Chapter 5B:Membrane Structure
& Function
2. Diffusion
1. Membrane Structure
3. Membrane Transport
1. Membrane Structure
Plasma MembraneAll cells are enclosed by a plasma membrane
• a selectively permeable barrier to “outside”Prokaryotic cell
Membrane
Eukaryotic cell
cholesterol
What are Membranes made of?
Biological membranes consist mainly of phospholipids, with some cholesterol (in animal cell membranes)and a variety of membrane proteins…
Hydrophilic head
Hydrophobic tails
Symbol
phosphategroup
Phospholipids form a Lipid Bilayer
Polar headsface “out”
Hydrophobictails face “in”
• all biological membranes are a lipid bilayer
cholesterol
phospholipidmicrofilamentsof cytoskeleton
(extracellularenvironment)
protein
Phospholipid Bilayer with
someCholesterol and Proteins
The Lipid Bilayer is Fluid
Phospholipids, cholesterol, membrane proteins can move freely in the bilayer
• consistency of the bilayer is like a viscous oil
• degree of “fluidity” depends on:
1) temperature2) types of “fatty tails” (saturated vs unsaturated)3) amount of cholesterol
2. Diffusion
Diffusion
Diffusion is the spontaneous movement of a substance from higher to lower concentration
• molecules dissolved in liquid move randomly• over time the net effect is equal dispersion
of the molecules (provided there is no barrier)
• aka “moving down concentration gradient”
Osmosis is the Diffusion of Water
• water undergoesa net flow from high to lowconcentration
• has powerfuleffects if a barrier is semi-permeable• large moleculescan’t diffuse,so water keepsdiffusing in
lowerconcentration
of solute
higherconcentration
of solute
equalconcentration
of solute
H2Osolutemolecule
selectivelypermeablemembrane
watermolecule
solute molecule withcluster of water molecules
net flow of water
Isotonic solution
(B) lysed (C) shriveled
(D) flaccid (E) turgid (F) shriveled
Hypertonic solutionHypotonic solution
Plantcell
Animalcell
(A) normalplasma
membrane
(plasmolyzed)
Osmosis can cause cells to swell, shrivel
Water will diffuse to where it is less concentrated!
3. Membrane Transport
“Small-scale” TransportCells accomplish membrane transport on a“small scale” (molecule by molecule) in 3basic ways:1) passive transport (simple diffusion)
• diffusion directly through the membrane bilayer
2) facilitated diffusion• diffusion with the help of specific membrane proteins
3) active transport• movement from low to high concentration• requires special membrane proteins and energy
Passive Transport across a Membrane• small, uncharged molecules can diffuse acrossa lipid bilayer (membrane) without “help”
O2
CO2
H2O
e.g.
Facilitated Diffusion via Protein Channels
• each channel isspecific forparticularion or molecule
• creates apore thatallows only1 type of molecule topass
• allow the diffusion of small polar or charged molecules
proteinchannel
smallmolecule
Specific transport proteins bind & transfer specificmolecules from high to low concentration
• transport proteins change shape upon binding of themolecule and release it on other side of membrane
Facilitated Diffusion with the help ofTransport Proteins
Active Transport
Substances can be moved from low to highconcentration across membranes via specific protein pumps (requires a source of energy such as ATP)
“Large-scale” TransportCells accomplish membrane transport on a“large scale” (in bulk) in 2 basic ways:1) exocytosis
• release of material packaged in membrane vesicles tothe outside of a cell
2) endocytosis• ingestion of large objects or large amounts of material
by enclosing within a membrane vesicle:
• PINOCYTOSIS• PHAGOCYTOSIS• RECEPTOR-MEDIATED ENDOCYTOSIS
Exocytosis
A general process for “releasing” materialfrom a cell (e.g., release of neurotransmitters).
Fluid outside cell
Cytoplasm
Protein
Vesicle
Pinocytosis
Plasmamembrane
Vesicle
Plasma membrane
• ingestion of fluid = PINOCYTOSIS (“cell drinking”)
PinocytosisCapture of extracellular fluid in vesicles.
Phagocytosis
EXTRACELLULARFLUID
Pseudopodium
CYTOPLASM
Foodvacuole
“Food” orother particle
Foodbeingingested
Phagocytosis (“cell eating”)
• how many single-celled organisms feed (e.g., amoeba)• how cells of the immune system destroy invaders
Capture of large extracellular particles in vesicles.
Coatedvesicle
Coatedpit
Specificmolecule
Receptor-mediated endocytosis
Coat proteinReceptor
Coatedpit
Material boundto receptor proteins
Plasma membrane
Receptor-mediated EndocytosisCapture of very specific substances in vesicles.
• receptors bind specific substances, enclosed in vesicles
Key Terms for Chapter 5B
• isotonic, hypertonic, hypotonic
• vesicle, pinocytosis, phagocytosis
• facilitated diffusion, active transport
• exocytosis, endocytosis, receptor-mediated end.
• plasma membrane, lipid bilayer
• diffusion, osmosis
• passive transport (simple diffusion)
• transport proteins, protein channels, pumps