Short Notes : The Fundamental unit of life


Cell is called the fundamental unit of life . A cell is  the fundamental and structural unit of life. All living beings are composed of the basic unit of life, i.e. cell.


The cell was discovered by Robert Hooke in 1665 when he saw honeycomb like structures that organisms and plants were made up of , he named these compartments as cells .However what Hooke actually saw was the dead cell walls of plant cells (cork) as it appeared under the microscope.. Later in 1839 Robert Brown discovered the nucleus of the cell


Cell theory states that

• All organisms are made up of cell .

• Cell is the fundamental unit of life .

• All new cells are formed from existing cells .


Cells come in all shapes and sizes.  cells are spherical in shape, rectangular , spiral or irregular in shape .

• Most of the cells are microscopic in size, i.e. it is impossible to see them with naked eyes. Some cells are fairly large, e.g. a neuron in human body can be as long as 1 meter. The egg of an ostrich is the largest known cell of a living animal and an average egg is 15 cm long and 13 cm wide.

A cell is enclosed in a membranous casing and is filled with a liquid substance which is called the cytoplasm. There are many cell organelles in a typical cell.


Cells can be of two types

1 Prokaryotic Cell – These cells lack nucleus and they are primitive cells  . Procaryotes are unicellular organisms . Example : blue green algae , archaebacteria etc

2 Eukaryotic Cell – These cells contain a nucleus . They are advanced and complete cells . Eukaryotes include all living organisms that are unicellular or multicellular  .

Example – bacteria , protozoan , human cells etc .


A cell is enclosed in a membranous casing and is filled with a liquid substance which is called the cytoplasm. There are many cell organelles in a typical cell.

Some of the main structures of a cell are as follows:

Cell wall: Cell wall is made of cellulose. It is somewhat hard but permeable to most of the substances. Cell wall is available in plant cells and in cells of bacteria and fungi. It is an outer covering of cell .


Plasma membrane: Plasma membrane is a semi-permeable membrane. It is composed of a second layer of lipid and protein.Functions of Plasma Membrane: Plasma membrane provides a container to the cytoplasm. It facilitates passage of various substances in and out of the cell.

Nucleus: Nucleus is covered by double membrane; called nuclear membrane. The fluid which is inside the nucleus is called nucleoplasm . Nucleus contains chromosomes which are important for the functioning of a cell. Chromosomes contain genes which are the carriers of genetic information. Nucleus plays an important role during cell division. Nucleus controls all the functions of the cell.

Mitochondria: Mitochondrion is a capsule-like structure. It is a double membrane structure.. Its inner membrane is projected into numerous finger-like structures; called cristae. Mitochondria are the sites of cellular respiration. After cellular respiration, energy is stored in the form of ATP (Adenosine triphosphate) in mitochondria.Functions of Mitochondria:- Cellular respiration is the main function due to which, mitochondria are also known as the ‘powerhouse of the cell’.

Endoplasmic Reticulum: Endoplasmic reticulum is a mesh-like structure which is composed of numerous tubes. It extends from the plasma membrane to the nuclear membrane. There are two kinds of endoplasmic reticulum, viz. smooth ER and rough ER. Rough ER has ribosomes on its surface which give it the rough appearance.

Function of ER: It serves as the transport channel in the cell. Substances are transported from cell membrane to cytoplasm and to nucleus and vice-versa. ER also serves the role of packaging many substances in the cell.

Golgi Complex: Golgi Complex was discovered by Camillo Golgi. It is composed of many sac-like structures which are stacked one above another. It  is responsible for packaging of various substances in the cell.

Lysosome: Lysosome are small sac-like structures and they are derived from Golgi complex. Lysosome contains digestive enzymes.

Functions of lysosome: The enzymes in the lysosome digest foreign particles and thus destroy them. Sometimes, the lysosome may burst open and its content ends up digesting the contents of the cell. The cell gets killed in the process. Due to this, lysosome is also called the ‘suicide bag of the cell’.

Ribosome: These are tiny dot like structures interspersed in the cytoplasm and also on the surface of Rough ER. Ribosome is responsible for protein synthesis.

Plastids: These are similar to mitochondria; in appearance. Plastids are found in plant cells. They are of two types, chromoplast and leucoplast. Colourful plastids are called chromomplast and colourless plastids are called leucoplast. Chloroplast is green in colour and is found in green parts of plants. Plastids too have their own DNA and ribosome.

Functions of Plastids: Leucoplasts are responsible for storing food; such as carbohydrates, protein and lipid. Chromoplasts impart various colours to the plant parts. A leaf of a plant is green in colour because of chloroplast. Chloroplast is the site of photosynthesis.

Vacuoles: These are fluid filled chambers and are often seen in many cells. Vacuoles are very large in plant cells. A plant cell usually has single but large vacuole. Such a vacuole fills almost the entire space inside the cell. Vacuoles are much smaller and very few in animal cells.



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