Organism – Definition, Types, Structure, Examples

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What is Organism?

  • An organism, derived from the Ancient Greek terms ὄργανον (órganon) meaning ‘instrument’ and -ισμός (-ismós), is a complex biological system that operates as an individual life form. Every organism, regardless of its complexity, is fundamentally composed of cells, which are the basic building blocks of life. These cells function in harmony, ensuring the survival and continuity of the organism.
  • The concept of an organism is rooted in the idea of the minimal functional unit of life. To be classified as an organism, an entity must exhibit three primary traits:
    • Noncompartmentability: An organism’s structure is such that dividing it would result in a loss of its functionality.
    • Individuality: The entity possesses genetic uniqueness, genetic homogeneity, and autonomy.
    • Distinctness: The genetic information within an organism must maintain an open-system, typically represented by a cell.
  • Organisms span a vast spectrum, from unicellular entities like bacteria and archaea to multicellular beings such as animals, plants, and fungi. Regardless of their complexity, all organisms share certain fundamental capabilities: reproduction, growth, maintenance, and responsiveness to environmental stimuli. For multicellular organisms, specialization into distinct tissues and organs often occurs during developmental stages.
  • The genetic blueprint of an organism is stored within its chromosomes. In simpler life forms like bacteria and archaea, chromosomes are typically circular strands of DNA. In more complex organisms, they are linear, threadlike strands. Specific segments of this DNA, known as genes, dictate the organism’s heritable and physical characteristics. These genes encode for essential biomolecules, such as proteins and RNA molecules. Proteins, in particular, play a pivotal role in biological processes, with many functioning as enzymes that catalyze vital reactions.
  • Mutations, or changes in the genetic code, can lead to the emergence of new traits. While some mutations may be detrimental or even lethal, others can confer advantages, driving evolution and natural selection. For instance, certain bacterial strains might evolve antibiotic resistance through genetic mutations, showcasing an organism’s capacity for adaptation and change.
  • Energy is indispensable for biological processes. Adenosine triphosphate (ATP) is the primary chemical energy currency utilized by organisms. Plants and certain other organisms convert light energy into chemical energy through photosynthesis. Alternatively, energy can also be derived from cellular respiration, a process that breaks down carbohydrates.
  • Metabolism encompasses the myriad of chemical reactions that sustain life. These reactions can be categorized into anabolism, which involves building complex molecules, and catabolism, which breaks down complex molecules. Organisms have intricate regulatory mechanisms to ensure these processes occur in a balanced manner, maintaining homeostasis.
  • Furthermore, organisms possess the ability to perceive and react to environmental stimuli. For example, animals have evolved senses like sight, smell, touch, taste, and hearing to detect changes in their surroundings and respond accordingly.
  • Reproduction is another hallmark of organisms. They can reproduce either sexually, involving the fusion of gametes, or asexually, where offspring are genetically identical to the parent. As organisms progress through their life cycle, they undergo various stages, from birth to maturity, and eventually reproduction.
  • In conclusion, an organism is a dynamic and complex entity, exhibiting a myriad of characteristics that distinguish it from inanimate matter. Its existence is a testament to the intricate and harmonious interplay of molecular, cellular, and environmental factors.
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Definition of Organism

An organism is a living entity composed of one or more cells, capable of growth, reproduction, and response to stimuli, functioning as an individual life form.

Structure of an Organism

Every organism, whether unicellular or multicellular, is fundamentally composed of cells, the basic structural and functional units of life. The intricate architecture and operations of an organism can be traced back to these microscopic entities.

