The Living World Class 11 Biology Notes | YoLearn.ai

Section 1

Key Biological Terms

Growth

Increase in mass and increase in number of individuals (for multicellular organisms). It is an intrinsic property in living beings, occurring from within.

Metabolism

The sum total of all biochemical reactions occurring inside a living organism. It involves catabolism (breaking down) and anabolism (building up) processes.

Consciousness

The ability of an organism to sense its surroundings or environment and respond to these environmental stimuli. It is a defining characteristic of living organisms.

Biodiversity

The total number and types of organisms present on Earth, representing the variety of life forms.

Nomenclature

The scientific system of naming organisms such that a particular organism is known by the same name all over the world.

Taxonomy

The scientific study of classification, identification, nomenclature, and description of organisms.

Systematics

The study of the diversity of organisms and all their comparative and evolutionary relationships. It includes taxonomy plus evolutionary history.

Taxon

A unit of classification representing a group of organisms occupying a particular category (e.g., species, genus, family, order).

Key (Taxonomic Aid)

A taxonomical aid used for the identification of plants and animals based on similarities and dissimilarities. It is based on contrasting characters generally in a pair called a couplet.

Defining Characteristics of Living Organisms

Understanding what makes something 'living' is fundamental. Biologists identify several characteristics, but not all are defining properties because some non-living things can exhibit them, or some living things might lack them. Here's a breakdown:

1. Growth: All living organisms grow. Growth is defined by an increase in mass and an increase in the number of individuals. In multicellular organisms, growth occurs by cell division. In plants, growth is continuous throughout their life, while in animals, it's limited to a certain age. However, non-living objects like mountains, boulders, and sand mounds also grow, but this growth is by accumulation of material on the surface (extrinsic growth). Living organisms grow from within (intrinsic growth). Thus, growth cannot be considered a defining property if we consider only the increase in mass.

1. Reproduction: It is the process by which living organisms produce offspring similar to themselves. Reproduction can be sexual or asexual. While reproduction is a characteristic of living beings, there are exceptions: mules, sterile worker bees, and infertile human couples cannot reproduce. Therefore, reproduction also cannot be an all-inclusive defining characteristic of living organisms.

1. Metabolism: This is the sum total of all chemical reactions occurring inside our body. Thousands of metabolic reactions occur simultaneously in all living organisms, from single-celled to multicellular. No non-living object exhibits metabolism. Metabolic reactions can be demonstrated outside the body in cell-free systems, which are neither living nor non-living but 'living reactions'. Therefore, metabolism is a defining feature of all living organisms without exception.

1. Cellular Organization: All living organisms are composed of cells. Whether unicellular or multicellular, the basic unit of life is the cell. Non-living entities do not exhibit cellular organization. Hence, cellular organization is a defining feature of life forms.

1. Consciousness: It is the ability to sense the surroundings and respond to environmental stimuli (physical, chemical, or biological). All organisms, from the simplest bacteria to the most complex humans, show consciousness. Humans are the only organisms that are aware of themselves, possessing self-consciousness. Therefore, consciousness is a defining property of living organisms.

In summary, the truly defining characteristics of living organisms are metabolism, cellular organization, and consciousness.

Key Points to Remember

• Defining Characteristics: Metabolism, Cellular Organization, and Consciousness are the universally accepted defining characteristics of life.
• Non-Defining Characteristics: Growth and Reproduction are characteristics but not defining because they have exceptions or can occur in non-living things (extrinsic growth).
• Binomial Nomenclature: Given by Carolus Linnaeus, it assigns each organism a two-part scientific name (Genus species).
• Rules of Binomial Nomenclature: Scientific names are italicized when typed, underlined when handwritten. The first word (genus) starts with a capital letter; the second (species) starts with a small letter.
• Taxonomic Hierarchy: The arrangement of organisms into a definite sequence of categories from Kingdom to Species (Kingdom → Phylum/Division → Class → Order → Family → Genus → Species).
• Species: The lowest taxonomic category, consisting of a group of individual organisms with fundamental similarities that can interbreed to produce fertile offspring.
• Taxonomic Aids: Tools like Herbarium, Botanical Gardens, Museums, Zoological Parks, and Keys facilitate identification and classification of organisms.
• Herbarium: A storehouse of collected plant specimens that are dried, pressed, and preserved on sheets.
• Botanical Gardens: Specialized gardens having collections of living plants for reference and identification.
• Museums: Have collections of preserved plant and animal specimens for study and reference.

Taxonomy vs. Systematics

Steps in Binomial Nomenclature Example

1. — For instance, the common mango tree.
2. — The first part of the name is the genus, which should start with a capital letter. For mango, the genus is Mangifera.
3. — The second part is the species epithet, starting with a small letter. For mango, it is indica.
4. — The entire scientific name should be italicized when printed (e.g., Mangifera indica) or underlined separately when handwritten (e.g., Mangifera indica).
5. — The name of the author (discoverer) may be written in an abbreviated form after the species epithet, e.g., Mangifera indica Linn. (for Linnaeus).

Exam Tip: Avoiding Common Traps

Examiners often set traps around the defining vs. non-defining characteristics of life. Always remember: growth and reproduction are not defining characteristics due to exceptions (e.g., sterile organisms, extrinsic growth in non-living things). Metabolism, cellular organization, and consciousness are the true defining features. Be precise with the rules of binomial nomenclature – capitalization and italics/underlining are often checked. Practice recalling the taxonomic hierarchy in order, both ascending and descending, as questions on intermediate levels are common.

Practice Questions with Solutions

• Q: Why is reproduction not considered an all-inclusive defining characteristic of living organisms? A: Because there are living organisms (e.g., mules, sterile worker bees, infertile human couples) that cannot reproduce, yet they are living.
• Q: What is the correct way to write the scientific name for humans? A: Homo sapiens (italicized) or Homo sapiens (underlined separately when handwritten).
• Q: Name two taxonomic aids that store preserved specimens. A: Museums and Herbaria store preserved specimens. Zoological parks and Botanical gardens store living specimens.
• Q: Arrange the following in correct ascending order of taxonomic hierarchy: Family, Class, Genus, Order, Species. A: Species → Genus → Family → Order → Class.

Frequently Asked Questions