Mass Transport in Plants

• Transpiration definition

  the loss of water in plants via the stomata by diffusion
• transpiration stream


  the movement of water in plants from the roots to the leaves
• Why is transpiration important?

  Is a way of cooling the plant

  It is helpful in the uptake of mineral ions

  The turgor pressure of cells provides support to the leaves and the stem of non-woody plants

  Enables photosynthesis, elongation and growth which all need water
• Role of the stomata


  Transpiration controlled by guard cells that surround stomata

  when the stomata are open there is a grater rate of transpiration

  stomata allow gaseous exchange and are mainly open during the day
• Transpiration-process

  water vapour diffuses from air spaces through a stoma by a process called transpiration, lowering the water potential

  water evaporates from a mesophyll cell wall into the air spaces creating a transpiration pull

  water moves through the mesophyll cell wall (apoplastic pathway) or out of the mesophyll cytoplasm into the cell wall (symplastic pathway)

  water leaves a xylem vessel through a non-lignified area

  water moves up the xylem vessels to replace the water lost from the leaf
• What affects the rate of transpiration?

  air movement, temperature, humidity, light intensity, amount of leaves, amount of stomata, water availability
• How does air movement affect the rate of transpiration?

  High air movement removes water vapour from the air surrounding the leaf which sets up a concentration gradient between the leaf and the air, increasing water loss
• How does humidity affect the rate of transpiration?

  more moisture in the air so the concentration gradient is weaker and less water is lost
• How does light intensity affect the rate of transpiration?

  Guard cells are responsive to light intensity when it is high they are turgid and the stomata open and allow water to be lost
• How does temperature affect the rate of transpiration?

  At higher temperatures particles have more kinetic energy so transpiration occurs at a faster rate as water molecules evaporate from the mesophyll and diffuse away faster that at lower temperatures
• How do assimilates move through the plant?

  through the process of translocation, from source to sink
• Transport system in plants

  1. Xylem tissue enables water and dissolved minerals to travel up the plant in the passive process of transpiration

  2. Phloem tissue enables sugars to reach all parts of the plant in the active process of translocation
• Transpiration


  1. Plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves

  2. Carbon dioxide enters, while water and oxygen exit through a leaf's stomata
• Transpiration


  1. Involves osmosis, where water moves from the xylem to the mesophyll cells

  2. Involves evaporation from the surface of mesophyll cells into intercellular spaces and the diffusion of water vapour down a water vapour potential gradient out of the stomata
• Investigating rate of transpiration


  Using a potometer where water lost by the leaf is replaced by water in a capillary tube
• Movement of water in the root

  1. Water enters through root hair cells and moves into the xylem tissue

  2. Movement occurs due to water potential gradient

  3. Root hair cells provide large surface area for water movement

  4. Minerals absorbed through root hair cells by active transport
• Pathways of water movement in the root

  • Symplast pathway - water enters cytoplasm and passes through plasmodesmata
  • Apoplast pathway - water moves through cell walls without passing through plasma membranes
• Water movement across the endodermis

  1. Encounters Casparian strip which cannot be penetrated

  2. Water must now enter the symplast pathway to cross the endodermis

  3. Continues down the water potential gradient from cell to cell until it reaches a pit in the xylem vessel
• Water moving in the xylem up the stem

  1. Removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient

  2. Root pressure pushes water into the xylem by osmosis

  3. Cohesion-tension theory - surface tension and cohesion of water molecules maintains the flow
• Phloem vessels


  • Tubes made of living cells involved in translocation of nutrients
  • Consist of sieve tube elements and companion cells
  • Sieve tube elements transport sugars in dissolved sap
  • Companion cells involved in ATP production for active processes
• Translocation


  1. Energy requiring process to transport assimilates like sucrose from sources (leaves) to sinks (roots, meristem)

  2. Involves active loading of sucrose into phloem, creating a diffusion gradient

  3. Sucrose diffuses into sieve tube elements, reducing water potential and causing water to enter by osmosis

  4. Water moves down the pressure gradient, carrying the sucrose
• Ringing experiments


  Removing bark and phloem, leaving just xylem - tissues above ring swell with sucrose, tissues below die
• Tracer experiments


  Growing plants in 14CO2 environment, then using autoradiography to trace movement of labelled sugars through the plant
• vascular bundle


  enables the transport of substances and provide structural support
• Vascular bundle in the stem

  Xylem is located on the inside in non-wooded plants

  Phloem is found on the outside of the vascular bundle

  Layer of cambium in between the xylem and phloem (meristem cells, produce xylem and phloem tissue)
• Vascular bundle in the roots

  Xylem and phloem

  Xylem vessels are arranged in an X shape in the centre

  X shape arrangement is surrounded by endodermis

  Inner layer of meristem cells known as the pericycle
• Features of xylem


  -long cylinders made of dead tissue with open ends-can form a continuous column

  -transport water and minerals, provide structural support

  -contain pits- enable water to move sideways between the vessels

  -thickened with lignin, in spiral patterns-enables the plant to remain flexible.