7.7 - Transport of Water in the Xylem

• what is transpiration?

  evaporation of water from the leaves
• what's the transpiration stream?

  flow of water through the plant
• factors affecting transpiration rate

  light intensity: the lighter it is the faster transpiration is as stomata are open for gas exchange for photosynthesis

  temperature: the higher it is the faster the transpiration rate as warmer water molecules have more energy so evaporate more easily

  humidity: the lower the humidity the faster transpiration rate due to a steeper concentration gradient

  Wind: the windier it is the faster the transpiration rate as it increases the concentration gradient
• movement of water out through stomata

  humidity of atmosphere is usually less than that of air spaces next to the stomata

  so there's a water potential gradient from air spaces through stomata to the air

  when stomata are open water diffuses out of the air spaces into air and is replaced by water evaporating from all walls of surrounding mesophyll cells
• morement of water across cells

  mesophyll cells lose water to air spaces by evaporation due to heat from the sun

  so they have a lower water potential than neighbouring cells

  so water enters mesophyll cells from the xylem via cell walls or via the cytoplasm

  Water potential gradient established that pulls water from the xylem across mesophyll and out into atmosphere
• movement of water up the xylem: cohesion-tension theory

  Water molecules form hydrogen bonds with each other so stick together (cohesion)

  they also form hydrogen bonds with cellulose on walls of the xylem so stick to it (adhesion) which pulls walls together making them narrower

  So water forms a continuous, unbroken column in the xylem

  so when water moves into mesophyll cells the water molecules are pulled up the xylem under high negative pressure due to transpiration - called a transpiration stream which is passive
• movement of water into the xylem

  transpiration pull causes root haircalls to take up water by osmosis (crosses cell membrane - cytoplasmic pathway) or by diffusion (doesn't cross cell membrane - apoplastic pathway)

  when water in cell wall meest the Casparian strip cwhich is impermeable to water it moves into the cytoplasm

  these then go into xylem due to transpiration pull
• how can plants control rate of transpiration?

  by changing the size of stomatal pores
• evidence to support cohesion-tension theory

  change in diameter of tree trunks according to rate of transpiration: daytime means more transpiration so more negative pressure so walls of xylem pulled more inwards so diameter shrinks

  if xylem is broken & air enters trees can't draw up water as column of water is broken so molecules no longer stick together

  when xyum is broken water doesn't leak out but is drawn in suggesting it's under tension
• adaptations of the xylem

  vessels are dead so can't actively more water

  no end walls forming continuous, unbroken tubes from roots to leaves which is needed for cohesion-tension theory of water flow

  have lignin in all walls so they die, also forms rings or spirals around the vessel which strengthens them so they can withstand tension