Viruses, prions, and viroids are excluded from the five kingdom classification system because they are not considered living organisms - they are acellular and lack a cellular structure. While viruses contain nucleic acids and reproduce within host cells, causing disease, they are classified as non-living because they do not have their own metabolism and require a host cell. Prions and viroids are also excluded from the five kingdom system because, like viruses, they are acellular particles that can cause infectious diseases in plants but lack cells of their own.

1. 3.4 STATUS OF VIRUSES
Viruses, Prions and Viroids
Dr. Syed Abdullah Gilani
Textbook of BIOLOGY
CLASS 9
FBISE, Islamabad Pakistan

2. • Why Viruses, Prions, and Viroids are excluded from the Five
Kingdom Classification System?
• Viruses are on borderline of living and non-living organisms
• They are excluded because they are NOT considered as organisms:
– Crystalline
– Acellular: do not have cellular organization (i.e., Basic structure and
Components of the Cell).
• They are considered as living because:
– Possess nucleic acids (DNA or RNA) enclosed in protein coat
• Viruses reproduce only in their host living cells causing diseases.

3. Prions and Viroids are NOT included in Five
Kingdom Classification because these are acellular
Prions Viroids
Differences
Composed of proteins
only
Composed of Circular
RNA only
Similarities
1. Both the particles cause infectious diseases in
certain plants
2. Acellular in nature