What is the function of structure E (cholesterol)?

a) Transport across the plasma membrane

b) Detection of environmental change

c) Stabilization of the phospholipids

d) Cell-cell communication

e) Structural support of the cell

The correct answer is c) stabilization of the phospholipids.

Cholesterol is a type of fat that is important for living organisms. Most people only hear about the bad side of cholesterol and how it can contribute to heart disease.

However, there is more than one type of cholesterol, and the reality is that it is an important part of animal cell membranes.

These molecules are actually modified from steroids and are considered to be a type of sterol. They do have an important function in cell membranes, helping to give stability to the phospholipids.

Cholesterol has both a hydrophilic and a hydrophobic part to the molecule, similar to the amphipathic nature of phospholipids.

The effect of cholesterol on the fluidity of a cell membrane depends on how long the hydrocarbon chains are and on the type of bonds formed.

Longer chains and single bonds make for a stiffer plasma membrane. Thus, the molecular structure of these molecules really can help to regulate the fluid nature of the membrane.

Phospholipids are the major component of cell membranes, into which both cholesterols and proteins are embedded and attached at various places.

Proteins help to move substances across the membrane and in some cases act as receptors for substances to bind to in signal transduction pathways.

Cholesterol

These are a form of lipid molecule which is essential to life and has both hydrophilic and hydrophobic components in the molecular structure.

The hydrocarbon tail repels water while the hydroxyl head does not. In this way, it is similar to the phospholipids of the membrane.

They are molecules which are modified from steroids to form a sterol that is known as cholesterol. A sterol is a molecule that includes both an alcohol and a steroid in its chemical makeup.

They are important in animal cells, helping to stabilize the phospholipids of the plasma membrane, and in fact, most of the cholesterol in a cell is found in the cell membrane. Small amounts may, however, be found in the endoplasmic reticulum and the mitochondria.

In vertebrate animals, such as humans, the cholesterol is mostly manufactured by the cells of the liver. It is also important in forming steroid hormones and even in the synthesis of bile acids.

These acids are used to form bile which is essential in the emulsification of fat, which greatly helps in digestion. It helps the lipids taken in as food to be broken into smaller pieces and thus increases the area for enzymes such as lipase, to act on.

Testosterone and estrogen are both hormones that are synthesized from cholesterol molecules, as is vitamin D. These are all substances that are important for the correct functioning of the human body.

Furthermore, cholecalciferol is the vitamin D3 that is made in the skin in the presence of sunlight from cholesterol molecules.

Function in cell membranes

Although the main molecule making up membranes is the phospholipids, cholesterol is also a crucial component of the structure. It is, in fact, a very important part of the plasma membrane of animal cells, and these molecules, in fact, provide stability to the membrane.

The amount of cholesterol present in the membrane also influences the degree of fluidity of the structure. Furthermore, the type of bonds that form between the chains and the length of the hydrocarbon chains also impacts how fluid the membrane is.

If the hydrocarbons have many double bonds or are relatively short, then they make the membrane more flexible and fluid, compared with long-chained molecules.

There are cholesterol-enriched domains which are areas in which there are many of these lipids present, forming what are called rafts. It is these regions of the membrane that may be responsible for cell signaling methods of signal transduction and for protein sorting.

Phospholipids

Much of the cell membrane is made up of phospholipids, which are molecules that are amphipathic. In other words, they have a hydrophobic and hydrophilic part.

The phosphate heads are hydrophilic and thus point outwards while the hydrophobic fatty acid tails point inwards.

These tails are made of several hydrogen and carbon atoms that are bonded together to form a long hydrocarbon structure. The nature of the bonding between the atoms also works to impact the fluidity of the membrane.

In addition, researchers have found that these molecules can move and often switch places with neighboring phospholipids. Thus, the membrane is described as adhering to a fluid mosaic model.

The phospholipids form a double bilayer and work in conjunction with other molecules to carefully regulate what substances enter and leave the cell.

Only a few substances are able to diffuse through the membrane into the cell. Most substances can only enter the cell by passing through special transport proteins that are embedded in the membrane at various points between the phospholipids.

Proteins

The cell membrane also has proteins present that are embedded in amongst the phospholipids and cholesterol molecules. Some of these act as receptor molecules to which substances bind, triggering a response in the cell.

Several of these proteins are integral proteins, many of which do interact with the phospholipids by virtue of their hydrophobic side chains. This allows for the secure attachment of the protein in the membrane.

Polypeptides in cell membranes are often important as receptors or as channels through which substances can pass. They can play a role in both passive and active transport of substances through the cell membrane.

Some of the proteins also act as receptors to which molecules bind, triggering various reactions that are part of signal transduction pathways.

References

  1. A Wisniewska, J Draus, WK Subczynski (2003).  Is a fluid-mosaic model of biological membranes fully relevant? Studies on lipid organization in model and biological membranes. Cellular and Molecular Biology Letters.
  2. K Simons, E Ikonen (1997). Functional rafts in cell membranes. Nature.
  3. Editors of Encyclopedia Britannica (2018). Lipoprotein. Retrieved from Encyclopedia Britannica.
  4. Harvard Medical School (2018). How it’s made: cholesterol production in your body. Retrieved from harvard.edu.
  5. Editors of Encyclopedia Britannica (2018). Cholesterol. Retrieved from Encyclopedia Britannica.

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