Vacuoles are intracellular organelles that are separated from the cytosolic environment by a membrane. They are found in many different cell types, both prokaryotic and eukaryotic, as well as in unicellular and multicellular organisms..
The term "vacuole" was coined by the French biologist Félix Dujardin in 1841, to refer to an "empty" intracellular space that he observed inside a protozoan. However, vacuoles are particularly important in plants and it is in these living beings that they have been studied in more detail..
In the cells where they are found, vacuoles perform many different functions. For example, they are very versatile organelles and their functions often depend on the type of cell, the type of tissue or organ to which they belong, and the life stage of the organism..
Thus, vacuoles can exert functions in the storage of energetic substances (food) or of ions and other solutes, in the elimination of waste materials, in the internalization of gases for flotation, in the storage of liquids, in the maintenance of the pH, among others.
In yeast, for example, vacuoles behave like the counterpart of lysosomes in animal cells, as they are full of hydrolytic and proteolytic enzymes that help them to degrade different types of molecules inside..
They are generally spherical organelles whose size varies with the species and the type of cell. Its membrane, known in plants as the tonoplast, has different types of associated proteins, many of them related to transport to and from the interior of the vacuole.
Vacuoles are found in a wide variety of organisms such as all land plants, algae, and most fungi. They have also been found in many protozoa, and similar "organelles" have been described in some species of bacteria..
Its structure, as expected, depends especially on its functions, especially if we think of the integral membrane proteins that allow the passage of different substances into or out of the vacuole..
Despite this, we can generalize the structure of a vacuole as a spherical cytosolic organelle that is composed of a membrane and an internal space (lumen).
The most outstanding characteristics of the different types of vacuoles depend on the vacuolar membrane. In plants, this structure is known as the tonoplast and not only acts as an interface or separation between the cytosolic and luminal components of the vacuole, but, like the plasma membrane, it is a membrane with selective permeability..
In the different vacuoles, the vacuolar membrane is crossed by different integral membrane proteins that have functions in the pumping of protons, in the transport of proteins, in the transport of solutions and in the formation of channels..
Thus, both in the membrane of the vacuoles present in plants and in that of protozoa, yeasts and fungi, the presence of proteins can be described as:
The interior of the vacuoles, also known as the vacuolar lumen, is a generally liquid medium, often rich in different types of ions (positively and negatively charged)..
Due to the almost generalized presence of proton pumps in the vacuolar membrane, the lumen of these organelles is ordinarily an acid space (where there are large amounts of hydrogen ions).
Much experimental evidence suggests that the vacuoles of eukaryotic cells derive from internal pathways of biosynthesis and endocytosis. The proteins inserted in the vacuolar membrane, for example, come from the early secretory pathway, which occurs in the compartments corresponding to the endoplasmic reticulum and the Golgi complex..
In addition, during the process of formation of vacuoles, events of endocytosis of substances from the plasma membrane, autophagy events and events of direct transport from the cytosol to the vacuolar lumen occur..
After their formation, all the proteins and molecules found inside the vacuoles arrive there mainly thanks to the transport systems related to the endoplasmic reticulum and the Golgi complex, where the fusion of transport vesicles with the vacuolar membrane.
Likewise, transport proteins located in the membrane of vacuoles actively participate in the exchange of substances between the cytosolic and vacuolar compartments..
In plant cells, vacuoles occupy, in many cases, more than 90% of the total cytosolic volume, so they are organelles that are closely related to cell morphology. They contribute to cell expansion and the growth of plant organs and tissues.
As plant cells lack lysosomes, vacuoles exert very similar hydrolytic functions, since they function in the degradation of different extra and intracellular compounds.
They have key functions in the transport and storage of substances such as organic acids, glycosides, glutathione conjugates, alkaloids, anthocyanins, sugars (high concentrations of mono, di and oligosaccharides), ions, amino acids, secondary metabolites, etc..
Plant vacuoles are also involved in the sequestration of toxic compounds and heavy metals such as cadmium and arsenic. In some species, these organelles also possess nuclease enzymes, which work to defend cells against pathogens..
Plant vacuoles are considered by many authors to be classified as vegetative (lytic) vacuoles or protein storage vacuoles. In the seeds, the storage vacuoles are the ones that predominate, while in the rest of the tissues the vacuoles are lytic or vegetative.
The contractile vacuoles of protozoa prevent cell lysis due to osmotic effects (related to the concentration of intracellular and extracellular solutes) by periodically eliminating excess water inside the cells when they reach a critical size (about to burst) ; that is, they are osmoregulatory organelles.
The yeast vacuole is of utmost importance for autophagic processes, that is, within it, recycling or elimination of waste cellular compounds occurs, as well as aberrant proteins and other types of molecules (which are labeled for their "Delivery" in the vacuole).
It works in the maintenance of cellular pH and in the storage of substances such as ions (it is very important for calcium homeostasis), phosphates and polyphosphates, amino acids, etc. The yeast vacuole also participates in "pexophagia", which is the process of degradation of entire organelles..
There are four main types of vacuoles, which differ mainly in their functions. Some with characteristics of some particular organisms, while others are more widely distributed.
This type of vacuole is the one found mainly in protozoan organisms, although it has also been found in some "lower" animals and in the phagocytic cells of some "higher" animals..
Its interior is rich in digestive enzymes capable of degrading proteins and other substances for food purposes, since what is degraded is transported to the cytosol, where it is used for various purposes..
In English they are known as "sap vacuoles”And are those that characterize plant cells. They are fluid-filled compartments and their membrane (the tonoplast) has complex transport systems for the exchange of substances between the lumen and the cytosol..
In immature cells, these vacuoles are small in size, and as the plant matures, they fuse to form a large central vacuole..
Inside they contain water, carbohydrates, salts, proteins, waste products, soluble pigments (anthocyanins and anthoxanthins), latex, alkaloids, etc..
Contractile or pulsatile vacuoles are found in many single-celled protists and freshwater algae. They are specialized in the osmotic maintenance of cells and for this they have a very flexible membrane, which allows the expulsion of liquid or the introduction of it.
To exercise its functions, this type of vacuoles undergo continuous cyclical changes during which they gradually swell (fill with fluid, a process known as diastole) until they reach a critical size..
Then, depending on the conditions and cellular requirements, the vacuole suddenly contracts (empties, a process known as systole), expelling all its contents into the extracellular space..
This type of vacuole has only been described in prokaryotic organisms, but differs from the rest of eukaryotic vacuoles in that it is not delimited by a typical membrane (prokaryotic cells do not have internal membrane systems).
Gas vacuoles or aerial “pseudovacuoles” are a set of small structures filled with gases that are produced during bacterial metabolism and are covered by a layer of proteins. These have functions in flotation, radiation protection and mechanical resistance.
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