In this chapter: characteristics of living organisms, diversity, taxonomy and systematics, binomial nomenclature, and the taxonomic categories from species to kingdom.What is "living"?
Biologists look for properties that separate living things from non-living ones. Some are shared by all living things, while others have exceptions. NEET often asks which properties are defining (no exceptions) and which are not.
| Property | Defining? | Why |
|---|---|---|
| Growth | No | Non-living things also "grow" (a mountain, a sand mound) by accumulating material on the surface. In living things growth is from inside. Plants grow throughout life; animals up to a certain age. |
| Reproduction | No | Mules, sterile worker bees and infertile couples do not reproduce, yet are alive. |
| Metabolism | Yes | All living organisms carry out chemical reactions. Isolated metabolic reactions in a test tube are not "living", but no living thing lacks metabolism. |
| Cellular organisation | Yes | The body is made of one or more cells. |
| Consciousness | Yes | The ability to sense the surroundings and respond to stimuli. Plants respond to light, water and temperature; even prokaryotes respond. |
Note: in unicellular organisms, growth and reproduction are the same event: the cell divides. Multicellular organisms grow by cell division and reproduce separately.Diversity and the need to classify
About 1.7 to 1.8 million species have been described so far, and many more remain unknown. Since the same organism has different local names in different places, scientists need one agreed name and one agreed way of grouping organisms.
- Identification: finding the correct name of an organism.
- Nomenclature: giving the correct scientific name.
- Classification: placing organisms into convenient groups (taxa) based on easily observable characters.
- Taxonomy: the process of characterisation, identification, nomenclature and classification.
- Systematics: classification that also takes evolutionary relationships into account. The word comes from Latin systema, "systematic arrangement". Linnaeus used Systema Naturae as the title of his book.
Binomial nomenclature
In this system, proposed by Carolus Linnaeus, every scientific name has two parts: the generic name (genus) and the specific epithet (species). Plant names follow the ICBN (International Code of Botanical Nomenclature); animal names follow the ICZN (International Code of Zoological Nomenclature).
The rules
- Names are in Latin or Latinised, whatever their origin.
- The first word is the genus; the second is the specific epithet.
- Printed names are in italics. Handwritten names are underlined, each word separately.
- The genus begins with a capital letter; the specific epithet with a small letter.
- The name of the author appears after the specific epithet, in abbreviated form. Example: Mangifera indica Linn. (described by Linnaeus).
Common trap: "Mangifera Indica" (capital I) or "mangifera indica" are both wrong. Only the genus is capitalised. A common question shows four spellings and asks which one is correct.Taxonomic categories (hierarchy)
Categories are arranged in a step-like order from the most specific to the most general. Each step is a taxon (plural: taxa).
Species → Genus → Family → Order → Class → Phylum (animals) / Division (plants) → Kingdom
- As we go up, the number of organisms in the group increases, and the number of characters they share decreases.
- Species: a group of individuals with fundamental similarities. Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion): indica, tuberosum and leo are specific epithets.
- Genus: related species. Potato and brinjal are both in Solanum; lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) are in Panthera.
- Family: related genera. Solanum, Petunia and Datura are in Solanaceae. Cats (Felis) and Panthera are in Felidae.
- Order: related families. Felidae and Canidae (dogs) are in Carnivora. Convolvulaceae and Solanaceae are in Polymoniales.
- Class: related orders. Primata (monkeys, apes, humans) and Carnivora are in Mammalia.
- Phylum: classes like fishes, amphibians, reptiles, birds and mammals form Chordata (notochord, dorsal hollow nerve cord). In plants the equivalent is Division.
Four organisms to know by heart
| Common name | Scientific name | Genus | Family | Order | Class | Phylum / Division |
|---|---|---|---|---|---|---|
| Man | Homo sapiens | Homo | Hominidae | Primata | Mammalia | Chordata |
| Housefly | Musca domestica | Musca | Muscidae | Diptera | Insecta | Arthropoda |
| Mango | Mangifera indica | Mangifera | Anacardiaceae | Sapindales | Dicotyledonae | Angiospermae |
| Wheat | Triticum aestivum | Triticum | Poaceae | Poales | Monocotyledonae | Angiospermae |
Common trap: mixing up which category is lowest. Species is the lowest; kingdom is the highest. Also note that houseflies are in order Diptera (two wings); Muscidae is the family.NEET focus
- Defining vs non-defining properties of life (growth and reproduction are not defining).
- Rules of binomial nomenclature, especially capitalisation and italics.
- The order of taxonomic categories and how shared characters change up the hierarchy.
- The four-organism table: family and order of man, housefly, mango and wheat.
Practice questions
Which one of the following is a defining property of living organisms?
- Growth
- Reproduction
- Cellular organisation
- Increase in mass
Show answer
The correct way of writing the scientific name of mango is:
- Mangifera Indica
- Mangifera indica
- mangifera indica
- Mangifera indica (not italic)
Show answer
As we move from species to kingdom in the taxonomic hierarchy, the number of common characters:
- Increases
- Decreases
- Remains the same
- First increases, then decreases
Show answer
Housefly belongs to the order:
- Muscidae
- Diptera
- Insecta
- Lepidoptera
Show answer
In unicellular organisms, which two processes are synonymous?
- Growth and metabolism
- Growth and reproduction
- Reproduction and response
- Metabolism and response
Show answer
The families Felidae and Canidae are placed together in the order:
- Primata
- Carnivora
- Mammalia
- Polymoniales