Characteristics and Classification of Living Organisms

The One Thing

Living organisms are identified by the life processes they carry out, while classification organizes and identifies them according to shared characteristics and evolutionary relationships. Scientific names, taxonomic hierarchies, and dichotomous keys provide standardized systems for comparing and identifying organisms.

Reactions, Processes and Experiments

Key Terms

• Movement: An action by an organism or part of an organism that changes its position or direction; plants may show movement through growth responses.
• Respiration: The chemical reactions in cells that break down nutrient molecules and release energy for life processes.
• Sensitivity: The ability to detect and respond to changes in the internal or external environment.
• Growth: A permanent increase in size and dry mass, caused by an increase in cell number or cell size.
• Reproduction: The production of new individuals, either sexually using genetic material from two parents or asexually from one parent.
• Excretion: The removal of toxic materials, substances in excess of requirements, and waste products of metabolism from the body.
• Nutrition: The taking in of materials needed for energy, growth, repair, and maintaining health.
• Classification: The organization of organisms into groups based on shared characteristics and relationships.
• Species: A group of organisms that can reproduce with one another to produce fertile offspring.
• Binomial naming: The scientific naming system in which each species has a genus name followed by a species name, for example Homo sapiens.
• Taxonomic hierarchy: The arrangement of organisms in ranked groups, commonly domain, kingdom, phylum, class, order, family, genus, and species.
• Dichotomous key: An identification tool made of paired statements or questions that lead to the name of an organism.
• Observable characteristic: A feature that can be detected or measured, such as the presence of wings, number of legs, leaf shape, or body covering.
• Vertebrate: An animal with a backbone or vertebral column.
• Arthropod: An invertebrate with jointed legs, a segmented body, and a hard external skeleton.
• Mammal: A vertebrate with hair or fur that feeds young with milk; mammals generally breathe with lungs and maintain a constant body temperature.
• Bird: A vertebrate with feathers, a beak, wings, and a hard-shelled egg; birds are warm-blooded and breathe with lungs.
• Reptile: A vertebrate with dry, scaly skin that usually lays shelled eggs on land and breathes with lungs.
• Amphibian: A vertebrate that commonly has moist skin, reproduces in water, and undergoes a life cycle involving aquatic larvae and terrestrial adults.
• Fish: A vertebrate adapted to living in water, usually with gills, fins, and scales.
• Flowering plant: A plant that produces flowers and seeds, with seeds often enclosed within fruits.
• Fern: A plant with roots, stems, and leaves that reproduces using spores rather than flowers and seeds.
• Moss: A small non-flowering plant without true roots, commonly reproducing by spores and requiring damp conditions.
• Fungus: An organism that obtains nutrients by secreting digestive enzymes onto food and absorbing the soluble products; fungi commonly have hyphae and produce spores.
• Bacterium: A microscopic single-celled organism without a nucleus, usually possessing a cell wall and circular DNA.
• Virus: A non-cellular infectious particle containing genetic material inside a protein coat; it reproduces only inside living cells.

Easily Confused

• Respiration and breathing: Respiration is a chemical process in cells that releases energy; breathing is the movement of air into and out of gas-exchange organs.
• Excretion and egestion: Excretion removes metabolic waste such as carbon dioxide and urea; egestion removes undigested food from the digestive tract.
• Growth and temporary increase in size: Biological growth is a permanent increase in dry mass; water uptake or food storage can increase size temporarily without constituting growth.
• Fungi and plants: Fungi do not photosynthesise; they obtain nutrients by external digestion and absorption.
• Bacteria and viruses: Bacteria are prokaryotic cells, whereas viruses are non-cellular particles and are generally not considered living because they cannot reproduce independently.
• Species and larger taxonomic groups: A species consists of organisms that can produce fertile offspring together; kingdoms and phyla are broader groups containing organisms with fewer characteristics in common.
• Observable and environmentally acquired characteristics: Reliable classification uses inherited, observable features rather than characteristics caused only by environmental conditions.
• Classification and identification: Classification organizes organisms into groups; a dichotomous key identifies an unknown organism by directing the user through paired choices.
• Vertebrates and invertebrates: Vertebrates have a backbone or vertebral column; invertebrates do not.
• Flowering plants and ferns or mosses: Flowering plants reproduce with flowers and seeds, whereas ferns and mosses reproduce using spores.

What Gets Asked

• State the seven characteristics of living organisms: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition, often remembered as MRS GREN. A common error is treating one characteristic alone as sufficient evidence that an organism is living.
• Distinguish respiration from breathing: Respiration is the cellular chemical process that releases energy, whereas breathing moves air into and out of gas-exchange organs.
• Distinguish excretion from egestion: Carbon dioxide and urea are excretory products; undigested food is removed by egestion.
• Complete or interpret a taxonomic hierarchy: The usual order is domain, kingdom, phylum, class, order, family, genus, species. More specific groups contain organisms with more shared characteristics and are likely to reflect closer relationships.
• Write or interpret a binomial name: The genus begins with a capital letter, the species name begins with a lower-case letter, and the complete name is italicized or underlined when handwritten. Homo sapiens is the named example.
• Use a dichotomous key: Begin with the first pair of statements, choose the option matching the specimen, and follow the indicated route. Choices must be based on clear, contrasting, observable characteristics rather than vague or overlapping descriptions.
• Compare classification evidence: Visible features can aid identification, but DNA and protein similarities, cell structure, biochemical features, and evolutionary history may provide stronger evidence of relationships.