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