Cells are complex entities. Far from being a singular mass, cells are composed out of many different parts and substructures, each specialized to perform some specific function. The various specialized structures are called organelles. If one imagines a eukaryotic cell as a house, the different organelles correspond to the different rooms of the house, each one serving some distinct purpose.
A vacuole is a cellular organelle present in all eukaryotic cells and some bacteria. Essentially, vacuoles are membrane-bound compartments that contain a liquid solution of enzymes. These membrane-bound compartments provide specialized environments for necessary biological processes. Vacuoles have many functions in cells, including removing waste products, isolating harmful substances, transporting proteins and other molecules, removing materials from the cell, and maintaining internal hydrostatic pressure in the cell. Vacuoles, along with the Golgi apparatus, lysosomes, and endoplasmic reticulum, form the endomembrane system of a cell which functions to modify, package, and transport lipids and proteins.
Vacuoles are made out of a phospholipid bilayer, the same kind of material that forms the outer cell membrane. Vacuoles are formed by the fusion of smaller vesicles and are basically larger versions of these. The phospholipid bilayer allows vacuoles to engulf particles by enclosing them inside the membrane. The exact mechanism for vacuole biogenesis is not currently understood, though it is thought that they form via the fusion of vesicles that are secreted by the Golgi apparatus.
Vacuoles are found in all eukaryotic organisms and some prokaryotic organisms. They are typically much more prominent in plant and fungal cells than animal and bacterial cells. The exact shape and function of the vacuole differ substantially depending on the type of cell.
Plant Vacuoles
Vacuoles in plant cells tend to be very large. Most plant cells have a single large central vacuole that occupies 30%-80% of the cell’s entire volume, depending on conditions. Plant vacuoles are surrounded by a membrane known as the tonoplast that separates the vacuole from the rest of the cell.
The main function of vacuoles in plant cells is to provide hydrostatic pressure to give the cell structure. Central vacuoles fill up with water which pushes on the thick plant cell walls. This turgor pressure causes the plant to stand upright and gives the cells strength and rigidity. When water levels are low, the central vacuole deflates, releasing its pressure on the cell walls causing the plant to wilt. Proteins in the vacuole membrane control the flow of water in and out through the membrane, which causes water to flow in via osmosis. The loss of water from a plant vacuole in a hypertonic solution is called plasmolysis while the influx of water in a hypotonic solution is called cytolysis. Turgor pressure from the vacuole also pushes all the other organelles against the cell wall which keeps organelles like plasmids and chloroplasts closer to light.
Plant cells also use vacuoles to regulate and maintain internal pH levels. When acidity in the cytoplasm is high, the vacuole will engulf lone protons (hydrogen ions) to decrease the pH of the solution. The concentration of protons in the vacuole creates an electrochemical gradient across the membrane which the vacuole can exploit to draw materials in. When waste materials are engulfed, the enzymes in the vacuole degrade the compounds and transport the waste materials out of the cell. Lastly, plants use vacuoles to store carbohydrates as an extra reserve when food production is low.
Fungal Vacuoles
Fungal cells also tend to have large pronounced vacuoles. Vacuoles in fungal cell perform many of the same function as in plant cells. Generally, fungal cells do not rely too much on vacuoles to give their cells structure and rigidity as they have cell walls made out of flexible chitin. Fungal cells typically have multiple smaller vacuoles per cell as opposed to the single central vacuoles in plants.
In many fungi, vacuoles are used to isolate and remove harmful materials from the cell, most often ions that can damage cellular structures. Fungal cell vacuoles work to isolate toxic ions such as strontium, cobalt (II) and lead from the cytoplasm to be removed from the cell. They are also involved in homeostasis, osmoregulation, and storage and transport of materials like carbohydrates, enzymes, amino acids, and proteins.
Animal Vacuoles
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In animal cells, vacuoles perform a more subordinate role as mediating storage units and carriers during exocytosis and endocytosis. Animal cell vacuoles are much smaller than plant or fungal vacuoles and animal cells generally have multiple vacuoles. Some animal cells do not have vacuoles as well.
