Plant organ system (2.3.2)

• Translocation is the movement of food substances made in the leaves up or down the phloem, for use immediately or storage.
• Transpiration is the loss of water of water vapour from the leaves and stems of the plant. It is a consequence of gaseous exchange, as the stomata are open so that this can occur
• ● Water also evaporates at the open stomata
• As water molecules are attracted to each other, when some molecules leave the plant the rest are pulled up through the xylem
• This results in more water being taken up from the soil resulting in a continuous transpiration stream through the plant
• Guard cells close and open stomata.
• ● They are kidney shaped ● They have thin outer walls and thick inner walls ● When lots of water is available to the plant, the cells fill and change shape, opening stomata (they are also light sensitive) ● This allows gases to be exchanged and more water to leave the plant via evaporation ● More stomata are found on the bottom of the leaf, allowing gases to be exchanged whilst minimising water loss by evaporation as the lower surface is shaded and cooler.