Chapter 04 Lecture Outline Movement Across Cell Membranes

Chapter 04 Lecture Outline* Movement Across Cell Membranes Eric P. Widmaier Boston University Hershel Raff Medical College of Wisconsin Kevin T. Strang University of Wisconsin - Madison *See Power. Point Image Slides for all figures and tables pre-inserted into Power. Point without notes. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 1

Diffusion • The movement of molecules from one location to another as a result of their random thermal motion. 2

Diffusion Equilibrium Fig. 4 -1 3

Diffusion Equilibrium Fig. 4 -2 4

Magnitude and Direction of Diffusion Fig. 4 -3 5

Diffusion Rate vs. Distance • Diffusion times increase in proportion to the square of the distance over which the molecules diffuse. • Diffusion is limited by distance. If a substance has to diffuse a long distance it is very slow and not an effective way to move solutes. 6

Diffusion through Membranes • The rates at which molecules diffuse across membranes, as measured by their permeability coefficients, are a thousand to a million times slower than the diffusion rates of the same molecules through a water layer of equal thickness. • Membranes act as barriers that considerably slow the diffusion of molecules across their surfaces. • The major factor limiting diffusion across a membrane is the hydrophobic interior of its lipid bilayer. 7

Diffusion through Membranes Fig. 4 -4 8

Diffusion through Membranes • Oxygen, carbon dioxide, fatty acids, and steroid hormones are examples of nonpolar molecules that diffuse rapidly through the lipid portions of membranes. • Remember that lipophilic (lipid-loving) substances move through easily. • Polar molecules and hydrophilic (water-loving) do not diffuse readily through the membranes. 9

Diffusion through Ion Channels • Ions such as Na+, K+, Cl–, and Ca 2+ all use specific protein channels to diffuse into and out of cells. • Channels are integral membrane proteins that span the lipid bilayer. • A single protein may have a conformation that looks like a doughnut, with the hole in the middle providing the channel for ion movement. • More often, several proteins aggregate, each forming a subunit of the walls of a channel. • Specificity is determined by pore size of the channel, charge, and binding sites. 10

Diffusion through Ion Channels Fig. 4 -5 11

Membrane Potential • Membrane potential is a separation of electrical charges that exists across plasma membranes. • The membrane potential provides an electrical force that influences the movement of ions across the membrane. • Remember that like charges repel and opposites attract. 12

Fig. 4 -6 13

Regulation of Diffusion through Ion Channels • Channels are regulated to control the movement of ions into and out of a cell. • Types of Gated channels are: – Ligand gated – Voltage gated – Mechanically gated 14

Mediated-Transport Systems • Many molecules (like glucose) are either too large and charged to get into the cell without help. • The protein transporters (also called carriers) bring these molecules into and out of cells by conformation changes. 15

Mediated-Transport Systems Fig. 4 -8 16

Transporters • Transporters are specific for their ligand. • Transporters do not move as many molecules as channels do because of binding and conformational shifts. • Transporters can be saturated. This means that there is a maximum flux of molecules that can be reached. 17

Facilitated diffusion Fig. 4 -9 18

Active Transport • Active transport uses energy to move molecules against the concentration gradient. • These transporters are often called “pumps”. • These pumps can also be saturated and use two types of energy sources: (1) The direct use of ATP in primary active transport (2) The use of an electrochemical gradient across a membrane to drive the process in secondary active transport 19

Active Transport Fig. 4 -10 20

Na+/K+ ATPase Fig. 4 -11 21

Primary Active-Transporters • The Na+/K+-ATPase primary active transporter is found in every cell and helps establish and maintain the membrane potential of the cell. • In addition to the Na+/K+-ATPase transporter, the major primary active-transport proteins found in most cells are: (1) Ca 2+-ATPase (2) H+-ATPase (3) H+/K+-ATPase 22

