The bilayer cell membrane is a membranous structure, which consists of two phospholipid layer. Thus, a cell membrane also refers to as a phospholipid bilayer membrane. A cell membrane is composed of many phospholipid molecules. A structure of a phospholipid molecule consists of hydrophilic head, a phosphate molecule, a glycerol molecule and two hydrophobic fatty acid tail.
The phospholipid layer makes the plasma membrane selectively permeable by creating a barrier between interstitial and cytosolic fluid. To know a detailed or well-labelled structure of cell membrane, a prevalent model was introduced named “Fluid mosaic model”. Before proceeding to the fluid mosaic model, let us study the definition, integral components and functions of the cell membrane.
Content: Bilayer Cell Membrane
- Composition of Bilayer Cell Membrane
- Functions of Cell Membrane
- Structure of Bilayer Cell Membrane
The bilayer cell membrane can define as the innermost layer after the cell wall, which consists of a two-layer of phospholipid molecules, embedded proteins and extracellular carbohydrate. It acts as a semi-permeable membrane, where the phospholipid layer provides a chemical barrier between the cytoplasm and the surrounding environment. It also refers as “Plasma membrane”. The cell membrane allows the exchange of only specific or selective molecules between the cell and surrounding, and thus it also refers to as “Selectively permeable membrane”. A cell membrane primarily composed of 40% lipid, 60% protein and 1-10% of carbohydrate of dry cell weight.
Composition of Bilayer Cell Membrane
The composition of the cell membrane includes three essential biomolecules, namely:
Lipid: Phospholipid and cholesterol molecules constitute the part of lipid.
It is a chemical substance or triglycerides, which make up the basic fabric of the cell membrane. In the year 1847, lecithin was the first phospholipid that has been identified in the living tissues. The structural configuration of phospholipid consists of one hydrophilic head with a phosphate group and two hydrophobic tails containing fatty acid.
The phosphate group and the chains of fatty acid are usually attached by a glycerol molecule. One chain of phospholipid molecule contains saturated fatty acid, whereas the other contains unsaturated fatty acid. Therefore, it shows an “Amphipathic property” by being both hydrophobic and hydrophilic.
Head of Phospholipid
A phospholipid molecule consists of the hydrophilic or polar head. Being hydrophilic, head of a phospholipid molecule is water-loving and readily interacts with water or polar solvents. Head of a phospholipid molecule contains a negatively charged phosphate group with an additional polar or charged alkyl or R-group.
The hydrophilic head is pointed outwards and allow the exchange of polar molecule inside and outside the cell. Therefore according to the “Like dissolve like” rule, the hydrophilic head will dissolve readily in water, by forming a hydrogen bond.
Tail of Phospholipid
A phospholipid molecule consists of a hydrophobic or non-polar tail. Being hydrophobic, the tail of the phospholipid molecule is water-hating, i.e. poorly interact with the water molecules. Thus, the polar molecules cannot cross the hydrophobic core easily. The tail of phospholipid is lipophilic, i.e. it readily interacts with the non-polar solvents. The tail of the phospholipid molecule contains two fatty acid chains of about 14-24 carbon atoms.
One tail of phospholipid is slightly bent due to the presence of unsaturated fatty acid with one or more cis-double bond. The other tail of phospholipid contains a long or straight chain of saturated fatty acid without cis-double bond. The tail is pointed inwards because of its hydrophobic nature and contains Vander Waal forces between them.
If a phospholipid consists of a small tail, it forms a small, Lipid monolayer sphere commonly refers to as “Micelle”. In contrast to this, if a phospholipid molecule consists of bulkier tails, then it forms a large, lipid bilayer sphere commonly refers to as “Liposome”.
It can define as another lipid constituent, which is present in the core of the cell membrane. Cholesterol attaches alongside with the hydrophobic tails of the phospholipid layer. Its structure consists of a polar head, planar steroid ring and non-polar hydrocarbon tail. The cholesterol molecules bind with the neighbouring phospholipid molecule via hydrogen-bond.
Cholesterol is also a very crucial component of the cell membrane, which provides strength and firmness to the cell membrane at a different range of temperature. It maintains the fluid phase of the cell membrane, at low temperature.
Cholesterol is composed of four fused rings of carbon with hydrogen atoms. These are hydrophobic, thus found between the hydrophobic tails of phospholipid.
Proteins: Channel and carrier proteins are the membrane proteins, which constitute a significant part of a protein.
These are also the major components of the cell membrane and are of two types namely,
Integral membrane proteins are the fixed and permanent structures incorporated within the cell membrane. It also refers as “transmembrane proteins”, which can be of three kinds:
Single or bitopic transmembrane α helix: It functions as a receptor molecule for the binding of selective molecules or ions. Bitopic transmembrane α helix also helps in the mechanism of cell recognition.
Polytopic transmembrane α helical bundle: It performs three specific functions like:
- Transporters: Acts as a carrier molecule for the active or passive transport of molecules or ions.
- Mediates enzyme catalysis: Acts as a helper molecule for the enzyme catalytic reactions.
- Receptor: Also acts as a receptor which allows binding of only specific molecules from the environment into the cytoplasm and induces a chemical reaction within the cell.
Thus, α helical bundle plays a dual role by functioning as both cell receptor and ion channel.
These protein molecules are temporarily attached to the cell membrane. Peripheral Protein attaches with the cell membrane via hydrophobic, electrostatic and non-covalent interactions.
It is the third major component of the cell membrane, which is present in the extracellular space and refers as Glycocalyx. It basically consists of 2-60 monosaccharide units and the structure of glycocalyx can be straight or branched.
Functions of Cell Membrane
A cell membrane performs many activities, which can be understood by knowing the individual role of the particular component.
Membrane Protein Functions
- Membrane proteins help in the cell to cell attachment or Intercellular joining.
- Plays a vital role in the metabolic pathway, which is catalyzed by the enzymes.
- Functions in the transportation of specific molecules or ions through active, passive and facilitated diffusion.
- Helps in cell recognition by inducing a chemical reaction within the cell.
- Also helps in the cell anchorage or attachment, as it consists of some attachment sites for the cytoskeleton and extracellular matrix.
- It makes the cell membrane “Semi or selectively permeable”, by functioning as a barrier between the cytoplasm and extracellular environment.
- It restricts the passage of unwanted molecules.
- It maintains the membrane fluidity, at low temperature.
- Cholesterol also prevents the hydrophobic fatty acid tails against solidification at a cold temperature.
- Prevents the entry of small molecules from entering into the cell membrane.
- It keeps the phospholipid molecules intact.
Structure of Bilayer Cell Membrane
Fluid mosaic is the best model to explain the structure of the cell membrane. It was introduced in the year 1972, by the two scientists named Singer and Nicolson.
This model represents a very detailed structure and components of the cell membrane, which consists of phospholipids, membrane proteins and carbohydrate molecules. It also refers as “Phospholipid bilayer model”. Now let us understand the reason why this model named so.
Fluid: The components of the cell membrane are flexible to move inside the phospholipid bilayer.
Mosaic: A cell membrane comprises of many different molecules like lipids, proteins and carbohydrates which altogether gives a “Mosaic” appearance to the cell membrane.
Therefore, a cell membrane has a complex structure which comprises of many components which we have discussed earlier.