Plant Osmoregulation

• Plant Structures

  • cuticle
  • upper epidermis
  • palisade mesophyll
  • spongy mesophyll
  • vascular bundles (xylem and phloem)
  • lower epidermis
  • guard cells
  • stoma
• Open Stomata

  • stoma opens when guard cells are turgid due to absorbing water via osmosis (usually during the day)
• Closed Stomata

  • stoma closes when the guard cells are flaccid (due to losing water via osmosis usually at night)
• Transpiration

  The evaporative loss of water from plants, usually through stomata (small pores) found on the surface of a plant, mostly on the underside of leaves

  • occurs because of the concentration gradient of water vapour between the inside and outside of the leaf
  • water vapour moves down the concentration gradient, from areas of high water content to areas of relatively low water content
• Factors that Increase Rate of Transpiration

  • light
  • humidity
  • wind
  • temperature
• Importance of Transpiration

  • supplies the water needed for photosynthesis
  • evaporation of water in the leaves that accompanies transpiration requires energy and therefore cools the leaves - heat is drawn out of the plant, into the water, then out into the external environment
  • transpiration stream is necessary for distributing mineral salts throughout the plant
• Gas Exchange

  • occurs through the open stomata (usually during the day, when sunlight is being used in photosynthesis)
  • turgor: a force that results from the water pressure inside plant cells and is maintained by osmosis - stomata opening and closing depends on changes in turgor
  • when water flows into guard cells by osmosis, turgor increases and they expand and pore opens - if guard cells lose water, opposite happens and pore closes