Cell Membrane Structure

Study Notes

The cell membrane, also called the plasma membrane, acts as a barrier separating the cell's interior from its external environment.
It's primarily composed of a phospholipid bilayer with embedded proteins and carbohydrates.
The ratio of lipids to proteins in the cell membrane is roughly 1:1, with 50% being lipids and 50% proteins.
However, the number of lipid molecules is significantly higher than protein molecules (50 times more).

The fundamental building block of the cell membrane is the phospholipid, which is an amphipathic molecule.
It has both hydrophobic (water-fearing, nonpolar) and hydrophilic (water-loving, polar) regions.
The hydrophilic head contains a phosphate group, while the hydrophobic tails are composed of fatty acids.
The membrane lipids organize into a continuous bilayer, with the hydrophobic tails shielded from the aqueous environment.
The hydrophilic heads face the watery environment inside and outside the cell.

There are three major classes of membrane proteins based on their relationship with the lipid bilayer:
- Integral proteins: These proteins are embedded within the lipid bilayer and protrude from both the extracellular and cytoplasmic sides of the cell membrane.
- Peripheral proteins: These proteins are associated with the surface of the cell membrane, either on the cytoplasmic or extracellular side. They do not penetrate the hydrophobic core of the membrane and are generally more loosely attached.
- Carbohydrates: These are located on the outer surface of the cell and are bound to either proteins (forming glycoproteins) or lipids (forming glycolipids).
- They can consist of 2 to 60 monosaccharide units and can be either straight or branched.

This model describes the cell membrane as a dynamic structure where various molecules (phospholipids, cholesterol, and proteins) are constantly moving.
This fluidity is crucial for maintaining the membrane's function as a barrier between the cell's interior and exterior.
The fluidity of a cell membrane is influenced by the membrane's lipid composition, the density of integral proteins, and temperature.

The structure of the fatty acid tails of phospholipids significantly influences membrane fluidity.
- Saturated fatty acids have no double bonds and are relatively straight.
- Unsaturated fatty acids contain one or more double bonds, leading to bends or kinks in their structure.
Long chains of saturated fatty acids interact strongly with each other, making the cell membrane stiffer.
In contrast, more unsaturated fatty acids within the lipid tails create less dense packing, increasing membrane fluidity.
At cooler temperatures, the straight tails of saturated fatty acids pack tightly, resulting in a denser and more rigid cell membrane.