What are Membrane Lipids?
Most animal cell membranes include lipid molecules, which make up around 50% of their mass. These molecules, which are amphiphilic (amphipathic), are grouped in a bilayer structure that resembles a two-dimensional sheet. Because these molecules have both a hydrophilic polar end and a hydrophobic nonpolar end, the lipids in cell membranes display two distinct properties. In most biomembranes, the hydrophobic core is 3–4 nm thick and is made up of hydrophobic chains of lipids in each leaflet or layer.
Phospholipids, cholesterol, and sphingolipids are the three main groups of membrane lipid molecules. The phospholipids, however, are the most prevalent type of membrane lipid.
Table of Contents
- What are Phospholipids?
- Structure of phospholipid bilayer
- Types of Phospholipids
- Arrangement in Membranes
- Properties of the phospholipid bilayer
- Functions of Phospholipids
What are Phospholipids?
Phospholipids are complex or compound lipids that have phosphate groups bonded to lipid molecules. Glycerol-3-phosphate derivatives known as phospholipids have esterified phosphoric acid and an organic alcoholic group on one end and the glycerol backbone connected to two fatty acids on the other.
Figure: A phospholipid bilayer showing the hydrophilic head of the glycerol backbone, the phosphate, and the polar head group, as well as the hydrophobic tails of the fatty acid chains.
Structure of phospholipid bilayer
The phospholipid bilayer is made up of two layers of phospholipids with hydrophilic polar heads facing outward and hydrophobic non-polar tails facing inward. This gives phospholipids their amphiphilic properties. A phospholipid molecule is structurally made up of two fatty acid tails, a head made up of glycerol (3-carbon alcohol), and a phosphate molecule. While the phosphate group is esterified to the terminal hydroxyl group in glycerol, the two fatty acyl chains are esterified to the two hydroxyl groups in glycerol. The two fatty acyl chains differ in their degree of saturation, or whether there are 0, 1, or 2 double bonds present, as well as the number of C atoms (often 16 or 18). Unexpectedly, the two forms of phospholipid fatty acids—one saturated and the other unsaturated—are what give the phospholipid membrane its fluidity and flexibility.
Types of Phospholipids
Based on the structure of their backbone, phospholipids may be divided into two groups, namely:
1. Glycerophospholipids (phosphoglycerides)/ Glycerol phospholipids
The structural component is glycerol. Different phosphoglycerides exist depending on the kind of their head group. As follows:
- Phosphatidylcholine (PC): It is the phospholipid that makes up the majority of the plasma membrane. Choline, a positively charged alcohol linked to the negatively charged phosphate group by an ether bond (C-O-C), is present in the head group.
- Phosphatidylserine (PS): Here, an ether bond holds the positively charged ethanolamine to the negatively charged phosphate group.
- Phosphatidylinositol (PI): Inositol is the head group.
The phosphate group, when bound to other head molecules such as hydrogen, and ethanolamine, it’s known as phosphatidic acid and phosphatidylethanolamine. Phosphatidic acid is considered to be the precursor to many phospholipids. Thus, it is the most fundamental one.
Members of the phosphoglyceride family known as plasmalogens have one hydrocarbon chain that is ester-bonded to glycerol. The other hydrocarbon chain, on the other hand, is joined to glycerol via an ether bond. The human heart and brain tissue contain a great deal of these.
2. Sphingophospholipids
The skeleton is made up of sphingosine. The amino alcohol sphingosine has a lengthy hydrocarbon chain. Sphingomyelins are phospholipids with an overall structure that resembles phosphatidylcholine quite a bit. Phosphocholine is joined to the hydroxyl group at the end of the sphingosine backbone in sphingomyelin. It belongs to both the phospholipid and sphingolipid families.
Arrangement in Membranes
In an aquatic environment, phospholipids are organised in a bilayer configuration with hydrophilic heads outside and hydrophobic tails within. Charged or uncharged polar groups on the hydrophilic head molecule interact electrostatically or establish hydrogen bonds with water to make it easy for it to dissolve in it. However, the uncharged and non-polar nature of the hydrophobic fatty acyl chains in the tail region prevents them from interacting with water. The phospholipids aggregate in a way that exposes their hydrophilic heads to water and conceals their hydrophobic tails in the interior, minimising the cost of free energy needed to reorganise water molecules when phospholipids are distributed in water. As a result, spherical micelles, bimolecular sheets, or bilayers are formed.
Properties of the phospholipid bilayer
- Amphiphilic phospholipids are lipids.
- Self-assembling phospholipids are possible. The hydrophobic effect, which keeps lipid chains away from water to form micelles or liposomes, is the main driving factor behind this feature.
- Hydrophilic solutes cannot diffuse across the lipid bilayer due to its hydrophobic core. However, the bilayer's inclusion of membrane proteins makes it easier for them to pass the impermeable barrier.
- The hydrophobic and Van der Waals interactions between the lipid chains stabilise the lipid bilayer's distinctive layout. The ionic strength and pH of the outside aquatic environment, however, vary substantially.
- One phospholipid bilayer in the lipid bilayer exhibits either rotational or lateral movement, whereas the other bilayers exhibit transverse movement in a "flip-flop" fashion.
- Phosphoglycerides improve membrane fluidity whereas sphingolipids and cholesterol diminish it. Temperature, the amount of cholesterol in the bilayer, the length and saturation of the fatty acid tails, and other parameters all have an impact on how fluid the biomembranes are. Pinocytosis and endocytosis are two cellular processes made possible by the fluid nature of phospholipids.
- Phospholipids, which are structural components of biological membranes, make it easier for membrane proteins to be anchored.
- The two leaflets of the bilayer's bilayer show an uneven distribution of phospholipids.
Functions of Phospholipids
Below are a few biological processes that are listed:
1. Most biological membranes, including cell membranes, are made up mostly of phospholipids, which serve as barriers to control how molecules enter and exit the cell.
2. When phospholipids are broken down by an enzyme, some byproducts serve as second messengers in the signalling process. For instance, phosphatidylinositol (4,5)-bisphosphate is divided by the enzyme phospholipase C into inositol triphosphate (IP3) and diacylglycerol (DAG). These two products may play a part in signal transduction.
3. Numerous cellular processes, including apoptosis, phagocytosis, and the control of mitochondrial function, are mediated by phospholipids.
4. The fluidity and flexibility of cellular membranes are aided by phospholipids. Overindulging in alcohol reduces the amount of phospholipids present in hepatic cells, decreasing their flexibility and increasing the risk of liver injury or cell damage. Therefore, it is advised to provide phospholipids to treat this disease.
5. For the metabolism and absorption of lipids, phospholipids are essential.
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