What is living? Practice Questions

Correct answer: Metabolism and consciousness

Metabolism and consciousness (response to stimuli) are defining properties because they occur in all living beings without exception. Growth and reproduction are not defining properties as some non-living things grow (accumulation of matter) and some living organisms like mules or worker bees do not reproduce.

Correct answer: Growth and cellular reproduction

For unicellular organisms, growth is defined as an increase in the number of cells, which is essentially the process of reproduction. Therefore, both terms are used interchangeably in the context of single-celled life forms.

Correct answer: Non-living objects like mountains and dunes also gain mass

Growth by increase in body mass is observed in non-living objects like snow mountains or sand dunes via surface accumulation. Since the criterion is met by non-living entities, it cannot be a defining characteristic of life.

Correct answer: It is the sum of all chemical reactions in a living body

Metabolism is the sum total of all biochemical reactions in a cell or organism and is a defining feature of life. While isolated reactions can be performed in a test tube (in-vitro), these are living reactions but not living organisms themselves.

Correct answer: The ability to sense and respond to environmental stimuli

Consciousness or irritability is the ability to sense and respond to physical, chemical, or biological stimuli. It is considered the most technically sophisticated feature found in all living organisms, from prokaryotes to eukaryotes.

Correct answer: Human beings only

While all living organisms possess consciousness (awareness of their environment), only human beings possess self-consciousness. This refers to the ability to be aware of oneself as an individual.

Correct answer: Continuously; only up to a certain age

Plants exhibit indeterminate growth, meaning they continue to grow throughout their life span via meristematic activity. Animals exhibit determinate growth, where cell division occurs only up to a specific age to reach a definitive size.

Correct answer: Mules, sterile worker bees, and infertile couples

While reproduction is a characteristic of living things, it is not a defining one because certain living organisms like mules and sterile worker bees are incapable of producing offspring. However, they are still considered living due to other defining traits like metabolism.

Correct answer: Neither living nor non-living, but living reactions

Metabolic reactions performed in-vitro (outside a living system) are not organisms, so they aren't 'living things'. However, because they involve the same biochemical processes found in cells, they are categorized as 'living reactions'.

Correct answer: Only seasonal breeders (plants and animals)

Many plants and animals are seasonal breeders whose reproductive cycles are triggered by the duration of light (photoperiod). This demonstrates how organisms respond to environmental cues.

Correct answer: They are supported by machines and lack self-consciousness

A comatose patient may be kept alive with machines and lacks self-consciousness, yet still shows some properties of life such as metabolism. Because they exhibit some defining features of life but not others, they represent a boundary case in the definition of life.

Correct answer: Self-replicating, evolving, and self-regulating interactive systems.

The biological definition of life emphasizes that organisms are dynamic, self-replicating systems capable of responding to external stimuli and evolving over generations through inheritance.

Correct answer: Reproduction

Reproduction is the biological process by which an organism produces offspring (progeny) that are biologically similar to themselves, ensuring the continuity of the species.

Correct answer: Asexual spores

Fungi primarily use asexual spores as a rapid means of dispersal and reproduction. These spores can withstand harsh conditions and germinate when they find a suitable substrate.

Correct answer: Planaria (Flatworms)

While many organisms show fragmentation, Planaria exhibit true regeneration where a fragmented organism regenerates the lost part of its body and becomes a new individual.

Correct answer: Deposition of sand on a dune

The deposition of sand on a dune is an external, physical process of accretion in non-living things. Protein synthesis, glucose breakdown, and ATP production are all biochemical (metabolic) processes occurring within living cells.

Correct answer: Submicroscopic molecular level

Biological organization starts at the submicroscopic molecular level, where inorganic and organic molecules interact to form organelles and eventually the cell, the basic unit of life.

Correct answer: Interaction with environment and progressive evolution

The ability to interact with the environment and undergo progressive evolution are fundamental traits that distinguish the complexity of life from non-living matter. While non-living things can move or 'grow', they do not evolve biologically.

Correct answer: Atoms → Molecules → Cells → Tissues → Organs → Organ systems → Organism

The hierarchy of life starts from atoms, which form molecules, which then organize into cells (the first living level), followed by tissues, organs, systems, and finally the complete organism.

Correct answer: Interactions among the constituent tissues

Emergent properties arise at higher levels of organization due to interactions between simpler components. Tissues interacting together give an organ its unique functional properties that no single tissue possesses on its own.

Correct answer: Increase in mass and increase in number of individuals

NCERT defines growth in living organisms by two twin characteristics: an increase in body mass and an increase in the number of individuals (or cells). In multicellular organisms, growth occurs through cell division, while in unicellular organisms, an increase in cell number directly represents population growth.

