Characteristics of Living Organisms and Classification of Life

The Unifying Characteristics of Living Organisms

In order for any entity to be classified as 'living,' it must fulfill eight specific criteria that apply to all living organisms. These essential life processes are summarized by the acronym MRS C GREN: Movement, Respiration, Sensitivity, Control, Growth, Reproduction, Excretion, and Nutrition. If an entity does not perform all of these processes, it is categorized as either dead or non-living. For instance, viruses are considered non-living particles or agents because they do not carry out these processes independently.

Nutrition in Living Organisms

All organisms must obtain food to provide the energy and raw materials required for growth and development. Energy is a prerequisite for carrying out vital life processes such as movement and excretion. Nutrition in plants is achieved through photosynthesis, where they utilize sunlight, carbon dioxide ($CO_2$), and water ($H_2O$) to produce oxygen ($O_2$) and glucose ($C_6H_{12}O_6$). In contrast, nutrition in animals involve the consumption of other living organisms. Animals must break down large, complex organic molecules into simpler molecules through the process of digestion. These simplified molecules are absorbed into the bloodstream and transported to cells for use in respiration, growth, and tissue repair.

Respiration and Energy Transfer

Respiration is a fundamental chemical process occurring in every living cell. It involves the release of energy from glucose. This can occur in the presence of oxygen, known as aerobic respiration, or in the absence of oxygen, known as anaerobic respiration. The chemical reactions involved in respiration ultimately result in the production of carbon dioxide ($CO_2$) and water ($H_2O$) as waste products. The energy released during this process is transferred and stored in the form of ATP (adenosine triphosphate). It is critical to distinguish respiration from gas exchange; gas exchange is the physical process of taking in oxygen and removing carbon dioxide, whereas respiration is the cellular chemical process that utilizes that oxygen to produce energy.

Metabolic Reactions and the Process of Excretion

Metabolism encompasses all the chemical reactions taking place inside living cells. Many of these metabolic reactions produce waste products, some of which are toxic and must be eliminated to prevent harm to the organism. Excretion is the formal definition for the removal of these toxic materials and metabolic waste substances. In animals, primary excretory products include carbon dioxide ($CO_2$) and water ($H_2O$) from aerobic respiration, along with urea, a nitrogenous waste resulting from the breakdown of proteins. In plants, the waste products vary: oxygen ($O_2$) is excreted as a byproduct of photosynthesis, while carbon dioxide ($CO_2$) is excreted during respiration. A common point of confusion is the difference between excretion and egestion; excretion involves waste produced by chemical reactions within cells, while egestion refers to the removal of undigested remains (faeces) that were never absorbed into the body during digestion.

Sensitivity and Response to Surroundings

Sensitivity refers to an organism's ability to detect and respond to stimuli in its environment, a trait essential for maximizing survival chances. In humans, this is managed by the nervous system and the endocrine system. The nervous system utilizes a complex network of receptors, neurones, and effectors to respond to stimuli via electrical impulses. The endocrine system uses chemical messengers called hormones, which are transported through the blood to trigger responses. Plants also respond to stimuli, though their processes are controlled by chemicals and are typically much slower. Key plant responses include geotropism, where roots grow downward in response to gravity, and phototropism, where shoots grow toward sunlight.

Movement and Locomotion

Movement is defined as an action by an organism that causes a change in position or place. While the movement of an entire organism from one place to another is called locomotion, plants exhibit movement despite being stationary. For example, flowers like sunflowers can change their orientation throughout the day to track the position of the sun.

Control of the Internal Environment (Homeostasis)

Living organisms must maintain their internal environment within narrow required limits, a process known as homeostasis. In humans, a primary example is thermoregulation, which is the control of body temperature. The optimum human body temperature is $37\,^{\circ}C$. If the temperature rises, such as during exercise, the body initiates cooling mechanisms like sweating or vasodilation. Another human homeostatic mechanism is osmoregulation, which is the control of internal water levels. Plants achieve homeostasis and maintain suitable temperatures through transpiration, where water evaporates from the stomata on the underside of leaves to facilitate heat loss.

Reproduction: Sexual and Asexual Processes

Reproduction is the process leading to the production of more individuals of the same kind, ensuring the survival of the population and the species. There are two primary types: sexual and asexual reproduction. Sexual reproduction involves the fusion of male and female gametes. In humans, these are the sperm and the egg; in plants, these are the pollen grain (male) and the ovule (female). The resulting offspring possess a mix of maternal and paternal DNA. Asexual reproduction involves only one parent and results in an exact clone with DNA identical to the parent. This can occur via mitosis. Single-celled organisms like bacteria or the protoctist amoeba reproduce asexually. Plants can reproduce asexually naturally through runners or artificially through cuttings.

