Organelle

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Definition

noun

plural: organelles

or·gan·elle, [ˌɔɹ.ɡənˈɛl]

A membrane-bound compartment or structure in a cell that performs a special function


Details

Overview

A cell is the structural, functional and biological unit of all organisms. It is a membrane-bound structure containing a cytoplasm and cytoplasmic structures. One of the cytoplasmic structures inside the cell is the organelle. An organelle is a membrane-bound compartment or structure in a cell that performs a special function.


Characteristics

Organelle literally means "little organs". As the body is composed of various organs, the cell, too, has "little organs" that perform special functions. In general, they are membrane-bound compartments or structures of a cell.


A eukaryotic cell contains many organelles, for example, the nucleus, endoplasmic reticulum, golgi apparatus, mitochondria, and chloroplast (plastid). However, not all these organelles are found in only one cell or in an organism. The chloroplast, for instance, is abundant in plant cells but not in animal cells. There are organelles that have their own DNA apart from the nucleus and are suggested to have originated from endosymbiotic bacteria according to the endosymbiotic theory. These organelles are mitochondria and plastids.


Prokaryotes, which were believed to have no organelles, have been recently described to possess their own sort of “organelles”. However, some references pertain to them as proteinaceous microcompartments rather than true organelles. Examples are carboxysome (a protein-shell compartment for carbon fixation in some bacteria), chlorosome (a light harvesting complex in green sulfur bacteria), magnetosome (found in magnetotactic bacteria), and thylakoid (in some cyanobacteria).


Some references are strict in their definition of an organelle. An organelle is one that which is surrounded by lipid bilayers. They are particularly nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplast (plastid). In this sense, ribosomes and nucleosomes are not regarded as organelles because they are not bounded by membranes. In the same way, lysosomes and vacuoles, would not qualify as an organelle because they are single-membrane bounded cytoplasmic structures. Other references, though, are less restrictive. An organelle is one that which acts as a specialized subunit inside the cell that performs a specific function. In this regard, there are two types of organelles: (1) membrane-bound organelles (included are double-membraned and single-membraned cytoplasmic structures) and (2) non-membrane-bound organelles (also referred to as biomolecular complexes or proteinaceous organelles). Examples of membrane-bound organelles are nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, lysosomes and vacuoles. Examples of non-membrane-bound organelles are ribosomes, spliceosome, vault, proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, photosystem I, ATP synthase, nucleosome, centriole, microtubule-organizing center, cytoskeleton, flagellum, nucleolus, stress granule, etc.


Inborn errors, Pathobiology, Genetics

The nucleus contains nuclear genetic material. Mutations involving the genes or the chromosome could lead to deleterious effects or genetic disorders. Mutations of the extranuclear genetic material in the mitochondria and chloroplasts could also result in pathological or dysfunctional conditions.


A metabolic disease due to defects in lysosomal function resulting in an abnormal accumulation of toxic materials in the cell is referred to lysosomal storage disease. Lysosomal storage diseases are hereditary. The dysfunctional lysosomal enzyme is caused by a particular defective gene as a result of mutation. Lysosomal storage diseases that have been identified so far are as follows: sphingolipidoses, ceramidase (e.g. Farber disease, Krabbe disease, etc.), galactosialidosis, gangliosides, alpha-galactosidase (e.g. Fabry disease, Schindler disease, etc.), beta-galactosidase, GM2 gangliosidosis (e.g. Sandhoff disease, Tay-Sachs disease, etc.), glucocerebroside (e.g. Gaucher disease), sphingomyelinase (e.g. lysosomal acid lipase deficiency), sulfatidosis, mucopolysaccharidosis, mucolipidosis, lipidosis (e.g. neuronal ceroid lipofuscinosis, Wolman disease, etc.), cholesterol ester storage disease, lysosomal transport disease, glycogen storage disease, etc. The symptoms may vary depending on the dysfunctional lysosomal enzyme involved.


Biological importance

Organelles carry out specialized functions. The major functions of double-membraned organelles are as follows:

Double-membraned organelles Characteristics Major functions
Nucleus The large, membrane-bounded organelle that contains the genetic material, in the form of multiple linear DNAmolecules organized into structures called chromosomes Responsible for maintaining the integrity of DNA and in controlling cellular activities such as metabolism, growth, and reproduction by regulating gene expression
Mitochondrion A spherical or rod-shaped organelle with its own genome Responsible for the generation of most of the cell's supply of adenosine triphosphate through the process of cellular respiration
Plastid A double membrane bound organelle commonly found within the cells of photosynthetic organisms, like plants Responsible for food storage and photosynthesis
Endoplasmic reticulum A membrane-bounded organelle that occurs as labyrinthine, interconnected flattened sacs or tubules connected to the nuclear membrane, running through the cytoplasm, and may well extend into the cell membrane Involved in protein and lipid syntheses, metabolism of carbohydrates and calcium concentration, drug detoxification, attachment of receptors on cell membrane proteins, and intracellular transport
Golgi apparatus An organelle that is comprised of membrane-bound stacks Involved in glycosylation, packaging of molecules for secretion, transporting of lipids within the cell, and giving rise to lysosomes


As for the other membrane-bound organelles, their primary functions are as follows:

Other membraned organelles Characteristics Major functions
Lysosome A single-membrane-bound cytoplasmic structure containing a large range of digestive enzymes Primarily for digestion and removal of excess or worn-out organelles, food particles, and engulfed viruses or bacteria
Vacuole A membrane-bound vesicle found in the cytoplasm of a cell, especially of plants Involved in providing structural support, intracellular secretion, excretion, storage, and digestion


The main functions of some of the non-membrane-bound organelles are as follows:

Non-membraned organelles Characteristics Major functions
Ribosome A minute, sphere-shaped particle composed of protein and ribonucleic acid (RNA) Serves as the site of protein synthesis
Nucleosome The basic structural unit of chromatin, and is made up of a coil of DNA wound around a histone core The basic structural unit of chromatin
Centriole A self-replicating, small, fibrous, cylindrical-shaped organelle, typically located in the cytoplasm near the nucleus in cells of most animals Involved in the process of nuclear division.
Cytoskeleton The lattice or internal framework of a cell composed of protein filaments and microtubules in the cytoplasm Involved in controlling cell shape, maintaining intracellular organization, and in cell movement


Supplementary

Etymology

  • New Latin organella, diminutive of Medieval Latin organum, organ of the body, from Latin, implement, tool


Further reading

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