This description of the cell membrane highlights the importance of cholesterol, which is found in every cell in the body.
The body is a massive collection of cells. Some cells are more specialised than others. The liver is a collection of liver cells, the brain is composed of nerve cells, muscles are composed of muscle cells and so on. Each cell is bounded by a membrane, which separates it from other cells and from the extracellular fluid surrounding it. The cell membrane is also known as the plasma membrane. Inside the cell are many structures known as organelles which have many different functions.
The cell membrane is mainly composed of lipids. Each lipid has a head which is comfortable in a watery (aqueous) environment. The lipids also have tails which shy away from water. The lipids arrange themselves such that their heads are always in contact with the watery fluids and the tails are tightly protected from water. So the lipids arrange themselves in a bilayer with the heads making up the innermost and the outermost layers of the membrane with the tails in-between. Notice the blue spheres (heads) in Figure 1.
Figure 1. The Plasma Membrane. From PennState Eberly College of Science, Introductory Biology 1, Accessed 10/10/2014
The membrane lipids are mainly phospholipids and cholesterol. In fact cholesterol makes up approximately 25% of the lipids. Each and every cell has cholesterol in its membrane. Figure 1. shows the cholesterol molecules inside the lipid bilayer. They control the flexibility and the fluidity of the membrane which in turn affects the ability of the membrane to move chemicals into and out of the cell. For the body to manufacture Vitamin D, the steroid hormones such as testosterone and oestradiol and the bile acids (for fat digestion), it requires cholesterol.
Also in the membrane are various types of proteins. Many are attached to sugars and are known as glycoproteins. Some span the width of the membrane and act as channels (like tunnels) for the transport of ions like sodium and potassium across the cell. This sort of transport requires energy. For the brain to send messages to the rest of the body and to send nerve impulses it requires energy. Contracting muscles require energy. The energy is supplied by the mitochondria. Some proteins float on the surfaces of the membrane acting as receptors for chemicals such as hormones. Some glycoproteins enable cells to adhere to each other in order to form tissues.
Structures inside the cell include mitochondrion, rough and smooth endoplasmic reticulum, Golgi apparatus and other organelles.
Figure 2. Organelles Inside the Cell
The organelles have specific functions. The mitochondria are responsible for energy production so cells requiring a lot of energy, like brain, muscle and liver cells have thousands of mitichondria. The endoplasmic reticulum and Golgi complexes synthesise and interact with membrane proteins and lipids. Separating these structures from each other is a watery fluid called cytoplasm. This is the reason for the lipid bilayer with the hydrophobic tails of the membrane lipids being protected from the watery cytoplasm and the extracellular fluids. Each cell component has specific functions which are beyond the scope of this discussion.
The message to be gained here is that the body is made out of cells and that each and every cell has a lipid membrane in which cholesterol makes up approximately 25% of the lipids. Cholesterol is a crucial molecule in the body. If it is true that cholesterol causes heart disease then everyone on the planet would be born with heart disease.