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BSC 114, Chapter 7

by: Hannah Tomlinson

BSC 114, Chapter 7 BSC 114

Hannah Tomlinson

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Principles Of Biology I
Kimberly Caldwell
Class Notes
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This 5 page Class Notes was uploaded by Hannah Tomlinson on Sunday September 25, 2016. The Class Notes belongs to BSC 114 at University of Alabama - Tuscaloosa taught by Kimberly Caldwell in Fall 2016. Since its upload, it has received 5 views. For similar materials see Principles Of Biology I in Biology at University of Alabama - Tuscaloosa.


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Date Created: 09/25/16
9/14 Chapter 7: Membranes Phospholipids  When phospholipids are placed in water, they spontaneously form membranes  These membranes exhibit selective permeability -Allows some substances to cross it more easily than others  These are the most abundant lipids in most membranes  They are amphipathic (contains hydrophobic and hydrophilic regions) The Fluid Mosaic Model  The membrane is a mosaic of protein molecules bobbing in the fluid bilayer of phospholipids  This means: -Membranes are not static sheets of molecules locked rigidly in place -The membrane is held together with weak hydrophobic interactions Fluidity in the Membrane  Most of the lipid and some of the protein can drift about randomly in the membrane (laterally)  Can be increased by the presence of unsaturated hydrocarbon tails, which results in kinks and the inability to pack closely together  Can be decreased by the presence of saturated hydrocarbon tails  The presence of cholesterol will reduce membrane fluidity because it restricts the lateral movement of phospholipids Membrane Fluidity and Temperature  Membranes remain fluid as temperature decreases until finally at some point the phospholipids settle into a closely packaged arrangement and the membrane solidifies  The temperature at which a membrane solidifies depends upon its lipid composition  The membrane will remain fluid at lower temperatures if it’s rich in unsaturated hydrocarbons  Plants increase their unsaturated phospholipids in the fall to prevent the membranes from solidifying during cold temperatures The Mosaic Part of the Model  The membrane is composed of many kinds of proteins floating around in the phospholipid bilayers  E.g.: 50 proteins in membranes of red blood cells  2 main types of proteins associated with membranes Membrane Proteins 1. Integral: transmembrane proteins with hydrophobic regions that span the membrane -They have hydrophilic ends that are exposed to solutions on either side of the membrane 2. Peripheral: not embedded in the lipid bilayer -Loosely bound to the surface of the membrane, often to the exposed portions of integral proteins -Attached to the cytoskeleton or ECM for structural support Structure of a Transmembrane Protein  Hydrophobic regions that span the membrane  Hydrophilic ends that are exposed to the cytoskeleton and the ECM  Called an integral protein Some Functions of Membrane Proteins  Transport proteins to carry substances across membrane  Enzymes: serve as catalysts (chemical agents that change the rate of reaction without being consumed by the reaction)  Receptor sites for signal transduction  Cell-cell recognition  Attachment to the cytoplasm or ECM  Called peripheral protein Traffic of Small Molecules Across a Cell Membrane  A lot of traffic moves both ways across membrane  Sugars, amino acids, and nutrients enter cells and waste products leave  Oxygen goes in and Carbon Dioxide goes out  Small ions (Na, K, Ca, Cl) shuttle in both directions selectively Hydrophobic Core of Membrane  In spite of the heavy traffic through cell membranes, there is selective permeability  The hydrophobic core impedes transport of ions and polar molecules  Hydrophobic molecules can dissolve in membrane and easily cross it  Larger molecules like glucose can’t pass  Proteins play a key role in regulating transport Transport Proteins  Involved with transporting ions and polar molecules across membranes  These proteins span the membrane and allow only certain classes of molecules or substances to pass across the membrane Channel Proteins  A specific type of protein that has a hydrophilic channel in the center to allow hydrophilic substances to pass through it  One example is a channel protein called aquaporins  Aquaporins allow water molecules (hydrophilic) to enter cells  Without aquaporins, only a tiny fraction of water molecules would enter cells  Ex. of passive diffusion, because no energy is required for this to occur Two Ways Items Cross Biological Membranes 1. Passive (simple) transport (called diffusion in biological membranes)  Osmosis  Facilitated transport 2. Active transport Diffusion  The tendency of molecules of any substance to spread out into the available space (kinetic energy)  Molecules have natural kinetic energy and are in motion—result in diffusion  Spontaneous process  A substance will diffuse from where it is more concentrated (a substance will diffuse down its concentration gradient)  Each substance will diffuse down its concentration gradient, unaffected by the concentration of others In Biological Membranes  Diffusion is called passive transport because a cell does not have to expend energy to make it happen  E.g.: oxygen and water will diffuse across a membrane by passive transport Osmosis  A special case of passive diffusion  The movement of water  If a solute cannot pass through a membrane but water can, the water will diffuse to the area of higher solute concentration Solute Concentrations  When comparing solutions with unequal solute concentration: -Hypertonic: the solution with a higher concentration of solutes -Hypotonic: the solution with a lower concentration of solutes  Isotonic: solutions of equal solute concentration Osmosis in Living Cells  Cells without rigid walls can’t tolerate anything other than isotonic environments  An animal cell in an… -Isotonic environment: the volume of water flowing across membrane will be stable -Hypertonic environment: the cell will lose water to environment; shrivel up and die -Hypotonic environment: the cell will take up water faster than it can leave; cell will swell and lyse Osmoregulation  Some animals living in hypo or hypertonic environments must adapt for such living conditions  Osmoregulation: the control of water balance -e.g.: paramecium Osmoregulation in Paramecium  Lives in pond water that is hypotonic to the cell  Water is continually entering the cell -2 adaptions  A plasma membrane that is much less permeable to water  A specialized organelle called a contractile vacuole that functions as a pump to force water out of the cell as fast as it enters by osmosis Facilitated Diffusion  The process whereby polar molecules and ions (which are impeded by the lipid bilayer of a membrane) are able to pass through with help of transport proteins  This is a spontaneous passage of molecules and ions down their concentration gradients  A transport protein is thought to alter its conformation to receive or release a solute in either direction relative to its concentration gradient  It is still f because no energy is required on the part of the cell  High concentration  Low concentration Active Transport  The movement of a substance across a biological membrane against its concentration or electrochemical gradient  This process requires -Energy input -Specific transport proteins  The movement of a solute from a less concentrated side to more concentrated side  A cell must expend its own metabolic energy  ATP and membrane proteins are required  Na-k pump: it pumps out 3 Na ions for every 2 K ions it lets in -Outside of the cell, Na is higher than inside, even though it is pumping out even more Na -Outside the cell, K is lower than inside the cell, even though it is pumping in more K Transport Review  Small molecules either pass directly though the lipid bilayer of a membrane or are pumped or carried across by transport proteins  Passive transpot -No energy required -Diffusion (simple and facilitated) -Can’t move against a concentration gradient  Active transport -Requires energy -Moves molecules against concentration gradient Traffic of Large Molecules  Exocytosis: the cell secretes macromolecules by the fusion of vesicles with the plasma membrane -e.g.: insulin is secreted from pancreatic cells and sent into the blood  Endocytosis: the cell takes in macromolecules by forming vesicles from the plasma membrane -Pinocytosis: the cell engulfs fluids -Phagocytosis: the cell engulfs food particles


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