The Plasma Membrane and Membrane Potential Chapter 3

Objectives Know the composition of the plasma membrane Understand the functions of the plasma membrane Explain how the various forms of membrane transport work Know the functions of the sodium-potassium pumps Know what a membrane potential is and how it is established

The Plasma Membrane Surrounds all living cells Composed of a phospholipid bilayer – Polar hydrophilic heads on the outside, nonpolar hydrophobic tails on the inside – Trilaminar appearance

Phospholipid Bilayer

Composition of Plasma Membrane Trilaminar structure Composition includes – Phospholipids – Proteins – Cholesterol – Carbohydrates Fluid mosaic model – Proteins and cholesterol embedded

Bilayer Function Provides structure and fluidity to the membrane Prevents hydrophilic substances from crossing the membrane

Membrane Protein Function Transmembrane (integral) proteins – Channels Leak or gated – Carrier (transport) Selectively transport substances across membrane – Docking-marker acceptors or receptors – Membrane-bound enzymes – Cell adhesion molecules (CAMs) Caherins Integrins

Membrane Carbohydrate Function Function as “self” markers Allow cells to identify themselves as belonging to you Allows cells to identify cells of the same type Used during tissue formation to ensure that the same type of cells are being used – Also ensure that tissues do not overlap

Cell Adhesion Plasma membrane involved in cell adhesion – Three ways: CAMs, ECM, and specialized junctions Extra cellular matrix – The “glue” that holds the cells together – Network of fibrous proteins embedded in gel-like fluid Collagen, elastin, fibronectin – Secreted by fibroblasts – Cellular regulation and protection Specilaized junctions – Desmosomes – Tight junctions – Gap junctions vivo.colostate.edu

Cell Adhesions Tight junctions Gap junctions bio.davidson.edu academic.brooklyn.cuny.edu

Membrane Transport Selective permeability – Plasma membrane controls what enter and exits the cell – Determined by two properties Size Solubility in lipids – Can be unassisted or assisted

Diffusion Diffusion (simple diffusion) – Net movement from an area of higher concentration to an area of lower concentration – Does not require energy, passive process Fick’s law of diffusion – Effects of factors that influence the rate of diffusion Magnitude of concentration gradient Surface area of membrane Lipid solubility of substance Molecular weight Distance across membrane

Osmosis – Diffusion of water across a selectively permeable membrane Osmotic pressure, osmolarity (milliosmoles/L) 300 mOsm normal in body fluids Hydrostatic pressure – Aquaporins – protein channels that allow water the diffuse in and out of cell – Tonicity refers to the effect the solution will have on cell volume Hypertonic – Water out of cell, cell shrinks Hypotonic – Water into cell, cell swells Isotonic – Water movement is at equilibrium, cell retains its normal shape

Osmosis

Effects of Tonicity lrn.org

Assisted Membrane Transport Two types – Carrier mediated Transport of small hydrophilic molecules Require a conformational change of as transport protein Depends on: specificity, saturation (Tm), competition – Vesicular transport Transport of large molecules or many molecules at a time Formation of vesicles needed

Facilitated Diffusion and Active Transport Facilitated diffusion – Similar to simple diffusion, but requires a carrier – Glucose and amino acids Active transport – Moves substances against their concentration gradient Requires energy Primary and secondary active transport

Sodium Potassium Pump

Secondary Active Transport Depends on primary active transport Symports – SGLT Antiports

Bulk Passage Endocytosis – Phagocytosis – Pinocytosis – Receptor-mediated Exocytosis

Membrane Potential Separation of opposite charges across the plasma membrane – Occur in thin areas adjacent to the membrane Electrical potential measured in mV Na+, K+, A- responsible for maintaining resting membrane potential faculty.irsc.edu

Membrane Potential K+ more concentrated in the ICF – If K+ diffuses out, the ICF becomes more negative – K+ attracted by negative charge, moves into cell – Equilibrium is reach, membrane potential will equal -90mV Na+ more concentrated in the ECF – Diffuses into the cell – Inside becomes more positive – Equilibrium potential of Na equals 60mv Must consider both at the same time – Resting membrane potential typically -70mV – K+ has more influence because membrane more permeable to it