Secondary Active Transport • Secondary active transport is distinguished from primary active transport by its use of an electrochemical gradient across a plasma membrane as its energy source. • Transporters that mediate secondary active transport have two binding sites, one for an ion (e. g. , Na+)and another for the cotransported molecule (e. g. , Glucose). 23

Secondary Active Transport Fig. 4 -13 24

Secondary Active Transport Remember that Cotransporters (symporters) move molecules in the same direction. Countertransporters (antiporters) move molecules in opposite directions. Fig. 4 -14 25

Membrane Transport Proteins Fig. 4 -15 26

Osmosis • The net diffusion of water across a membrane • Facilitated by channel proteins called aquaporins • Aquaporin expression and insertion into the membrane varies among cell types. These are especially important in the kidney. 27

Osmolarity • The total solute concentration of a solution is known as its osmolarity. • One osmol is equal to 1 mol of solute particles. • So a 1 M solution of glucose has a concentration of 1 Osm (1 osmol per liter), whereas a 1 M solution of sodium chloride contains 2 osmol of solute per liter of solution. • A liter of solution containing 1 mol of glucose and 1 mol of sodium chloride has an osmolarity of 3 Osm. • Although osmolarity refers to the concentration of solute particles, it also determines the water concentration in the solution because the higher the osmolarity, the lower the water concentration. 28

Tonic solutions • Isotonic, hypotonic, and hypertonic solutions: – Isotonic solutions have the same concentration of nonpenetrating solutes as normal extracellular fluid. – Hypotonic solutions have a lower concentration of nonpenetrating solutes as normal extracellular fluid. – Hypertonic solutions have a higher concentration of nonpenetrating solutes as normal extracellular fluid. 29

Extracellular Osmolarity & Cell Volume Fig. 4 -19 30

Endocytosis & Exocytosis Fig. 4 -20 31

Endocytosis • Movement of molecules into the cell via vessicles. • There are three general types of endocytosis that may occur in a cell: 1. Fluid endocytosis (pinocytosis) 2. Phagocytosis 3. Receptor-mediated endocytosis 32

Forms of Endocytosis Fig. 4 -21 33

Exocytosis • Movement of molecules out of the cell via vessicles. • Exocytosis performs several functions for cells: 1. Provides a way to replace portions of the plasma membrane that endocytosis has removed 2. Adds new membrane components to the membrane 3. Provides a route by which membrane-impermeable molecules (such as protein hormones) the cell synthesizes can be secreted into the extracellular fluid 34

Epithelial Transport • Paracellular pathway: diffusion between adjacent cells • Transcellular pathway: movement into a cell, through the cytosol, and exit across the opposite membrane 35

Epithelial Cell terms • One surface of an epithelial cell generally faces a hollow or fluid-filled chamber, and the plasma membrane on this side is referred to as the apical or luminal membrane. • The plasma membrane on the opposite surface, which is usually adjacent to a network of blood vessels, is referred to as the basolateral membrane (also known as the serosal membrane). 36

Paracellular Transport • Diffusion through the paracellular pathway is limited by the presence of tight junctions between adjacent cells. The tight junctions form a seal around the apical end of the epithelial cells. • Although small ions and water can diffuse to some degree through tight junctions, the amount of paracellular diffusion is limited by the tightness of the junctional seal and the relatively small area available for diffusion. • The permeability of the paracellular pathway varies in different types of epithelia, with some being very permeable and others very tight. 37

Transepithelial Transport of Na+ Fig. 4 -22 38

Transepithelial Transport of Organic Solutes Fig. 4 -23 39

Transepithelial Osmosis Fig. 4 -24 40

Clinical Case Study • The patient in this study suffered from severe hyponatremia. • Instead of pure water, what should she have consumed at her rest stops? 41

Clinical Case Study • The patient in this study suffered from severe hyponatremia. • Instead of pure water, what should she have consumed at her rest stops? • Gatorade, Power. Ade, or any of the sports electrolyte solutions that are on the market. • This type of fluid loss is also why doctors tell you that after severe vomiting or diarrhea you should use Pedialyte for kids. 42