Vacuole

Like any other cell organelle, vacuoles are present in the cell cytoplasm. In the plant cells, vacuoles lie at the center, whereas they are located peripherally in animal cells. Generally, they lie close to the cell’s powerhouse, chloroplast in plant cells, and mitochondria in animal cells as their primary function is storage.

It is a sac-like structure surrounded by a single membrane called tonoplast that separates the vacuolar contents from the cell’s cytoplasm.  The membrane shares a structural resemblance to the plasma membrane. Though the structural component phospholipid is common, its type slightly varies from one cell type to another. Tonoplast also contains protein embedded in them, which help to transport molecules across the membrane and provide structure.

The shape and size of a vacuole depend on the need of a cell. For instance, the central vacuole in a plant cell occupies around 30 to 90 percent of the area within a cell when it stores water.  This amount varies as the storage and support needs of the plant change.

Vacuoles contain a wide range of hydrolytic enzymes, sugars, salts, and acids.  In case of plant cells, it contains nitrogenous compounds such as alkaloids and anthocyanin pigments.

The pH level of a vacuole depends on the content it holds. For instance, the pH of plant vacuoles may be as high as 9 to 10 due to large quantities of alkaline substances or as low as 3 due to the accumulation of several organic acids like citric, oxalic, and tartaric acids.

Vacuoles mainly help in the storage, and waste disposal in a cell. They store various molecules, including enzymes, waste products, water, and even food material, depending on the cell type.

Here are the main roles of vacuoles in a plant cell:

Storing Water and Reserve Food: The vacuoles can intake protons from cell sap, increasing proton concentration inside them. The cell transports materials in and out without using water, utilizing this concentration gradient. This process saves water and enables the plant to survive longer during drought.

Maintaining pH of the Cell: A part of the tonoplast acts as a proton pump and uses energy from adenosine triphosphate (ATP) to pump H⁺ions into the vacuoles from the cytoplasm. Vacuoles maintain their internal acidic medium (pH about 5.0) by this process. This lowered pH aids the activity of lytic enzymes that help to break down different molecules.

Maintaining Turgor Pressure: The tonoplasts in vacuoles regulate turgor pressure by maintaining a balance of ions and water, causing the vacuole to swell against the cell wall. This pressure helps the plant cells to be turgid and stand upright.

Adjusting Size of the Cell: The size of a vacuole determines the size of a particular cell, which in turn affects the size and shape of different parts of the plant.

However, both in plant and animals cells, the exact function of a vacuole can depend on the type of cell it resides in.

In Plant and Fungal Cells

Plant and fungi both contain vacuoles. Among them, plant cells generally contain one large vacuole in each cell, called a central vacuole. It is so enormous that it may consume up to 90% of cell space.  On the other hand, fungal cells contain multiple small vacuoles. A thin membrane called tonoplast lines the vacuoles in both, helping it retain its structure and shape.

Vacuole in plants and fungal cells perform similar functions in some ways. Thoughit mainly acts as a water reservoir, it also stores some small molecules, such as polyphosphate, amino acids, several ions, and other small molecules. Also, as these cells do not contain lysosomes, vacuoles help them to break down materials.

Why do plant cells only have a large central vacuole

The large central vacuole in plants contains water that provides the cell’s shape and maintains the pressure of the cell wall from inside, helping the plant stand erect.

In Animal Cells

As already mentioned, vacuole is not always present in animal cells.However, depending on the function and need of the cell, they may contain multiple small vacuoles in each. Unlike plants, it is not surrounded by tonoplast. Also, contrary to plants, vacuoles in animal cells do not break down or degrade larger molecules, as they possess lysosomes.

Vacuoles in animal cells do not help in providing structure to the cell. Instead, they help in the transportation of several substances into and out of the cell. The vacuoles usually provide two types of transport: Exocytosis and endocytosis. In exocytosis, vacuoles accumulate waste products and actively transport them out of the cell. On the other hand, in endocytosis, cells actively transport several nutrients, toxins, cell debris into themselves with the help of vacuole. During these processes, vacuoles are where the substances are broken down or stored while transporting.

In Protist Cells

Protists such as Amoeba, Paramoecium, Giardia possess a specialized vacuole called a contractile vacuole. They mainly use these vacuoles to regulate the water content in their cells (osmoregulation).

Freshwater protists tend to absorb too much water into their cells, which may lead to cell rupture. In that case, these contractile vacuoles prevent them from rupturing by expelling the excess water out of the cell by continuous contraction.

While some protists may have a single vacuole, others may have multiple.

In Amoeba, the vacuole changes position based on its movements. The vacuoles also collect the waste produced by the cell. On the other hand, in flagellates such as Euglena, the contractile vacuole remains stationary within the cell.

In Bacteria Cells

All types of bacteria do not have vacuoles, but for those that do, they are primarily used for storage. In some species of sulfur bacteria, vacuoles are especially large, taking up to 98% of the cell’s area. They often use these vacuoles to store nitrate ions for later use.

Some cyanobacteria also possess vacuoles that are permeable to gases. These bacteria control their buoyancy by moving gases into and out of the vacuole.