Characteristics of Living Organisms and Biological Organization

Evaluation of Living and Non-living States

Identifying the living nature of an entity is not always possible through simple observation of external features. Several instances illustrate the complexity of life. A hen egg, even if it appears inanimate, can produce a chick with full living features if incubated for a few weeks. Similarly, a tissue removed from an organism can be stored under special preservation methods for extended periods; when budded into another organism using appropriate techniques, it will manifest living characteristics. Furthermore, DNA isolated from a fossil thousands of years old can be used via gene technology to obtain new organisms with ancient characteristics. These examples demonstrate that living and non-living cannot always be separated by a clear line. While certain features are used to identify life, they may not be found in all living units simultaneously, though all living units show at least one or a few of these characteristics.

Fundamental Characteristics of Living Organisms

Living organisms are defined by a specific set of common characteristics. These include cellular organization, nutrition, respiration, irritability and co-ordination, excretion, movement, reproduction, and growth and development.

Cellular Organization and Biological Hierarchy

Cellular organization varies between unicellular and multicellular organisms. A unicellular organism consists of cytoplasm and organelles enclosed by a plasma membrane. Examples observed under a light microscope include Chlamydomonas, Euglena, Amoeba, and Paramecium. Even at this level, organelle-level organization is present. Multicellular organisms begin as a single cell called a zygote, which is the product of fertilization between an egg and a sperm. As the zygote undergoes cell division, it develops into an embryo and eventually a complex multicellular organism.

The structural hierarchy of life progresses from the simplest unit to the most complex. The cell is considered the smallest structural and functional unit of life. A group of cells modified to perform a specific function is known as a tissue. A collection of tissues forms an organ, such as the heart, eye, or tongue. Multiple organs work together to form a system, such as the blood circulatory system, and these systems collectively form the organism. For example, in the human body, organization includes skin cells, nerve cells, bone cells, muscle cells, white blood cells, red blood cells, and gamete cells. In plants, cellular organization includes pollen grains, xylem cells, phloem cells, epidermal cells, palisade cells, guard cells forming stomatal pores, and root hair cells.

Nutrition and Energy Acquisition

Nutrition is the process of obtaining energy and materials required for the maintenance of life, cell growth, and the repair of worn-out structures. Organisms are classified based on their mode of nutrition. Autotrophic organisms produce their own food. This is subdivided into photoautotrophics, which use light energy (most plants), and chemoautotrophics, which obtain energy from chemical reactions (most bacteria). Photosynthesis is the specific process where plants produce food inside chloroplasts using chlorophyll. The chemical equation for photosynthesis is represented as:

$6CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{ ext{light energy}} C_6H_{12}O_6 + 6O_2$

Food produced in leaves is stored in stems, roots, and fruits. Heterotrophic organisms, including animals, cannot produce their own food and must utilize organic compounds from plants or other living origins. This relationship is often visualized through a food web, where autotrophics serve as the base for heterotrophics, and saprophytes eventually decay organic matter.

Cellular Respiration and Experimental Evidence

Respiration is the biochemical process by which stored food is transformed into energy inside cells for metabolism. While cellular respiration cannot be seen directly, it is often observed through respiratory movements like inspiration (intake of $O_2$) and expiration (removal of $CO_2$).

Activity 02 demonstrates that $CO_2$ is a byproduct of respiration. In an experimental setup involving five bottles, air is drawn through KOH (potassium hydroxide) in bottle A to remove atmospheric $CO_2$. The air then passes through lime water in bottle B, which stays clear. After passing through bottle C containing a living organism like a frog or germinating seeds (green gram, paddy, maize, or bean), the air travels to bottle D containing lime water. The lime water in bottle D turns milky, confirming the release of $CO_2$ by the organism.

Activity 03 demonstrates the absorption of $O_2$ during respiration. Germinating seeds are placed in a flask with a small tube of KOH to absorb any released $CO_2$. As the seeds absorb $O_2$ from the flask's air, the volume of gas decreases, creating a vacuum that draws a column of colored water up a connected U-tube. Control setups where the volume of absorbed $O_2$ equals the released $CO_2$ (without KOH) show no change in water level. This experiment assumes that the initial volume of $CO_2$ in the flask is negligible and that the volume of $O_2$ absorbed is equal to the volume of $CO_2$ released.

Irritability and Co-ordination

Organisms possess the ability to respond to internal or external environmental changes, a characteristic known as irritability. A change strong enough to elicit a response is called a stimulus. Stimuli include light, sound, chemicals, and mechanical or physical vibrations, which are detected by organs like the eye, ear, nose, tongue, and skin. Co-ordination is the communication between different organs during a response, involving nerves, muscles, and hormones. Examples include humans closing their ears in response to loud noise, or insects flying toward or away from light. Plants also exhibit irritability; Mimosa leaves fold when touched, and the leaves of Thora, Tamarind, and Sesbania fold at night in response to changes in light.

Metabolism and Excretion

Metabolism refers to the sum of all chemical and physiological activities within a cell. It is divided into anabolism, the synthesis of complex compounds from simple ones (storing energy), and catabolism, the breakdown of complex compounds into simple ones (releasing energy). Excretion is the removal of waste products produced during these metabolic processes. Main excretory materials include urea, salts, $CO_2$, and water. Humans utilize kidneys for nitrogenous excretion and exhale $CO_2$ and water vapor. Plants excrete $CO_2$ during respiration and $O_2$ during photosynthesis through stomata and lenticels.

Movement and Locomotion

Movement is essential for obtaining food, protection, and reproduction. It can involve the whole organism (locomotion) or specific parts. Unicellular organisms move using structures like cilia, flagella, or pseudopodia. Multicellular organisms use muscles and specialized organs like fins, wings, or legs. Plants show movement through growth; shoot tips grow toward light (phototropism) and root tips grow toward gravity (geotropism). Organelles within cells also exhibit movement. These movements occur in response to stimuli such as light, chemicals, gravity, temperature, and touch.

Reproduction, Growth, and Development

Reproduction ensures the continuation of a species. It occurs in two forms: sexual and asexual. Sexual reproduction involves the union of gametes (sperm and egg) to form a zygote. Asexual reproduction involves a single organism producing identical offspring, such as the vegetative propagation of plants.

Growth is defined as the irreversible increase in the dry mass of an organism. In unicellular organisms, growth is the increase in cell size and volume. In multicellular organisms, it involves an increase in cell number via cell division. Development refers to the increase in complexity through cell differentiation. The three steps of growth and development are the irreversible increase in cell size, the increase in cell number, and cell differentiation. An auxanometer is used to measure plant growth by magnifying the slow growth of a shoot apex via a pulley and indicator system.

The Nature of Viruses and Borderline Cases of Life

Certain entities do not easily fit the definition of living or non-living. Some non-living things show life-like features, such as the growth of crystals or rocks and the movement of waves. Conversely, living entities like bacteria or yeast can be dried into a powder yet remain viable.

Viruses are acellular forms and are approximately $1/1000^{ ext{th}}$ the size of a bacterium, visible only under an electron microscope. They consist of a nucleic acid (DNA or RNA) enclosed in a protein capsid. Viruses lack organelles for metabolism and are only active inside a host cell. Their only living characteristic is reproduction; when a virus infects a host, it releases its nucleic acid which then multiplies to produce new viral particles. Viral diseases include banana bunchy top and curly leaf of chilies in plants, and influenza, common cold, Dengue, and AIDS in animals.