Vacuoles

In addition to a nucleus, mitochondria, Golgi bodies, endoplasmic reticulum, and ribosomes, a plant cell contains small droplets of watery solution called the cell sap. The droplets are called vacuoles. Vacuoles are almost universally present in plant cells and are surrounded by a membrane that separates their contents from the rest of the cytoplasm. Most young, meristematic cells, which are continuously dividing, have numerous small, round or drawn out vacuoles (although the cells of cambium, also actively dividing cells, seem instead to have a large vacuole). These vacuoles coalesce as the cells mature so that older cells tend to have a single, large vacuole. The vacuole may occupy most of the volume of the cell.

The concentration of substance dissolved in the vacuolar sap is often far greater than that in the surrounding cytoplasm and may be great enough to come out of solution and to form minute crystals. For example, analysis of the vacuolar sap of Nitella, a green algae, discloses a tenfold greater concentration of calcium ion in the vacuole than in the surrounding cytoplasm, and a one hundredfold greater concentration of potassium ion.

What is suspended in the water of a vacuole? Sugars, salts, pigments, and some enzymes. The composition of cell sap varies in different plants and also varies under changing conditions. Water may compose up to 98 percent of cell sap. Sodium, potassium, magnesium, and calcium ions are also generally present. Carbohydrates, nitrogenous compounds, proteins, amino acids, and probably waste products from the cell metabolism are commonly present. There may also be dissolved gases, specifically oxygen and carbon dioxide. Enzymes such as diastase and invertase are also common, and both acetic acid and formic acid are thought to be present in all living cells. Finally, many vacuoles contain anthocyanins, or colored pigments. The yellow, pink, and blue of flowers are often imparted by anthocyanins residing in vacuoles. Red cabbage and beets also owe their color to pigments in vacuoles.

The membrane that bounds the vacuole is similar to the cell membrane. Both are selectively permeable; both allow the free passage of water; and both have a certain control over what solutes are allowed to pass.

Chrysophyta algae have unicellular, flagellated cells, each possessing a reservoir at the attached end of the flagella. Just beneath the reservoir are one or more contractile vacuoles, which can discharge their contents out of the cell. These pulsating vacuoles alternately contract and expand. In this way, they can both get rid of waste products and regulate water content.

Are vacuoles alive? While they are certainly not passive, the contents of vacuoles are not chemically active; vacuoles are therefore not considered living. Yet, another interesting question arises: Is any constituent of a cell alive when considered by itself, isolated from the rest of the cell?

Vacuoles play an important part in the sexual process observed in Spirogyra, a green algae . The conjugation of Spirogyra is described in Chapter 13. Two filaments of Spirogyra come to lie side by side when conjugation bridges form between adjacent cells. One of the filaments takes on the role of female (being a receiver), and the other adopts the role of male (giving its substance). The protoplasts of the male cells migrate across the bridges and unite with the protoplasts of the female cells. As a result the receiver cells have twice as much cytoplasm as before conjugation; but they got twice as large. The vacuoles play a significant role here, functioning as pumping stations; they pump water out of the female cell. So much water is removed that when the two protoplasts unite, the resulting zygote does not even fill the cell.