Growth and Development

Growth is defined as a permanent increase in size. In animals, this involves an individual growing from a zygote into an adult, often accompanied by changes in body proportion and shape. In plants, growth is an ongoing process throughout the individual's entire life, with the continuous formation of new shoots, leaves, and branches year after year.

Common Features of Eukaryotic Organisms

Living organisms are categorized into five main groups based on shared features: Plants, Animals, Fungi, Protoctists, and Prokaryotes. The first four groups—Plants, Animals, Fungi, and Protoctists—are eukaryotic organisms (eukaryotes). Eukaryotic organisms can be multicellular or single-celled, but their defining feature is that their cells contain a nucleus with a distinct, bound membrane.

Characteristics of Animals, Plants, Fungi, and Protoctists

Animals are multicellular eukaryotes that lack cellulose cell walls and chloroplasts, meaning they cannot photosynthesize. They feed on organic substances, often store carbohydrates as glycogen, possess nervous coordination, and are capable of movement. Plants are multicellular eukaryotes with cell walls made of cellulose and chloroplasts for photosynthesis; they store carbohydrates as starch or sucrose but lack nervous coordination. Fungi can be multicellular (like Mucor) or single-celled (like yeast). Multicellular fungi feature thread-like structures called hyphae, organized into a mycelium. They have cell walls made of chitin, lack chloroplasts, and feed via saprotrophic nutrition (secreting extracellular digestive enzymes onto food and absorbing the resulting molecules). Protoctists are a diverse group of mostly microscopic, single-celled organisms. Some resemble animal cells (e.g., Plasmodium), while others resemble plant cells with cell walls and chloroplasts (e.g., Chlorella), and they lack nervous coordination.

Common Features of Prokaryotic Organisms

Prokaryotes, such as bacteria, differ significantly from eukaryotes. They are always single-celled and lack a nucleus. Their genetic material is found in the cytoplasm as a single, circular chromosome of DNA. Prokaryotic cells are substantially smaller than eukaryotic cells and lack mitochondria or other membrane-bound organelles. Bacterial cells consist of a cell wall, cell membrane, cytoplasm, and plasmids (small loops of DNA). While most bacteria feed on other living or dead organisms (saprobionts/decomposers), some can photosynthesize using chlorophyll-like substances and specific enzymes, despite lacking chloroplasts. Examples include Lactobacillus bulgaricus, a rod-shaped bacterium used in yoghurt production, and Pneumococcus, a spherical bacterium that causes pneumonia.

Pathogens and Pathogenic Diversity

A pathogen is any organism that causes disease in another organism. Pathogenic groups include bacteria, fungi, protoctists, and viruses. Pathogenic bacteria like Pneumococcus cause pneumonia, an inflammation of the lungs resulting in fever, coughing, and shortness of breath. Pathogenic fungi are more common in plants, though some Mucor species are pathogenic. Pathogenic protoctists include Plasmodium falciparum, which causes malaria and is spread by mosquitoes, leading to fever, chills, and fatigue.

The Nature and Impact of Viruses

Viruses are not considered living organisms because they do not carry out the eight life processes independently; they only display reproduction, and even then, they must hijack a host cell's metabolic pathways to replicate. Viruses are parasitic particles, always smaller than bacteria, and can infect every type of living organism. They lack a cellular structure, consisting instead of a protein coat surrounding a single type of nucleic acid (either DNA or RNA).

Specific examples include the Tobacco Mosaic Virus (TMV), the first virus isolated by scientists, which infects over 150 plant species (including tomatoes and cucumbers). TMV causes a mosaic pattern of leaf discoloration by infecting chloroplasts, which reduces photosynthesis and crop yield; it can survive in soil for up to $50\,\text{years}$. The Influenza virus infects human airway cells, causing flu symptoms like high temperature and body aches. The Human Immunodeficiency Virus (HIV) can lead to AIDS (Acquired Immunodeficiency Syndrome). HIV attacks the immune system, particularly cells in the lymph nodes, and can remain hidden for years. It is spread through direct sexual contact, exchange of bodily fluids (such as sharing needles), or from mother to child during birth or breastfeeding. While there is no cure, antiretroviral drugs can slow or halt the progression to AIDS.