(Mass transport in plants)

• XYLEM STRUCTURE (for transporting water + mineral ions): 

  • VESSEL ELEMENTS: The xylem vessels= long tubes of cells that run up the stem of plants. Vessel elements= dead cells of the xylem.
  • END WALLS: The vessel elements are stacked on top of each other. There are no cell walls at the end of each vessel element, which creates a continuous tube for water to flow through 
  • LIGNIN: The walls of the xylem are lined with lignin- a waterproof polymer that reinforces the walls of the vessel elements for structural support
• XYLEM WATER MOVEMENT ACROSS A LEAF :

  • EVAPORATION: Mesophyll cells lose water by evaporation (due to heat from the sun)
  • NEIGHBOURING CELLS: The cells have a lower water potential -> water enters by osmosis from neighbouring cells
  • NEIGBOURING CELLS 2: The neighbouring cells have lower water potential -> they take water from their neighbouring cells via osmosis
• COHESION TENSION THEORY (how water & inorganic ions travel up the xylem):

  1)TRANSPIRATION: Some of the water in the leaves is used in photosynthesis. Most of the water in the leaves evaporates in transpiration
• COHESION TENSION THEORY (how water & inorganic ions travel up the xylem):

  2) TENSION: The loss of water from the leaves creates tension in the xylem. Tension= the formation of hydrogen bonds between water molecules & the sides of the xylem vessel elements. Water in the xylem is pulled up by the tension towards the leaves
• COHESION TENSION THEORY (how water & inorganic ions travel up the xylem):

  3) COHESION: Cohesion= individual water molecules also form hydrogen bonds with each other. When water molecules are pulled up the xylem, other molecules of water are also pulled up due to cohesion. The combination of cohesion & tension together continuously pull water up to replace water that has been lost in the leaves by transpiration
• COHESION TENSION THEORY (how water & inorganic ions travel up the xylem):

  4) DIFFUSION IN THE ROOTS: When water is pulled up the stem, water potential at the bottom of the plant decreases. Water diffuses into the roots via osmosis down its water potential gradient
• XYLEM COHESION TENSION THEORY

  
• INVESTIGATING TRANSPIRATION RATE

  
• INVESTIGATING TRANSPIRATION RATE:

  1)ASSEMBLE THE POTOMETER: Potometer= estimates the vol of water taken up by a plant in a given time. It's filled with water & a cutting of a shoot is placed inside. The shoot must be cut & placed into the potometer while underwater to ensure no air enters the xylem
• INVESTIGATING TRANSPIRATION RATE:

  2) FORM THE AIR BUBBLE: Remove the end of the capillary tube from the water beaker. Wait for a bubble of air to form in the capillary tube. Place the capillary tube back into the water. The air bubble is used to calculate the vol of water used by the shoot
• INVESTIGATING TRANSPIRATION RATE:

  3) RECORD BUBBLE MOVEMENT:

  Mark the starting position of the air bubble. Use a stopwatch to record the distance moved by the bubble in a given time period
• INVESTIGATING TRANSPIRATION RATE:

  4) CALCULATE TRANSPIRATION RATE: Calculate the rate of movement of the bubble per hour. The rate of bubble movement= (equal to) the transpiration rate)
• INVESTIGATING TRANSPIRATION RATE:

  5)CHANGE THE VARIABLE: The experiment can be repeated with changed variables eg temp, light (but only 1 variable should be changed per repeat to compare environmental factors accurately)
• TYPES OF POTOMOTERS:

  
• Phloem

  STRUCTURE (for transporting sugars):

  • SIEVE TUBE ELEMENTS: are the 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. They are living cells that contain a cytoplasm but no nucleus. The walls of sieve tube elements are made of cellulose. 
• Phloem

  STRUCTURE (for transporting sugars):

  • COMPANION CELLS: are connected to sieve tube elements via plasmodesmata, which allow the cytoplasm to be shared between companion cells & sieve tube elements. Companion cells have a nucleus
• Phloem

  STRUCTURE (for transporting sugars):

  • SIEVE PLATES: are 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 (process where assimilates eg sucrose, amino acids are transported from leaves to other parts of the plant): 
• TRANSLOCATION: 

  • TRANSFER OF SUCROSE (FROM PHOTOSYNTHESISING TISSUE -> SIEVE ELEMENTS):

  1. Sucrose is produced by photosynthesis in chloroplasts
  2. Sucrose diffuses into companion cells, down a conc gradient by facilitated diffusion
  3. H ions are actively transported from companion cells into spaces in cell walls (uses ATP)
  4. The hydrogen ions diffuse down a conc gradient through carrier proteins into sieve tube elements
  5. Sucrose are transported along with the H ions (co transport) into sieve tube elements
• TRANSLOCATION: 

  • MASS FLOW OF SUCROSE (IN SIEVE TUBE ELEMENTS):

  1. sucrose transport into sieve tube -> lower water potential
  2. xylem= higher water potential now -> water moves from xylem into sieve tube elements by osmosis -> hydrostatic pressure between them
  3. The respiring cells (sink): sucrose used up (in respiration) or converted (into starch for storage) -> now have  a low sucrose conc
  4. Respiring cells low sucrose conc -> sucrose actively transported into them from sieve tube -> lower water potential -> water moves into them by osmosis -> hydrostatic pressure= high at source & low at sink
• TRANSLOCATION: MASS FLOW OF SUCROSE

  Mass flow of sucrose down the hydrostatic gradient in sieve tubes to restring
• INVESTIGATING MASS TRANSPORT IN PLANTS:

  • RINGING: can be used to investigate mass transport in the phloem. Phloem vessels= located outside the xylem vessels in the stem. If a ring is cut around the outside of a stem, this halts the transport in the phloem but allows transport in the xylem to continue. Eventually the tissue above the rings swells & growth stops below the rings as sucrose cannot be transported (shows the phloem transports sugar)
• INVESTIGATING MASS TRANSPORT IN PLANTS:

  • TRUNK DIAMETER: During day= more tension in xylem -> trunk shrinks in diameter (more transpiration). During night= less tension in xylem -> trunk diameter increases (less transcription)
  • TRACING: Sucrose produced during photosynthesis can be tracked by exposing leaves to carbon dioxide containing radioactive carbon. The plant is frozen in liquid nitrogen & placed onto photographic film. The pathway travelled by the radioactive sucrose down the phloem can be traced
• INVESTIGATING MASS TRANSPORT IN PLANTS:

  • EVIDENCE FOR MASS TRANSPORT: Tracing & ringing experiments provide evidence for the mass transport flow theory. Both types of investigation show a link between phloem & sucrose transport
  • APHIDS: Evidence that mass transport occurs in the phloem (rather than the xylem) as aphids’ mouthparts penetrate the phloem which extract sucrose
• EVIDENCE AGAINST MASS FLOW:

  • DIRECTION OF FLOW: Sap can move up or down the phloem vessels. The hydrostatic pressure gradient that's set up by water entering the source & leaving at the sink doesn't explain how sap can move in both directions
  • SIEVE PLATES: Are the pores between sieve tube elements. Increased hydrostatic pressure is required for water to flow through the pores. This means sieve plates should be a problem for mass flow
  • LIVING PHLOEM CELLS: Phloem cells= living & xylem cells= non living, the purpose of phloem cells being living is unclear