Vacuoles: Definition, Function, Structure

In microbiology, vacuoles are one type of microscopic cellular structure called an organelle. Both plant and animal cells can contain vacuoles, but vacuoles are far more prevalent in plant cells. They are also much larger in plant cells and often take up a great deal of space within the cell.

Animal cells do not always have a vacuole, and most never have a large vacuole, because it would cause harm to the cell and disrupt the functioning of the rest of the cell. Animal cells may instead have several very small vacuoles.

Vacuoles have multiple functions in both cell types, but they play a particularly important role for plants.

In animal cells, they are small and typically transport materials into and out of the cell. In plant cells, vacuoles use osmosis to absorb water and swell until they create internal pressure against the cell wall. This provides cell stability and support. There are many different types of vacuoles, but they all share similar structure and abilities across cells.

A vacuole is a kind of organelle called a vesicle. What differentiates vacuoles from other kinds of vesicles is its relative size and longevity. The vacuole is a sac surrounded by a single membrane called a ‌tonoplast‌.

This vacuole membrane structurally resembles the permeable plasma membranes that surround every cell. The cell membrane is constantly regulating what travels in and out of the cell and what must stay out or in; it uses protein pumps to push matter in or out, and protein channels to allow or block matter entrances or exits. These pumps often work on creating gradients across the vacuolar membrane to induce transport.

Like the plasma membrane of a cell, the tonoplast also regulates the inflow and outflow of molecules and microbes with protein pumps and protein channels. The tonoplast does not regulate the entrances and exits to cells, however, but instead acts as the guard for what kinds of matter are allowed passage to and from vacuoles.

Vacuoles have the ability to change their function to serve the needs of the cell. To do so, their main strategy is to change their size or shape. For example, plant cells often have a large vacuole that takes up a sizable portion of the space within the cell because the vacuole is storing water. The central vacuole in plant cells frequently occupies anywhere from 30 to 90 percent of the area within a cell. This amount changes as the storage and support needs of the plant change.

Eukaryotic cells include all cells that have a nucleus and other membrane-bound organelles. Eukaryotic cells engage in cell division by the processes of mitosis and meiosis. By contrast, ‌prokaryotic‌ cells are typically unicellular organisms lacking any membrane-bound organelles, and which asexually reproduce through binary fission. All animal and plant cells are eukaryotic cells.

There are a great many number of plant and animal species. Furthermore, for any individual plant or animal, there are typically a number of different organ systems and organs, each with their own types of cells.

A cell's particular needs for the very adaptable vacuole depend on that cell's job and on the environmental conditions in the plant or animal body at any given time. A few of these vacuole functions include:

•Space to store water, nutrients, or other materials like amino acids
•Providing a barrier for substances that need to be separated from the rest of the cell
•Removing, destroying or storing toxic substances or waste products to protect the rest of the cell
•Removing improperly folded proteins from the cell

Lysosomes are another type of cell organelle that can be considered a counterpart to the vacuole. Instead of storing necessary material, the lysosome uses enzymes to break down harmful or obsolete material.

Plants make use of vacuoles differently than animals or other organisms. The unique functions of vacuoles in plant cells help plants to do many things, such as grow upward on firm stalks, stretch toward sunlight and acquire energy from it, and protect themselves from predators and droughts.

Mature plant cells commonly contain one large vacuole that uses more cell volume than any other organelle. The plant cell vacuole consists of the the tonoplast, which forms a sac around a fluid called ‌cell sap‌. Cell sap contains water and a number of other substances. These can include:

•Salts
•Enzymes
•Sugars and other carbohydrates
•Lipids
•Ions

The cell sap can also contain toxins that the vacuole has helped to remove from the rest of the cell. These toxins can operate as a self-defense mechanism for some plants against herbivores.

The concentration of ions in the cell sap is a useful tool for moving water in and out of the vacuole via osmosis. If the ion concentration is higher within the vacuole, water moves through the tonoplast into the vacuole. If the ion concentration is higher in the cytoplasm outside of the vacuole, water moves out of the vacuole. The vacuole enlarges or shrinks as water moves into or out of it.

The process of osmosis to manage the size of the vacuole results in a desirable amount of internal pressure on the cell wall. This is known as ‌turgor pressure‌, and it stabilizes the cell and increases the structure of the plant. Increasing the turgor pressure of the vacuole can also help stabilize the cell during periods of cell growth. The large vacuole also serves the function of maintaining cell structure, by crowding other organelles into their optimal locations within the cell.

While plant vacuoles are easily identifiable because of the large amount of space they take up inside the cell, animal cells would not benefit from a large central vacuole. This is especially true because animal cells do not have a cell wall to provide counter-pressure to the turgor pressure of a large vacuole, and the animal cells would eventually burst. Animal cells may have no vacuoles, or they may have several vacuoles, depending on the cell's function and needs.

Instead of operating as structural elements, vacuoles in animal cells are small and spend most of their time providing transportation into and out of the cell for various organic materials. There are two kinds of transportation that the vacuoles provide: ‌exocytosis‌ and ‌endocytosis‌.

‌Exocytosis‌ is the method by which vacuoles move materials out of the cell. These materials are often unwanted materials such as waste, or molecules that are destined for other cells or the extracellular fluid. During exocytosis, vacuoles prepare some molecules to release signals that will be received by other cells, which will retrieve those molecules.

‌Endocytosis‌ is the inverse process of exocytosis, in which vacuoles help to bring organic matter into the animal cell. In the case of signaling molecules that were packaged and released by the vacuole of a cell, a vacuole of a different cell can receive the molecule and bring it into the cell.

Endocytosis is an important function for the vacuole in animal cells because it contributes to immunity from contagious disease. Vacuoles can bring bacteria and other microbes into cells while keeping the rest of the cell safe. Inside the vacuole, enzymes work on breaking down the dangerous pathogens.

Vacuoles also protect animals from illness and danger in the same way by breaking down potential food-borne and other toxins, with the barrier of the tonoplast keeping the offending molecules from the rest of the cell.