Cell membrane is defined as the outer most covering of the cells of living beings.
It acts as a protective layer to the cell. Besides it encloses all the cell components and cytoplasm to give a defined volume, shape to the cell. Just like skin is the body, cell membrane is to the cell.
It is also termed as plasma membrane as it is permeable to selective substance.
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 and lacks cell wall.
This cell membrane has diverse functions in cell physiology and growth.
It is engaged in the important stages of the cell like cell division, reproduction and even death by necrosis or apoptosis.
There are different types of cells and the cell membrane has small modification as the requirements of particular living being or cell.
The functions of cell membrane and its structure are quite interesting and worth study.
Cell membrane structure
The structure of cell membrane is quite complex, rigid (plant cell) or flexible (animal cell) 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 impart smoothness, softness elasticity and permeability.
The phosphate groups in the membrane determine its hydrophobic and hydrophillic properties. In general the membrane is hydrophillic 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 main function of 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 tare and also 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, 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 blood. proteins in cell membrane surface act as targets for drug, hormone to produce their action.
Even in nerve cells, there are chemo receptors on surface which are attached to membrane. They excite the cell by chemical stimulation.
e) Cell division:
It plays an important 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 of cell division in smaller organisms like bacteria, fungi, sponges. The partition of cell wall commences from cell membrane.
f) Movement: Cell membrane is actively involved in 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 forms 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 membrane, the male and female gametes are exchanged in between cells.
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. Exocitosis is a process to push out something from within. This is seen in smaller cells for expulsion of waste from 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.