Composition of the Cell Membrane and Functions
The main functions of cell membrane are
- cell Protection
- Transport and
Cell membrane definition can be read as “the outermost membranous covering of the cell which is made of lipid bilayer with selective permeability.”
cell membrane is the outer most part of the cell present in all the living beings.
Since t is a membranous in nature, it is called as a cell organelle.
It acts as a protective layer to the cell and gives a defined shape and volume.
Just like the skin is to our body, the cell membrane is to the cell.
It is also termed as plasma membrane as it is permeable to selective substance.
There are different types of cells, and the cell membrane has small modifications as per the requirements of particular cell.
Composition of the Cell Membrane
The cell membrane is composed of protein, lipids and carbohydrates. Their proportion in the membrane of human red blood cell is
Protein in the membrane: They form the significant part of cell membranes. They provide mechanical structure to the cell. They pass through the membrane by acting as carriers and channels for transport. Further, they also act as receptors, enzymes and even antigens.
There are two types of proteins in the cell membrane as
- Integral membrane proteins
- Peripheral membrane proteins (also called extrinsic membrane proteins)
Integral proteins comprise of 70% of total membrane protein. They are water-insoluble and are associated with lipids and oligosaccharides (carbohydrates) to form glycolipids and glycoproteins respectively.
Examples of peripheral proteins include α-spectrin, β-spectrin, ankyrin, actin, etc.
Examples of integral proteins include glycophorin.
Lipids in cell membrane: There are three types of lipids viz. phospholipids, cholesterol and galactolipids. The phospholipids present include phosphatidylinositol, phosphatidylethanolamine and sphingomyelin.
Carbohydrates in the membrane: Oligosaccharides are main carbohydrates in the membrane. Further, there are also carbohydrate monomers like hexose, hexosamine, fucose, etc.
The structure of cell membrane is quite complex and robust. It is very effectively designed for diverse roles and conditions of the environment.
It is a phopsho-lipid bi-layer with proteins in between. It is made of glycolipids (i.e., carbohydrate and lipid combination) and glycoproteins (i.e., carbohydrate and protein combination).
Glycoproteins in the cell membrane contribute to its rigidity while the lipid bilayer of cell membrane imparts smoothness, softness, elasticity and permeability.
The phosphate groups in the membrane determine its hydrophobic and hydrophilic properties. In general, the membrane is hydrophilic to the external environment and hydrophobic inside towards cytoplasm.
Check out the video below for visual demonstration on cell membrane structure.
8 Cell membrane functions
The primary function of the cell membrane is to keep the cell safe. So it protects from injuries, absorbs required substances and also checks on leakages from cell. It supports to maintain cell homeostasis. Lets see in detail.
a) Acts as a protective coat: Cell membrane is the only barrier to animal cell to control the transport of selective substances into and also outside the cell. Further, it undergoes wear and tear and even external stress.
b) Selective barrier: The membrane is semi-permeable. I.e. it is selectively permeable to water and essential substances. Helps in absorption of nutrients & oxygen from external supply like blood or plasma. Further, it helps in excretion of waste from cell into blood or plasma.
c) Inter-cellular bridges: Cell membrane forms inter-cellular bridges in plants, fungi and also animal cells. In animals the blood circulation takes the nutrition and oxygen to the deeper corners of tissues. This make it easy for the cell membranes of individual cells to absorb. But in plants and fungi, this is not the case. They have no circulation in between cells. The only option is inter-cellular bridges. These help the transfer of material from one cell to another. In animal cells also there are inter-cellular bridges for the same purpose.
d) Receptor for signals: Cell membrane has many receptor on its surface. When a hormone is released in the body, the receptor on the cell surface gets the signal and passes the information to the internal cell mechanism. Thus hormones control the body physiology through signalling of receptor on the surface of cell membrane.
For example, the pancreas secretes insulin into the blood. This insulin acts on the insulin receptors on the cell to let the membrane take up glucose. Hence in non-insulin dependent diabetes, the problem is not lack of insulin. But the non-responsiveness of receptors on the cell surface to take up the glucose from the blood. Proteins on the surface of cell membrane act as targets for drug and hormones to produce their action.
Even in nerve cells, there are chemoreceptors on the surface which are attached to the membrane. They excite the cell by chemical stimulation.
e) Cell division:
It plays an essential role in cell division by initiating the cleavage process. It leads to the cleavage of cell in binary fission.
This is the most common for cell division in smaller organisms like bacteria, fungi, sponges. The partition of cell wall commences from the cell membrane.
f) Movement: Cell membrane is actively involved in the movement. In small animals like protozoa like amoeba, the membrane forms pseudopodia to move. In bacteria, there is flagella attached to membrane which help in movement.
g) Eating & engulfment: In small microbes like amoeba, membrane helps in cell eating. Amoeba eats solid material by phagocytosis. It drinks liquid by pinocytosis (cell drinking) formed by membrane.
In higher animals, white blood cells, especially macrophages eat up bacteria and other harmful microbes by phagocytosis. In all these cases membranes form a pouch-like sac to eat or drink by the cell.
h) Reproduction: In bacteria, membrane form pilli which help in sexual reproduction. While in protozoans like paramecium, it forms a bridge of conjugation for sexual reproduction. Through these structures present on the membrane, the male and female gametes are exchanged in between cells.
i) Immune Defense: The lysosomes are present near the inner surface of the cell membrane.When a pathogen or toxin tries to gain entry into the cell through the membrane, they are destroyed. The lysosomal enzymes inside these organelles are released to destroy the microbes.
j) Other organelles: It is also part of other cell organelles like the Golgi apparatus, endoplasmic reticulum etc.
In plants, it lies immediately below the cell wall. This cell wall gives protection from harsh impact. While in animals it is the outer most cover as the cell wall is absent.
This cell membrane has diverse functions in cell physiology, reproduction, growth and death.
It is engaged in the critical stages of the cell-like cell division, reproduction and even death by necrosis or apoptosis.
Also, it helps to maintain cell volume by osmosis, exocytosis and also give defined shape and size to the cell. Osmosis is a phenomenon where liquid flows from a region of low concentration to higher one. Exocytosis is a process to push out something from within. This is seen in smaller cells for the expulsion of waste from the cytoplasm.
The cell membrane is absent in the bone cells and cartilage cells. Perhaps they do not require as the cell is covered by a hard matrix substance.