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💡 Broad first, then fine
Scientists start with big, easily seen features and then look closer — right down to DNA — to decide how organisms are truly related.
How does nutrition classify organisms?
Nutrition
Makes or takes food
1. Autotroph
Makes its own food
Example:
Green plants
2. Heterotroph
Depends on others
Example:
Animals, fungi
How does cell structure classify organisms?
Cell structure
Nucleus or not?
1. Prokaryote
No true nucleus
Example:
Bacteria (Monera)
2. Eukaryote
Has a true nucleus
Example:
Plants, animals
How does cell number classify organisms?
Cell number
One cell or many?
1. Unicellular
Made of one cell
Example:
Amoeba, bacteria
2. Multicellular
Made of many cells
Example:
Plants, animals
What criteria are used to classify living organisms?
Criteria to Classify
External features - shape, size, body plan Mode of nutrition - autotroph or heterotroph Cell structure - prokaryote or eukaryote Genetic similarity - studied using DNA
- External features (shape, size, body organisation) and internal structures (skeletons, organs, tissues).
- Mode of nutrition (autotrophic or heterotrophic), cell structure (uni/multicellular, eukaryote/prokaryote, cell wall) and ecological role (producer, consumer, decomposer).
- Reproduction (asexual and/or sexual) and genetic similarity (inherited features, studied using DNA); similar features suggest a common ancestor.
NCERT Question 3 — Observe two different animals in
Observe two different animals in your immediate environment. What features help you distinguish between them? How do these features help place them into different groups?
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NCERT Question 4 — How would a scientist justify
How would a scientist justify choosing cellular organisation as a more fundamental characteristic for the basis of classification rather than the presence of xylem and phloem?
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⏸▶ Pause and Ponder
1. If many organisms share common features, could they also share a common ancestry? Yes — similar inherited features suggest that organisms have evolved from common ancestors.
Important Definitions
- Mode of nutrition — whether an organism is autotrophic (makes its own food) or heterotrophic (depends on others for food).
- Ecological role — the part an organism plays in its ecosystem — producer, consumer or decomposer.
- Genetic similarity — similarity in inherited features, which scientists study in detail using DNA.
