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## MCAT

### Unit 6: Lesson 1

Cell membrane overview

# Fluid mosaic model: cell membranes article

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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 zoom in on the cell membrane, 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 only single bonds between them. As a result, the chains are straight and easy to pack tightly. Unsaturated fats are chains of carbon atoms that have double bonds between some of the carbons. The double bonds create kinks in the chains, making it harder for the chains 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.
These kinks play a role in membrane fluidity because they increase the space between the phospholipids, making the molecules harder to freeze at lower temperatures. In addition, the increased space allows certain small molecules, such as COstart subscript, 2, end subscript and Ostart subscript, 2, end subscript, to 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 (e.g. oxygen and carbon dioxide): These molecules 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 (e.g. water): These molecules can also pass through the lipid bilayer without the help of proteins, but they do so with a little more difficulty than the molecule type above. Recall that the interior of the phospholipid bilayer is made up of the hydrophobic tails. So, it's not easy for water molecules to cross, and it is a somewhat slower process.
3. Large, nonpolar molecules (e.g. carbon rings): These rings can pass through but it is also a slow process.
4. Large, polar molecules (e.g. simple sugar - glucose): The size and charge of large polar molecules make it too difficult to pass through the nonpolar region of the phospholipid membrane without help from transport proteins.
5. Ions (e.g. start text, N, a, end text, start superscript, plus, end superscript): Similarly, the charge of an ion makes it too difficult to pass through the nonpolar region of the phospholipid membrane without help from transport proteins.

## 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.

## Want to join the conversation?

• The article asks what makes a cell membrane fluid and then talks about three points that influence the fluidity of the cell, one of them being cholesterol. Towards the end of the paragraph it says that without cholesterol, the phospholipids get closer together, then a sentence or two later it says that without cholesterol phospholipids get farther apart. Is this a typo, or will phospholipids do both depending on the environment and condition they are in?
• The key is temperature. Without cholesterol, the phospholipids will get closer together in a cold environment. The cholesterol acts as a kind of spacer to prevent them from getting too close. Conversely, in hot temperature, the phopholipids spread too far apart without cholesterol. The phospholipids want to be near the cholesterol molecules, causing them to be closer together.
• are all membranes made of phospholipid bilayers
• Mostly yes, but some archaea that live at high temperatures us a monolayer, basically the lipid tails go through the whole membrane and are attached to head groups on either side. If you google it, some diagrams will make it clear.
Maybe other exceptions also exist, but that is the only one that comes to mind.
• How does phospholipid movement (flipping, flopping, scrambling) in the bilayer contribute to the survival of a cell?
• This is actually a super cool question, never thought of it. I can think of several possible answers:
1) Signaling: though these videos don't mention it much, there are many different kinds of phospholipids and they can be useful for signaling and telling other cells what type of cell this is. e.g. some phospholipid types are only on the outside membrane. One type of phospholipid, phosphatidylserine, is usually present more on the outside than inside. We know from observation that if it's present in equal numbers on the outer and inner leaflets, the cell is dying. So there might be some signaling going on.
2) To remove potentially dangerous molecules. Sometimes atoms or molecules generate radicals which are highly reactive. Maybe switching a radical from the outer to inner leaflet allows for safe removal or modification of the radical by the cell. It's better to contain a danger inside a cell so the cell can just die instead of losing the cell (in cases that the cell can be regenerated like in skin).
3) To increase the size of the membrane. Maybe phospholipids are produced inside the cell and they need to be flipped to the outer membrane to increase its size. The opposite could also be true, moving phospholipids into the inner leaflet to decrease the overall size of the outer membrane.
• How does nonpolar molecules move across the membrane ?
Because in order to move through it they first encounter the hydrophillic part ( or lypphobic ) part of the plasma membrane, they need to first cross this barrier then only they can move.
• There are two principal methods discussed in the videos. 1) Random movement. sometimes phospholipids acquire enough energy to just move. This depends entirely on factors like temperature, whether there's cholesterol nearby, and whether the phospholipid has saturated or unsaturated tails. 2) Proteins can either actively or passively catalyze movement of a phospholipid across the membrane. Active proteins use ATP to catalyze the amount of energy that's required to move a molecule through a space it doesn't want to cross. Passive proteins use ion gradients to power the same switch.
(1 vote)
• Do trans fatty acids tend to pack tightly together at room temperature, or are they more liquid (like unsaturated fatty acids) at room temperature?
• ​According to medicalexpress.com, trans fats are what we call "semi-solid" at room temperature. That just means that it is pretty viscous (it flows slowly).
• Can someone tell me the nitty gritty bits of the role cholesterol has on the membrane?
• Cholesterol is important because it controls the fluidity of the membrane.

At low temperatures: due to the limited movement, the membrane would become less fluid without cholesterol. However, because cholesterol is there, it does not. Why? I like to think of it as cholesterol "gets in the way" of the phospholipids packing together, since it is hydrogen bonded to them.

At high temperatures: Without cholesterol, the membrane could become too fluid. However, cholesterol raisings its melting point, thus preventing this.

Cholesterol has these properties because it is a special type of lipid known as a steroid.Therefore, it has a rigid 4-ring structure. These rigid rings interact with the phospholipids around them and limit their movement due to their rigidity, thus preventing the membrane from undergoing any exxtreme changes in fluidity.

Hope this helps! :)
• Do cell membrane apply force to hold the organelles
(1 vote)
• Organelles are held in place by the cytoskeleton, and the cytoskeleton pushes out on the cell membrane, holding it open in a globular shape.
• The article says the cell membrane has 2 types of proteins but it's missing the lipid bound protein (in-between the two phospholipid leaflets) mentioned in the "cell membrane proteins" video.

The article says there are 5 types of molecules but only 4 are listed.

The article says "The kinked shape of cis-unsaturated fats make it more difficult to pack tightly." What about trans-unsaturated fats?