The Living World

What is Life?

Defining Characteristics of Living Organisms

1. Growth

• Living organisms grow by increasing cell number (cell division) and cell size.
• In plants, growth is continuous and occurs throughout life.
• In animals, growth is limited to a certain age (determinate).
• Non-living things can also "grow" (e.g., a crystal), but only by external addition — not from within.
• True growth = increase in mass from inside (intrinsic growth).

2. Reproduction

• All living organisms reproduce — they produce offspring of their own kind.
• Reproduction can be asexual (single parent) or sexual (two parents).
• However, mules, worker bees, and infertile individuals are still alive, so reproduction is not an absolute criterion for life.

3. Metabolism

• The sum total of all chemical reactions occurring in the body is metabolism.
• Anabolism = biosynthetic reactions (building up)
• Catabolism = breakdown reactions (energy release)
• Metabolism is the most universal and defining characteristic of life — no non-living thing metabolises.
• Isolated metabolic reactions in a test tube are living reactions but do not constitute life by themselves.

4. Cellular Organisation

• The cell is the basic structural and functional unit of life.
• All living organisms are made of one or more cells.
• Viruses are debated — they are not cellular yet replicate inside host cells.

5. Response to Stimuli (Irritability)

• Living organisms sense and respond to environmental changes (light, temperature, chemicals, etc.)
• This property is called irritability or sensitivity.
• Plants respond to light (phototropism), touch (thigmotropism), and gravity (gravitropism).
• Animals respond via the nervous system.

6. Homeostasis

• Living organisms maintain a stable internal environment despite external changes.
• Examples: body temperature regulation, blood pH maintenance, osmoregulation.

7. Consciousness / Cognition

• Living organisms are aware of their surroundings at some level.
• Even single-celled organisms respond to stimuli.
• This is considered the most defining, but also most debated, property of life.

Biodiversity

• Biodiversity = the variety of life on Earth.
• An estimated ~8.7 million species exist; only ~1.7–1.8 million have been formally described and named.
• The discipline dealing with naming, describing, and classifying organisms is taxonomy.
• The full account of organisms: cataloguing → characterisation → identification → classification → nomenclature.

Nomenclature

• Binomial nomenclature (proposed by Carolus Linnaeus) assigns every organism a two-part Latin name:
  • Genus (first word, capitalised)
  • Species (second word, lowercase)
  • Both italicised when printed; underlined when handwritten.
• Example: Homo sapiens, Mangifera indica, Panthera leo
• ICBN (International Code of Botanical Nomenclature) — plants
• ICZN (International Code of Zoological Nomenclature) — animals

Taxonomy & Systematics

• Father of Taxonomy: Carolus Linnaeus
• Father of Indian Botany: William Roxburgh

Taxonomic Hierarchy (from broadest to narrowest)

Taxonomic Aids

Species Concept

• Species = a group of organisms that can interbreed and produce fertile offspring, and share a common gene pool.
• The Biological Species Concept was formulated by Ernst Mayr.

Key Points to Remember

1. Metabolism is the most fundamental and exclusive property of life.
2. Consciousness (awareness of surroundings) is the most unique characteristic — even plants show it at a basic level.
3. Growth and reproduction alone do not define life (crystals grow; mules don't reproduce but are alive).
4. Taxonomy answers what an organism is; systematics answers how it is related to others.
5. The basic unit of classification is the species; the basic unit of life is the cell.