In general, vacuoles in animal cells are much less pronounced than those in plant and fungal cells. This is because animal cells have less of a need to store water than plant or fungal cells. Animal cells lack a cell wall and instead rely on an internal cytoskeleton to give them shape and structure.
During exocytosis, materials absorbed by the Golgi apparatus are placed inside vacuoles which transport the materials outside the cell membrane into the extracellular matrix. Endocytosis is a similar process in which materials near the outer membrane diffuse into the cell and are then carried to their destination. In both exocytosis and endocytosis, the vacuoles are essentially storage vesicles that contain, transport, and dispose of proteins, lipids, and other molecules.
Vacuoles in animal cells also serve to isolate and remove harmful pathogens. “Phagocytosis” is the process in which bacteria, viruses, and other harmful things are engulfed by cells. When these materials make contact with the vacuolar membrane, it invaginates and pinches off, leaving the materials trapped within. Afterward, lysosomes release their enzymes to degrade any harmful pathogens.
Protist Vacuoles
The contractile vacuole of a Paramecium. The vacuole expels water by contracting via the assistance of motor proteins. Credit: Katyddd via WikiCommons CC-BY SA4.0
A “protist” is any eukaryotic organism that cannot be clearly classified as an animal, plant, or fungus. As such, kingdom Protista functions as a kind of “waste bin” taxon that contains organisms that do not really fit anywhere else. Because of this, characteristics of protists vary greatly and many organisms classified as protists are not evolutionarily related.
Many protists contain a specialized vacuole called a contractile vacuole. A contractile vacuole, as the name implies, functions mainly to expel water from the cell via contraction. It does this through to phases. The process of water entering the contractile vacuole is called diastole and the process of the expulsion of water from the contractile vacuole is called systole. The most well understood contractile vacuoles are found in the protist genera Paramecium, Amoeba, and Trypanosoma. The exact number of contractile vacuoles differ depending on the species. In protists, the contractile vacuole, along with associated structures such as membrane folds, tubules, and small vesicles, are know as the spongiome or alternatively the contractile vacuole complex (CVC). Most of the time, contractile vacuoles are found in protists that lack cell walls, though exceptions to this rule exist.
Bacterial Vacuoles
Though not as common, vacuoles are found in some species of bacteria. Bacterial vacuoles do not serve any one specific function and instead do different things in different species. For example, in many bacteria, vacuoles simply act as storage containers that contain nitrates, ions, and carbohydrates.
Cyanobacteria have evolved a particularly nifty trick with their vacuoles. In cyanobacteria, vacuoles are freely permeable to gases. Cyanobacteria can control the amount of gas in the vacuole to control their buoyancy in water. If they need to go up, they let more gas in. If they need to go down, they let the gas out.
Other Functions Of Vacuoles
One more important function performed by vacuoles in many cells is facilitating the process of lysis–i.e. programmed cell death. Typically, vacuoles contain solutions of enzymes that can break down organic substances. When it is time for a cell to die, the vacuole along with the lysosomes expand, breaking their membranes and releasing their enzymes into the cell cytoplasm. These released enzymes facilitate the breakdown of the cell membrane.
Discovery Of Vacuoles
Vacuoles were actually the very first kind cellular organelle to be discovered and were first observed by the Dutch biologist Antonie van Leeuwenhoek in 1676. Leeuwenhoek was the inventor of the microscope and used his paradigm-shifting invention to examine minute details of cells. At the time, Leeuwenhoek did not realize vacuoles were separate organelles and considered them a nondescript structure of cells.
Vacuoles were first identified as a separate cellular organelle in 1776 by Italian biologist Lazzaro Spallanzani who observed the organelles in protozoa. Spallanzani incorrectly identified vacuoles as a kind of respiratory organ. in 1841, Félix Dujardin gave these membrane-enclosed spaces found in protozoa the name “vacuole.”
To sum up, a vacuole is a cellular organelle found in all eukaryotic organisms and a number of prokaryotes. Vacuoles consist of a membrane-enclosed solution of enzymes. the main function of vacuoles is to assist the endomembrane system in the isolation, transport, and expulsion/introduction of substances in the cell.
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