Cell Membrane, Biology

7/30/2019 Cell Membrane, Biology

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Plasma Membrane

or thePlasmalemma.

Year 12


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Places found :-

Around animal cells and plant cells.

Vesicles for transport of substances in / outof cells =Endo/Exocytosis.

Endoplasmic Reticulum (ER)


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Functions

To control the movement ofsubstances into and out of cells

To keep the cells contents together andseparate from other cells

To allow communication betweencells


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Structure.

A phospholipid bilayer about 7-10nm thick.

Under the light microscope it appears as a thin

line and until the advent of the electron

microscope the structure could not be studied.


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Phospholipids

A special type of lipid

2 fatty acid chainsmaking a hydrophobic

tail. Chains have no charge

and are non-polar /insoluble.

Phosphate group ishydrophilic = has acharge so is soluble.


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History of

Packing :-

1930-40 Danielli and Davson studiedtriglyceride lipid bi-layers over water surfaces.

Found they arranged themselves with polar headsfacing outwards = forming droplets

However, if they add proteins they formed sheetsover water as this decreased the surfacetension.

NB. The polar pore.


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1950 Robertson Model

Did not see the spaces for pores on electronmicrographs.

He said that the rail road track was due to the

binding of Osmium tetroxide to proteins andpolar groups on the surfaces of the membrane.


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1966 Lenard and Singer

Noted over 30% of proteins were twistedinto an alpha helix (secondary structure)

More likely the proteins were spherical than

flat. Major turning point :- Freeze Fracturing

technique

Clearly showed bumps in membrane asproteins.


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Proteins are embedded in the bilayer.

Transmembrane proteins pass through the bilayer Others may be inserted at the cytoplasmic or

exterior face = Extrinsic / Intrinsic


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Singer and Nicholson 1972

Aided by electron microscope Singer and

Nicholson proposed the fluid mosaic model

Fluid = moving ; mosaic = made of pieces A bilipid layer with proteins + Cholesterol

molecules between the phospholipids and

polysaccharides attached to the proteins or

lipids and constantly moving.


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The Fluid Mosaic Model


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Cholesterol

Cholestrol helps to keep the lipid layer free to

move.


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Membrane proteins

Create pores through which water and water

soluble chemicals can pass

Act as carriers in active transport

Form receptor sites for hormones

Are important in cell recognition


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Glycoproteins and Glycolipids.

All proteins with sugar side chains are

involved in cell cell signalling and self

recognition.

This is important in the immune system

response and recognising bacterial invasion

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Cell Membrane, Biology