Plant transport

• Why do multicellular plants need transport systems? (4)

  Need water, minerals and sugars to live and also need to get rid of waste substances.

  
  

  Multicellular plants have a low SA:V, however they are relatively big with a high metabolic rate.

  
  

  Exchanging substances by direct diffusion would be too slow to meet their metabolic needs.

  
  

  So plants need transport systems to move substances to and from individual cells quickly.
• What is the function of the xylem?

  To transport water and mineral ions in solution to all parts of the plant.

  
  

  To provide structural support.
• what is the function of the phloem?

  To transport sugars in solution up and down the plant.
• How are xylem vessels adapted to thei function? (4)

  No end walls on cells, making an uninterrupted tube that allows water to pass up through the middle easily.

  
  

  cells are dead so they contain no cytoplasm so water can pass through easily.

  
  

  cell walls are thickened with lignin - helps support xylem vessels and stops them from collapsing inwards. Amount of lignin increases as cell gets older.

  
  

  water and ions move in and out of the vessels through small pits where theres no lignin.
• How are the structure of the sieve tube elements in the phloem tissue adapted to their function? (5)

  Living cells that form tubes for transporting solutes throught the plant.

  
  

  Joined end to end to form sieve tubes,

  
  

  'Sieve' parts are perforated end walls which allow solutes to pass through.

  
  

  have no nucleus, few organelles and a thin layer of cytoplasm, to make space for maximum transport of sucrose.

  
  

  cytoplasm of adjacent cells connected through the holes in the sieve plates.
• What is the role of companion cells?

  How is their structure adapted to their function?

  Lack of nucleus and few organelles in sieve tube elements mean they are not self sufficient. Companion cells carry out the living functions of both themselves and their sieve cells, e.g provides energy for active transport of solutes.

  
  

  Companion cells have lots of mitochondria, a large nucleus and a dense cytoplasm.
• Mass flow (source)


  Solutes are loaded into the phloem via active transport.

  
  

  This results in a low water potential in the sieve tube elements.

  
  

  Water from the xylem and the companion cells diffues into the sieve tubes via osmosis, resulting in a HIGH PRESSURE inside the sieve tubes at the source end of the phloem.
• Mass flow (sink)


  Solutes load into the companion cells at the source end of the phloem.

  
  

  As water potential is higher in the sieve tubes it diffuses out back into the xylem and the companion cells via osmosis.

  
  

  This results in a LOW PRESSURE at the sink end of the phloem. There is a pressure gradient. Source end has higher pressure and sink end has lower pressure.

  
  

  Solutes move down their pressure gradient from te source into the sink.
• Active loading.


  H+ actively transported outside of companion cell into surroundings. Energy provided by breakdown of ATP. Higher concentration of H+ outside cells than inside.

  
  

  Therefore H+ wants to diffuse back in. It does this by going through a co-transport protein in plasma membrane of companion cell.

  
  

  Sucrose goes down the co-transport protein with the H+ ions at the same time.

  
  

  Same method used to load sucrose into sieve tubes form the companion cells.
• How does light affect transpiration rate?

  More light = higher transpiration rate.

  
  

  Stomata open when at higher light intensity, so CO2 can diffuse into leaf for photosynthesis.

  
  

  When its dark, the stomata are suually closed, so theres little transpiration.
• How does temperature affect transpiration rate?

  Higher temperature = higher transpiration rate.

  
  

  Low temperatures decrease the relative humidity in the surrounding air. Higher water potential in leaf than outside leaf - water diffuses down it's water potential gradient from the leaves to te air.

  
  

  Warmer water molecules have more kinetic energy so they evaporate from the leaf faster.
• How does humidity of the surrounding air affect transpiration rate?

  lower humidity = faster transpiration rate.

  
  

  lower potential outside of plant than in so water will diffuse by osmosis to the outside from the leaf.
• How do wind levels affect transpiration rate?

  More wind = higher transpiration rate.

  
  

  more wind decreases humidity of the air surrounding the stomata. increases water potential gradient so more water will diffuse out.
• How are cacti (a xerophyte) adapted to it's surroundings? (3)

  Thick waxy layer on epidermis - reduces water loss by evaporation because the layer is waterproof.