Correct answer: Filamentous algae and Protonema of mosses

Fragmentation is the primary asexual method in filamentous algae (e.g., Spirogyra) and the protonema stage of mosses, where the organism breaks into pieces that each develop into a new individual. While Planaria shows regeneration, it is not classified primarily as fragmentation in NCERT.

Correct answer: Because organisms like mules and sterile worker bees do not reproduce

A defining characteristic must apply universally to all living things. Since some living organisms — such as mules, sterile worker bees, and infertile humans — do not reproduce, reproduction fails to meet that universal criterion and cannot be the single all-inclusive defining characteristic of life.

Correct answer: Living reactions but not living things

Isolated metabolic reactions in cell-free (in vitro) systems involve the same biochemical processes as in vivo reactions, so they qualify as 'living reactions.' However, because they lack cellular organization — the structural hallmark of life — they are not considered living things themselves.

Correct answer: Cellular level

Life's properties emerge at the cellular level, where molecular components interact in an organized system to produce characteristics like metabolism, growth, and response to stimuli. Although biological matter exists at sub-microscopic and atomic levels, those levels alone do not constitute the 'living state' — at minimum, a complete cell is required.

Correct answer: Non-living growth happens by accumulation of material on the outer surface

Growth in non-living objects (e.g., mountains, sand dunes) is extrinsic: material simply accumulates on the outer surface. Growth in living organisms is intrinsic, occurring from within through cell division and biosynthesis. This distinction is central to NCERT's definition of life.

Correct answer: Metabolism

Metabolism is the sum total of all chemical reactions — both anabolic (constructive) and catabolic (destructive) — occurring in living organisms. It is the defining characteristic of the living state: no non-living object exhibits metabolism, and disruption of metabolism leads to death.

Correct answer: Ability to sense their surroundings and respond to environmental stimuli

Consciousness — defined as the ability to sense and respond to physical, chemical, or biological stimuli — is considered the most technically sophisticated defining property of all life forms, from bacteria reacting to chemicals to complex animals responding to their environment.

Correct answer: Human beings

While all living organisms possess some form of consciousness (awareness of and response to their environment), self-consciousness — the reflective awareness of one's own existence, thoughts, and identity — is a uniquely human trait, making human beings the only organisms known to exhibit it.

Correct answer: They arise as a result of interactions among the constituent cells

Emergent properties at the tissue level are not inherent in individual cells; they arise from the organized interaction and coordination of those cells. For example, a cardiac muscle cell can contract, but the pumping of blood is an emergent property of the coordinated tissue.

Correct answer: Has lost the property of self-consciousness

A brain-dead comatose patient maintained by life-support machines still exhibits cellular metabolism, but has permanently lost self-consciousness — the ability to sense and respond to environmental stimuli. This makes such patients a boundary case that challenges standard definitions of the living state.

Correct answer: Plants grow continuously throughout life; animals grow only up to a certain age

Plant growth is indeterminate: meristematic tissues in roots and shoots allow continuous growth throughout the plant's life. Animal growth is determinate: it occurs only up to a specific developmental age, after which somatic growth stops even though cell replacement continues.

Correct answer: Self-sustaining metabolism

Metabolism — the complete set of anabolic (synthesis) and catabolic (breakdown) reactions running inside an organism — is the property unique to living things. Non-living objects have mass, a definite chemical composition, and molecular motion, but they cannot carry out self-sustaining internal chemical transformations.

Correct answer: Living systems

Living systems are defined as self-replicating, evolving, and self-regulating interactive open systems capable of responding to external stimuli. These three properties — self-replication, evolution, and self-regulation — together distinguish living systems from all non-living matter, including objects that may possess only one of these traits.

Correct answer: Absence of self-consciousness despite ongoing biological processes

A comatose brain-dead patient retains cellular metabolism supported by machines but has permanently lost self-consciousness. Because metabolism (a key living criterion) persists while consciousness (another key criterion) is absent, placing them strictly in the 'living' or 'non-living' category creates a philosophical and technical challenge.

Correct answer: Planaria

NCERT specifically highlights Planaria (flatworms) as exhibiting true regeneration, where even a small fragment can regenerate into a complete new organism. While Hydra shows budding and Starfish shows arm regeneration, Planaria is the textbook example of true whole-body regeneration.

Correct answer: Reproduction

In unicellular organisms such as bacteria or unicellular algae, the organism increases in number by cell division (binary fission). Since every act of division simultaneously constitutes growth (an increase in cell number) and reproduction (producing new individuals), the two processes are synonymous in unicellular life.

Correct answer: It is present in all living organisms, from prokaryotes to eukaryotes

Biology uses consciousness in a restricted sense: the capacity to sense the environment and respond to it. Understood that way the property has no exceptions among living things — a prokaryote moving along a chemical gradient shows it just as a mammal with a nervous system does. A property shared by every living organism and no non-living one is precisely what makes it defining rather than merely common.