What’s so Special about the Cell Membrane?
Topics Covered: cell membrane, phospholipid, selective permeability, cholesterol, protein, glycoprotein, cell-to-cell recognition
Every cell, whether it’s eukaryotic or prokaryotic, has its own cell membrane. Cell membrane, also known as the plasma membrane, is so important for every organism in this world. So, what’s so special about the cell membrane? In this post, we will cover the structure and functions of the plasma membrane that makes it so essential for life.
The eukaryotic plasma membrane consists of phospholipid bilayer. A phospholipid is a type of lipid that is amphipathic, which means it has both hydrophilic (as its name implies, it means water loving) and hydrophobic (water fearing) parts. The polar, hydrophilic heads are exposed to water and the nonpolar, hydrophobic tails are facing away from the water, thus creating a bilayer of phospholipids. This bilayer contributes to the selective permeability of the plasma membrane; that is, the cell membrane only allows certain molecules to pass through the membrane!
The plasma membrane is also known as the fluid mosaic model. It is called fluid because the molecules are constantly changing position fluidly, and it is called mosaic because it is composed of different kinds of molecules, including proteins, carbohydrates, and cholesterol!
Let’s first talk about proteins in plasma membranes. Integral proteins are proteins that are embedded through the phospholipid bilayer, while peripheral proteins are proteins that are loosely bound to the surface (aka “peripheral” region) of the membrane. Now, these proteins play important roles in the cell membrane, such as transport (allowing charged or big molecules to pass through membrane), enzymatic activity, and signal transduction (sending signals)
Cholesterols are embedded in the interior of the phospholipid bilayer and help with stabilizing the membrane. When the temperature changes, cholesterols help membranes to resist changes in fluidity. At high temperatures, cholesterol makes the membrane less fluid, and at low temperatures, cholesterol makes the membrane more fluid (retain its fluidity).
Lastly, carbohydrates bond to other macromolecules and are attached on the external surface of the plasma membrane. When they covalently bond to lipids, they become glycolipids, and when they covalently bond to proteins, they become glycoproteins. And these attached carbohydrates serve as “identification flags” that allow cells to distinguish one type of cell from another, a cool ability that is known as the cell-to-cell recognition!
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