  1. Cellular Composition:
    • Unicellular Organisms: These are organisms that consist of a single cell, which performs all the essential functions to sustain life. Both prokaryotic and certain eukaryotic organisms fall under this category.
    • Multicellular Organisms: Comprising multiple cells, these organisms exhibit cellular specialization where different cells perform distinct roles. This specialization leads to the formation of tissues, organs, and organ systems, each with a specific function.
  2. Cellular Components:
    • Plasma Membrane: This lipid bilayer, embedded with proteins, encloses every cell, acting as a barrier and regulator for substances entering or exiting the cell.
    • Cytoplasm: A gel-like substance within the cell membrane, the cytoplasm houses various organelles and structures essential for cellular functions.
    • Organelles: These are specialized structures within eukaryotic cells that perform specific tasks. Notable organelles include:
      • Nucleus: The control center of the cell, it contains DNA, the genetic blueprint that guides protein synthesis.
      • Endoplasmic Reticulum and Ribosomes: These structures are involved in protein synthesis and maturation.
      • Golgi Apparatus: It labels and directs proteins to their respective destinations.
      • Mitochondria: Often termed the powerhouse of the cell, mitochondria generate ATP, the cell’s energy currency. They possess their own DNA, supporting the Endosymbiotic theory which suggests that mitochondria were once independent prokaryotic cells.
      • Chloroplasts: Present in plant cells, these organelles facilitate photosynthesis. Like mitochondria, they too have their own DNA.
  3. Cell Functionality:
    • Reproduction: Cells reproduce through processes like binary fission, mitosis, or meiosis, ensuring the continuity of life.
    • Metabolism: Cells undergo various metabolic pathways to extract and utilize chemical energy from organic molecules.
    • Response: Cells can detect and react to internal and external changes, such as alterations in temperature or nutrient levels.
  4. Tissues and Systems in Multicellular Organisms:
    • Tissues: A collection of similar cells working together forms a tissue. In animals, there are four primary tissue types: epithelium, nervous tissue, muscle tissue, and connective tissue.
    • Organs and Organ Systems: Multiple tissues collaborate to form organs, each with a unique function. These organs further group into organ systems, like the digestive or respiratory system, to perform coordinated tasks.

In essence, the structure of an organism is a testament to the complexity and efficiency of cellular organization and function. From the singular capabilities of unicellular organisms to the intricate coordination in multicellular entities, the cell remains central to the mysteries and marvels of life.

Types of Organisms

Organisms, the fundamental units of life, can be broadly categorized based on their cellular structure and organization. This classification provides insights into the evolutionary complexity and functional capabilities of different life forms. Here, we delve into the primary distinctions:

  1. Prokaryotes and Eukaryotes:
    • Prokaryotes: These are primitive organisms lacking a defined nucleus and other membrane-bound organelles. Their genetic material is dispersed within the cytoplasm. DNA replication and transcription in prokaryotes occur directly within the cytoplasm. Examples include bacteria and archaea.
    • Eukaryotes: Eukaryotic cells are characterized by the presence of a well-defined nucleus enclosed by a nuclear envelope. This compartmentalization ensures regulated genetic processes. Eukaryotes also possess other membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus, contributing to their larger size compared to prokaryotes. DNA replication and transcription predominantly occur within the nucleus. Examples encompass protists, fungi, plants, and animals.
  2. Single-celled and Multicellular:
    • Unicellular Organisms: These organisms consist of a single cell that performs all vital functions independently. While bacteria and archaea are prokaryotic unicellular organisms, certain fungi and protists are eukaryotic unicellular entities.
    • Multicellular Organisms: Comprising multiple cells, these organisms exhibit a higher level of organization. Cells with similar functions group together to form tissues. For instance, in animals, tissues like nervous and muscle tissues have specialized roles. In plants, tissues can be meristematic or permanent. Multiple tissues collaborate to form organs, such as the heart or lungs in animals and leaves or roots in plants. In more complex animals, organs further organize into systems like the digestive or cardiovascular system, each with a specific function.

In summary, the vast diversity of life can be understood by examining the cellular organization of organisms. From the simplicity of unicellular prokaryotes to the intricate systems of multicellular eukaryotes, the spectrum of life showcases evolution’s marvel and the intricate interplay of structure and function.

Taxonomic Classification of Organisms

The vast diversity of life on Earth necessitates a structured system for classification. This system, known as taxonomy, categorizes living organisms based on shared characteristics and evolutionary relationships. The major taxonomic ranks, as proposed by Carl Woese, are as follows:

