Chapter 8 – Cell - Structure and Functions
Cell:
● A cell is defined as the basic smallest structural and functional unit of an organism. They are complex units which are microscopic in nature.
● The term ‘cell’ was first coined by Robert Hooke in the year 1665 when he noticed boxes or compartments in a slice of cork under microscope.
● An organism can be made up of a single cell unit and they are called unicellular organisms like amoeba, paramecium, etc.
● The single cell unit of unicellular organisms perform all the functions necessary for life like growth, digestion, respiration, excretion, and reproduction by itself.
● An organism made up of many cells is called as a multicellular organism like human beings, plants etc.
● In the multicellular organisms a group of special cells perform the same function and are termed as tissues. Similar tissues form an organ and organs form an organism. Thus each cell or group of cells has a specific function to perform.
● In general the cells exhibit a wide variety of shapes, sizes, and number and these can vary from organism to organism.
● A cell can be as big as a hen’s egg which can be seen by naked eye and as small as 0.1 - 0.5 micrometres in a bacteria.
● A cell can be of multiple shapes. It can be spherical like the red blood cells or spindle shaped like the muscle cells or branched like the nerve cells.
● The size of a cell is not related to the size of the organism, it is the function of that cell which is important. The nerve cells perform the same function of transferring messages whether it is in an elephant or a rat.
Parts of a cell:
The cell has basically three main parts, which are the cell membrane, cytoplasm, and the nucleus.
a) Cell membrane:
- This is the basic component of a cell that encloses the cytoplasm and nucleus and gives shape to the cell.
- This is also known as the plasma membrane and it separates the cells from each other.
- It is porous in nature to allow transport of substances in and out of the cell.
- The plants have an additional thick outer layer known as the cell wall.
b) Cytoplasm:
- The jelly-like substance which is present between the cell membrane and the nucleus is called as the cytoplasm.
- There are various other components of a cell which are present in the cytoplasm called organelles. They are as follows:
● Mitochondria
● Endoplasmic reticulum or the ER
● Ribosomes
● Lysosomes
● Vacuole
● Golgi body
● Plastids which are small coloured bodies in the cytoplasm in plants. The green coloured ones are called chloroplasts.
● Centrosome which is present only in animal cells.
c) Nucleus:
- This is the dense spherical component that is present in the centre of the cell.
- This is the most important part of a living cell as it controls all the activities of the cell.
- It also contains thread like structures called chromosomes which carry the hereditary material in the genes.
d) Nuclear membrane:
- The nucleus of a cell is separated from the cytoplasm by a porous membrane called nuclear membrane which functions to transport substances between the cytoplasm and the nucleus.
e) Nucleolus:
- There are small spherical bodies present in the nucleus which are known as nucleolus.
Types of Cell
a) Prokaryotic cells:
The cells that lack a nuclear membrane for the nucleus are called prokaryotic cells, like that of bacteria and blue green algae.
b) Eukaryotic cells:
The cells that have a defined nuclear membrane for the nucleus are called eukaryotic cells. All organisms except bacteria and blue green algae are eukaryotic cells.
Comparison of Animal and Plant cell:
