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Carbohydrates are crucial for protecting cells and for differentiating host cells from intruders. In this lesson, we’ll explore what carbohydrates are, as well as their important functions in the cell membrane.

The Cell Membrane

A cell membrane is a busy place.

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Like a city wall, it surrounds the cell and allows the cell to maintain its environment. The membrane is a barrier, but it allows important molecules to pass into and out of the cell, just as travelers can pass into and out of city gates. We can find proteins, lipids, and carbohydrates in the membrane.Carbohydrates help to protect the cell from the outside world.

They are also involved in differentiating host (friendly) cells from intruder (enemy) cells in the immune response. Let’s take a closer look at carbohydrates and what they do in the membrane.

What Are Carbohydrates?

Carbohydrates are made from sugar molecules. Simple carbohydrates include glucose and fructose, which are monosaccharides (one sugar molecule).

They can join up to form sucrose, a disaccharide (two sugar rings).Complex carbohydrates are starches found in foods like wheat, potato, and beans, and are often made up of a large number of sugar molecules bound together (polysaccharides).

Protection for the Cell

Let’s take a closer look at the role of carbohydrates. One of the most important functions of carbohydrates is to form a structure called the glycocalyx.

This is a coat around the cell. If the cell membrane is like a city wall, then the glycocalyx is another outer wall, which is used as a first layer of defense.In bacteria the glycocalyx is known to be especially strong. It allows bacteria to adhere together, creating a biofilm.

One bacterium alone with its glycocalyx is tough. But if bacteria join together and present a unified, tough glycocalyx coat (a biofilm), they are stronger.Biofilms are resistant to harmful factors, which makes it difficult for us humans to combat bacterial infections and plaque, as they contain biofilms. Humans throw cleaning products, like bleach, at biofilms in the bathtub and still have trouble clearing bacteria from unwanted areas.The glycocalyx also has important functions in humans. It allows cells on the inside of blood vessels to withstand the strong flow of liquid across their surfaces. It protects microvilli in the gut, which absorb nutrients, and the glycocalyx even aids in the breakdown of food for this absorption by holding digestive enzymes in its coat.

Cell Recognition

Carbohydrates in the membrane also play a role in cell recognition. Carbohydrate chains carry a signature, like a flag, that says what organism a cell belongs to (host or intruder).Let’s go back to our city analogy. If a knight visits a city, he will have a crest on his shield or armor.

If this crest is the same one as the city’s, he’ll be let in. If it’s not, he might be attacked. The same thing happens with cells. Intruder cells that don’t have the same crest as the host cell can trigger the immune response and be attacked. In the case of a cell, the crest is made (in part) of a carbohydrate.

Different Carbohydrates in the Membrane

Just like the mythical beast ‘chimera’ that can be part-lion and part-snake, types of molecules sometimes mix together.

So, we can add some new types of molecules to our picture of the cell membrane.Glycolipids, glycoproteins, and proteoglycans are all components of the cell membrane.

  • Glycolipids are a carbohydrate plus a lipid. (‘Glyco’ means ‘sugar’ and refers to a carbohydrate because a carbohydrate is composed of one or more sugar rings). Glycolipids help maintain the stability of the cell membrane and facilitate cell-cell interactions.
  • A glycoprotein is a mix of carbohydrate and protein but is mostly protein. Glycoproteins are a component of some antigens, for example the signal (the knight’s crest) that marks another cell as either foreign or familiar.

    This is crucial for the immune system to respond to invaders.

  • A proteoglycan is a mix of carbohydrate and protein but is mostly carbohydrate. Glycoproteins and proteoglycans form the glycocalyx, that strong ‘cell coat’ we’ve been talking about.

Lesson Summary

The cell membrane is populated with proteins, lipids, and carbohydrates. Carbohydrates are made up of sugar molecules and can be joined to protein (glycoproteins and proteoglycans) or lipids (glycolipids). Carbohydrates protect the cell by forming a glycocalyx, which is especially strong in bacteria and enables biofilms to form. The glycocalyx is also important in humans; it protects cells in arteries and veins from the force of blood flow, protects microvilli in the gut from harsh, acidic conditions and aids in the digestive process.

Carbohydrates have important roles as antigens, allowing the cell to differentiate other cells as either host or intruder.

Review Notes

Carbohydrates play an important role in cell membranes
Diagram of a cell membrane
Carbohydrates are formed from sugar molecules and are found in starch in foods like potatos and beans
Carbohydrates create a glycolax, which protects cells from the force of the blood flow, protects microvilli in the stomach, and aids in the digestive process
Carbohydrate chains helps identify what organism a cell belongs to
The types of carbohydrates in the cell membrane are glycolipids, proteoglycans, and glycoproteins

Learning Outcomes

When you are done, you should be able to:

  • Describe the cell membrane
  • Discuss the importance of carbohydrates in the cell membrane and in cell recognition
  • List the components of the cell membrane

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