Lipid Bilayer: Definition, Structure & Function

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  • 0:00 Plasma Membrane and…
  • 0:28 Phospholipids
  • 1:07 The Need for Lipid Bilayer
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: John Williams
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.

Lipid Bilayer
lipid bilayer

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