The kangaroo rat of North America doesn't need to drink or store water. Sound impossible?

It lives on hard dry seeds and makes its own water as it digests its food.

Fairy shrimp eggs can survive for 100 years without water and still hatch after a rainfall. They mature and lay new eggs before the pools dry up again.

Plants

Osmosis and plant cells

Although the osmotic principles apply equally to plant and animal cells, a different set of terms is currently applied to the osmotic relationship of plant cells. Water potential is a measure of the tendency of water to leave a solution. Pure water is designated a water potential of zero. As the solute molecules in a solution tend to prevent the water molecules leaving it, the solution will have a lower water potential than pure water. Its value will therefore be less than zero, i.e. negative. The more concentrated the solution, the more negative is its water potential.

For practical purposes a plant cell can be considered as a solution of salts and sugars in the vacuole surrounded by a partially permeable membrane (tonoplast, cytoplasm and plasma membrane) and a slightly elastic but completely permeable cell wall. A plant cell therefore has a more negative water potential than pure water and will draw in water when surrounded by it. This entry of water forces the living part of the cell, known as the protoplast, against the cell wall. In effect, the water in the vacuole is being subjected to a pressure from the cell wall. This pressure is referred to as the pressure potential. In a turgid plant cell this is a positive value, although in the xylem of a transpiring plant it is negative. The water potential of a cell is changed by the presence of solute molecules. This change is referred to as the solute potential. As solute molecules invariably lower the water potential, its value is always negative. The relationship between these three terms is given as:

water potential = solute potential + pressure potential

Water for the support of plants

Water is almost impossible to compress (press into a smaller volume). This feature of water allows it to be used as a form of support by many smaller plants. Although such plants do not have a skeleton, they do have cells with strengthened walls the cell is filled with a gel-like substance called cytoplasm. The cytoplasm contains a lot of water. Plant cells also contain a large vacuole (a fluid-filled structure bound by a membrane). The vacuole holds a lot of water. If the plant cell takes up more water, the cytoplasm and the vacuole fill with water and become swollen. This causes the contents of the cell to push against the cell wall. These cells are described as being turgid, and turgid cells make the plant stiffer and better able to stand up to forces such as wind.

The importance of water is clearly demonstrated by a plant that has not had enough water because it soon starts to wilt. As the cell lose water, the plant loses its support and the leaves begin to collapse. If the plant is quickly given some water, the cell can recover. However, if a plant is allowed to remain without water for too long, the cell will be permanently damaged and the plant will die.

Water is a rare resource in many parts of the world. Deserts are places where there is very little water for almost all of the year. When it eventually does rain, it usually comes all at once in heavy downpours, often causing floods. In many other climatic areas of the world there are rainy seasons and dry seasons. In these places, water has to be used carefully and stored for use in the seasons.

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