Armed with the knowledge of lipids, as well as carbohydrates and proteins, we can now explore the structure of plasma membranes, specifically in the context of the fluid-mosaic model. Phospholipids have a hydrophilic (water loving) head, and hydrophobic (water repelling) tails. This results in the formation of a phospholipid bilayer (double layer), which forms the basis for the plasma membrane.
The name of fluid-mosaic model comes from:
Fluid = the arrangement of proteins contained in the membrane is always changing
Mosaic = the proteins present are spread around in a mosaic-like fashion.
It’s pretty isn’t it? The proteins are crucial to cell communication as well as the selective permeability of the membrane. The glycoprotein (sugars/carbohydrates attached to a protein) side chains act as receptors. The layer of glycoproteins and polysaccharides on the surface of plasma membrane is termed the glycocalyx.
Lipid soluble stuff such as vitamins A, D and K, as well as oxygen and carbon dioxide, can pass freely though the membrane. Cholesterol can be part of the membrane to restrict the movement of other components.
The main properties of molecules that determine how they may be transported across a membrane are solubility, size and charge.
Large molecules can’t cross the membrane, charged molecules also can’t, and naturally, lipid-repelling (or water-attracting) molecules can’t. Conversely, small molecules can cross the membrane barrier, alongside molecules with no charge (nonionised) as well as lipophilic (hydrophobic) molecules.
It’s important to understand the role of microvilli. These are elongations of plasma membrane which increase the surface area available for reaction or absorption.
Additional functions of the plasma membrane, enabled by its various components, include transport via channels such as aquaporin, reaction sites via enzymes and presenting antigens (unique chemical groups that identify the cell, key to communication and immune function).
Aquaporins allow the passage of water molecules in a single file. They prevent the passage of other small ions dissolved in water because they have a positive charge in the middle of the channel that repels them.