What is Living? 

Living refers to being alive and possessing the qualities of life, including growth, reproduction, metabolism, responsiveness to stimuli, and adaptation to the environment. Living organisms like plants, animals, fungi, and microorganisms exhibit these characteristics. Living things are distinct from non-living matter because they have the capacity for self-sustaining processes that maintain their existence and enable them to interact with their surroundings.

1.0Characteristics of Living 

There are five main characteristics of living, which are as follows: 

Growth 

• Growth, defined as the increase in mass or overall size of a tissue, organism, or its parts, is a fundamental characteristic of living organisms. It encompasses both an increase in mass and an increase in the number of individuals, making it a twin feature of growth. This irreversible process permanently enlarges an organ, its parts, or even individual cells. Growth is intricately tied to metabolism, where the balance between synthetic (anabolic) and destructive (catabolic) processes determines its occurrence.
• In situations where anabolism exceeds catabolism, growth occurs. Conversely, when catabolism surpasses anabolism, there is a phenomenon known as "Degrowth" or negative growth.

Growth can be categorised into two types:

• Intrinsic Growth: This type of growth originates from within living organisms' bodies.
• Extrinsic Growth: Contrary to intrinsic growth, extrinsic growth involves the accumulation of material on any external body surface. This type of growth is observed in non-living entities.

Furthermore, growth can be classified into two overarching categories:

• Indeterminate Growth: Also known as unlimited growth, this type of growth occurs continuously throughout the lifespan of organisms. It is primarily observed in plants, where growth is not restricted by age.
• Determinate Growth: Alternatively termed limited growth, this type of growth is restricted to a certain age or stage of development. It is prevalent in animals, although certain tissues undergo cell division to replenish lost cells.

In many higher plants and animals, growth and reproduction are mutually exclusive events, indicating a trade-off between these processes. While both living and non-living entities exhibit growth, intrinsic growth, originating from within the organism's body, serves as a defining property of living organisms.

Reproduction

• Reproduction, the process of producing new individuals or progeny, is a defining characteristic of living organisms. In multicellular organisms, reproduction involves the production of offspring that possess features similar to those of their parents. Conversely, in unicellular organisms such as bacteria, unicellular algae, or Amoeba, reproduction entails an increase in the number of cells, effectively making growth and reproduction synonymous in these organisms.
• Importantly, reproduction is not observed in non-living objects, which distinguishes it as a feature exclusive to living organisms. However, it's worth noting that certain living organisms, such as mules, sterile human couples, or worker bees, may not engage in reproduction, indicating that it cannot solely serve as a defining characteristic of life.

Reproduction occurs through two primary mechanisms :

• Asexual Reproduction: This form of reproduction does not involve gametic fusion, fertilization, or meiosis. Various methods of asexual reproduction exist:
• Asexual spores: Common in algae and fungi.
• Budding: Observed in organisms like yeast and Hydra.
• Fragmentation: Found in filamentous algae, fungi, and the protonema of moss plants.
• True Regeneration (Morphallaxis type): Fragmented organisms regenerate lost body parts, effectively forming new organisms (e.g., Planaria).
• Note: Regeneration (Epimorphosis type) involves repairing or regressing lost body parts, as seen in organisms like starfish and lizards.

Sexual Reproduction: This process entails the formation of gametes through meiosis, followed by fertilization to produce offspring. Sexual reproduction involves gametes formed through meiosis, with fertilization leading to the formation of progeny.

Metabolism

• Metabolism is the collective term for all the chemical reactions happening inside our bodies. Every living thing, from tiny single-celled organisms to complex multicellular creatures, has a metabolism. Non-living objects don't have a metabolism.
• Metabolism is what makes living organisms unique, as it's a universal trait with no exceptions. We've learned a lot about the chemical reactions in our bodies, so we can replicate many of them in a lab using cell-free mediums or test tubes.
• When we perform these reactions outside of a living organism, like in a test tube, they're neither alive nor not alive. Even though these reactions aren't alive, they mimic the ones that happen inside living organisms. It's important to remember that metabolism includes all the chemical reactions in our bodies, not just a select few.
• Living things are composed of various chemicals that carry out numerous reactions and create other chemicals within their bodies. Metabolism is present in all living beings, including plants, animals, fungi, and microorganisms.

2.0Cellular Organization  

• A Cell is the basic unit of life. All living organisms are composed of cells, with some existing as single-celled entities known as Unicellular Organisms, while others, like humans, consist of many cells, classified as multicellular organisms.
• Unicellular organisms can exist and carry out essential life functions independently. Anything less than the complete structure of a cell cannot sustain independent life. Therefore, the cell is the fundamental building block responsible for the structure and function of all living organisms. Thus, the cellular organization emerges as a defining characteristic of all living beings.

3.0Consciousness

• The ability to perceive and respond to environmental stimuli is termed consciousness. Consciousness is the most conspicuous and technically intricate trait among all living organisms. We perceive physical, chemical, or biological stimuli through specialized sense organs. Plants exhibit sensitivity and reactions to external factors such as light, water, temperature, other organisms, and pollutants.
• All organisms, from prokaryotes to complex eukaryotes, display consciousness in response to environmental cues. Examples include:

Plants flower in specific seasons (photoperiodism).

Animals breed only during particular seasons (seasonal breeders).

Organisms managing chemical intake.

• Human beings possess an exceptionally high level of consciousness, owing to our highly developed nervous system and advanced communication abilities, known as self-consciousness.
• Humans swiftly respond to external stimuli and can anticipate and prepare for environmental changes. Moreover, humans possess the capacity to modify their surroundings to some extent. This pinnacle of consciousness, self-consciousness, is unique to humans and not observed elsewhere.

4.0Emergent Properties of Life and Homeostasis

• All living phenomena are the result of intricate interactions among various components.
• Tissue properties do not solely reside within individual cells but emerge from interactions among these cells.
• Similarly, the characteristics of cellular organelles do not solely stem from their molecular constituents but arise from interactions among these molecules.
• These interactions lead to emergent properties at higher levels of organization.
• This phenomenon is evident across all levels of organizational complexity.
• Therefore, living organisms are characterized as self-replicating, self-organizing, evolving, and self-regulating interactive systems capable of responding to external stimuli.