Plasma membrane
Table of Contents
Plasma membrane
In all living organisms the cell is surrounded by plasma membrane.
Since the discovery of the cell, scientists considered the cell to be a thin membrane
but its presence was proved only after the availability of the electron microscope.
Structure-
Plasma membrane is a living thin elastic and characteristically permeable membrane.
It is 70A° thick.
Based on chemical analysis, it is made up of 75 percent phospholipids and 25 percent proteins.
In 1972, Singer and Nicholson presented the “Fluid Mosaic Model“, according to which protein-like ice cubes float in the oceanic fluid.
“a number of protein ice bergs floating in the Sea of lipids”
According to this model, plasma membrane is a two-layered lipid structure
which is made up of two types of proteins, outer protein and inner protein.
The plasma membrane is flexible due to the presence of lipids and proteins.
Function –
(1) Diffusion-
The movement of molecules from higher concentration to lower concentration is called diffusion.
Diffusion of molecules, ions and water occurs.
(2) Osmosis-
The movement of water molecules from a solution of lower concentration to a solution of higher concentration through the plasma membrane is called osmosis.
In exo-osmosis, the molecules of solvent (water) inside the cell move outside the cell.
In endo-osmosis, solvent (water) molecules from outside the cell move inside the cell.
(3) Active Transport –
The transport of molecules or ions against the concentration gradient using “ATP” energy is called active transport.
(4) Exocytosis-
In this process the substances inside the cell come out through the plasma membrane.
Since in this process the cell expels substances, hence it is called cell vomiting.
1. R.B.C of mammalian animals. The nucleus is not found in it.
2. Camel and llama are such mammals whose R.B.C. The nucleus is found in.
3. Nucleus is not found in the sieve tubes of phloem tissue in plants.
(5) Endocytosis-
The process of solid or liquid particles coming inside the cell is called endocytosis.
(a) Pinocytosis –
The process of movement of particles along with liquid from the plasma membrane into the cell is called pinocytosis.
The vesicle containing fluid particles surrounded by membrane is called Pinosome.
(b) Phagocytosis –
The process of solid particles passing through the plasma membrane into the cell is called phagocytosis.
The vacuole containing solid particles surrounded by membrane is called phagosome.
(6) To give definite shape to the cell.
(7) Acts as a barrier between external substances and the substances present inside the cell.
(8) Helps in food intake, excretion and circulation in unicellular organisms.
Cell wall
A covering is found around the plant cells which is called cell wall.
It is formed from the secretion of living fluids. It is made up of two levels.
(1) Primary wall-
It is made of cellulose.
And it is flexible and thin.
(2) Secondary wall-
It is also made of cellulose and is hard and thick.
The primary and secondary walls are connected to each other by the middle lamella.
This middle lamella is made of calcium and magnesium pectate.
Function –
(1) To provide firmness and strength to the cell.
(2) Not allowing the cell to burst in the state of expansion of the cell.
3) To provide mechanical support to the cell.
1. Smallest cell organelle – Ribosome.
2. The largest organelle in plants is plastid.
3. The largest organelle in animals is mitochondria.
4. The largest cellular structure – nucleus.
(4) The fine pores present in the cell wall allow the fine filaments of plasmoderma to enter,
through which life contact is maintained between two adjacent cells.
(5) It is permeable to water and substances.
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