membrane structure and function

factors that affect fluidity of membranes

• saturation of phospholipid hydrocarbon tails: inc. saturation decreases fluidity as the tails are packed closer together (unsaturated = kinks)

• cholesterol = temperature buffer; stabilizes membrane at temps of 37; hinders solidification at low temps by wedging itself between phospholipid tails and decreases fluidity at high temps by pulling phospholipid tails closer together

integral vs. peripheral proteins (both membrane proteins)

integral proteins = embedded in cell membrane; penetrate hydrophobic core bc of nonpolar amino acids; many transmembrane proteins which span across the lipid bilayer

peripheral proteins = loosely bound to surface of membrane, usually bound to integral proteins

six major functions of membrane proteins

• transport

• enzymatic activity

• signal transduction

• cell-cell recognition

• intercellular joining

• attachment to cytoskeleton and extracellular matrix

role of membrane carbohydrates in cell to cell recognition

glycoproteins and glycolipids (membrane carbohydrates) on the cell membrane identify cells

selective permeability

nonpolar molecules diffuse easily in the fatty acid core

polar/ionic do not diffuse easily because the inside of the bilyaer is made of hydrophobic fatty acid tails

channel vs carrier protein

channel: forms hydrophilic pore through a cell’s phospholipid bilayer down concentration gradient; only transport water soluble molecules, polar molecules, ions

carrier: bind to specific molecules → change shape to shuttle molecules across; down and against concentration gradient; transport both water soluble and insoluble molecules

passive transport

• from high to low concentration, does not require ATP

diffusion: movement of particles from an area of high concentration to low concentration; passive bc no atp required - molecules use their own kinetic energy ; spontaneous bc of random thermal motion

facilitated diffusion: passive transport aided by proteins such as channel proteins (aquaporins ) and carrier proteins; usually carrying polar or ionic molecules

osmosis: diffusion of water across a semipermeable membrane down a concentration gradient

active transport

• uses energy to move solutes against their gradients

cell maintains internal concentration of solutes different from surroundings (ex. salty banana)

two forces that combine to produce an electrochemical gradient

ions concentration and attraction of ion to opposite charge

bulk transport (how large molecules are transported across the cell membrane): endocytosis and exocytosis

• endocytosis = cell membrane folds inward to engulf large molecules in a membrane bound vesicle; requires energy

• exocytosis = transport vesicle containing large molecules travels to the plasma membrane and fuses w/ outer membrane → opens and releases contents out; requires energy

phagocytosis vs pinocytosis

• both are forms of endocytosis

• phagocytosis: takes in large solid matter; special immune cells engulf through pseudopodia and the membrane forms a food vacuole

• pinocytosis: cell ‘drinking’, takes in solutes by any cell

how electrogenic pump creates voltage across a membrane

actively transports ion against their concentration gradients

for every cycle, the pump moves a diff number of positive charges out of the cell than it brings in

there is a net negative charge inside the cell and net positive charge outside the cell which establishes electric potential aka voltage

ex. sodium potassium pump: 3 Na⁺ out, 2 K⁺ in → net negative interior, positive exterior

ex. proton pump: found in plants, fungi, and bacteria'; H⁺ out of the cell → net negative interior, positive exterior

cotransport

• active transport driven by concentration gradient

• a membrane protein couples the movement of one substance moving down electrochemical gradient to drive another substance against its concentration gradient

• ex. proton pump → H⁺ out of the cell; sucrose-H⁺ cotransporter: allows H⁺ to move down its concentration gradient if it brings sucrose along with it (against concentration gradient)

receptor-mediated endocytosis

• ligands bind to receptor proteins, coated pit is formed which then forms a coated vesicle, the vesicle fuses w/ endosome and the cargo is delivered toward lysosomes (transports ligands)

• ex. clears ldl (‘bad’ cholesterol) from the bloodstream using ldl receptors

how does a concentration gradient represent potential energy

drives diffusion - as a result of diffusion particles will move spontaneously from one place to another = work

how does the fluid mosaic model of membrane structure explain why

• actual membranes adhere more strongly to water than do artificial membranes composed only of phospholipids

• membrane proteins aren’t very water-soluble

• freeze-fracture electron microscopy shows protein particles interspersed in a smooth matrix

• actual membranes contain cholesterol and proteins

• membrane proteins have both polar and nonpolar sections in order to puncture all the way through a membrane

• cholesterol in membrane prevents it from freezing which allows proteins to move around

how do hydrophobic molecules cross cell membrane

can cross cell membrane easily due to the cell membrane’s hydrophobic interior

how do aquaporins facilitate the passage of water

aquaporins allow entry of up to 3 billion water molecules per second; if there were no aqauporins only a fraction of that amount would diffuse

• funnel water into a cell faster than the cell membrane alone would permit water to move by preventing ions from entering the water channels and lining up the water molecules