Explain why viruses are not considered living organisms, with reference to their structure and dependence on host cells.

Viruses are not considered living organisms because they lack cellular organisation – they are not composed of cells but are simple particles consisting of nucleic acid surrounded by a protein coat (and sometimes a lipid envelope). They cannot carry out metabolism independently; they have no enzymes or metabolic pathways of their own. Replication is only possible inside a living host cell, where the virus hijacks the host’s biosynthetic machinery to produce new virions. Although viruses can respond to their environment by evolving through mutation and natural selection, they do not meet the full set of criteria for life such as homeostasis, growth and independent response to stimuli.