CELL MEMBRANES AND CELL MEMBRANES AND TRANSPORTTRANSPORT

The fluid mosaic model of The fluid mosaic model of membrane structuremembrane structure

ALBIO9700/2006JK

Phospholipids Phospholipids

• Form a single layer with heads in water Form a single layer with heads in water and tails projecting out of the waterand tails projecting out of the water

• Can form stable structures called Can form stable structures called micellesmicelles if shaken up with water if shaken up with water

• Two-layered structures can from in Two-layered structures can from in sheets (sheets (bilayersbilayers))

ALBIO9700/2006JK

ALBIO9700/2006JK

ALBIO9700/2006JK

Structure of membranesStructure of membranes• The phospholipid bilayer is visible using the The phospholipid bilayer is visible using the

electron microscope (> x100 000)electron microscope (> x100 000)• 7nm wide7nm wide• Also contains proteinsAlso contains proteins• Model for the structure is called Model for the structure is called fluid mosaic fluid mosaic

modelmodel• ‘‘fluid’ – the individual phospholipid and protein fluid’ – the individual phospholipid and protein

molecules move around within their layermolecules move around within their layer• ‘‘mosaic’ – describes the pattern produced by mosaic’ – describes the pattern produced by

the scattered protein molecules when the the scattered protein molecules when the surface of the membrane is viewed from abovesurface of the membrane is viewed from above

ALBIO9700/2006JK

ALBIO9700/2006JK

Features of the fluid mosaic Features of the fluid mosaic modelmodel

• Membrane is bilayer of phospholipid molecules that Membrane is bilayer of phospholipid molecules that move about by diffusion within their own monolayermove about by diffusion within their own monolayer

• Phospholipid tails point inwards and form a non-Phospholipid tails point inwards and form a non-polar hydrophobic interior. Heads face the aqueous polar hydrophobic interior. Heads face the aqueous medium surrounding membranemedium surrounding membrane

• Some phospholipid tails are saturated and some Some phospholipid tails are saturated and some unsaturated. The more unsaturated, the more fluid unsaturated. The more unsaturated, the more fluid the membranethe membrane

• Most protein molecules float in phospholipid layersMost protein molecules float in phospholipid layers• Some proteins are embedded in the outer layer, Some proteins are embedded in the outer layer,

some in the inner layer and some span the whole some in the inner layer and some span the whole membranemembrane

• Thickness = 7nmThickness = 7nm• Many proteins and lipids have short , branching Many proteins and lipids have short , branching

carbohydrate chains attached to the external carbohydrate chains attached to the external surface, forming surface, forming glycoproteinsglycoproteins and and glycolipidsglycolipids

• presence of cholesterolpresence of cholesterolALBIO9700/2006JK

Features of the fluid mosaic Features of the fluid mosaic modelmodel

ALBIO9700/2006JK

ALBIO9700/2006JK

Roles of components of cell Roles of components of cell membranesmembranes

• PhospholipidsPhospholipids– Form bilayer (basic structure of membrane)Form bilayer (basic structure of membrane)– Act as a barrier to most water-soluble substancesAct as a barrier to most water-soluble substances

• CholesterolCholesterol– Have hydrophilic heads and hydrophobic tailsHave hydrophilic heads and hydrophobic tails– Regulates the fluidity of the membrane, Regulates the fluidity of the membrane,

preventing it from becoming too fluid or too rigidpreventing it from becoming too fluid or too rigid– Important for the mechanical stability of Important for the mechanical stability of

membranesmembranes– Hydrophobic regions help to prevent ions or polar Hydrophobic regions help to prevent ions or polar

molecules from passing through the membrane molecules from passing through the membrane (myelin sheath around nerve cells – leakage of (myelin sheath around nerve cells – leakage of ions would slow down nerve impulses)ions would slow down nerve impulses)

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• ProteinsProteins– Transport proteinsTransport proteins: provide hydrophilic : provide hydrophilic

channels or passageways for ions and polar channels or passageways for ions and polar molecules to pass through the membranemolecules to pass through the membrane

– EnzymesEnzymes: catalyse the hydrolysis of molecules : catalyse the hydrolysis of molecules such disaccharides (small intestines)such disaccharides (small intestines)

– Protein in membranes of organellesProtein in membranes of organelles (mitochondria and chloroplasts): involved in the (mitochondria and chloroplasts): involved in the processes of respiration and photosynthesis processes of respiration and photosynthesis

• GlycolipidsGlycolipids and and glycoproteinglycoprotein– Protein and lipid molecules with short Protein and lipid molecules with short

carbohydrate chainscarbohydrate chains– Form hydrogen bonds with the water molecules Form hydrogen bonds with the water molecules

and so help to stabilise the membrane structureand so help to stabilise the membrane structure– Act as Act as receptor moleculesreceptor molecules, binding with , binding with

hormoneshormones or or neurotransmitters neurotransmitters (chemicals (chemicals that enable nerve impulses to pass from one that enable nerve impulses to pass from one nerve cell to another)nerve cell to another)

ALBIO9700/2006JK

ALBIO9700/2006JK