All cells are completely surrounded by a membrane that separates them from their environment. Kind of like how your skin completely covers your body. This cell membrane is sometimes called the plasma membrane, and it acts as a barrier, regulating what gets in and out of the cell. Because the membrane lets some things through, but not others, we say that the membrane is semi-permeable, or selectively permeable. If we zoom in on the membrane, we can see that it is made of several molecular parts.
Most of the membrane is made of molecules called phospholipids. Phospholipids have a head made of phosphate and two tails made of fatty acids. The heads are hydrophilic, meaning that they are attracted to water, or literally, water-loving.
The tails, however, are hydrophobic, meaning that they repel water. Literally, hydrophobic means water-fearing. Because of these properties, phospholipids form two layers known as a phospholipid bilayer, with the hydrophobic tails facing each other and the hydrophilic heads facing the watery external environment and the watery cytoplasm inside of the cell. So the water lovers get to be next to water and the water fearers can hide away from it. A perfect arrangement.
The phospholipid bilayer is the main component of the membrane and it is largely responsible for making the membrane semi-permeable. Basically, the smaller and less charged something is, the easier it is to pass through the phospholipid bilayer. Bigger, more charged things have a harder time. A second membrane component is cholesterol. Although people often think of cholesterol as a bad thing, it's actually a very important part of cell membranes, where it helps to stabilize the membrane and make it better at controlling what can pass through it.
Proteins are another important part of the membrane. They do many functions, but one of the most important ones is transporting materials across the membrane, especially if they have a hard time passing through the phospholipid bilayer. A fourth membrane component is carbohydrates. These are often involved in cell identification. Blood types, for example, are the result of membrane carbohydrates attached to proteins.
These help you to identify blood that belongs to you and distinguish it from foreign invaders. All of these components work together to make the plasma membrane that surrounds all cells, a critically important barrier needed for all life on Earth. If you enjoyed this video, please like it, subscribe, and check out the other free games, quizzes, and interactive learning experiences at Bioman Biology.