Plasma membrane/cell membrane

PLASMA MEMBRANE/CELL MEMBRANE

Nature and Occurrence:

The cytoplasm and nucleus are enclosed within the living membrane known as cell or plasma membrane.
This is the outermost covering of the cell that separates the contents of the cell from its external environment.
It is porous in nature hence it is semi-permeable in nature.
Due to this property it controls the entry and exit of some specific materials in and out of the cell. It also prevents movement of some other materials. Thus it is known as a selectively permeable membrane.
This movement across membrane can occur by the process known as diffusion and osmosis.

Diffusion:

Diffusion is process by which solute particles can move from a region of higher concentration to the region of lower concentration. For example in living system oxygen enters the cell and carbon dioxide is released out by the process of diffusion.
Outside the cell, the concentration of oxygen is higher so it tends to enter inside the cell via diffusion. Now as a result of cellular respiration carbon dioxide is produced which accumulates inside thus increasing the internal concentration of carbon dioxide which then tends to diffuse out of the cell.
Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.
Sometimes diffusion can occur via some channel or carrier proteins than it I known as facilitated diffusion.
While in some cases it occurs by using energy rich molecules. Such process is known as active transport system whereas when it doesn’t use energy lie in simple and facilitated diffusion than it is known as passive transport.

Diffusion in living cell:

Osmosis:

Water also obeys the law of diffusion. The movement of water molecules through such a selectively permeable membrane from a region of its higher concentration to the region of lower concentration is called osmosis.
Direction in which the water molecule moves will depend upon the concentration of solute in surrounding medium. On this basis solution can be of three types-hypotonic, hypertonic and isotonic.

Hypotonic: If the medium surrounding the cell has a higher water concentration than the cell then it is known as a hypotonic (dilute) solution.
If cells are placed in such medium than water molecules tends move from outside to inside and the process is known as endosmosis.
Hypertonic: - If the medium has a lower concentration of water than the cell, i.e., if it is a very concentrated solution then such a solution is known as a hypertonic (concentrated) solution.
If cells are placed in such medium than water molecules tends move from inside to outside so the cells will shrink and the process is known as exosmosis.
Isotonic: - If the medium has exactly the same water concentration as that of the cell, then there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution.
If cells are placed in such a medium then they will remain in equilibrium condition showing equal amount of water outside and inside both. Hence if cells are to be isolated and kept in in vitro condition in laboratory then it should be placed in isotonic solution.

This process can help in the absorption of water from surroundings as in roots in plants or it can also help in removal of excess of water from the cell by exosmosis as during urine formation in animals.

Structure of plasma membrane:

The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.
The structure of the plasma membrane can be observed only through an electron microscope.
It is a bilayer structure that means lipid molecules are arranged in two layers.
There are two types of proteins in the membrane on the basis of their position: intrinsic and extrinsic.
Intrinsic proteins are those which are located within the two layers of lipids while extrinsic proteins are those which are located on outer and inner surface of the membrane.
Lipids provide strength to the membrane while proteins can serve as enzyme receptor and they can also help in the transport of material across the membrane.
The boundary of the plant cell is the cell membrane covered by another thick covering called the cell wall.

Function:

Selective exchange of material:

It selectively controls the entry and exit of materials in and out of the cells.

Barrier:

Protection:

Flow of material:

It allows flow of materials and information between organelles of cell and different cells also.

Endocytosis/exocytosis:

Since it is flexible so it can change shape thus helps unicellular organism (amoeba) to capture their food (endocytosis) as well as release the waste material from the cell (exocytosis).