in order to allow them to perform unique functions. Organs are a collection of tissues, and the cells that make up an organ are specialized themselves to enable the organ to complete its unique function1.
Within the human body, there are 79 different organs, many of which are vital organs that are essential for survival, such as :
-Liver and etc
In higher organisms, a collection of organs form organ systems. There are ten organ systems that are found in higher animals including:
These can also exist in lower organisms; however, their systems are much less complex. In plants, the organs are the roots, stem and leaves, whereas their seeds, spores and flowers act as their reproductive structures. All basic life-sustaining processes and functions in organisms are performed by organs4.
An organelle, can be thought of as the functional analogue to an organ in a cell, and single-cell organisms such as bacteria5. They are tiny, specialized cellular structures, that operate like organs by performing specialized and specific functions in the cell.
They are fixed in the cell cytoplasm, and in eukaryotes, a membrane encloses the organelles. Eukaryotic cells have organelles :such as
-Endoplasmic reticulum (ER)
Being located at two very different scales, there is naturally a pronounced difference in size between the two structural units. An organ is a macroscopic structure, operating at a much larger size scale than organelles, and as such are much larger in size.
As there are 79 different organs in the body, there is no standard size of an organ, as they vary depending on their location and function. However, they can often occupy very large spaces in the body, should they be combined to form organ systems, such as the digestive organ system.
Here, a single organ and organ system can extend through large areas in the body, such as the gastrointestinal tract and the accessory organs that are required for digestion (pancreas, salivary glands, gallbladder, liver and the tongue). The entire system stretches from the head where the tongue is, all the way to the lower torso and hips at the end of the large intestine7.
An organelle is significantly smaller than an organ, and operates at a smaller cellular scale, making it a microscopic structure. Where an organ is often arranged into much larger systems composed of larger structural components, an organelle simply consists of a singular cell.
Again, there is no standard size to a single organelle, with each varying depending upon its individual function. One of the largest organelle is the cell nucleus, which can reach up to 10um in size. The Golgi complex, is typically just 7nm thick in each layer, while the endoplasmic reticulum can be as little as 0.2um in thickness in each layer6,8.
With such pronounced size differences between the two structures, there is naturally a very notable difference in their compositions.
As discussed, there are 79 different organs in the human body. Each individual organ is a recognizable structure, that performs a specific function, with each made of several different types of tissue, and as such, contain several different types of cells.
All organs are composed of a main tissue type, ‘sporadic’ tissues, stroma and parenchyma9. The main tissue is unique to each specific organ. In the heart for example, myocardium is its main tissue and the sporadic tissues are the, blood vessels, nerves and connective tissues10.
The main tissues that make up an organ tend to have common embryologic origins. Some organs have much more complex compositions, while others are far simpler. The gallbladder for example, while a much simpler organ than the heart, still contains different cell types, such as a resistant lining to the irritant effects of bile, and muscle cells that contract to expel the bile11.
While many organs can have very complex compositions, with a multitude of components, organelle are very simple structural units composed of a simple cell. Individual organelles are enclosed within their own separate lipid bilayers.
There is no standard composition for an organelle, as their composition depends upon their individual functions within the cell.
The nucleus for example, is enclosed by a nuclear envelope which is comprised of a set of two lipid bilayers. These lipid bilayers have protein channels called nuclear pores that form holes in the nuclear envelope12.
The composition of an organelle is also determined by the definition that is used to describe an organelle.
Many cell biologists specify an organelle as synonymous with the term cell compartment. However, other biologists have limited the term organelle to simply include those which contain Deoxyribonucleic acid (DNA), that have originated from former autonomous organisms, microscopic in nature, that were acquired via endosymbiosis5,13,14.
Under this definition, there would be two broad organelle classes: mitochondria (in almost every eukaryote) and plastids (such as in plants, algae and some protists).
With this more restrictive definition of structures that are membrane bound, there are some components of the cell that will not qualify as an organelle. However, the use of organelle as a term to refer to non-membrane bound structure (such as a ribosome) is still common practice.
As such some texts have delineated between non-membrane and membrane-bound organelles. Organelles that are non-membrane bound, are also referred to as large biomolecular complexes, that are essentially large macromolecule assemblies carrying out specific specialized functions, however are lacking a membrane boundary.
Therefore, the individual composition of an organelle Is dictated by its underlying structure, and whether or not it contains DNA.
Being different sizes, both organs and organelle perform very different processes within the body. An organ is responsible for performing life processes within the body of higher organisms. Seven processes are common for all living things, which all organs in the body contribute to providing15. These are:
Each organ contributes to these processes in its own individual way, or as part of a larger organ system, such as one of the ten outlined in the introduction.
For example, the lungs operate to provide oxygenation of the blood, which is then circulated through the body by the heart, and removes carbon dioxide that is present in the blood.
Kidneys remove any metabolic waste and any fluid that is in excess in the circulating blood flow.
The brain organ is the control hub for the entire body, coordinating other organs within the body through its nervous system. It is responsible for a range of traits including general perception, thoughts, memory storage and feelings.
The liver acts as the primary storage of food, and any chemicals like drugs or toxins are detoxified within the liver1,16.
While organs provide processes at the larger, macro level within the body, organelles provide processes at the cellular level, within each individual cell.
These processes vary greatly depending upon the individual organelle.
Endoplasmic reticulum (ER) for example, translates and folds any new proteins (within the rough ER) and lipid expression (within the smooth ER).
The mitochondria are responsible for the energy production within a cell, through glucose oxidation and adenosine triphosphate release6.
An Organ and organelle are two different structural units, that are highly specialized in order to perform unique functions at different scales and in different locations.
Organs exist exclusively in higher organisms such as animals and plants.
An organelle can be thought of as the functional analogue to an organ, in cells within the body and single celled organisms like bacteria.
Given this difference in size scales, organs are therefore macroscopic structures, while organelles are structurally microscopic. T
he primary difference between an organ and an organelle is their organization and composition.
Author: Alex Hammond
Alexander Hammond hold a first-class master’s degree in Ecology. He has conducted a number of international research projects in Indonesia, Belize and the UK, in the areas of Marine Biology, Terrestrial Ecology and Conservation. Several of his research reports have been published.