Biological Organisms

Biology studies organisms and their life-sustaining processes. But what exactly are living organisms? How do we distinguish living organisms like mosses and elephants from nonliving things like rocks and smartphones? 

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Table of contents

    In the following, we will define biological organisms, identify their key characteristics, discuss how they are classified, and touch on how they interact with each other and their environment in biological communities.

    What is the meaning of biological organisms?

    Biological organisms are individual living entities that share key characteristics or functions, including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.

    Although a biological organism is an individual being, in nature it interacts with other organisms in a biological community.

    What are the characteristics shared by biological organisms?

    Think of a plant, a fungus, an animal, or a bacteria. Biological organisms, or living beings, are so diverse that sometimes it is difficult to identify which characteristics define them. Do all these entities really share some basic traits? Let's look at the main characteristics biologists use to define a biological organism.


    Biological organisms are organized and coordinated structures made up of one or more cells, which are tiny structures we consider as the fundamental unit of life.

    Each cell is incredibly complex: at the fundamental level, it is composed of atoms. These atoms make up molecules. These molecules come together to form complex compartmentalized cell structures called organelles.

    Then, in multicellular organisms, multiple cells come together to form tissues, which then form structures with specialized functions called organs, which, in turn, work together in organ systems.

    Response to stimuli

    Stimuli (singular: stimulus) are things that can elicit a response from a living organism.

    Organisms can respond by moving toward the stimulus; this is called a positive response. They can also respond by moving away from the stimulus; this is called a negative response.

    For example, plants exposed to light stimuli might respond by bending toward the light.


    Organisms can replicate themselves by passing on their genetic information to their offspring. By passing on their genetic information, the offspring will belong to the same species and have similar traits.

    Growth and development

    Organisms grow and develop, meaning their structures and functions change over time. This change is determined by a combination of the genetic information passed on to the individual organism as well as its environment.

    The organism acquires materials or energy from its environment to allow for such changes to take place.


    Organisms require multiple complex regulatory mechanisms to coordinate their internal processes, such as transporting nutrients and responding to stimuli.


    Homeostasis is the ability of organisms to maintain internal balance while responding to external conditions.

    Organisms need to maintain homeostasis because their internal structures function optimally within a set of internal and external conditions.

    For example, proteins can break down or misfold when exposed to high temperatures and pH levels. For this reason, the human body needs to maintain temperatures close to 37 °C (or 98.6 °F).

    Energy processing

    Organisms need an energy source to carry out their metabolic processes. Some organisms might produce their own food by capturing energy from the sun and converting it to chemical energy, while other organisms might obtain energy by eating other organisms.

    Do all biological organisms need oxygen? What are aerobic and anaerobic biological organisms?

    Considering how we often hear that we need oxygen to live, you might think that all biological organisms need oxygen. However, for the first two billion years of the Earth’s existence, the atmosphere contained no free molecular oxygen (O2).

    Based on the fossil record, 3.5 billion-year-old microbial mats found in hot springs and hydrothermal vents are the earliest known organisms on Earth. These microbes were anaerobic, which means they did not require oxygen. Over time, other anaerobic organisms emerged, including cyanobacteria which took up water during photosynthesis and released oxygen as a by-product.

    That means we can trace the production of the world’s first free molecular oxygen to the emergence of these photosynthetic cyanobacteria about 2.6 billion years ago. With this, oxygen slowly accumulated in the atmosphere, enabling the evolution of other more complex life forms, including aerobic organisms (including us humans) that require oxygen to live.

    Classification of biological organisms

    Biological organisms can be classified into three groups called domains: bacteria, archaea, and eukarya. This classification is illustrated in the phylogenetic tree.

    A phylogenetic tree shows the evolutionary relationships among organisms through a diagram with branches and nodes.

    The nodes represent the points in evolutionary history when an ancestor forms two new, distinct species, while the length of each branch corresponds to the amount of time that elapsed since the split.

    Take some time to review the phylogenetic tree to better understand the unity and diversity of biological organisms.

    The organisms that comprise bacteria and archaea are prokaryotic, meaning they are single-celled or colonial organisms that lack membrane-bound organelles. Instead of being enclosed in a nucleus, their DNA is organized into a single circular chromosome. As prokaryotes, they reproduce through fission, a process where an individual cell replicates its chromosome and splits into two distinct cells.

    On the other hand, the members of domain eukarya are single-celled or multicellular organisms with eukaryotic cells, which means they have membrane-bound organelles, including a nucleus that separates their DNA from other parts of the cell. Unlike prokaryotes, eukaryotes have multiple linear chromosomes. Unlike prokaryotes, some eukaryotes can reproduce sexually.

    What are the three domains of life? What are examples of biological organisms from each domain?

    Now that we have cited important similarities and differences among the three domains let us take a closer look at their characteristics and cite some examples.

    Domain Bacteria

    Bacteria are a highly-diverse group of prokaryotic organisms that we can encounter in our everyday lives. Individual bacteria have three basic shapes:

    • Coccus: spherical

    • Bacillus: rod-like

    • Vibrio, spirillum, or spirochete: curved

    Bacteria are so small that the average rod-shaped individual is about 2 micrometers long and half a micrometer wide, while the average spherical bacterium is around 1 micrometer in diameter.

    Because of their size, we need to use microscopes to examine their internal and external structures.

    Escherichia coli is an example of a bacillus bacteria. It is typically found in the intestines of humans and other animals. While many are harmless, some strains of E. coli are pathogenic. The consumption of water contaminated with these strains of E. coli can cause diarrhea and other gastrointestinal illnesses.

    Streptococcus pneumoniae is an example of a coccus bacteria. It is one of the most common causes of bacterial pneumonia, which can affect one or more regions of the lungs.

