Membrane organization & function

Lecture 20Ch 11 pp365 - 386

Fantastic Facts• Every cell on Earth has a membrane around it!• No membrane = no cells = no life as we know it!• Two layers of lipids = lipid bilayer• Just 50 atoms thick = which is 5nm• Prevents entry and exit of most molecules, except

under controlled conditions - nutrients have to enter & waste has to leave

• Has gates to permit certain other molecules to enter; receptors to detect the environment

• Can grow and shrink without effect• Piercing of this membrane does not cause it to

pop, or tear. It simply reseals!

• Bacteria has just one membrane - plasma membrane (cell membrane)

• Eukaryotes have many internal membranes too - these offer each organelle the ability to concentrate certain chemicals.

• ALL natural membranes are made of lipids and proteins.

Fantastic Facts

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The Lipid Bilayer• Each membrane lipid molecule has two

very different properties - a hydrophilic head, and one or two hydrophobic tails

• Most abundant lipids = phospholipids - the head is lined to the rest of the molecule through a phosphate group.

• PHOSPHATIDYLCHOLINE is the most abundant of all = choline, plus phosphate, plus two long hydrocarbon chains…

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Amphipathic nature

• A molecule which has two faces - a water loving side and awater hating side.

• The water hating parts force water molecules to form cages…

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The lipid bilayer

• The lowest energy structure which is formed by amphipathic molecules….

• …. a lipid bilayer

• …Or a sphere with water inside and outside.

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No Edges

• Any tears are quickly repaired by either– The exclusion of the water molecules– The formation of small vesicles– NO FREE edges are permitted by the free

energy of the system– Therefore, the only way a large collection

of these membranes can exist without edges is as sacs - exactly what the plasma membrane is!

No flip-floppers please

• A single lipid molecule will rarely flip from one surface of the molecule to the other - no flip-flopping

• However, the lipid molecule is free to move within its own layer relatively freely

• Spin at RT about 30,000 rpm around themselves!

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Saturated fats!

• The properties of different membranes are dictated by their composition…– The length of the hydrocarbon tail (14 to 24 carbon

atoms - the shorter the more fluid)– The number of double bonds (fewer = saturation) -

the more saturation the more rigid– Hydrogenated margarines– Cholesterol = more rigid membrane = bad– THE FLUID NATURE OF THE MEMBRANE IS

RELATED TO ITS FUNCTION

‘Two faced’

• If you look at the membrane from above, it will have a different composition of both fats and lipids then when viewed from the bottom. It is said to be asymmetrical

• FLIPPASES - permit the manufacture of lipid membranes….

11_17_asymmetic.dist.jpgDifferent composition on different faces

Where is membrane made?

In place or,

Outside the cell or,

Inside the cell????

ANSWER:

It is made as part of the endoplasmic reticulum (ER)

system and then moved to the cell surface

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REMEMBER ME PLEASE!

Membrane Proteins

• Most functions of the membrane are performed by membrane proteins

• Up to 50% of the mass of the membrane in animal cells can be protein

• 50:1 ratio of lipids to proteins

• P’s have many functions…

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Remember these 4 categories of membrane P’s

(TARE the membrane)

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Integral membrane proteins

Peripheral membrane proteins

How P’s are associated with the membrane - remember

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Transmembrane proteins are alpha-helixes

How do P’ cross the membrane? Via a helical domain

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However, larger structures may involve more complex arrangements too. Here we have a cylinder of 5 helical domains which form a central channel.

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Beta-barrels are formed to make even largee pores, because the sheets cannot be bent too drastically. E.g. Porin proteins of bacteria.

Plasma membrane is flimsy

• The membrane itself is not that strong• Nature uses proteins to strengthen it.• These structures composed of a meshwork of

fibrous proteins are known as CELL CORTEX• These proteins are complexed to the plasma

membrane via other proteins• Great example is the SPECTRIN in the red

blood cells of humans…

11_30_blood.cells.EM.jpgThe complex shape of these cells is critical to their function.

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If one were to look from the inside of the cell towards the cell membrane, one would see a complex scaffold of proteins everywhere inside the cytoplasm just below this membrane…

The reinforced concrete model = invented by Nature before manThe reinforced concrete model = invented by Nature before man

Cells’ surface is coated with ‘PAINT’

• The outer surface lipids of many plasma membranes is complexed with sugars - GLYCOLIPIDS = most cells are sugar coated!

• The same is true of the proteins of the outer membrane– Some of these are short sugars - oligosaccharides, they form

glycoproteins– Others are longer and form proteoglycans

• These ALL constitute the CARBOHYDRATE LAYER (Sugar layer)– Serves a Protective layer – confers functional properties

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Very important functions

Permits:

• Cell-cell communications

• Cell-cell recognition

• Cell behaviour

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White blood cell rolling brakes…

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Proteins can ‘roam’ around plasma membranes too, just like lipids

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Proteins are generally free to roam around. Except in circumstances where they are anchored within the cell

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TIGHT JUNCTIONS prevent the migration of proteins to other regions of the cell - more about these later.