  1. Domains:
    • Archaea and Eubacteria: Both these domains encompass prokaryotic organisms, which lack membrane-bound organelles. However, they differ in certain genetic and metabolic pathways. Archaea, in particular, exhibit genetic features more akin to eukaryotes than to eubacteria, warranting their classification into a separate domain.
    • Eucarya: This domain includes eukaryotic organisms, characterized by the presence of a defined nucleus and other membrane-bound organelles.
  2. Kingdoms within Eucarya:
    • Protists: These are diverse organisms, ranging from single-celled to multicellular entities. They predominantly inhabit aquatic environments and lack specialized tissue structures. Examples encompass protozoans, algae, and slime molds.
    • Fungi: Fungi are heterotrophic eukaryotes, distinguished by their filamentous structures called hyphae. These organisms lack chlorophyll, preventing them from photosynthesizing. Their classification further extends to perfect and imperfect fungi based on their reproductive spore types.
    • Plants: Plants are autotrophic eukaryotes, possessing chloroplasts that facilitate photosynthesis. They synthesize their own food using carbon dioxide, water, and sunlight. Their life cycle often alternates between sporophyte and gametophyte phases, especially in tracheophytes.
    • Animals: Animals are multicellular heterotrophs, relying on other organisms for sustenance. They possess specialized sensory organs to perceive environmental stimuli and lack cell walls. Their structural support primarily comes from their skeletal systems.
  3. Subsequent Taxonomic Levels: Following the domain and kingdom levels, organisms are further classified into phyla, classes, orders, families, genera, and species. Each subsequent level represents a more specific grouping, culminating in the species level, which denotes organisms that can interbreed and produce fertile offspring.

In essence, taxonomic classification provides a systematic approach to understanding the relationships and characteristics of diverse life forms. Through this hierarchical structure, scientists can trace evolutionary lineages and discern the shared ancestry of organisms.

Phylogenetic tree
Phylogenetic tree

Evolution of Organisms

The origin and subsequent evolution of life on Earth is a subject of profound scientific interest and remains a topic of ongoing research and debate. The evolutionary trajectory of organisms is a complex tapestry woven over billions of years, and while many aspects are well-understood, numerous mysteries persist.

  1. Origins of Life:
    • Abiogenesis Hypothesis: This theory posits that life arose from non-living matter through a series of chemical processes that spanned billions of years. The early Earth is often conceptualized as a “primordial soup”, an aqueous environment teeming with a mixture of compounds, notably RNA.
    • RNA World Hypothesis: This theory emphasizes the central role of RNA in the early stages of life. RNA, with its dual capability to store genetic information and catalyze chemical reactions, is believed to have been the cornerstone of primordial life. Recent discoveries by NASA, which identified nucleobases in meteorites, lend credence to the idea that the precursors of RNA (and DNA) might have extraterrestrial origins.
  2. Emergence of Prokaryotes: The first life forms were likely simple, devoid of organelles, and thus classified as prokaryotes. The evolution of organelles, such as mitochondria and chloroplasts, is believed to have resulted from symbiotic relationships between smaller and larger cells. The 70S ribosomes of mitochondria, which bear resemblance to those of prokaryotes, further support this theory.
  3. Last Universal Common Ancestor (LUCA): The concept of LUCA postulates a singular organism from which all extant life on Earth descended. Estimated to have existed between 3.5 to 3.8 billion years ago, during the Paleoarchean era, LUCA represents a pivotal point in evolutionary history.
  4. Phylogenetic Relationships: The evolutionary tree, or the tree of life, is a graphical representation of the evolutionary relationships among organisms. It traces the divergence of species and highlights common ancestry. While the tree converges on LUCA, it’s worth noting that not all scientists concur with this singular origin theory. Figures like Jean-Baptiste Lamarck proposed alternative theories suggesting multiple origins of life.
  5. Advent of Multicellularity: The transition from unicellular to multicellular organisms is believed to have occurred approximately 600 million years ago. Geological records indicate periods of rapid diversification, such as the Ediacaran and Cambrian explosions, where a plethora of life forms emerged. The Ediacaran biota, for instance, is thought to have consisted of both unicellular and multicellular entities. By 2016, it was estimated that Earth was home to about 1 trillion species.

In conclusion, the evolutionary narrative of organisms is a testament to the dynamic and intricate processes that have shaped life on Earth. From the enigmatic origins of life to the vast biodiversity we observe today, the evolutionary journey is a captivating saga of adaptation, diversification, and survival.

Examples of Organism

Here are 10 examples of organisms across various domains and kingdoms:

  1. Escherichia coli (E. coli): A common bacterium found in the intestines of many animals, including humans. It is often used in scientific research.
  2. Saccharomyces cerevisiae: Commonly known as baker’s yeast or brewer’s yeast, it is used in baking and brewing.
  3. Arabidopsis thaliana: A small flowering plant related to mustard and cabbage, often used as a model organism in plant research.
  4. Drosophila melanogaster: Commonly known as the fruit fly, it is a widely used model organism in genetic research.
  5. Homo sapiens: Humans, the species to which all modern human beings belong.
  6. Canis lupus familiaris: Domestic dogs, which have been bred into numerous breeds with various sizes, shapes, and personalities.
  7. Felis catus: The domestic cat, a small carnivorous mammal kept as a pet by humans worldwide.
  8. Gallus gallus domesticus: Domestic chicken, one of the most common domesticated birds and a significant source of food (meat and eggs) for humans.
  9. Panthera leo: The lion, often referred to as the “king of the jungle,” is a large carnivorous feline found in parts of Africa and South Asia.
  10. Aptenodytes forsteri: Known as the Emperor penguin, it is the tallest and heaviest of all living penguin species and is native to Antarctica.

