Transpiration

• Define Xylem:

  A non-living plant transport vessel that carries water and dissolved mineral ions from the roots
• Describe the features of the xylem:

  • long continuous columns - dead cells with open ends
  • Pits (bordered pits) = allows water to move sideways through adjacent vessels, allows water to leave the xylem and pass into the living parts of the plant
  • lignin = thickens the walls, kills the cells, waterproof - withstands the pressure of water, provides strength, prevents collapsing, forms spiral patterns = stretches as the plant grows, allows some flexibility
• Name the features of the xylem vessels:

  • long continuous columns
  • bordered pits
  • lignin
• Define Bordered pits:

  Gaps in the cellulose cell wall that form when lignification is not complete which allows water to move sideways through adjacent vessels
• Xylem vessels are hollow: empty
• Define Transpiration:

  The loss of water vapour from the plant's stem and leaves via diffusion and evaporation
• Outline the factors that affect the rate of transpiration:

  • Light intensity
  • Temperature
  • Humidity
  • Air movement
  • Water availability
  • Number of leaves
  • Number/size/position of stomata
  • The presence of a waxy cuticle
• Outline reasons why transpiration is important:

  • maintains cell turgidity
  • transports mineral ions up the plant
  • cools the surface of the plant
  • supplies water for growth/ cell elongation/ photosynthesis
• Name the processes taken by water to leave the leaf:

  1. osmosis
  2. evaporation
  3. diffusion
• What is the equipment used to measure the rate of transpiration?

  Potometer
• Water molecules are attracted to each other by the forces of cohesion
• Water molecules are attracted to the sides of the xylem vessels by the forces of adhesion
• Water molecules have strong cohesive forces thanks to their ability to form hydrogen bonds with one another
• The narrower the xylem, the bigger the impact of adhesion
• What aids the movement of water up the xylem against the force of gravity?

  • cohesion
  • adhesion
  • root pressure
• Water molecules adhere to the walls of the xylem
• What makes the walls of the xylem narrower?

  • lignin
  • the tension caused by the water molecules being pulled up
• What causes root pressure?

  the upwards movement of minerals via active transport
• Formula for volume of cylinders:

  $𝚷r^2l$
• General formula for Rate of Transpiration:

  volume/time
• The rate of transpiration is the volume of a cylinder divided by the time taken. $\pi r^2l / time$

  What does r represent?

  the radius of the capillary tube
• Define Adhesion:

  The attraction of water molecules to the impermeable walls of xylem tissue
• Adhesion is the formation of hydrogen bonds between carbohydrates in the xylem vessel walls and water molecules
• Large multicellular animals need a transport system for oxygen and carbon dioxide.

  Large multicellular plants do not need a transport system for oxygen and carbon dioxide, Why?

  Plant cells have a low metabolic rate
• What waterproof polymer lines the walls of the xylem?

  Lignin