The Living World

Chapter overview

Biology is the study of living organisms. The living world is diverse, organised, and dynamic. This chapter introduces biodiversity, classification, taxonomy, systematics, species, taxonomic hierarchy, and binomial nomenclature.

1. What is living?

Living organisms show several features:

• Growth
• Reproduction
• Metabolism
• Cellular organisation
• Consciousness
• Response to stimuli
• Adaptation

2. Growth

Growth means increase in mass and number of cells.

In multicellular organisms:

$$\text{Growth} = \text{increase in cell number} + \text{increase in cell size}$$

But growth alone cannot define life, because non-living things like crystals can also grow by external deposition.

3. Reproduction

Reproduction is the production of new individuals of the same kind.

Types:

Amoeba binary fission:

Note: Reproduction is not a defining property of all living organisms because sterile organisms, worker bees, and mules may not reproduce.

4. Metabolism

Metabolism is the sum of all chemical reactions occurring inside the body.

$$\text{Metabolism} = \text{Anabolism} + \text{Catabolism}$$

Anabolism:

$$\text{Simple molecules} \rightarrow \text{Complex molecules}$$

Catabolism:

$$\text{Complex molecules} \rightarrow \text{Simple molecules} + \text{energy}$$

Example of respiration:

$$C_6H_{12}O_6+6O_2 \rightarrow 6CO_2+6H_2O+\text{energy}$$

Metabolism is a defining feature of living organisms.

5. Cellular organisation

All living organisms are made of cells.

Cell theory summary:

• All living organisms are composed of cells.
• Cell is the basic unit of life.
• New cells arise from pre-existing cells.

6. Consciousness

Consciousness means the ability to sense surroundings and respond to environmental stimuli.

Examples:

• Plants bend towards light.
• Humans respond to sound, smell, touch, and temperature.
• Organisms respond to chemicals, light, water, gravity, and predators.

Stimulus-response flow:

7. Biodiversity

Biodiversity means variety of living organisms present on Earth.

Levels of biodiversity:

8. Need for classification

Classification is needed because:

• There are millions of organisms.
• It makes study easier.
• It helps identify organisms.
• It shows relationships among organisms.
• It helps understand evolution.

9. Taxonomy

Taxonomy is the science of identification, nomenclature, and classification of organisms.

Main steps:

10. Systematics

Systematics studies diversity and evolutionary relationships among organisms.

It includes:

• Identification
• Nomenclature
• Classification
• Evolutionary relationships

11. Species concept

Species is the basic unit of classification.

A species is a group of organisms that:

• have similar characteristics,
• can interbreed naturally,
• produce fertile offspring.

Example:

$$\textit{Homo sapiens}$$

12. Taxonomic hierarchy

Taxonomic hierarchy arranges organisms in different ranks.

$$\text{Kingdom} \rightarrow \text{Phylum/Division} \rightarrow \text{Class} \rightarrow \text{Order} \rightarrow \text{Family} \rightarrow \text{Genus} \rightarrow \text{Species}$$

Mnemonic:

King Philip Came Over For Good Soup
Kingdom Phylum Class Order Family Genus Species

13. Example of taxonomic hierarchy

Human classification:

Mango classification:

14. Binomial nomenclature

Binomial nomenclature gives every organism a two-word scientific name.

Format:

$$\text{Scientific name} = \text{Genus} + \text{Species}$$

Example:

$$\textit{Homo sapiens}$$ $$\textit{Mangifera indica}$$

Rules:

• The biological name has two words.
• First word is genus.
• Second word is species.
• Genus starts with capital letter.
• Species starts with small letter.
• Scientific name is italicised when printed.
• When handwritten, both words are underlined separately.

15. Taxonomical aids

Taxonomical aids help in identification and classification.

16. Herbarium

A herbarium is a storehouse of collected plant specimens that are dried, pressed, and preserved on sheets.

Herbarium sheet usually contains:

• date of collection,
• place of collection,
• local name,
• scientific name,
• family,
• collector's name.

17. Biological key

A key is used for identification based on contrasting characters.

Example:

18. Common mistakes

• Treating growth as the only defining feature of life.
• Forgetting that metabolism is a defining property of living organisms.
• Confusing taxonomy and systematics.
• Writing species name with capital letter.
• Not italicising scientific names.
• Mixing order of taxonomic hierarchy.
• Using phylum for plants instead of division.

19. Quick revision table