Cell Membrane

Topic Description

The cell’s boundaries are determined by a defining structure known as the cell membrane. This organelle’s structure is actually made up of two layers of molecules known as phospholipids. It is the behavior of these molecules when surrounded by water that creates the membrane itself. In order for things to enter the cell or exit the cell, they must first pass through the cell membrane. A closer look at the cell membrane reveals that it is made of billions of phospholipid molecules. You will notice that the cell membrane is not a rigid structure, but rather a fluid, moving, structure made out of a double layer of special molecules called phospholipids.(Phospholipid bilayer) Each phospholipid is an amphipathic molecule composed of a hydrophilic (water-loving) phosphate head, and a hydrophobic pair of fatty acid tails (water-hating). It is this behavior of the bipolar phospholipids in water that cause the bilayer to form. To avoid contact with water, the hydrophobic lipid tails of phospholipids will face the lipid tails of other phospholipids forming a bilayer. This arrangement not only eliminates the hydrophobic tails’ contact with water, but it also results in the hydrophilic phosphate heads facing outward, putting them in contact with water on each side of the membrane. In order to “seal” the lipid tails away from the water, this bilayer must completely close to create a circle, and a three Dimensional sphere. This now creates an internal environment (inside the cell) and an external environment (outside the cell). In addition, the cell membrane contains glycolipids and sterols. One important sterol is cholesterol, which regulates the fluidity of the cell membrane in animal cells. When there is less Cholesterol, membranes become more fluid, but also more permeable to molecules.