Video: Cell membrane

Why did castles have walls? Well, for protection, obviously. But that was not their only function. Gates controlled what merchants brought in and out of the city. Messages were often exchanged at the city walls, and flags on the wall identified if the ruler was a friend or a foe. Wondering what this has to do with physiology?

Let's talk about the cell membrane.

The cell membrane, also called the plasma membrane, is a thin, flexible structure that surrounds the cell and defines its boundaries. It separates the cytosol within the cell from the extracellular fluid outside the cell.

The structure of cell membranes is described by the fluid mosaic model. Mosaic because cell membranes look like a sea of lipids, dotted with proteins and carbohydrates. Fluid because most of the cell membrane molecules are not fixed in their location, but instead shift continuously between different places. So they can be where the cell needs them when it needs them.

Typically, proteins make up approximately 50 percent of the weight of the cell membrane. Lipids consist about 45 percent, and carbohydrates represent less than 5 percent. However, these proportions change significantly based on the functions of the cell.

Lipids, proteins, and carbohydrates have different roles within the cell membrane. Let's look at their structure and function one by one, starting with lipids.

Phospholipids are the most common lipids found in the cell membrane. These molecules have a polar phosphate group as the head and two chains of non-polar fatty acids as tails. The head of the phospholipids is hydrophilic, meaning that it is attracted to water. On the other hand, the fatty acid tails are hydrophobic, so they repel water.

Since they have both water-loving and water-repelling regions, phospholipids are considered amphipathic molecules. This feature of the phospholipids can help us understand how the cell membrane works.

The phospholipid heads, being hydrophilic, orients themselves to point towards water. Cells are immersed in the extracellular fluid, so the outside of the cell membrane is made up entirely of phospholipid heads. But cells also have water in their intracellular fluid, so there is water on both sides of the membrane. Because of this, the phospholipids organize themselves into two adjacent layers, forming a phospholipid bilayer.

Here, the heads point outwards towards the intracellular or extracellular fluid to be as close as possible to their beloved water. While the hydrophobic tails huddle inside the core of the cell membrane to complain about how much they despise water.

The inner hydrophobic layer is also what makes the cell membrane a selectively permeable barrier. This means that some molecules can cross the cell membrane freely, while others can't.

Substances that can freely pass through the cell membrane include small, non-polar molecules like oxygen and carbon dioxide; small polar molecules like water, which are small enough to squeeze between the phospholipids; and other hydrophobic molecules like steroid hormones. However, highly charged substances like ions, or large molecules like sugars and proteins, cannot freely cross the cell membrane.

Another important lipid found in the cell membrane is cholesterol. This molecule is lodged between phospholipids to regulate how fluid and flexible the cell membrane is. This is important to maintain optimal physical characteristics across a range of conditions; for instance, when temperature changes.

These were the main functions of lipids. Next, let's move on to discuss proteins.

Proteins of the cell membrane can be classified into integral and peripheral. Integral proteins typically span the entire cell membrane, so they are in contact with both the intracellular and extracellular fluid. In contrast, peripheral proteins attach to integral proteins, or to phospholipids, either inside or outside the cell membrane.

Remember when we said that the cell membrane blocks some substances? These substances can be transported across the membrane by integral proteins like channels or carriers. This is how the cell strictly controls the concentration of ions in its cytosol, and when ions enter or exit from the cell.

Since ions have an electrical charge, controlling how they flow in and out of the cell helps cells regulate the difference in electrical charge across their membrane. This is very important for nerve function, for example.

Integral proteins can also function as receptors. Upon binding with specific chemicals called ligands, receptors initiate intracellular pathways that activate specific cellular processes. Receptor proteins are used to receive chemical messengers. It's one of the ways cells communicate.

Other types of proteins can be integral or peripheral. These include enzymes, which split up or combine molecules to speed up metabolic processes; structural proteins, which anchor the cell membrane to the cytoskeleton to maintain the shape of the cell; and adhesion proteins, which hold the cells of the tissue together and limit the passage of substances between them. Some sources include adhesion proteins in the structural proteins group.

All right, now that we've learned about proteins, let's move on to our final group -- carbohydrates.

Carbohydrates attach to proteins, forming glycoproteins, and to lipids, forming glycolipids. The carbohydrates stick out from the cell and form a coating called a glycocalyx, which helps with many of the functions already discussed. Its main function, however, is to help cells recognize each other. This is important for immune responses where our immune system needs to identify the cells of invading organisms.

Before we sign off, let's quickly recap the main functions of the cell membrane: isolation of the cytosol from the extracellular fluid, transport of substances inside and outside of the cell, communication between cells, catalysis of chemical reactions, adhesion to neighboring cells, structural support, and cell identification.

Going back to our wall, can you see now how cell membranes make our cells work like tiny castles? Just don't expect them to use pigeons to send each other messages.

Continue to explore the physiology of the cell membrane with our quizzes and specialized visuals. Let's learn together!