2.4: Plasma Membranes

The most likely first component of cells to exist would have been the cell membrane. The membrane was just a thin layer surrounding an empty cell interior. However, that ancient cell membrane is almost indistinguishable from those of the cells in our bodies. Topic 2.4 is about the cell membrane and all the unique bits and bobs that make it work.

Vocab List

• Fluid mosaic model
• Phospholipid
• Bilayer
• Membrane proteins
  • Integral
  • Peripheral
• Glycoproteins
• Glycolipids
• Cholesterol

Written Explanation

Fluid Mosaic Model:

The fluid mosaic model describes the structure of the cell membrane as a fluid (moving/shifting) mosaic (arrangement of components). The components are primarily phospholipids, proteins, and carbohydrates.

Phospholipids:

Phospholipids are two tailed lipids with a phosphate group attached to their head. For the most part, you don't need to know their chemical structure, but you do need to know that the (phosphate) head is polar/hydrophilic, and the two fatty acid tails are nonpolar/hydrophobic. These properties are what cause cell membranes to form and stay together.

Essentially, the hydrophilic part of the phospholipid will face towards water, and the hydrophobic part will be pushed around. Eventually, phospholipids will find one of a few stable forms. Take note of which parts of the membrane are polar/hydrophilic and nonpolar/hydrophobic.

As seen in the diagram above, liposomes form a protective layer around an inner cavity, which is the reason they are used in all known cells. This layer is actually called a bilayer because it is formed of two layers of phospholipids (specifically, a phospholipid bilayer). The fatty acid tails of the phospholipid can also be either saturated or unsaturated, and the more unsaturated fatty acid tails there are in the bilayer, the more space and fluidity there will be within the fluid mosaic model. This is due to the bending of the fatty acid as a result of the double bond formed between two of the carbons.

Membrane proteins:

The cell membrane isn't just phospholipids though. It also contains several different components that make the membrane more useful. Firstly, integral proteins are proteins that span across the entire membrane and can be used as channels for molecules or signal receivers. Peripheral proteins are proteins that are only on one side of the membrane, and can help communicate signals within and between cells.

Other membrane components:

Additionally, the cell membrane also contains glycoproteins and glycolipids. Glycoproteins are proteins with a polysaccharide attached to their end. The protein part of a glycoprotein sits in the membrane, and the polysaccharide sticks out and is used for cell to cell communication. Glycolipids are very similar to glycoproteins, they just have a lipid in the membrane instead of a protein.

Lastly, cholesterol is a steroid (type of lipid) which sits inside the plasma membrane and regulates the rigidity of the membrane. At high (hotter) temperatures, the cell membrane is more flexible, and in that case cholesterol makes it more rigid. At low (colder) temperatures, the cell membrane is more rigid, and in that case cholesterol makes it more flexible.

Note: the "heads" on the cholesterol represent a hydroxyl group (-OH), not a phosphate group like in phospholipids