1. Cell-Single celled organisms: In these organisms, a single cell usually carries out all the body processes. Examples of organisms that exist as single cell are Amoeba, Euglena and Paramecium.
2. Tissue: Some organisms exist as a tissue which performs all the life functions example is the Hydra. Details about hydra are explained further down in the tutorial.
CLASSIFICATION OF LIVING THINGS
BIOLOGICAL ASSOCIATIONS / RELATIONSHIPS
IN LIVING ORGANISMS
3. Organ: Some organisms exist as an organ like the organ of storage in the onion bulb,. Another example of plant organ can be found in rhizome. Heart is an animal organ which is part of a system in the body of multicellular organisms.
4. System: Mammalian system and system in flowing plants, reproductive system, excretory system etc.
5. Complexity of organization in higher organisms, the advantages and disadvantages
In unicellular or single-celled organisms, the cell carries out the whole of the life functions on its own while in multicellular or many celled organisms, there are various levels of organization that exist among them.
Individual cell in addition to performing a particular function as well work jointly with other cells for the wellbeing of the entire organism. Thus there is division of labour among all the cells that the organism is composed of and each of the cells is dependent on the other to function effectively.