The lipid bilayer is important for maintaining the shape of a cell and for selective permeability. Therefore, it is vital for the survival and function of the cell. This article explains what the lipid bilayer is and how its structure contributes to cell viability.
Plasma Membrane and Lipid Bilayer
Cells are outlined by a plasma membrane. The plasma membrane is responsible for protecting the interior of the cell and for selective permeability, which means it monitors what materials enter and exit the cell. The ability of the plasma membrane to determine which materials can move in and out of a cell is due to the molecules found in the membrane and the presence of a lipid bilayer. Let’s discuss the properties of the lipid bilayer.
Phospholipids are the primary molecules found in the plasma membrane. A phospholipid is unique in that it has a hydrophilic region called the head, and a hydrophobic region known as the tail. The hydrophilic region can interact with water due to the presence of a phosphate group which is polar, like water. If something is hydrophilic, that means it likes water. Conversely, the hydrophobic region of the molecule does not interact with water because it is not polar. If something is hydrophobic, that means it does not like water. A molecule that has both partial hydrophilicity and partial hydrophobicity is classified as amphipathic.
The Need for a Lipid Bilayer
The interior of the cell is primarily made of water. Likewise, the exterior of the cell is usually surrounded by watery fluid. This means that the plasma membrane could not possibly consist of just one layer of phospholipids.
This is because the hydrophobic (or water fearing) tail region would have to interact with one of the watery regions inside or outside of the cell. So instead, the cells have evolved to have two layers of phospholipids.The bilayer creates a ‘sandwich’ style arrangement, where the hydrophilic heads of each layer face the watery environment inside and outside of the cell. This means that the hydrophobic tails are confined to the middle, creating a hydrophobic region between the two layers of heads. This allows for the plasma membrane to be stable in this dual watery environment.
So how does this sandwich-like bilayer contribute to selective permeability? Polar molecules, which are hydrophilic, are not able to pass through the hydrophobic interior of the phospholipid bilayer.
This means that they either don’t enter the cell or they have to be transported into the cell by using alternative means such as protein channels. Protein channels are proteins that provide a passage through the lipid bilayer, and in the case of polar molecules, allow them to transport across the hydrophobic interior. Materials, such as sugars and ions, enter the cell this way.On the other hand, non-polar molecules (which are hydrophobic) are able to move freely through this hydrophobic region of the membrane and therefore do not require protein channels. These molecules diffuse or move from areas of high concentration to low concentration–in and out of the cell.
As non-polar molecules cross the lipid bilayer, the polar phospholipid heads offer little resistance, allowing free diffusion. Oxygen and carbon dioxide are two gases that move through the membrane in this manner.
The lipid bilayer is arranged in two layers of phospholipids with the hydrophilic heads forming the outer edges and the tails forming the interior. In this arrangement, the bilayer has a hydrophobic core that prevents the passage of polar molecules while allowing the relatively free diffusion of non-polar molecules. This is important because it allows the bilayer to select which molecules it will allow into and out of the cell.
Key Points of the Lesson
|The Lipid Bilayer||Function||Structure|
|Control the movement of molecules into and out of the cell||Two layers, like a sandwich, of phospholipids with a hydrophobic core|
After completing this lesson, you should be comfortable with these tasks:
- Describe the lipid bilayer
- Explain how the lipid bilayer functions to protect the cell