Ch 7 Membrane Structure and Function

- Boundary between intracellular compartments, living cells, and abiotic environment

– Selectively permeable– Some molecules cross membranes more readily than others

Cellular membrane-overall functions

Predominant constituent: phospholipids• Amphipathic molecules: hydrophobic AND hydrophilic

Membrane Structure

Dispersed protein components

Membrane organization and properties described by:

Fluid Mosaic Model

Singer and Nicolson 1972

LE 7-2

Hydrophilichead

Hydrophobictail

WATER

WATER

Organization of membrane phospholipids

Mosaic: something made of small pieces

LE 7-3

Hydrophilic regionof protein

Hydrophobic region of protein

Phospholipidbilayer

Mosaic: Proteins dispersed among phospholipids in membrane:

• Freeze-fracture studies of the plasma membrane

• Frozen membrane split along the middle of the phospholipid bilayer using a knife

• Imaged by EM

Supports mosaic part of model

LE 7-4

Knife

Cytoplasmic layerExtracellular layer

Cytoplasmic layer

Plasmamembrane

Extracellular layer

Proteins

The Fluidity of Membranes

• Phospholipids move laterally within the bilayer

• Some membrane proteins also drift laterally

• Rarely does a phospholipid flip-flop transversely across the membrane

LE 7-5a

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Movement of phospholipids

• Cool temp: membranes switch from fluid to more solid state

• Solidification depends on type of lipid

• What property of lipids would favor liquid versus solid state?

Effects of Temperature on membranes

LE 7-5b

ViscousFluid

Unsaturated hydrocarbontails with kinks

Membrane fluidity

Saturated hydro-carbon tails

Degree of saturation of fatty acid tails

• Tends to moderate effects of temp. on membrane state

• At warm temperatures (such as 37°C), restrains movement of phospholipids

• At cool temperatures, maintains fluidity by preventing tight packing

Steroid cholesterolalso component of membranes

LE 7-5c

Cholesterol

Cholesterol within the animal cell membrane

Can drift within the bilayer– Proteins much larger than lipids--> move more slowly

• Cell fusion studies support fluidity of membrane proteins

Movement of membrane proteins

LE 7-6

Membrane proteins

Mixedproteinsafter1 hourHybrid cell

Human cell

Mouse cell

Membrane Proteins and Their Functions

• Proteins determine most of the membrane’s specific functions

• Peripheral membrane proteins – not embedded– attached to extracellular or cytoplasmic

surface

• Integral membrane proteins– penetrate the hydrophobic core of bilayer– often span the membrane

LE 7-7

Fibers ofextracellularmatrix (ECM)

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton

Cholesterol

Integralprotein

Peripheralproteins

CYTOPLASMIC SIDEOF MEMBRANE

EXTRACELLULARSIDE OFMEMBRANE

Glycolipid