An organism is a living system that functions as an individual entity. All organisms are capable of growth, active metabolism, and responding to changes in their environment. They also possess the ability to reproduce, ensuring the continuation of their species. Life forms range from microscopic single cells to massive multi-cellular beings.
Defining Life: Prokaryotes and Eukaryotes
The fundamental division in the biological world separates all life into two primary groups: prokaryotes and eukaryotes.
Prokaryotes
Prokaryotic cells are characterized by their simple structure and the absence of a true nucleus. Their genetic material, the DNA, is concentrated in a region called the nucleoid, but it is not enclosed by a membrane. Prokaryotes also lack internal membrane-bound compartments, or organelles.
Eukaryotes
The defining trait of a eukaryote is the presence of a true, membrane-bound nucleus that houses the cell’s genetic information. These cells contain numerous specialized, membrane-bound organelles, such as mitochondria and the endoplasmic reticulum, which compartmentalize various cellular functions. This complex internal architecture makes up all multi-cellular life forms as well as many single-celled ones.
Examples from the Prokaryotic Domains: Bacteria and Archaea
The prokaryotes are divided into two distinct domains: Bacteria and Archaea.
Bacteria
Bacteria are ubiquitous, found in nearly every environment on Earth, from soil and water to the digestive tracts of animals. Many species play an important role in nutrient cycling, such as Rhizobium bacteria, which fix atmospheric nitrogen into a form plants can use. Other bacteria, like cyanobacteria, are photosynthetic and were responsible for oxygenating Earth’s early atmosphere.
Archaea
Archaea are genetically and biochemically unique, often thriving in environments considered too extreme for most other life. Many archaea are extremophiles, including methanogens that produce methane gas and halophiles that require extremely high salt concentrations to survive. Their cell walls lack peptidoglycan, and their cell membranes have a different lipid structure, allowing them to withstand harsh conditions like the high temperatures of volcanic hot springs.
The Essential Eukaryotes: Fungi and Protists
Fungi and Protists are diverse groups within the Eukaryote domain not classified as plants or animals.
Protists
Protists are mostly single-celled and include plant-like organisms, such as algae and diatoms, which are photosynthetic and form the base of many aquatic food chains. Animal-like protists, or protozoans, include amoebas and paramecia, which move and capture other organisms for food.
Fungi
Fungi are heterotrophs, obtaining nutrients by absorbing organic material from their surroundings. Unlike plants, fungal cells are encased in cell walls made of chitin, the same substance found in insect exoskeletons. This domain includes familiar forms like mushrooms, as well as microscopic molds and yeasts, like Saccharomyces cerevisiae used in baking. Fungi are ecologically important as primary decomposers, breaking down dead organic matter and recycling nutrients back into the ecosystem.
The Most Familiar Organisms: Plants and Animals
Plants and Animals constitute the two most visible and complex kingdoms of multi-cellular life.
Plants
Plants are defined by their autotrophic nature, producing their own food through photosynthesis using sunlight, water, and carbon dioxide. Their cells contain specialized organelles called chloroplasts and possess rigid cell walls for structural support. The plant kingdom encompasses non-flowering varieties, such as mosses and ferns, alongside the vast majority of flowering plants, including grasses and trees.
Animals
Animals are multi-cellular heterotrophs, obtaining energy by consuming other organisms. A defining feature is their mobility, allowing them to actively search for food and mates, a trait facilitated by muscle and nerve tissues. The animal kingdom encompasses invertebrates, such as insects, worms, and mollusks, which lack a backbone. The vertebrates, the sub-group with a spinal column, include fish, amphibians, reptiles, birds, and mammals, all displaying complex organization and behavior.