Membrane structure and function

Phospholipids

Membranes are made of fat (lipids)

Hydrophilichead

Hydrophobictails

Hydrophilichead

Hydrophobictail

WATER

WATER

Bilayer

Bilayer

Biological membranes are fluid mosaics of lipids and proteins

Hydrophilic regionof protein

Hydrophobic region of protein

Phospholipidbilayer

Fluid Mosaic Model of Singer and Nicolson (1972)

Freeze fracture (electron microscopy)

Evidence for fluid mosaic model

Knife

Cytoplasmic layerExtracellular layer

Cytoplasmic layer

Plasmamembrane

Extracellular layer

Proteins

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Movement of phospholipids

Membrane fluidity

ViscousFluid

Unsaturated hydrocarbontails with kinks

Membrane fluidity

Saturated hydro-carbon tails

Cholesterol

Cholesterol within the animal cell membrane

Fibers ofextracellularmatrix (ECM)

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton

Cholesterol

Integralprotein

Peripheralproteins

CYTOPLASMIC SIDEOF MEMBRANE

EXTRACELLULARSIDE OFMEMBRANE

Glycolipid

Membrane Proteins

Fibers ofextracellularmatrix (ECM)

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton

Cholesterol

Integralprotein

Peripheralproteins

CYTOPLASMIC SIDEOF MEMBRANE

EXTRACELLULARSIDE OFMEMBRANE

Glycolipid

Membrane Proteins

EXTRACELLULARSIDEN-terminus

C-terminusCYTOPLASMICSIDE

Helix

Transmembrane protein

EnzymesSignal

ReceptorATP

Transport Enzymatic activity Signal transduction

Membrane Proteins: functions

Glyco-protein

Cell-cell recognition Intercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)

Membrane Proteins: functions

• Passive transport

• Active transport

• Bulk transport

Moving stuff around

Diffusion

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Diffusion of one solute

Passive Transport

• Passive transport is the diffusion of a substance across a biological membrane

• No energy required

Passive Transport

Lowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can

H2O

Osmosis

Osmosis

Diffusion of water across a membrane

Animalcell

Lysed

H2O H2O H2O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H2O

Shriveled

Getting across the membrane

Rather directly

Exocytosis

Some material can diffuse through membranes on their own

Diffusion

Passive

Some materials diffuse through only with help

Diffusion

Passive

Some materials must be pumped through membranes

Against Diffusion

Active (not passive)

Diffusion Facilitated diffusion

Passive transport

ATP

Active transport