IGCSE Biology: Characteristics of Living Organisms
The Unifying Characteristics of Living Organisms
For any entity to be classified as 'living,' it must fulfill a specific set of criteria common to all living organisms. These life processes are often summarized by the mnemonic acronym MRS C GREN: Movement, Respiration, Sensitivity, Control, Growth, Reproduction, Excretion, and Nutrition. If an entity does not perform all eight of these processes, it is considered either dead or non-living. Viruses serve as a primary example of non-living particles or agents because they do not independently carry out these essential functions.
Nutrition and Respiration
Nutrition is the process by which organisms obtain food to provide the energy necessary for critical life functions such as movement, respiration, and excretion. Organisms are fundamentally divided into two categories based on how they obtain nutrients: autotrophs and heterotrophs. Plants are autotrophic, meaning they create their own food using sunlight, carbon dioxide, and water to produce oxygen and glucose through photosynthesis. The term 'autotroph' is derived from 'auto' (self) and 'trophic' (feeding). Animals, conversely, are heterotrophic, meaning they consume other living organisms to obtain energy. They must break down large, complex molecules into simpler ones through the process of digestion. The term 'heterotroph' comes from 'hetero' (different) and 'trophic' (feeding).
Respiration is a chemical reaction occurring within the cells of all living organisms to release energy from glucose. This can occur in two forms: aerobic respiration, which requires the presence of oxygen, and anaerobic respiration, which occurs in the absence of oxygen. These reactions ultimately produce energy transferred in the form of , with carbon dioxide and water generated as waste products. It is vital to distinguish respiration from gas exchange; gas exchange is the physical process of moving oxygen into cells and carbon dioxide out, whereas respiration is the internal chemical use of oxygen to release energy.
Excretion and Response to Surroundings
Excretion is defined as the removal of toxic materials and substances from an organism that are products of metabolic reactions. Metabolism refers to the chemical reactions taking place inside living cells. In animals, major waste products include carbon dioxide and water from respiration, as well as urea, a nitrogenous waste resulting from the breakdown of proteins. Humans utilize specific organs for excretion, such as the lungs for carbon dioxide and the kidneys for urea and excess water. It is important to differentiate excretion from egestion: excretion removes waste generated by cellular chemical reactions, while egestion refers to the removal of undigested remains (faeces) that were never absorbed by the body.
Plants also excrete waste products. During the day, oxygen produced from photosynthesis is excreted, while at night, carbon dioxide from respiration is excreted. Water is also excreted as a byproduct of respiration and other chemical processes. Sensitivity refers to an organism's ability to detect and respond to stimuli in its environment, a trait essential for survival. In humans, the nervous system uses receptors, neurones, and effectors to respond via electrical impulses, while the endocrine system uses hormones (chemical messengers) in the blood. Plants respond more slowly using chemicals. Examples of plant responses include geotropism, where roots grow downward in response to gravity, and phototropism, where shoots grow toward sunlight.
Movement, Control, and Homeostasis
Movement is a definitive action by an organism causing a change in position or place. While animals typically engage in locomotion (moving from one place to another), plants are generally stationary but can change their orientation. A notable example is sunflowers, which track the sun to change their orientation throughout the day. Control, or homeostasis, is the ability of an organism to maintain a constant internal environment within required limits despite external changes.
In humans, homeostasis includes thermoregulation, the maintenance of an optimum body temperature of . If body temperature rises, such as during exercise, the body initiates control mechanisms like sweating or vasodilation to return to the optimum. Other human homeostatic processes include glucoregulation (control of blood glucose levels) and osmoregulation (control of water levels). Plants maintain their temperature through transpiration, where water evaporates from the stomata on the underside of leaves, facilitating heat loss to prevent overheating.
Reproduction and Growth
Reproduction is the process leading to the production of more individuals of the same kind, ensuring the survival of the population and species. Sexual reproduction involves the fusion of male and female gametes (sperm and egg in humans; pollen and ovule in plants) to form a zygote. The resulting offspring contain a combination of maternal and paternal DNA. Asexual reproduction involves only one parent and results in an exact clone with identical DNA, often through mitosis. Examples include bacteria, amoeba, and plants using runners, tubers, or budding.
Growth is defined as a permanent increase in size. In animals, this involves an individual growing larger between the zygote and adult stages, often with significant changes in proportion or shape. In plants, growth is an ongoing process throughout their entire lifespan, as they continually form new shoots, leaves, and branches year after year.
Common Features of Eukaryotic Organisms
Living organisms are classified into five kingdoms: Animals, Plants, Fungi, Protoctists, and Prokaryotes. The first four kingdoms are composed of eukaryotic organisms, or eukaryotes. These organisms can be multicellular or single-celled and are characterized by cells containing a nucleus with a distinct membrane. In animal cells, key structures include the nucleus (DNA storage), cytoplasm (the site of chemical reactions like anaerobic respiration), the cell membrane (controls substance entry/exit), ribosomes (protein synthesis), and mitochondria (aerobic respiration site). Animals are multicellular, lack cell walls and chloroplasts, feed on organic substances, and store carbohydrates as glycogen.
Plants are multicellular eukaryotes with cellulose cell walls and chloroplasts containing chlorophyll for photosynthesis. They store carbohydrates as starch or sucrose and lack nervous coordination. Fungi can be multicellular or single-celled (like yeast). Multicellular fungi, such as Mucor, consist of thread-like hyphae organized into a network called a mycelium. They have chitinous cell walls, lack chloroplasts, and practice saprotrophic nutrition by secreting extracellular enzymes onto decaying matter to absorb digested 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).
Common Features of Prokaryotic Organisms
Prokaryotic organisms, known as prokaryotes, belong to their own kingdom and differ significantly from eukaryotes. They are always single-celled, substantially smaller, and lack a membrane-bound nucleus; instead, their genetic material (circular DNA) is found in the cytoplasm. Bacteria are the primary example of prokaryotes. They possess a cell wall, cell membrane, cytoplasm, and plasmids (small loops of DNA), but they lack mitochondria and other membrane-bound organelles.
Examples of bacteria include Lactobacillus (a rod-shaped bacterium used in yogurt production) and Pneumococcus (a spherical bacterium that causes pneumonia). Bacteria feed in various ways: some perform photosynthesis using chlorophyll despite lacking chloroplasts, while others feed on living or dead organic matter. Those that feed on dead matter are known as saprobionts or decomposers.
Pathogens and Viruses
A pathogen is any microorganism that causes disease in another organism, including various bacteria, fungi, protoctists, and viruses. Pathogenic bacteria include M. tuberculosis, which causes tuberculosis in humans and is often linked to poor sanitation. Pathogenic fungi are common in plants, such as Black Sigatoka in bananas, which creates black streaks on leaves and prevents photosynthesis, eventually killing the leaf. In humans, fungal pathogens cause cattle ringworm and athlete's foot. Pathogenic protoctists include Plasmodium falciparum, which is spread by mosquitoes and causes malaria.
Viruses are unique because they are not considered living; they only reproduce by taking over a host cell's metabolic pathways. They are small parasitic particles with a protein coat and either DNA or RNA. Examples include the Tobacco Mosaic Virus (TMV), which prevents chloroplast formation in plants and can survive in soil for 50 years. Human Immunodeficiency Virus (HIV) leads to AIDS by attacking the immune system; it is spread via bodily fluids or from mother to child. Influenza viruses (A, B, and C) infect airway cells, causing flu symptoms, with Influenza A being the most common global cause.