    Domain Archaea

    Archaea are also prokaryotic organisms but have molecular characteristics that set them apart from bacteria. These include the following characteristics:

    • Their membrane lipids are composed of branched hydrocarbon chains attached to glycerol by ether linkages (Fig. 2).

    • Their cell walls do not have peptidoglycan, a substance typically found in bacteria cell walls.

    • Their ribosomal RNA (a molecule that forms the protein-synthesizing organelle called a ribosome) is different from those of bacteria and eukarya.

    Another distinguishing feature of archaea is their ability to live in extreme environments, which can be inhospitable for other living organisms.

    For example, Pyrolobus fumarii was found living in hydrothermal vents where temperatures can go up to 113 °C (235 °F), representing the upper limit of life.

    On the other hand, species of Picrophilus were found growing in extremely acidic soils in Japan, where the pH can go as low as 0.

    Domain Eukarya

    As mentioned earlier, organisms under the domain eukarya are different from archaea and bacteria mainly due to the presence of membrane-bound organelles like the nucleus.

    You might find references that identify four kingdoms under the domain eukarya, namely:

    • Plantae (or Plants)are multicellular organisms that produce their own food by photosynthesis and absorption. Their cells have cell walls and are typically organized into tissues.

      • Plants include mosses, ferns, conifers, and flowering plants.

    • Animalia (or Animals) are multicellular organisms that do not carry out photosynthesis and obtain nutrients by eating and digesting other organisms.

      • Examples of animals include sponges, insects, birds, and humans.

    • Fungi are unicellular or multicellular organisms with cell walls. Their cells are not organized into tissues. They do not undergo photosynthesis; instead, they absorb nutrients in their dissolved form from the environment.

      • Examples of fungi include yeasts, molds, mildew, and mushrooms.

    • Protista (or protists) are mostly unicellular, but some are colonial and multicellular species. They are diverse in terms of their feeding patterns, reproduction, and life cycles.

      • Examples of protists include algae, slime molds, and dinoflagellates.

    It is important to note that the classification of eukaryotes has been changing in the past years due to recent findings revealing genetic and evolutionary relationships among eukaryotes.

    An emerging hypothesis dissolves the kingdom Protista and divides eukaryotes into four supergroups: excavata, SAR, Archaeplastida, and unikonta. This classification was proposed because DNA evidence shows that some protists are more closely related to plants, animals, or fungi than to other protists. As such, all of these supergroups include protists.

    For example, Archaeplastida includes red algae, green algae, and plants because they share a common ancestor: a cell that devoured a photosynthetic cyanobacterium. On the other hand, unikonts include animals, fungi, and some protists, which are grouped together because of their shared ancestry.

    What is a biological community of interacting organisms?

    Organisms interact with each other on various levels. For instance, we usually make the distinction between individuals, populations, and species, which form a biological community. But there are also ecosystems, so, what is the difference between all these biological levels?

    Individuals of a species that live together in a specific area are collectively called a population.

    For example, all the pine trees in a specific forest can be considered one pine population.

    When different populations of living organisms inhabit and interact in the same area, they are called a community.

    For example, all trees, insects, and animals in the same forest form a forest community.

    The sum of all living organisms and nonliving components of their physical environment constitute an ecosystem.

    For example, the forest is an ecosystem consisting of living organisms (such as plants and animals) and nonliving things (such as water, wind, and soil).

    The collection of all ecosystems on Earth is called the biosphere. The biosphere represents all the zones of life.

    Biological Organisms - Key takeaways

    • Biological organisms are individual living entities that share key characteristics or functions, including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.
    • Biological organisms share many characteristics including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.
    • Aerobic organisms require oxygen, while anaerobic organisms do not.
    • Biological organisms can be classified into three groups called domains: bacteria, archaea, and eukarya.
    • Organisms interact with each other on various levels: population, community, ecosystem, and biosphere.


    1. Zedalis, Julianne, et al. Advanced Placement Biology for AP Courses Textbook. Texas Education Agency.
    2. Reece, Jane B., et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
    3. Kaiser, Gary. “1.3: Classification - The Three Domain System - Biology LibreTexts.” Biology LibreTexts, 24 Dec. 2015.
    4. Encyclopedia Britannica. “Bacteria - Diversity of Structure of Bacteria.” Accessed 17 Sept. 2022.
    5. Encyclopedia Britannica. “Archaea | Definition, Characteristics, and Examples.” Accessed 17 Sept. 2022.
    6. OpenStaxCollege. “Groups of Protists – Biology.” Accessed 20 Aug. 2022.
    7. Georgia Tech Biological Sciences. "Prokaryotes: Bacteria and Archaea | Organismal Biology.” Accessed 17 Sept. 2022.
    8. Briggs, George M. “Chapter 1: Organisms – Inanimate Life.” Accessed 17 Sept. 2022.
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    Frequently Asked Questions about Biological Organisms

    What is the simple definition of biological organisms?

    Biological organisms are individual living entities that share key characteristics or functions, including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.

    What are 5 examples of biological organisms?

    5 examples of biological organisms are E. coli bacteria, ferns, humans, mushrooms, and algae.

    Are humans aerobic organisms?

    Humans are aerobic organisms, which means we need oxygen to live. 

    How are biological organisms classified?

    Biological organisms are classified into three groups called domains: bacteria, archaea, and eukarya. This classification is based on their evolutionary relationships. 

    What is a biological community of interacting organisms and their physical environment?

    A biological community of interacting organisms and their physical environment constitute ecosystem.

    Test your knowledge with multiple choice flashcards

    What is the distinct cellular feature that makes the Opisthokonta group stand out from other entities in the cellular world?

    What does the term "Opisthokonta" mean and what does it signify in microbiology?

    What role does gene duplication play in the evolution of Opisthokonta?


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