Unicellular and Multicellular Organisms

Unicellular and Multicellular Organisms

Living things are made of small units called cells. All the functions and processes in the body of living things are brought about with the help of these microscopic cells.

Some organisms are made up of a single cell and these are called unicellular organisms, whereas, the organisms that are made of many cells are called multicellular organisms.

Amoeba, Paramecium and Euglena are unicellular while, fish, frog, lizard, bird and man are multicellular.

Unicellular organism

Unicellular organisms are small, usually microscopic, cannot be seen with naked eye. They are aquatic, simplest and most primitive of all animals. They perform all their physiological activities by the special structures present inside the body called organelles.

We know Amoeba is an unicellular organism It does all the activities like digestion, locomotion, respiration and reproduction.

It swallows food from the water and the food is digested in the food vacuole. Contractile vacuoles help in excretion. Respiration is by simple diffusion through the body surface. They have finger-like structures called Pseudopodia, (false foot) which help in movement or locomotion.

Amoeba

Paramecium is also a unicellular organism which lives in water and move with the help of cilia.

Paramecium

Euglena is an unicellular animal which moves with a flagellum.

Euglena

Multicellular organisms:

Majority of organisms we see around us, including animals are multicellular. In such organisms, different functions are carried out by different groups of cells or organs in their body. E.g. Jelly fish, Earth worm, snails, fish, frog, snakes, pigeon, tiger, monkey and man.

Table 2.2 The Differences between Unicellular and Multicellular Organisms.

Unicellular Organisms

• They are made up of single cell.

• The single cell of the organism can perform all the functions of life.

• These organisms are generally very small (microscopic) in size.

• They lack tissues, organs and organ systems.

• Growth occurs by an increase in the size of the cell.

eg. Amoeba, Paramecium and Euglena.

Multicellular Organisms

• They are organisms are made up of many cells.

• Division of labour exists among cells. Different cells are specialized to perform different functions.

• They are mostly large in size.

• They are composed of tissues, organs and organ systems.

• Growth occurs by an increase in the number of cells by cell division.

eg. Earthworms, Fish, Frogs, Lizard and human beings.