Lecture 6: Membranes and Transport

rutgers genbio

important features of membrane structure

fluid mosaic model, phospholipids, proteins, carbohydrates

membrane components

primarily phospholipids; carbohydrates; proteins

how does a phospholipid bilayer form?

spontaneously

why does phospholipid bilayer form spontaneously?

amphipathc structure

amphipathic

polar/nonpolar sides

how is phospholipid bilayer held together?

hydrophobic interactions (van der waals)

what do membrane proteins do?

determine many membrane functions

transmembrane

span membrane

can membrane proteins move?

some membrane proteins can move, some cannot

functions of membrane proteins

transport; enzymatic activity; signal transduction; cell-cell recognition; intercellular joining; attachment to the cytoskeleton and extracellular matrix (ECM)

carbohydrates

polysaccharides attached to protein or lipid

glycoprotein

polysaccharide attached to protein

glycolipid

polysaccharide attached to lipid w

function of carbohydrates in membrane components

cell identification (blood types A, B, O) f

fluid mosaic model

membrane components can move laterally within one layer of the membrane (lipids, proteins, carbs); plasma membrane moves dynamically, changing constantly, moving; not rigid, fluid; different parts come together, freely moving structure

passive transport

does not use metabolic energy (ATP); moves with the gradient like a bike downhill

active transport

does use metabolic energy (ATP);moves against the gradient like bike uphill; low concentration to high concentration

parts of passive transport

simple diffusion, osmosis, facilitated diffusion

diffusion

tendency for molecules of a substance to fill available space; small gases (O₂, CO₂, N₂); small nonpolar molecules (including hydrocarbons); small polar uncharged molecules (including H₂O)

dynamic equilibrium

no net movement at equilibrium; different substances diffuse independently

osmosis

diffusion of H₂O across selectively permeable membrane

“salt sucks”

water diffuses from lower → higher [solute] or from higher → lower [H₂O]
(high salt concentration draws water out of cells)

tonicity

ability of a solution to cause a cell to gain or lose H₂O

isotonic solution

[solute] outside cell = [solute] inside cell

(no net H₂O movement)

hypertonic solution

[solute] outside cell > [solute] inside cell

hypotonic solution

[solute] outside cell < [solute] inside cell

what cannot diffuse directly across a membrane?

large molecules (just too big); not-small polar molecules (hydrophilic e.g. glucose); ions (charged—even H⁺)

facilitated diffusion

large molecules or ions (H⁺, Ca⁺², Na⁺)
transport proteins
channel proteins → (ie: ion channel)
carrier proteins →
specific to what each protein is transporting

how is active transport facilitated? 

proteins (carriers or pumps) or bulk transport of molecules

sodium potassium pump

3 Na⁺ out, 2 K⁺ in; establishes electrical gradient

exocytosis

vesicle fuses with plasma membrane to release contents from cell → primary mechanism for growing plasma membrane; waste, proteins, and secretory products “out”

endocytosis

material taken into cell by forming vesicles derived from plasma membrane

phagocytosis

cell engulfs large particle (non-specific); “cellular eating”

pinocytosis

ingestion of fluid and dissolved material (non-specific); “cellular drinking”