Unicellular and multicellular organisms

Unicellular organisms (e.g., bacteria, protists, unicellular fungi) consist of only one cell. That single cell carries out all life processes such as nutrition, exchange of substances, and reproduction.
Multicellular organisms (e.g., plants, animals, humans, fungi, algae) are made up of a large number of cells. Their cells are specialized – each type of cell has a particular function (e.g., nerve cells transmit impulses, muscle cells enable movement).
In multicellular organisms, cells communicate and cooperate with each other.

In Short

Unicellular organism = one cell only (bacterium, yeast).
Multicellular organism = many cells → tissues → organs → organ systems → organism.
The cell is the fundamental unit of structure and function in all living beings.

Scientists believe that multicellular organisms originated when cells, after dividing, did not separate but stayed together.
Over time, these cells became specialized for different functions, which led to a division of labor among them.

There is a hierarchy:

Cell – the basic unit of structure and function of all living things.
Tissue – a group of similar cells with the same structure and function.
Organ – made of different tissues working together to perform a function (e.g., heart, leaf).
Organ system – a group of organs working together for more complex functions (e.g., digestive system in humans).
Organism – the whole living being, made of all organs and systems.

Plants have vegetative organs (root, stem, leaf) and reproductive organs (flower, fruit, seed).
Animals and humans have various organs connected into systems (e.g., digestive, respiratory, circulatory systems).
Both plants and animals have organs and systems of organs, but they differ in structure and function.