- Identify the structure and function of phospholipids.
- Describe the fluid-mosaic structure of the cell membrane.
The cell membrane protects the cell and regulates the movement of materials in and out of the cell, bringing in nutrients and disposing of wastes while preventing harmful materials from entering and keeping needed materials inside.
The cell membrane is a bilayer, a structure consisting of two layers of molecules. This bilayer is composed of phospholipids which are lipids with the usual glycerol backbone but instead of three fatty acids have two fatty acids and a phosphate and nitrogen group. The phosphate chain forms a "head" while the two fatty acids form a "tail".
The electric charge in the molecule is unevenly distributed making the head polar and the tail non-polar. The phosphorous polar head is attracted to polar water molecules and the non-polar hydrocarbon tail is repelled from water molecules. Through a combination of attraction and repulsion, phospholipids tend to form spherical bilayers with the tails facing each other and the heads forming the inner and outer surface of the sphere. This structure is called a phospholipid bilayer.
The Fluid-Mosaic Membrane ModelEdit
The cell membrane contains a mosaic of different components scattered throughout it, such as protein molecules. The phospholipid molecules and some of the proteins can drift sideways in the bilayer giving the membrane a fluid consistency. This description of the cell membrane is called the fluid-mosaic membrane model.
Animal cell membranes contain cholesterol, a lipid which allows the membrane to function in a wide range of temperatures. At high temperatures it helps maintain rigidity in the oily membrane bilayer. At low temperatures it keeps the membrane fluid, flexible, and functional, preventing cell death from a frozen membrane. Cholesterol also makes the membrane less permeable to most biological molecules.
The shapes of the membrane proteins vary according to their function, and each type of cell has a characteristic arrangement of proteins in its membrane. For example, a human red blood cell includes 50 different protein types arranged in a pattern only other cells from humans with the same blood type will "recognize".