Cell Membrane : Structure and Function



Cell Membrane : Structure and Function

 

Cell Membrane Structure:

- Early Theory: After chemically analyzing cell membranes, scientists discovered that the two main ingredients in the membrane were – Proteins and Phospholipids.

They then proposed that the Proteins were sandwiched between two layers of phospholipid molecules. (as illustrated below)

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- Problem: Since the phospholipid tails are hydrophobic (hate water) this theory did not explain for how water (hydrophilic) could travel freely through the membrane.

 

"Fluid Mosaic Model":

- Accepted Theory: There is a double layer of phospholipids but the proteins are scattered through the membrane (mosaic). These embedded proteins float (raft around) among the semi-fluid phospholipids (fluid). Many of these embedded proteins (seen in brown below) span from one side of the membrane straight through to the other.

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- Water travels through special protein pores called Aquaporins.

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The proteins have polar and non-polar regions from the amino acid R-Groups, which accounts for their placement among the phospholipid bilayer. R-groups that are hydrophobic point toward the fatty acid chains while R-groups that are hydrophilic point toward the inner channel where water flows through.

- Some of the proteins and phospholipids have carbohydrate chains attached to them.

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- Protein + carbohydrate chain = Glycoprotein

- Phospholipid + Carbohydrate chain = Glycolipid

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- These carbohydrate chains function as cell identification markers. Forming somewhat of a fingerprint on the outside of the cell. A body will attack cells with the wrong

fingerprint (like a foreign cell, cancer cell, or infected cell)

Example: Organ rejection after a transplant.

 

Cell Membrane Function:

 

- Impermeable – nothing passes through

- Permeable – most things pass through

- Semi-permeable – smaller molecules pass through but not larger ones. Like a Strainer/Sieve

 

- The cell membrane is described as being SELECTIVELY PERMEABLE (differentially permeable)-- only certain desirable molecules can pass through, size is not the determining factor. The membrane has mechanisms to allow for the transport of various types of desirable molecules.

 

Example : Protein molecules are too large to fit through the various protein pores, but they can be transported across the membrane with vesicle formation.

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