Vacuole (plants)

There is more to a vacuole than the eye can see
The name vacuole has its origins in the Latin word vacuus meaning 'empty' and this unfortunately is how vacuoles appear in many slide preparations and photographs. The fact that vacuoles are fluid filled and that different vacuoles within the same cell can contain different chemicals is not normally visible.

Flexible space but never empty space
A membrane barrier called a tonoplast limits each vacuole. This membrane is remarkable in that it can surround a small amount of fluid and then, after a short amount of time, during which water is taken in, stretch to become an organelle occupying as much as 95% of the cell by volume. And all this happens without the tonoplast losing its integrity as an active membrane. In this process all the other organelles in the cell are pressed, without damage, against the firm cellulose cell wall.

The state of plant cell vacuoles indicates whether you need to water your garden
A cell in which the vacuole contains all the water it needs is said to be in a turgid state.
A state of wilt shows a shortage of water and a cell is said to have lost its turgor.
A plant wilting on a hot summer afternoon may 'pick up' in the evening but a plant wilting in the evening or morning needs water!

Vacuoles assist with growth
The relatively high hydrostatic pressure produced by vacuoles also assists in cell elongation but only when the cell wall is made soft enough for extension to take place.

Chemicals help create 'cell pressure'
Chemicals in the vacuole forming a concentrated solution create the hydrostatic pressure produced within plant cells. Some of these chemicals form ions and the effect of this system is to create a high osmotic pressure. It is this high osmotic pressure that has the power to 'pull in' water molecules through the tonoplast until the cell is turgid.

The vacuolar membrane is a selective membrane
The vacuolar membrane or tonoplast is a selective membrane and the passage of chemicals through it is controlled in both directions. Water can pass in and out freely but other small molecules are retained within the vacuole.

Molecules too require entry tickets
Many proteins arriving at the cytoplasmic surface of the vacuole are synthesised by the membrane-bounded ribosomes of the rough endoplasmic reticulum and transported to the vacuole via the Golgi apparatus. In the Golgi apparatus they are given a combined 'address label and ticket' The 'address' part of the label on a protein directs it to the vacuole and the 'ticket' portion helps it gain admission.

The vacuolar membrane also works as a proton pump
Part of the vacuolar membrane works as a proton pump and uses energy from adenosine triphosphate (ATP) to pump H+ ions into the contents of the vacuole. This maintains acid conditions inside it.

Keeping your waste on site can attract and deter
Plants, unlike animals, do not have a well-developed excretory system but they do have vacuoles and vacuoles provide safe storage space.
When chemicals are produced in plants they can be temporarily or permanently stored. This is often done in vacuoles. The list of chemicals is extensive and includes the pigments in flower petals, latex, digitalis in foxglove, resins, alkaloids such as opium and the chemicals in garlic. The pigments producing autumn colours are thought by some to be waste products to be lost at leaf fall by deciduous plants.

Pigments in petals are clearly attractive and intimately linked to the pollination process. Some chemicals in plants are distasteful and act as a deterrent to some animals. This can give them some degree of protection from being eaten.

Vacuoles - an endowment for the next generation
Proteins, fats and carbohydrates can be safely stored in the vacuoles of storage cells in seeds for many years for utilisation when germination takes place. Vegetative reproduction by tubers, rhizomes and bulbs depends to a large extent on the storage of food material in vacuoles for the next generation.

Vacuoles and lysosomes have similar functions
Vacuoles in plant cells are in some respects the equivalent of lysosomes in animal cells. The environment inside a vacuole is slightly acid (pH about 5.0) whilst for the rest of the cytosol it is slightly alkaline (about pH 7.2). Under these conditions acid hydrolase enzymes in vacuoles break down large molecules sent there for disposal. These 'breakdown products' are then retained within the vacuole, which acts as a depository, until the plant is eaten, the leaves fall, or the plant dies.

Summary

• A vacuole is often considered to be the plant equivalent of a lysosome in animal cells. From the point of view of its ability to break down large molecules under acid conditions, this is certainly the case.
• Vacuoles have the facility to contribute to the rigidity of the plant; to cell elongation and to the processing and storage of waste products. This makes them unique and a distinct organelle in their own right.