Human Physiology-Chapter 4 summary
Human Physiology-Chapter 4 summary BIOL 2213
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This 6 page Class Notes was uploaded by Celine Notetaker on Thursday January 21, 2016. The Class Notes belongs to BIOL 2213 at University of Arkansas taught by Dr. Hill in Fall 2014. Since its upload, it has received 20 views. For similar materials see Human Physiology in Biology at University of Arkansas.
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Date Created: 01/21/16
Chapter 4 – Movement of Molecules across Cell Membrane Diffusion: the movement of molecules from one location to another as a result of their random thermal motion. It is a movement from an area of high concentration to an area of low concentration. Diffusion Rate vs. Distance: Diffusion time increases in proportion to the square of the distance over which the molecules diffuse. Diffusion occurs very quickly in close proximities but quickly slows down the father away you get. Humans account for this through the circulatory system, which allows nutrients to be exchanged through the alveoli → plasma → red blood cells → interstitial fluid → cells Diffusion through Membranes: The rate at which molecules diffuse across membranes are a thousand to a million times slower than the diffusion rates of the same molecules through a water layer of equal thickness. As the concentration of molecules between the extracellular fluid and intracellular fluid becomes similar, the diffusion rate slows. In other words – the larger the concentration gradient, the faster the diffusion rate. Factors that affect diffusion are: 1. Concentration Gradient 2. Solubility in Membrane Lipids 3. Presence of Membrane Ion Channels 4. Area of the Membrane 5. Electrical Forces acting on the Ions Diffusion through the Lipid Bilayer: Oxygen, carbon dioxide, fatty acids, and steroid hormones diffuse rapidly through the lipid portions of membranes because they are nonpolar. On the other hand, polar molecules do not move easily through membranes. Nonpolar molecules have large permeability constants. Finally, most organic molecules that make up the intermediate stages of metabolic pathways (like in the electron transport chain) are polar and therefore do not diffuse across the lipid bilayer. This is desirable because those certain organic molecules need to stay in their respective place. Diffusion of Ions through Protein Channels: Ions, being polar, must use ion channels to pass through plasma membranes. Most cells have the same permeability of nonpolar molecules. However, most cells differ on their permeability to ions. Ion channels are made of one or several integral proteins. Ion channels show specificity, meaning certain integral proteins only let certain ions pass. Specificity is caused by channel diameter and the charged and polar surfaces of the integral proteins. Membrane Potential: This is the separation of electrical charge that exists across the plasma membrane. The magnitude of potential difference is measured in millivolts. A “volt” is like electrical pressure. The membrane potential provides an electric force that facilitates the movement of ions across the membrane. Similarly, the electrochemical gradient is a collective term referring to 1) membrane potential (electrical difference) and 2) the concentration difference of molecules or ions. Regulation of Diffusion through Ion Channels: The process of opening and closing ion channels is called channel gating. A channel protein can therefore be opened or closed at any given time. Three factors affect how long or how often channels (transmembrane integral protein channels) are open: 1. LigandGated Channels – molecules may bind to channel proteins to produce an allosteric or covalent change in the shape of the channel protein, thereby allowing ions to flow through the opening. 2. VoltageGated Channels – changes in membrane potentials can cause movement of charged portions of channel proteins, altering its shape. 3. MechanicallyGated Channels – physically deforming a protein channel (such as stretching in the skin) so the membrane can affect the conformation of channel proteins Mediated Transport Systems: Protein channels and diffusion do not account for all movement of molecules. Some molecules, like amino acids and glucose, are too polar to diffuse and too large to pass through ion channels. Certain integral proteins called transporters move these substances through a membrane. The transporter is open on the outside, allowing a substance to bind to it. Once bound, the protein changes shape and opens towards the inside, releasing the substance. 1. Facilitated Diffusion – this is a type of diffusion that facilitated by a transporter protein. Because no ATP is used, facilitated diffusion is incapable of producing a higher concentration on one side. Diabetes occurs when insulin is not available so that there are not enough glucose transport proteins in muscle and adipose tissue. 