Module 3

• Phloem


  Plant vessels responsible for transporting sugars
• Sieve tube elements


  Cells that make up the phloem vessel
• The vessel consists of many sieve tube elements that run up the stem on top of one another
• Sieve tube elements


  Living cells that contain a cytoplasm but no nucleus
• The walls of sieve tube elements are made of cellulose
• Companion cells


  Connected to sieve tube elements via plasmodesmata
• Plasmodesmata


  Allows the cytoplasm to be shared between companion cells and sieve tube elements
• Companion cells


  Have a nucleus
• Sieve plates


  At either end of the sieve tube elements
• Sieve plates


  Have large pores that allow sap to move through the sieve tube elements
• Sieve plates


  Allow sugars to be transported through the phloem
• Translocation


  1. Active transport of sucrose

  2. Diffusion of sucrose

  3. Osmosis

  4. Unloading of sucrose

  5. Osmosis
• Active transport of sucrose

  1. The leaves are called the source

  2. Sucrose is actively transported into the companion cells in the phloem

  3. Energy is provided for active transport from mitochondria in the companion cells
• Diffusion of sucrose


  1. The loading of sucrose into the phloem causes the concentration of sucrose to increase

  2. Sucrose diffuses from the companion cells into sieve tube elements
• Osmosis


  1. The increase in the concentration of sucrose in the phloem causes water potential in the phloem to decrease

  2. Water diffuses into the phloem by osmosis
• Unloading of sucrose


  1. At the bottom of the phloem, sucrose concentration is low because it is being used up in the cells

  2. The cells at the bottom of the phloem are called the sink

  3. Sucrose diffuses out of the phloem and into the sink cells

  4. This lowers the water potential of sink cells
• Osmosis


  1. Water diffuses down its water potential gradient out of the phloem by osmosis

  2. The diffusion of water into the phloem at the source and out of the phloem at the sink creates a hydrostatic pressure gradient

  3. The hydrostatic pressure gradient allows mass transport of sucrose from source (leaves) to sink
• Xylem


  Plant vessels responsible for transporting water and mineral ions
• Vessel elements


  Cells of the xylem that are dead
• Xylem vessels are long, tubes of cells that run up the stem of plants
• The vessel elements in the xylem are dead
• The vessel elements are stacked on top of one another
• There are no cell walls at the end of each vessel element
• This creates a continuous tube for water to flow through
• Lignin


  A waterproof polymer that lines the walls of the xylem
• Cohesion-tension theory


  Water and inorganic ions travel up the xylem through cohesion and tension
• Transpiration


  1. Some of the water in the leaves is used in photosynthesis

  2. Most of the water in the leaves evaporates
• Tension


  1. The loss of water from the leaves creates tension in the xylem

  2. Tension is the formation of hydrogen bonds between water molecules and the sides of the xylem vessel elements

  3. Water in the xylem is pulled upwards by this tension towards the leaves
• Cohesion


  1. Individual water molecules form hydrogen bonds with each other

  2. When water molecules are pulled up the xylem, other molecules of water are also pulled upwards due to cohesion
• Diffusion in the roots

  1. When water is pulled up the stem, the water potential at the bottom of the plant decreases

  2. Water diffuses into the roots via osmosis down its water potential gradient
• Xerophytic plants


  Specialised to dry habitats where water availability is low
• Xerophytic plants


  • Have fewer stomata
  • Have sunken stomata
  • Have fine hairs covering the epidermis
  • Have curled leaves
  • Have a waxy cuticle
• Fewer stomata


  Avoid dehydration
• Sunken stomata


  Trap moist air, decreasing the water potential gradient
• Hairs


  Trap moist air, decreasing the rate of water loss
• Curled leaves


  Shelter stomata from wind, reducing water loss
• Cuticle


  Waxy and waterproof, reduces evaporation of water
• Mammalian gas exchange


  Air enters the body through the trachea and travels through a highly branched system
• Trachea


  Entrance to the lungs
• Ridges of cartilage surround the front of the trachea to provide protection and structure