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The Plasma Membrane and the Fluid Mosaic Model

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Let's take a look at a cell. One thing that you will find common among all cells is the cell membrane - whether it's an animal cell, a plant cell or a bacterial cell. While this layer may only be ten nanometers thick, it has an intricate molecular structure designed for efficient transport of material into and out of the cell. This property is known as selective permeability, the control of the passage of materials across the cell membrane.

The cell membrane is designed in a way that substances having certain properties are unable to enter or leave the cell (this movement across the cell boundary is known as transport). The fluid mosaic model describes how substances, mainly cholesterol, phospholipids and proteins, slide freely in the membrane.

Image result for fluid mosaic model

First, let's start with the phospholipid bilayer. A phospholipid a complex lipid with a "head" and a "tail". The head is made of one polar/hydrophilic phosphate group and a glycerol molecule. The tail is made two nonpolar/hydrophobic fatty acid chains. A fatty acid is a long chain of hydrocarbons (one carbon atom bonded to two hydrogen atoms) with a carboxyl group at one end. Glycerol is an alcohol with three carbon atoms.

Image result for phospholipid

The phospholipids of the cell membrane are arranged in a bilayer (picture below). The hydrophilic (water-loving) head faces outward, where it has contact with the aqueous solution both inside and outside the cell. The hydrophobic (water-fearing) tail easily interacts with nonpolar molecules, but it interacts poorly with water. Because of this, the fatty acid tails are tucked away in the interior of the membrane to avoid any interactions with water. The chemical nature of the phospholipid bilayer only allows lipid-soluble molecules and some small molecules to freely pass across the membrane; ions and large polar molecules cannot.

Image result for phospholipid bilayer

                                                                                                                    ________
Image Credit:
(1) “Cell Membrane Detailed Diagram.” Wikimedia Commons, 31 Jan. 2007, commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg.

 (2)  OpenStax, CNX. “Phospholipid.” Wikimedia Commons, 27 May 2016, commons.wikimedia.org/wiki/File:Figure_05_01_02.jpg.

 (3) OpenStax. “Phospholipid Bilayer.” Wikimedia Commons, 18 May 2016, commons.wikimedia.org/wiki/File:0302_Phospholipid_Bilayer.jpg.
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