What is the Phospholipid Bilayer?
You’ve already learned about the cellular membrane found in all cells, which separates the environment outside the cell from the cytoplasm inside the cell. If you were to zoom in on the structure of the cellular membrane, you would see something called the “phospholipid bilayer”.
As you can probably guess, the phospholipid bilayer is composed of molecules called phospholipids. The structure of a phospholipid looks something like a microscopic tadpole with two tails. The “head” of the tadpole is polar, and so attracted to water (or “hydrophilic”), while the two “tails” of the tadpole are nonpolar, and so repelled by water (or “hydrophobic”).
Since there is water both inside and outside a cell, the phospholipids arrange themselves into two rows with the “heads” facing outwards (towards either the cytoplasm or the exterior of the cell) and the “tails” facing inwards towards each other. This is why the membrane is called a phospholipid bilayer, because it is made up of two layers of phospholipids.
As you already know, the cellular membrane plays a vital role in determining what substances are able to enter or exit a cell. The phospholipid bilayer is what gives the cellular membrane the ability to do this.
Small molecules like oxygen and carbon dioxide can move through the phospholipid bilayer with ease, but the phospholipids are so tightly packed that large molecules cannot pass through on their own. In order for large molecules to move through the cellular membrane they must pass through protein channels embedded in the phospholipid bilayer that work as gates. Some of these gates only allow specific substances through them, while others are less selective but require energy to open (this is called “active transport”).
In addition to giving a cell structure and regulating entry and exit through the membrane, the phospholipid bilayer also plays an important role in cell recognition. Structures on the outer surface of the cellular membrane called “antigens” are what allow cells to recognize that they belong to the same organism. You can think of antigens as nametags that tell cells they are on the same team so they don’t attack each other.
The phospholipid bilayer also plays a role in protecting the cell, by keeping out unwanted materials and invaders.
BioNinja (artist). (2016). Phospholipid Structure Med [digital art]. Retrieved from https://ib.bioninja.com.au/standard-level/topic-1-cell-biology/13-membrane-structure/phospholipid-bilayer.html