  
  

  Spines instead of leaves - reduces surface area for water loss.

  
  

  Closes stomata at hottest times of the day when transpiration rates are the highest.
• How is marram grass (a xerophyte) adapted to it's surroundings? (4)

  Has stomata that are sunk in 'pits' so that theyre sheltered from the wind - slows down transpiration rate.

  
  

  Layer of hairs around epidermis that traps moist air around the stomata - reduces water potential gradient between air and leaf so slows transpiration rate.

  
  

  Rolls their leaves in hot/windy conditions to trap moist air to slow down transpiration.

  
  

  Also reduces exposed surface area for losing water and protects stomata from wind.
• How are water lillies (a hydrophyte) adapted to its surroundings? (4)

  Hydrophytes need to adapt to low oxygen levels.

  
  

  Large air spaces between their leaves - allows leaves to float on surface of water - more light for photosynthesis.

  
  

  Air spaces in the root and stem allows oxygen from the floating leaves to move to the parts that are underwater.

  
  

  Stomata usually only present on upper surface of the leaves to maximise gas exchange.

  
  

  Have flexible leaves and stem - helps to prevent damage from water currents.
• Symplast pathway


  Goes through living part of ceslls - the cytoplasm

  
  

  Cytoplasm of neighbouring cells cnnect through plasmodesmata. Water moves in symplast pathway via osmosis.
• Apoplast pathway


  goes through non-living parts of the cells - the cell walls.

  
  

  walls are very absorbent and water can simply diffuse through them as well as pass through the spaces between them.

  
  

  The water can carry solutes and move from areas of high hydrostatic pressure to low hydrostatic pressure along a pressure gradient (example of mass flow)
• Transport of water from roots into xylem.

  water transported through apoplast pathway. When it reaches the ENDODERMIS cells in the root, it is blocked by waxy CASPARIAN STRIP. Now water has to take symplast pathway.

  
  

  This is useful, as water has to move through a cell membrane, which are partially permeable. Able to control whether or not substances in the water can get through.

  
  

  Both pathways are used but mainly apoplast pathway is used as it has the least resistance.

  
  

  Once past this barrier, water moves into the xylem.
• Transport of water from xylem to the leaves.


  Xylem vessels transport water all around the plant.

  
  

  At the leaves, water leaves the xylem and moves into the cells mainly by apoplast pathway.

  
  

  Water evaporates between the cell walls in the spaces between the cells and the leaf.

  
  

  When stomata open, water diffuses out of the leaf to the surrounding envioments down it's water potential gradient. (transpiration)
• Mechanism of the transpiration stream (cohesion)

  Water evaporates from the leaves at the top of the xylem via transpiration.

  
  

  This creates tension (suction) which pulls more water into the leaf.

  
  

  Water molecules are cohesive (stick together) so when some are pulled into the leaf, others follow. This means that the whole column of water in the xylem moves upwards.

  
  

  Water enters the stem through the root cortext cells and the cycle (transpiration stream) continues.
• Mechanism of the transpiration stream (adhesion)

  adhesion is partly responsible for movement of water.

  
  

  As well as being attracted to eachother, water molecules are atracted to the walls of the xylem vessels.

  
  

  This helps water rise up through the xylem vessels.
• How to dissect plant stems for a practical

  1) cut a thin cross section of the stem (transverse or longitudinal) with a scalpel.

  
  

  2) use tweezers to place the samples in water until you come to use them. This helps them from drying out.

  
  

  3) Stain the sample e.g. toluidine blue O (TBO) an leave for 1 minute.

  TBO staisn the lginin of the xylem vessels blue-green. This will let you to see the position of the xylem vessels and examine their structure.

  
  

  4) rinse off the sections in water and mount each one onto a slide.
• What does a potometer do?

  https://www.youtube.com/watch?v=HfeRBh-Hxlk

  measure the rate of transpiration.
• What are the practical considerations that need to be acknowledges when using a potemeter to measure the rate of transpiration?

  Plant shoot must be fresh and healthy.

  
  

  Plant shoot must be cut underwater.

  
  

  No air bubbles in the apparatus.

  
  

  leaves must be dry.

  
  

  Only one variable can be changed at a time. all other variables must be kept constant.