This is Online Notes on Chapter = 5 ( THE FUNDAMENTAL UNIT OF LIFE) Part 2 for
preparation of CBSE BOARD Examination, NTSE etc.
Cell Shape :
Cells are of variable shapes and sizes. Their shape is according to the function. Generally cells are spherical but they may be elongated (nerve cell), branched (pigmented), discoidal (RBC), spindle-shaped (muscle cell) etc.
Different kinds of cell found in the human body.
Cell Size :
Size of cell is variable depending upon the type of organism.
Some are microscopic while some are visible with naked eyes.
Their size may vary from 0.2 mm to 18 cm.
• Size of a typical cell in a multicellular organism ranges from 20-30 mm.
• The largest cell is ostrich egg (15 cm in diameter with shell & 8 cm in diameter without cell).
• The longest cell is nerve cell (upto 1 m or more).
• Smallest cells so far known are PPLOs e.g., mycoplasma (0.1 mm in diameter).
• Human egg is 0.1 mm in diameter.
Components of Cell
There is an occurrence of division of labour within a cell as they all got certain specific components called ‘Cell organelles’.
Each of them perform a specific function.
The three basic components of all the cells are :
(i) Plasma membrane
(ii) Nucleus
(iii) Cytoplasm
Cell Membrane :
(a) Cell membrane is also called as plasma membrane or plasma
lemma.
(b) It is the limiting boundary of each cell which separates the
cytoplasm from its surroundings.
(c) It is found in both plant as well as animal cells.
(d) It is the outermost covering of a cell in case of animals and lies
below the cell wall in case of plants.
(e) It is made up of proteins and lipids where proteins are sandwiched between bilayer of lipids.
(f) Plasma membrane is selectively permeable in nature. It allows or permits the entry and exit of some materials in and out of the cell.
(g) Singer and Nicholson gave the fluid mosaic model of plasma
membrane. According to them, it consists of a protein layer
sandwiched between two layers of lipids. It is in quasifluid state.
It is 75A thick.
(h) It is flexible and can be folded, broken and reunited.
Functions of Plasma Membrane :
(a) It regulates the movement of molecules inside and outside the
cell.(b) It helps in maintaining the distinct composition of the cell.
(ii) Transportation of molecules across the Plasma Membrane :
This can be done by following ways :
• Diffusion : Movement of solutes or ions from higher concentration to lower concentration is called as diffusion. It does not require energy therefore, it is called as passive transport.
• Osmosis : The movement of solvent or water from higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called as "osmosis" Or the movement of solvent or water from lower concentration to higher concentration of solution through a semipermeable membrane is called as osmosis.
• Osmosis can also be called as ‘Diffusion of solvents’.
• Endomosis : Movement of solvent into the cell is called as Endomosis.
• Exosmosis : Movement of solvent outside the cell is called as Exomosis.
Types of Solutions on the Basis of Concentration:
(a) Isotonic Solution : When the concentration of the solution
outside the cell is equal to the concentration of cytoplasm of the
cell, it is called as isotonic solution.
(b) Hypertonic Solution : When the concentration of the solution
outside the cell is more than the inside the cell. Due to this, cell
looses water and becomes plasmolysed.
(c) Hypotonic Solution : When the concentration of the solution
outside the cell is lesser than that of cytoplasm of cell. Due to
this, cell swells up and bursts
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