What is an Organism?

It refers to an individual that can exist on their own. It can carry out all the necessary functions which are important to lead a life. The organism can be simple and complex. A simple organism has a quite simple system to study and has evolved less, whereas a complex organism is always much more evolved. In a complex one, many systems are synchronized together to carry out life, while only one system performs all the vital functions in the simpler ones.

Biology is the science which deals with the study of biological beings, their life, and their systems. It has different branches which are associated with the study of an organism and its biological system. For instance, gynecology is the study of the female reproductive system.

Human physiology is a specific biology category that includes maintaining homeostasis, the amount of nutrients in the body, the type of diet, and the nutrition required for an organism's growth. Homeostasis is the maintenance of body temperature and body fluids respective to external conditions. In humans, the hypothalamus is primarily associated with maintaining the body's homeostasis. Thus, biology is a broad spectrum that includes various studies.

Categories of an Organism

An organism can be either a eukaryote or a prokaryote.


  • Prokaryotes comprise bacteria and archaea.
  • They are considered the primitives on Earth. They are simpler than eukaryotes.
  • They lack a nucleus and other membranous structures.
  • The DNA (deoxyribonucleic acid) of prokaryotes is circular and is in direct contact with the cytoplasm.
  • A prokaryotic cell has three regions: cell envelope, cytoplasmic region, and flagella.
  • The cell envelope comprises the cell membrane and the cell wall. It gives rigidity to the cell and demarcates the interior of the cell to the exterior environment.
  • The cell wall of prokaryotes is composed of peptidoglycan that prevents the cell from bursting and expanding.
  • The cytoplasmic region has genomic DNA, inclusions, and ribosomes.
  • Prokaryotic cells have extrachromosomal DNA called plasmids.
  • DNA is condensed to form a nucleoid.
  • Prokaryotes have cellular appendages such as flagella and pili. These structures project outwards of the membrane to facilitate the movement of the cell.


  • The eukaryotes include fungi, slime molds, animals, algae.
  • The size of the eukaryotic cell is 15 times larger than a prokaryotic cell.
  • The compartmentalization is the main distinguishing feature in both cells. The eukaryotes have much compartmentalized spaces for membranous organelles.
  • The plasma membrane is similar to that of prokaryotes.
  • The cell wall is sometimes present.
  • The eukaryotic DNA is linearly associated with histone proteins.
  • DNA is folded and compacted to fit inside the membrane-bound. The mitochondria in the cell have their DNA.
  • Some types of eukaryotic cells have cilia that work as sensory receptors antennae. The motile cilia and flagella assist in movement and are more complex than the prokaryotes.

Types of Organisms

Generally, scientists classify living organisms into three domains and six kingdoms, although the classification has changed throughout history. There are now three recognized domains or the broadest classification of the organism. These are archaea, bacteria, and eukarya.

  1. Bacteria

These are the unicellular microorganisms that lack membrane-bound organelles. Their genetic material is not surrounded by a nuclear membrane and floats freely within the cytoplasm. The bacteria primarily divide through a process called binary fission.

2. Archaea

These are bacteria-like organisms that thrive in extreme conditions. For example, halophiles that live in high salt concentration areas.

3. Eukarya

These organisms have a membrane-bound nucleus as well as organelles. The DNA is present inside the nucleus. These organisms have highly specialized organelles that are used to carry out the cell's different functions.

Organisms and Environment

All organisms can adapt to various environmental conditions because of genetic variations. These variations also increase the probability of their survival. The polar bear possesses those character traits that help it adapt to Antarctica's extremely cold climate. Moreover, the dense fur coats of polar bears save them from cold as well as predators.

What Makes up an Organism?

The biological hierarchy system describes the basic composition of an organism:


In biology, the study of cells is called cytology. A cell is the basic functional and structural unit of life. The organisms can be made of single-cell like an amoeba (unicellular) or trillions of cells like humans(multicellular). Each cell in an organism is specific for a function, like the cells in the stomach produce gastric acid and the heart cells pump blood throughout the body. Many cells form a group of tissue that performs a specific function.


The multicellular organisms have collective cells which perform a specific function in the body. Tissues are a group of cells that work together to perform a particular function in the body. They make a series of networks or a system to coordinate with the different tissues in the body, forming a tissue system. The word tissue is derived from the Latin word 'weave.' Plants and animals have various kinds of tissue systems that differ in their work and composition according to the different requirements.

The different tissues present in animals are:

1. Connective Tissue

The connective tissue is made of an extracellular matrix, just like blood has plasma. It helps maintain the position of organs in the body. Examples of connective tissue are ligament, tendon, bone, adipose, and areolar. Ligaments do the bone-to-bone attachment. Their rigidity helps maintain the body structure. The bones store minerals and need calcium to maintain their structure. Further classification of the connective tissue includes fibrous, skeletal, and fluid.

2. Epithelial Tissue

The epithelial tissue covers the skin surface of various organs. They have semi-permeable tight junctions that provide a barrier between the organ and the external environment. The epithelial tissues are specialized to perform excretion, absorption, and secretion. Certain epithelial tissues are simple squamous, simple cuboidal, ciliated, stratified keratinized, stratified non-keratinized. 

"Types of epithelium"
Open Stax College, CC BY 3.0 | https://upload.wikimedia.org/

3. Nervous Tissue

The nervous tissue is a part of the central nervous system (CNS) and peripheral nervous system (PNS). The brain and the spinal are part of the CNS. The cranial and spinal nerves form the peripheral nervous system and transmit information to and from the brain. The nervous tissue function is to maintain the signaling of the transmitters and information from the brain to the body's reflexes. It has two components: neuron and neuroglia. The neurons are nerve cells that conduct nerve impulses. The different types of neuroglial cells in the nervous system are oligodendrocytes, microglial cells, ependymal cells, and astrocytes. These cells provide support to the neurons.

4. Muscle tissue

The muscle forms the contractile tissue of the body. Its function is to produce motion and force. It is categorized into skeletal, cardiac, and smooth tissues. The cardiac muscle is found in the heart. The contraction of this muscle helps pump blood throughout the body. The smooth muscle is found in the inner linings of visceral organs. The skeletal tissue is attached to the bone (providing structure, stability, and rigidity) and generates movement.


A structure formed by multiple tissues is an organ. The organ has relatively more specificity than the tissues. In humans, the major organs are:

  • Stomach: It aids in the digestion of food and other parts of the digestive system such as the mouth, esophagus, small intestine.
  • Heart: It pumps oxygenated blood to all the tissues and deoxygenated blood to the lungs.
  • Lungs: Its function is to inhale and exhale oxygen and carbon dioxide, respectively, enabling breathing.
  • Kidneys: They detoxify the body by excreting all the toxic products or substances outside the body in urine.
  • Brain: It is the master organ that regulates the functioning of other organs and coordinates of whole body's metabolism.
"Various organs of the digestive system"
OpenStax College, CC BY 3.0 | https://upload.wikimedia.org/

Organ system

When the organs work in a synchronized manner, they form an organ system. Certain organ systems are the endocrine system, reproductive system, urinary system, lymphatic system, cardiovascular system, nervous system, respiratory system, skeletal system.

"Parts of the nervous system"
Cenveo, CC BY 3.0 | http://learn.neurotechedu.com/

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

  • Bachelor of Science in Biology
  • Bachelor of Science in Microbiology
  • Master of Science in Microbiology
  • Master of Science in Microbial Biotechnology
  • Master of Science in Molecular Genetics
  1. Types of organisms
  2. Prokaryotic genome
  3. Eukaryotic genome
  4. Reproduction in organisms

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