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MEMBRANE ADVANCE LEARNER PAGE Fluidity Permeability Types of lipids Glycolipids Membrane protein Membrane transport Ion pores Active transport by carrier protein Passive transport by carrier protein

Fluidity Membrane is a two dimesional fluid Phenomena that makes the membrane to look like a fluid Lateral diffusion of one layer over the other in the lipid array Flip-flop motion of each array Flexion motion of monomer Rotation of monomer The above phenomena can be explain pictorially as follows

Fluidity depends on the compostion and temperature Shorter the tail length, more the cis double bond and higher the temperture less the fluidity What is the use of fluidity?

Fludity helps in intercellular transport and is essential factor that decides the surivival of the particular organism/cell.

This can be explained by an example

in organism like Bacteria, Yeast is usually forced to survive at different temperatures. So at low temperatures they produce fatty acids with more cis double bonds.

Permeability of the membrane

For the cell activity it

needs various types of components

. These components should get absorbed into the cell through the membrane. At the same time membrane

should not permit the substance that is not necessary

. Permeablity of a membrane is a very important process and hence a membrane has to be very selective and specific in its action.

For example, in eucariotic plasma membrane, Cholestrol is present in large amount (1:1 with phospho lipid molecule). This enhances the permeanblity barrier of the lipid bilayer.

Mobility or fluctuation of tail is decreased leading to less phase transistion

Various types of lipids

Phosphatidycholine

Sphingomyelin

Phosphatidylserine

Phosphotidylethanolanime

Phosphatidylethanolamine Phosphatidylserine Phosphatidylcholine Sphingomyelin

The first two molecules are present in the innersphere of the membrane

The third and the fourth are present in the outer sphere of the membrane

Glycolipids

Present in the extracellular portion of lipid bilayer

Galactocerebroside GM1 ganglioside Sialic acid (NANA)

Glycolipids constitutes 5% of total cell plasma membrane

Gangliosides are very important and complicated

Function may be like

Protecting the cell from harsh conditions like low pH and degradative enzymes

Helping the cell to bind with extra cellular matrix and

Protecting from electrical shock

Membrane Protein

Specific function of membrane depends on the protein present in that membrane

Protein contitutes 25- 75% of the membrane

50 lipid molecule : 1 protein molecule

How the protein is present in the membrane?

Trans membrane proteins extend across bilayer by helices.

It can be further classified into single pass (1) or multipass (2).

Other proteins are attached through a fatty acid chain or a prenyl group (3, 4)

MembraneTransport

So for we have talked about permeability, now its time to see how transport occurs.

Transport of various species across a synthetic membrane can be visualized as follows.

But in real membrane the transport of ions and other molecule occurs and membrane proteins are mostly responsible for that.

Membrane transport is feasible by two types of proteins, namely Carrier proteins and Channel proteins.

The following figure differentiates the two types of proteins

Transport of charged species can occur in two ways

(i) Active transport - against the gradient

(ii) Passive transpot - along the gradient

This can be easily understood by the following pictorial representation

Only carrier proteins perform active transport

Passive transport is performed by channel proteins as well as carrier proteins

Ion Pores

Ion pores are small hydrophobic molecules that dissolve in lipid bilayers and increase their permeability to specific inorganic ions.

eg. Vealinomycin, A23187 and A232187

There are two types are ion pores

(i) Channel former

(i) Mobile carrier

The function of ion pores can be represented by the following picture

Active transport by Carrier protein

Active transport of carrier protein can be defined in three differenent ways

(i) Uniport

(ii)Symport

(iii) Antiport

These can be picturized as follows

Passive transport by carrier protein

Do you want to know more about membranes?

Follow these links

http://www.library.advance.org

http://www.dcn.davis.ca.us/~carl/cellhome.htm

http://www.mblab.gla.ac.uk

http://www.lenti.ned.umn.edu

http://www.auhs.edu/netbiochem/