The term means "empty space". But in the cell they are many membrane bound sacs with little or no inner structure. Plant cells have very large distinct vacuoles. In fact this organelles often dominates the inside if the plant cell crowding all other organelles toward the cell wall. The membrane surrounding the plant cell vacuole is called the tonoplast. This a very active, dynamic membrane.

Plant cells use their vacuoles for transport and storing nutrients, metabolites, and waste products. In a sense, the vacuole can be regarded as equivalent to the extra cellular space of animals. The simple space-filling function of the vacuole is of great importance to plants, which capture energy from the sun rather than move to capture food. The mechanical stability provided by the combination of a cell wall and turgor pressure allows plant cells to grow to a relatively large size, so they generally occupy a much larger volume than animal cells. However, producing large cells by filling them with cytoplasm would be costly both on terms of maintenance and initial synthesis. The majority if plant cells accumulate water in their vacuoles as they become larger through tugor driven cell-wall expansion (usually in many small vacuoles that then coalesce to form a large vacuole.

NB The large vacuole in such cells means that mature plant cells have a large ratio of surface to cytoplasmic volume. With the cytoplasm, forming a thin layer pressed against the cell wall.

In photosynthetic cells for example, chloroplasts are arranged in a thin layer of cytoplasm at the cell periphery facilitating exchanges.

(NB It is the plant cells cytoskeleton that organizes the cytoplasm however.)

Vacuoles can store many types of molecules, in particular essential substances that are potentially harmful if present in bulk in the cytoplasm. For example in the appropriate plant vacuoles of certain specialized cells contain such interesting products as rubber and opium. Even ordinary molecules such as Na⁺ are found in these organelles. The salts add to the osmotic activity of the vacuole therefore contributing to the turgor pressure.

Eg. Nittella sp (alga) Their tonoplasts have Na⁺ pumps located in the membrane which maintain low [Na⁺] in the cystol and approximately 5 times higher [Na⁺] in the vacuole.

Since the vacuole occupies a much greater volume than the cytoplasm most of the cellular Na⁺ is in the vacuole. The different permeability properties of the plasma membrane and tonoplast govern the different solute compositions of the cytoplasm and the vacuole.

Other molecules stored in vacuoles are involved in the interactions of the plant with animals or with other plants.

Eg. Pigment (colours) for insect attraction - flower petals.

Defense chemicals - poisonous alkaloids, etc.

Small vacuoles found in many freshwater organisms. These vacuoles periodically expand within the cell, filling with water (from cytoplasm) and then suddenly contract, expelling its contents to the exterior of the cell. This mechanism probably removes the water which continually enters the cell by osmosis from the environment, or with food.