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This lesson looks at the details of the cell membrane. It focuses on the structure of the membrane and cholesterol’s role in maintaining a healthy, strong cell membrane.

Cholesterol in the Cell Membrane

When you sleep in a cold room, you might have a couple of layers of blankets that you roll up in to keep warm. The blankets help protect your body from the cold; this is similar to the role the cell membrane plays when it ‘rolls up’ around a cell.

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Your body is made up of trillions of cells, which are small units working together to create organisms – like you! If you travel inside the cell, you’ll find it’s filled with fluid called plasma and many small parts that are vulnerable to the outside world. The cell membrane, also called the plasma membrane, surrounds the cell and protects what is inside from the outside environment. This lesson looks at the cell membrane in detail and focuses on cholesterol, which is one of the components found in the cell membrane.The cell membrane is described as a fluid mosaic. This is because the structure of the membrane is flexible and fluid, and also made up of a variety of molecules.

Four main molecules make up the mosaic structure of the cell membrane. They are phospholipids, cholesterol, proteins, and carbohydrates. Each of these molecules gives the cell membrane unique characteristics due to the way the molecules interact with each other. To understand cholesterol’s role in the membrane, let’s look at the basic structure of a cell membrane.

Structure of the Cell Membrane

Phospholipids are the molecules that make up most of the membrane structure. A single phospholipid molecule has two parts, a head that is attracted to water and a tail that is repelled by water.

The head is referred to as hydrophilic, or water-loving, while the tail is called hydrophobic, or water-hating. Both the outside and inside of a cell are aqueous. This causes the phospholipids to orient themselves into layers when in water solutions, just like vegetable oil does when mixed with water.

This aqueous environment causes two phospholipids to stack together like magnets. They attract tail to tail with the heads facing outwards. Then these two molecule structures join side-by-side with two other molecular structures. This creates a two-layer membrane that surrounds the cell. The cell is just like you sleeping rolled up in a quilt and sheet. The outside ”blanket” of the membrane has heads pointed out and tails facing in. The inside ”blanket” has heads pointing towards the plasma and tails facing in.

The tails that face in bond weakly to each other. The bond is similar to the static of your sheets, allowing for a temporary bond to the quilt. These two layers don’t lock together but instead, end up sliding past one another like the quilt and the sheet. It’s important to the vitality of the cell that the membrane layers remain fluid.

Role of Cholesterol

Cholesterol is similar to phospholipids in regards to having hydrophilic and hydrophobic portions. This would be referred to as being amphiphilic.

The hydrophilic portion of the cholesterol bonds to the hydrophilic heads within the membrane layers. The hydrophobic regions of the cholesterol fit in between the tails of the membrane layer. The cholesterol interacts with the tails of the membrane and gives the membrane unique properties.

It assists with stability of the membrane, keeps the membrane from becoming solid at cooler temperatures, and helps anchor molecules, like protein, in the membrane. Let’s take a look how cholesterol does this.The phospholipids are too flexible and, by themselves, would be insufficient in making a strong membrane. At body temperature, 37C, cholesterol actually gums up the membrane and reduces the fluidity.

This has its benefits because cholesterol gives the membrane some integrity and strength. The reaction between the cholesterol and the phospholipids keeps the hydrophobic tails in place, making the two sheets stable and stronger.In cold temperatures, the cholesterol has the opposite effect. In cold temperatures the tails of the phospholipids in the cell membrane begin to solidify, like bacon grease turns to thick lard when cooled. But cholesterol comes to save the day. By keeping the tails from packing together too tightly, cholesterol gives insulation between the tails and maintains fluidity of the membrane.The last role of cholesterol is to help anchor molecules like proteins in the membrane.

Proteins are very large and sometimes are larger than the thickness of the membrane itself. Due to their size, the phospholipids shift and drift around the protein, causing these large molecules to slide around. Imagine keeping a small ball in place between the quilt and sheet you’re rolled up in. The cholesterol stabilizes the phospholipid tails, keeping them in place, and making them more of an anchor to hold the protein in place.

Lesson Summary

In this lesson, you learned that the role of cholesterol in the cell membrane is to maintain stability, anchor other molecules, and keep the membrane fluid in cold temperatures. The cell membrane is composed of two layers of phospholipids and is a fluid structure that’s composed of four main molecules.

Phospholipids and cholesterol work together because they are both amphiphilic, meaning they have hydrophobic and hydrophilic parts.

Learning Outcomes

Your result at the end of the video should be to:

  • Define cells
  • Recall the function of the cell membrane
  • List the four main molecules that make up a cell membrane
  • Paraphrase the structure of a cell membrane
  • Explain the role of cholesterol in the cell membrane
  • Understand what ampiphilic means

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