Characteristics of Life
The word “life” is both familiar and simple. We associate life with specific images of people, plants, or animals, but can you define exactly what makes something alive? Biologists do. Cells are the basis of all living things. Because all cells are constructed with the same set of organic molecules, they share certain characteristics in common.
Despite vast differences in organism form and function, living things require an input of energy and raw materials, or nutrients. In terms of energy, living organisms fall into one of two categories, producers or consumers. Producers such as plants can absorb energy from sunlight or other inorganic energy sources. Producers convert the energy into a more accessible form, storing chemical energy in the bonds of organic molecules. Consumers such as humans acquire energy by eating food, organic energy sources such as plants and other animals. Why do living things need energy? Energy is consumed while performing all the activities that define life.
Using energy to sense and respond to changes in the external environment, living organisms maintain homeostasis, a balance in their internal environment. Responses to an environmental signal may be immediate such as withdrawing from a negative stimulus, or complicated such as adaptations, the long-term adjustments to stimuli. In humans, increasing pigmentation of the skin is an adaptation to ultraviolet sunlight, protecting body cells from continued ultraviolet exposure. On a long timescale, populations of living organisms evolve, or accumulate heritable changes in response to environmental pressure.
All living organisms grow and reproduce. Using energy to synthesize complex molecules, living cells grow in size and divide, creating more cells. Complex living organisms develop, a process in which some cells specialize into tissues, organs, and organ systems, each performing unique functions within an organism. Whether unicellular or multicellular, simple or complex, living organisms reproduce, creating a new organism with the same genetic information. Offspring inherit genetic information from parents, grow and reproduce, and continue the chain of life.
Although the basic characteristics of life are universal, the unique ways in which the activities are accomplished help scientists organize living things into a classification system. Organisms that exist as a single cell grow and reproduce differently than organisms made of thousands of cells working together. Plants absorb energy directly from the sun, converting light energy for use or storage, while animals eat food to absorb already usable energy. At the most basic level, scientists classify organisms into three domains: Bacteria, Archaea, and Eukarya. Within the domains are additional levels of classification. This classification system enables scientists to study nature in an organized, productive manner.
Bacteria and archaeans are prokaryotic, unicellular organisms. The term prokaryotic denotes organisms that store hereditary material, DNA, directly in the cellular fluid, or cytoplasm. Prokaryotes live in nearly every imaginable habitat on Earth and include both producers and consumers.
Unlike prokaryotes, organisms in the domain Eukarya store DNA in a nucleus, a sac inside the cell. With unicellular and multicellular forms ranging from microscopic paramecia to gigantic redwood trees, eukaryotes are the most structurally diverse domain. Although originally organized into groups by physical characteristics, scientists now classify organisms based on a number of characteristics, including ancestral history, which is measured by genetic similarity, or relatedness. Classification is depicted in a family tree, with branches where groups of organisms evolved in a different direction from other groups.
Traditionally, Eukarya has been divided into 4 kingdoms: protists, fungi, plants, and animals. Within each kingdom, organisms are divided into smaller groups called phyla (singular, phylum). Each phylum is divided into classes, then orders, and then families. Within each family are genera (singular, genus), small groups of organisms that share many genetic traits in common.
Each genus includes one or more species. In species that reproduce sexually, members of the same species are defined by the ability to produce offspring with one another. Species names are written in italics, and include the genus and specific name. For example, Grus americana refers to a species of whooping cranes found in the United States. Grus is the genus name, and the G is always capitalized. Grus americana is the complete species name; americana is not used independently to refer to the whooping crane because the term means nothing alone. The word americana is actually part of several other species names in other genera, including Periplaneta americana, the American cockroach!
In the domain Eukarya, protists were originally defined as a single kingdom because of the primitive appearance compared to organisms in other kingdoms. Currently, many scientists classify protists in several different groups based on evidence that protists are too unrelated to form a single kingdom. Members of the kingdom fungi are defined by a specific trait: how they acquire energy. Rather than ingesting and disassembling food, fungi digest food outside their bodies by releasing enzymes onto the food. After enzymes digest food externally, fungi absorb the small molecules, eliminating the need for grinding mechanisms or an internal digestive organ. Dead organisms are the main food source for many fungi. By decomposing dead organic matter, fungi provide a valuable service to other living organisms including humans!
Plants are multicellular eukaryotes and the primary food source for land animals. As producers, nearly all plants absorb light energy for photosynthesis, converting carbon dioxide gas into glucose, an organic food source. Animals are multicellular consumers, unable to make food from inorganic sources. Species in the animal kingdom vary widely, from sponges at the bottom of the ocean to humans in high-rise office buildings.
In this activity, you will explore the classification of an animal, plant, and bacterium from domain to species.