Fluid mosaic model: cell membranes article

It may seem like the human body is made up of a chaotic mix of random parts, but that’s not the case. The liquid nutrients, cell machinery, and blueprint information that make up the human body are tucked away inside individual cells, surrounded by a double layer of lipids.
The purpose of the cell membrane is to hold the different components of the cell together and to protect it from the environment outside the cell. The cell membrane also regulates what enters and exits the cell so that it doesn’t lose too many nutrients, or take in too many ions. It also does a pretty good job of keeping harmful things out.
A drawing showing a part of a cell membrane magnified to see the molecules that it is comprised of.

What’s it made up of?

The cell membrane is primarily made up of three things: 1. Phospholipids 2. Cholesterol 3. Proteins
A drawing showing the three main cell membrane components and how they are arranged in a cell membrane.

1) Phospholipids

There are two important parts of a phospholipid: the head and the two tails. The head is a phosphate molecule that is attracted to water (hydrophilic). The two tails are made up of fatty acids (chains of carbon atoms) that aren’t compatible with, or repel, water (hydrophobic). The cell membrane is exposed to water mixed with electrolytes and other materials on the outside and the inside of the cell. When cellular membranes form, phospholipids assemble into two layers because of these hydrophilic and hydrophobic properties. The phosphate heads in each layer face the aqueous or watery environment on either side, and the tails hide away from the water between the layers of heads, because they are hydrophobic. Biologists call this neat assembling characteristic “self-assembly”.
A drawing showing the structure of a phospholipid.

2) Cholesterol

Cholesterol is a type of steroid which is helpful in regulating molecules entering and exiting the cell. We’ll talk about this in more depth later, but for now remember it’s part of the cell membrane.

3) Proteins

The cell is made up of two different types, or “classes”, of proteins. Integral proteins are nestled into the phospholipid bilayer and stick out on either end. Integral proteins are helpful for transporting larger molecules, like glucose, across the cell membrane. They have regions, called “polar” and “nonpolar” regions, that correspond with the polarity of the phospholipid bilayer.
Polar and nonpolar refer to the concentration of electrons on a molecule. Polar means the electrons are not evenly distributed, making one side of the molecule more positively charged or negatively charged than another side. Nonpolar means the electrons are evenly distributed, so the molecule is evenly charged across the surface.
The other class of protein is called peripheral proteins, which don’t extend across the membrane. They can be attached to the ends of integral proteins, or not, and help with transport or communication.
A drawing showing the various proteins that are part of the cell membrane. It also shows where cholesterol is present within the cell membrane.

What makes the cell membrane fluid?

The fluid mosaic model of the cell membrane is how scientists describe what the cell membrane looks and functions like, because it is made up of a bunch of different molecules that are distributed across the membrane. If you were to look at a cell membrane using a microscope, you would see a pattern of different types of molecules put together, also known as a mosaic. These molecules are constantly moving in two dimensions, in a fluid fashion, similar to icebergs floating in the ocean. The movement of the mosaic of molecules makes it impossible to form a completely impenetrable barrier.
There are 3 main factors that influence cell membrane fluidity:
  1. Temperature: The temperature will affect how the phospholipids move and how close together they are found. When it’s cold they are found closer together and when it’s hot they move farther apart.
Drawing showing the influence of cholesterol at varying temperatures on a cell membrane.
  1. Cholesterol: The cholesterol molecules are randomly distributed across the phospholipid bilayer, helping the bilayer stay fluid in different environmental conditions. The cholesterol holds the phospholipids together so that they don’t separate too far, letting unwanted substances in, or compact too tightly, restricting movement across the membrane. Without cholesterol, the phospholipids in your cells will start to get closer together when exposed to cold, making it more difficult for small molecules, like gases to squeeze in between the phospholipids like they normally do. Without cholesterol, the phospholipids start to separate from each other, leaving large gaps.
  2. Saturated and unsaturated fatty acids: Fatty acids are what make up the phospholipid tails. Saturated fatty acids are chains of carbon atoms that have single bonds between them. This makes them straight and easy to pack tightly. Unsaturated fats are chains of carbon atoms that have some double bonds between them. Double bonds create kinks in the chain, making them not as easy to pack tightly. There are two possible kinks that can occur:
  1. Cis-unsaturated fats, where both sides of the chain remain on the same side
  2. Trans-unsaturated fats, where the sides of the chain are opposite from each other
The kinked shape of cis-unsaturated fats make it more difficult to pack tightly.
Drawing showing saturated fatty acids are easier to stack compared to unsaturated fatty acids, which are difficult to stack because of the kinks in their carbon chains.
This plays a role in membrane fluidity, because the kinks increase the space in between the phospholipids, making them harder to freeze at lower temperatures. Also, some molecules, like CO2_2 and O2_2, require small spaces between the phospholipids so that they can cross the membrane quickly and easily.

What can go through the cell membrane?

Phospholipids are attracted to each other, but they are also constantly in motion and bounce around a little off of each other. The spaces created by the membrane’s fluidity are incredibly small, so it is still an effective barrier. For this reason, and the ability of proteins to help with transport across the membrane, cell membranes are called semi-permeable.
There are 5 broad categories of molecules found in the cellular environment. Some of these molecules can cross the membrane and some of them need the help of other molecules or processes. One way of distinguishing between these categories of molecules is based on how they react with water. Molecules that are hydrophilic (water loving) are capable of forming bonds with water and other hydrophilic molecules. They are called polar molecules. The opposite can be said for molecules that are hydrophobic (water fearing), they are called nonpolar molecules. Here are the 5 types:
  1. Small, nonpolar molecules (ex: oxygen and carbon dioxide) can pass through the lipid bilayer and do so by squeezing through the phospholipid bilayers. They don't need proteins for transport and can diffuse across quickly.
  2. Small, polar molecules (ex: water): This is a little more difficult than the molecule type above. Recall that the interior of the phospholipid bilayer is made up of the hydrophobic tails. It won’t be easy for the water molecules to cross, but they can cross without the help of proteins. This is a somewhat slower process.
  3. Large, nonpolar molecules (ex: carbon rings): These rings can pass through but it is also slow process.
  4. Large, polar molecules (ex: simple sugar - glucose) and ions: The charge of an ion, and the size and charge of large polar molecules, makes it too difficult to pass through the nonpolar region of the phospholipid membrane without help.

Consider the following:

What happens when there is a problem with the cell membrane’s ability to uptake/export important molecules or communicate? There are many diseases associated with problems in the ability of the phospholipid bilayer to perform these functions. One of these is Alzheimer’s disease, characterized by brain shrinkage and memory loss. One idea explaining why Alzheimer’s disease occurs is the forming of plaque sticking to the phospholipid bilayer of the brain neurons. These plaques block communication between the brain neurons, eventually leading to neuron death and in turn causing the symptoms of Alzheimer’s, such as poor short-term memory.
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