Students who have started doing Biology often find the concept of cell membrane a boring topic. What kind of wonder can one get from a big oval and a slightly smaller one? Well, just like a Kubrick film, where the deceptively simple things often hide an intense complexity, the topic of 'Membrane Structure and Function' is an important filed in molecular cell biology, and has stimulated intense research worldwide. This time, I will talk about the 'Fluid Mosaic Model' of cell membrane, an iconic concept that has influenced the idea of membrane trafficking, for which a Nobel Prize of Physiology has been awarded to its founding thinkers in 2013.
Section 1 Phospholipids
A phospholipid molecule is the basic building block of the cell membrane. It is made up of 2 parts with different chemical properties:
(a) a polar, hydrophilic (water-loving) head containing a phosphate group bonded to glycerol; and
(b) a non-polar, hydrophobic (water-hating) tail containing fatty acids.
A compound like phospholipid is amphilphilic because it contains both a hydrophilic component and a hydrophobic component. As we will see, this peculiar character is essential for the formation of bilayer structure on the cell membrane.
The bilayer is formed because the ‘head’ and ‘tail’ have fundamentally different chemical properties and they try to avoid each other – by aggregating with their corresponding ‘friends’ with similar solubility properties.
Thus, the hydrophilic heads of the phospholipid molecules appear on the outside of the membrane, where an aqueous environment is present, while the hydrophobic ‘tails’ orientate inside the double layer, to get away from water. As a result, a bilayer is formed.
Section 2 The Fluid Mosaic Model
The fluid mosaic model is a concept proposed by Singer and Nicholson in the early 1970s, to account for the structure of cell membrane and how that is related to its function to protect, give shape, and regulate the transport in and out of the cell.
So why bother with two words that are not interesting, and the fun factor does not increase when they are put together? Well, 'fluid' and 'mosaic' succinctly describe the properties of cell membrane. Do not be fooled by the apparent observation that the cell membrane is stable like a castle wall in a boring chapter in your Biology textbook. The fact is, the cell membrane is a dynamic system and a lot of biological processes are taking place there every single moment in your life.
Singer and Nicholson proposed that the cell membrane is made up of phospholipid and proteins, and the phospholipid molecules are arranged in a bilayer structure. It is important to note that these phospholipid molecules can move laterally, giving the fluid character. For the embedded protein molecules, they are interspersed among the phospholipid molecules, and that formed a mosaic pattern.
It is worthwhile to note at this stage that the proteins can interact with the phopsholipids in many different ways. While some proteins are embedded on one side of the membrane bilayer, others are 'membrane-spanning', such as carrier proteins and channel proteins.
Thus, the proteins that are attached to the surface of the phospholipid bilayer are called peripheral proteins, and those embedded half-way in the bilayer are integral proteins.
Section 3 Membrane Proteins
There are many types of proteins interacting with the cell membrane. Let's take a look at a few key types.
Channel proteins provide channels across the membrane for transporting some ions, like a molecular tunnel connecting the outside and inside of the cell.
Carrier proteins are involved in active transport to carry substances across the membrane. The process requires energy (in the form of ATP) and is against a concentration gradient.
There are many receptors on the surface of the cell, which receives the chemical messenger molecules outside the cell. Antigens are also present for cell recognition. In many cases, these are glycoproteins, where carbohydrates are attached to the proteins. The carbohydrate branching and pattern found on this glycoproteins are often useful for the recognition process (glyco-code), and that is a complex and hot research field.
Enzymes are also present as biological catalysts for various processes on the membrane.
Cholesterol is also found on the cell membrane and they are responsible for the membrane fluidity, which is a more complex topic and I have dealt with in a separate article.
by Ed Law
