The Chemistry of Life

• Water is essential to all known forms of life; many of its unique properties, like high surface tension and the ability to form weak bonds called hydrogen bonds, make it an essential part of life's chemistry. Plant cells are basically bags of water and dissolved substances like DNA and proteins; the water provides a fluid medium wherein the biochemical processes known as life can take place. The first use your plant has for water, then, is to maintain a healthy solute concentration inside its cells.
  
  

Transpiration

• Plants have microscopic pores in the undersides of their leaves. These stomata, as they are called, enable the plant to inhale carbon dioxide (CO2) and exhale oxygen (O2). However, the plant also loses water through these pores as part of a process called transpiration. This process accounts for the majority of water usage in a plant; about 95 percent of the water your plant drinks is lost through transpiration. This doesn't mean that transpiration is undesirable, however; in fact, transpiration helps generate the force needed to pull water up through the plant from its roots. Nonetheless, if the plant continually loses more water than it can replace, it may eventually wilt and die.
  
  

Turgor Pressure

• A constant supply of water also helps maintain turgor pressure in plant cells. Plant cells are enveloped in a sturdy cell wall. When water ascends through the xylem, the water has a lower concentration of dissolved solutes than the fluid inside the cells, so it will tend to diffuse into the plant cells. The cell wall counteracts this "osmotic pressure" so that the cell doesn't burst. The tension or pressure created by water diffusing inward helps to keep the plant rigid and erect against the force of gravity.
  
  

Photosynthesis

• Your plant also uses water as part of photosynthesis, in which water molecules serve as electron donors. The water molecule is split apart in the process, yielding hydrogen ions and oxygen gas. The oxygen gas diffuses out through the plant's stomata and into the atmosphere. Only roughly five percent of the water your plant drinks is used in photosynthesis or carbohydrate synthesis. Consequently, although photosynthesis is another important reason why your plant needs water, the single most important reason is transpiration.