Photosynthesis

• When your body oxidizes glucose to yield carbon dioxide and water, energy is released -- in other words, the products of this reaction (carbon dioxide and water) have lower energy than the reactants (glucose and oxygen). In photosynthesis, a plant is carrying out this reaction in reverse. Since the forward reaction releases energy, it takes energy to make the reverse reaction happen. The plant obtains the energy it needs to power photosynthesis by absorbing sunlight with the aid of pigments like chlorophyll. This absorbed energy drives a complex series of chemical reactions that synthesizes glucose from the starting materials carbon dioxide and water.
  
  

Transpiration

• Plants need water for other reasons. Some water is consumed in photosynthesis, but this amount is fairly small compared to the water lost to transpiration, which is a process analogous to sweating in humans. Plants take in carbon dioxide through microscopic leaf pores called stomata, but they also lose water to evaporation through these pores. This water loss, coupled with water's high surface tension and the narrow size of the vessels through which water ascends the stem, helps to draw water up through the plant from the roots, ensuring a continual supply. In response to drought stress, however, many plants will temporarily close their stomata to reduce water loss.
  
  

Nutrients

• The continual flow of water upward from the plant's roots also transports nutrients -- potassium, nitrates, phosphates and all kinds of other trace minerals essential to the plant's growth. Plants need nitrogen, phosphorus, sulfur and other elements to synthesize the complex molecules essential to all life, and they also need metal ions like magnesium that help proteins and other molecules do their jobs inside plant cells. Drawing water upward by transpiration helps the plant "suck up" the nutrients it needs as well.
  
  

Turgor Pressure

• Water diffuses across cell membranes in the direction of increasing solute concentration. Since solutes are more concentrated inside cells than outside of them, water diffuses into plant cells from the stem so that the plant cells start to swell. This swelling pressure is called turgor pressure. It's balanced by the cell wall, which counteracts the pressure to keep the plant cell at a constant size. Turgor pressure nonetheless helps to keep the plant firm and upright. When water is scarce, turgor pressure drops, and plant leaves start to wilt.
  
  

Considerations

• Not all wavelengths of light are equally effective for photosynthesis. The pigments plants use have specific absorption spectra, meaning they absorb some wavelengths of light much more efficiently than others. Green light, for example, is mostly reflected by chlorophyll, which is why healthy plants look green -- their cells have abundant chlorophyll. Plants can make better use of light in the violet-blue and red regions of the spectrum. The collection of wavelengths most effective for photosynthesis in a plant are called its action spectrum.