step in the secretory pathway 2. Endocytosis occurs when a cell engulfs nearby parts of the environment, forming new internal vesicles from parts of the plasma membrane  Phagocytosis = cell eating  Cell engulfs a particle by extending pseudopodia and packing a food vacuole around it  Attach to lysosome to digest the particle  Examples: Amoeba or white blood cell eating bacterium  Pinocytosis = cell drinking  Cell engulfs a small volume of liquid into vesicles  Receptor mediated endocytosis is when the cell internalizes tiny sections of membrane where ligands have bound to receptors  Example: uptake of cholesterol from blood


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