The cell membrane is the outermost part of the cell present in all living beings.
It can be defined as “a lipid bilayer membrane covering the cell which is made of with selective permeability.”
It is also termed as a plasma membrane, and it is permeable to selective substances.
Since there are different types of cells, the cell membrane has small variations as per the requirement of those cells.
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 content in the cell membrane
They form a 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 the 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 the main carbohydrates in the membrane. Further, there are also carbohydrate monomers like hexose, hexosamine, fucose, etc.
The structure of the 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 the cell membrane imparts smoothness, softness, elasticity, and permeability.
The phosphate groups in the membrane determine the hydrophobic and hydrophilic properties. In general, the membrane is hydrophilic to the external environment and hydrophobic inside towards the cytoplasm.
Check out the video below for a visual demonstration of the cell membrane structure.
8 Cell membrane functions
The main functions of the cell membrane are
- Cell Protection
- Transport and
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 cells. It supports to maintain cell homeostasis. Let’s see in detail.
a) Acts as a protective coat
The cell membrane is the only barrier to the 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 the absorption of nutrients & oxygen from an external supply like blood or plasma. Further, it helps in the excretion of waste from cells into blood or plasma.
c) Inter-cellular bridges
The 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 makes 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 the intercellular 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
The cell membrane has many receptors 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 signaling of receptors on the surface of the 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 the cell membrane act as targets for drugs 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 the cell wall commences from the cell membrane.
The cell membrane is actively involved in the movement. In small animals like an amoeba (protozoa), the membrane forms pseudopodia to move. In bacteria, there are flagella attached to the membrane, which helps in movement.
g) Eating & engulfment
In small microbes like an amoeba, the membrane helps in cell eating. Amoeba eats solid material by phagocytosis. It drinks liquid by pinocytosis (cell drinking) formed by the 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.
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, it is destroyed.
The lysosomal enzymes inside these organelles digest the contents of the pathogen leading to destruction.
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 outermost cover as the cell wall is absent.
Read the “difference between plant cell and animal cell.”
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 a 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.