2. Active Transport – this uses energy to move a substance uphill across a membrane, or against the substances electrochemical gradient. Active transport uses the same binding mechanism as facilitated diffusion, as well as specificity and saturation, but uses energy to do so. a. Primary Active Transport – Hydrolysis of ATP provides energy for primary active transport. One of the best examples is the Na /K ATPase pump. This transporter moves sodium ions outside and potassium ions into the cell. Intracellular ATP and 3 sodium bind to the transporter. ATP is hydrolyzed to ADP, providing a phosphate to cause covalent modulation, changing the protein’s shape to open to the outside. Once opened, 2 extracellular potassium bind to the protein, which dephosphorylates the protein, causing it to open back to the inside. b. Secondary Transport – Instead of using ATP, this transport system uses the + electrochemical gradient for an energy source. Ions (Na ) move down their concentration gradient, coupled to the transport of another molecule, such as glucose or amino acids. Therefore, these proteins have 2 active sites. In addition, Na or other ions always flow into the cell, because of the established gradient set up by primary transport. i. Cotransport – This is when the movement of a molecule goes into the cell, in the same direction as Na + ii. Countertransport – This is when the movement of an molecule goes out of the cell, in the opposite direction of Na + Osmosis: The net diffusion of water across a membrane. The degree to which water concentration is decreased depends on the number, not the chemical properties, of certain substances. Water always flows down the concentration gradient. There are several terms to know regarding osmosis: 1. Osmolarity – The total solute concentration of a solution 2. Osmol – One osmol is equal to 1 mol of solute particles a. For example, 1 M of glucose has a concentration of 1 Osm (1 osmol per liter) while 1 M sodium chloride has a concentration of 2 Osm. b. The higher the osmolarity, the lower the water concentration. Water always flows in the direction of low Osm to high Osm. 3. Aquaporins: a family of membrane channel proteins that form channels through which water can diffuse. Membranes and Osmosis: If two compartments of different solute concentrations are separated by a permeable membrane, then water and solutes will both diffuse down their concentration gradient. However, if the membrane is only permeable to water, then the water/solutes will still reach equilibrium, but there will be a volume differential between the compartments. However, compartments are not infinitely expandable, as in this example. In a selectively permeable membrane, the cells are not infinitely expandable and will cause a pressure increase 1. Osmotic Pressure – The pressure that must be applied to a solution to prevent the net flow of water across the membrane when a solution containing solutes is separated from pure water by a semipermeable membrane. Tonic Solutions: 1. Isotonic – when the water concentrations are the same inside and outside the cell. There is no change in cell size. 2. Hypotonic – extracellular fluid has a nonpenetrating solute concentration lower than found in cells. Therefore, water concentration is higher outside the cell, so water rushes in, swelling the cell. 3. Hypertonic – extracellular fluid has a nonpenetrating solute concentration lower than found in cells. Therefore, water concentration is lower outside the cell, so water rushes out, shrinking the cell. Endocytosis and Exocytosis: 1. Endocytosis – moving material into the cell by meaning of pinching off a small pocket of membrane. There are 3 general types of endocytosis: a. Fluid endocytosis – an endocytotic vesicle encloses a small volume of extracellular fluid. The solutes can be anything. b. Phagocytosis – cells engulf bacteria or large particles such as cell debris from damaged tissues. Extensions of the plasma membrane called pseudopodia fold around the surface of the particle and engulf it. c. Receptormediated Endocytosis – this is when endocytosis specifically takes in certain molecules. The substance binds to a particular protein and then the plasma membrane pinches inward to bring the substance in. i. Clathrin is a protein that forms a cagelike structure that leads to the aggregation of ligandbound receptors into a localized region of the membrane. This forms a depression, called the clathrincoated pit, which then invaginates and pinches off to form a clathrincoated vesicle. 2. Exocytosis – exocytosis pinches off part of the plasma membrane to move substances outside the cell. It serves 2 purposes: a. It provides a way to replace portions of the plasma membrane that endocytosis has removed. b. It provides a route by which membraneimpermeable molecules (such as protein hormones) that the cell synthesizes can be secreted into the extracellular fluid. Epithelial Transport – Epithelial cells line hollow organs or tubes and regulate the absorption or secretion of substances across the surfaces. 1. Apical Membrane – the surface of an epithelial cell that faces the hollow or fluid filled chamber (lumen) 2. Basolateral Membrane – the plasma membrane on the opposite surface, which is adjacent to a network of blood vessels More Epithelial Transport – there are two pathways by which a substance can cross a layer of epithelial cells: 1. Paracellular Pathway – facilitates the diffusion between adjacent cells of the epithelium. These pathways are limited by the presence of tight junctions. 2. Transcellular Pathway – facilitates the movement into an epithelial cell across either the apical or basolateral membrane. a. This pathway utilizes diffusion and mediated transport to move substances. The permeability characteristics of the apical and basolateral membranes are not the same. These membranes contain ion channels and different kinds of transporters for mediated transport. As a result, the epithelial cells can move substances from areas of low concentration to areas of high concentration, such as actively moving substances into the blood from the small intestine. Summary of Types of Transport: 1. Non Protein Facilitation a. Simple Diffusion (Water) b. Endocytosis i. Fluid Endocytosis ii. Phagocytosis iii. ReceptorMediated Endocytosis c. Exocytosis 2. Protein Facilitation a. Mediated Transport i. Requires Energy 1. Primary Active Transport 2. Secondary Active Transport a. Cotransport b. Countertransport ii. No Required Energy 1. Facilitated Diffusion b. NonMediated Trasnport i. Channels 1. Aquaporins 2. Ion Channels a. Ligand Gated b. Voltage Gated c. Mechanically Gated
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