Phospholipids: The unsung heroes in biomoleucles!

Hi friends, I am back. This time I thought I will discuss phospholipids which are the main components of cellular membranes. And there are 2 reasons for that:

This will be a continuation article from the previous post where I emphasized on the significance of self-organization processes and molecules which take part in such processes. Previous post here: Information-driven processes versus Self-Organization processes in Biology
Secondly, I am going to give a talk on our work at my institute next Thursday. And it is related to the phase transition in lipids. ;)

So let us start with a small animation from Molecular Dynamics simulations highlighting a single DMPC lipid. I will explain what a DMPC molecule is in the coming section.

Cellular Membranes

Most of us naively classify lipids or fats as unhealthy in daily life. But let me tell you that these molecules are very vital for the existence of life on this earth. As I mentioned in the previous article here, although lipids not mentioned in the central dogma of biology it deserves the equal status of importance. Why? Simply because cells won't exist if there were no lipids! As simple as that. And cells are the building blocks of life. We classify living organisms based on cellular features. We have single cell organisms like bacteria, archaea, yeasts and multicellular organisms like fungi, humans, plants etc. We also know that the information content which gives us our identity is coded in our genome which is stored in sequence form of DNA molecule chains. And proteins are the workhorses which execute the instructions in this genetic codes. Yes, I have really oversimplified the exact biology. Still, this is a first-order approximation which will work well. Now think about a real cell. So, first of all, any cell requires a protection to cover itself from the external world. Otherwise, the cell constituents will flow outside and life will not be possible. Here comes the cellular membrane. Cellular membranes in living organisms are mostly made up of lipids(phospholipids and glycolipids), cholesterols, some protein molecules and few carbohydrates. But calling the membrane a protective cover is not proper in reality because there are a lot of transport and diffusion happening through the cell membranes. Like nutrients go into the cell and waste is expelled. Also, membranes with the aid of different proteins act like a sensor too. So the real picture is a bit more complicated. Please see the below figure:

From Wikimedia Commons, License: CC BY-SA 3.0

To further enhance your understanding, I invite you to watch this TED-ed youtube video. This will give you a kick starter to the amazing world of membranes.

And in today's article, I will talk about phospholipids which come in many varieties. Specifically, I will focus on DMPC molecules.

Phospholipids

First of all, let us start from the anatomy of a phospholipid. There are numerous kinds of phospholipids like shown below:

From Wikimedia Commons, License: CC0 1.0 Universal (CC0 1.0)

DMPC (1,2-Dimyristoyl-sn-glycero-3-phosphocholine) membrane

As you might have imagined phospholipids has phosphate group which is linked to other groups. If it is linked to choline group, as shown above, we classify the lipid as phosphocholine (PC for short). Glycerol group which is connected to phosphate part via the phosphorous then connects to the long fatty acid chains. The long fatty acid chains are abundant in carbon atoms which are non-polar and thus hydrophobic. (In some PC lipids unlike DMPC, carbon chains have double bonds in them we call it unsaturated fatty acids, which can contribute to kink angles in the lipid tails and fluidity of the membrane.) And the head group comprising of charged phosphate is hydrophilic. So these phospholipid molecules are amphiphilic in the sense that they have water-loving hydrophilic heads and water-hating hydrophobic long carbon tails.

These lipids can form different structures like micelles, vesicles(made up of bilayer lipids) etc based on the environmental factors like pH, temperature etc. There can also be phase transitions in lipids.

Phase transitions in DMPC bilayer lipids

The phases of DMPC lipids can be divided into four. They are:

fluidy disordered state,
crystalline ordered state,
Ripple state and
fluidy ordered state

See this figure from an open-access article From biological membranes to biomimetic model membranes:

If you simplify what happens is this: Firstly as the temperature is less the lipids are more ordered and in gel state (crystalline ordered state). As the temperature increases and reaches a pre-transition temperature there are 2 states co-existing in the lipid, one ordered state, and another disordered fluid state. This is an intermediate Ripple state. As you keep on increasing temperature, the lipid becomes more disordered and fluid in the state especially when it reaches main transition temperature. Now there is one more intermediate state called liquid-ordered state which is attained with the help of addition of cholesterol. In the real cell membranes, its fluidity is mainly regulated by cholesterol. These molecules have the capacity to keep the lipid in fluid mode but keeps an order too in the system. In my opinion, cholesterol adds a negative feedback mechanism to the lipid bilayer for maintaining stable, but fluid state.

A Simulation Video

So this is a classical molecular dynamics simulation which we did and it comprises of a protein segment called NOGO66 which interacts with a DMPC lipid bilayer. And the water molecules are not shown for clarity.

For further reading :

[1]: Physical Biology of Cell by Rob Philips et al.. [This is a great book.]
[2]: From biological membranes to biomimetic model membranes

If you like my content please upvote and resteem it. Below are few of my recent posts, you may find interesting:

I am currently doing three article series:

Evil genius!: My experiments with electronics and other DIY stuff!
Classical Molecular dynamics: Focusing on teaching and setting up a molecular dynamics simulation of atomic systems. (Dormant mode)
Visually Explained!: Explaining technical concepts visually.

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