These examples represent a diverse range of organisms from microorganisms to plants and animals, showcasing the vast biodiversity present on Earth.


Which of the following is a unicellular organism?
a) Frog
b) Pine tree
c) Amoeba
d) Tiger

Which domain includes organisms with cells that have a nucleus?
a) Archaea
b) Bacteria
c) Eukarya
d) None of the above

Which of the following is NOT a characteristic of all living organisms?
a) Movement
b) Respiration
c) Photosynthesis
d) Growth

Which organism is known as the “king of the jungle”?
a) Elephant
b) Cheetah
c) Lion
d) Gorilla

Fungi are:
a) Autotrophic
b) Heterotrophic
c) Photosynthetic
d) None of the above

Which of the following organisms is a marsupial?
a) Kangaroo
b) Elephant
c) Dolphin
d) Ostrich

Which organism is known for its ability to regenerate lost body parts?
a) Starfish
b) Frog
c) Snake
d) Sparrow

Which of the following is a flightless bird?
a) Sparrow
b) Penguin
c) Eagle
d) Parrot

Which organism is a primary producer in the ocean ecosystem?
a) Shark
b) Whale
c) Plankton
d) Dolphin

Which of the following organisms has a three-chambered heart?
a) Human
b) Fish
c) Frog
d) Bird


What is an organism?

An organism is any individual living entity that can carry out all basic life processes. This includes growth, reproduction, response to stimuli, and maintenance of homeostasis.

How are organisms classified?

Organisms are classified into a hierarchical system starting with domains, followed by kingdoms, phyla (or divisions for plants), classes, orders, families, genera, and species.

What’s the difference between unicellular and multicellular organisms?

Unicellular organisms are made up of a single cell, like bacteria and amoeba, while multicellular organisms consist of multiple cells, like plants, animals, and fungi.

Do all organisms need oxygen to survive?

No, not all organisms require oxygen. Some organisms, known as anaerobes, can survive and thrive in environments without oxygen.

What is the smallest known organism?

The smallest known organisms are certain bacteria known as mycoplasmas. Some species can be as small as 0.2 micrometers in diameter.

How do organisms reproduce?

Organisms can reproduce either sexually, involving the fusion of gametes (like sperm and egg in animals), or asexually, where an individual can reproduce without the involvement of another individual.

What is a genetically modified organism (GMO)?

A GMO is an organism whose genetic material has been altered using genetic engineering techniques. This is often done to produce food crops that are resistant to pests or can tolerate herbicides.

Why is biodiversity important for organisms?

Biodiversity ensures resilience in ecosystems, allowing them to recover from disturbances, resist diseases, and adapt to changes. It also provides a variety of resources for human survival, including food, medicine, and raw materials.

What is the difference between prokaryotic and eukaryotic organisms?

Prokaryotic organisms, like bacteria, lack a nucleus and membrane-bound organelles. Eukaryotic organisms, like plants, animals, and fungi, have a nucleus and other membrane-bound organelles.

How do organisms adapt to their environment?

Organisms adapt to their environment through the process of evolution. Over time, beneficial traits that help an organism survive and reproduce in its environment become more common in a population.


  1. NASA. (2011, January 1). NASA Researchers: DNA Building Blocks Can Be Made in Space.
  2. Doolittle, W. F. (2000, February). Uprooting the tree of life. Scientific American, 282(2), 90-95.
  3. Glansdorff, N., Xu, Y., & Labedan, B. (2008). The last universal common ancestor: emergence, constitution and genetic legacy of an elusive forerunner. Biology Direct, 3, 29.
  4. Bowler, P. J. (2003). Evolution. The History of an Idea (3rd ed., pp. 90-91).
  5. The History of Animal Evolution. (2000, January 1).
  6. NSF – National Science Foundation. (2016, January 1). Researchers find that Earth may be home to 1 trillion species. Retrieved from
  7. Resnick, B. (2018, February 8). Tardigrades, explained. Vox.

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