How can a single-celled organism carry out all its life processes when billions of cells are required to perform similar functions in multicellular organisms like us? [Pg. No. 236]

A single-celled organism can carry out all life processes because its one cell is a complete living unit. The cell membrane, cytoplasm, nucleus and other cell parts perform functions such as nutrition, respiration, excretion, movement, growth and reproduction within the same cell. Since the organism is very small, substances can easily enter, move inside and leave the cell. In multicellular organisms like humans, the body is large and complex, so different cells become specialised for different functions, such as nerve cells for messages, muscle cells for movement and blood cells for transport. Thus, unicellular organisms show all life processes in one cell, while multicellular organisms show division of labour among many cells.