Mass T Plants

• What is the xylem

  A tube in plants that is responsible for moving water and dissolved ions around the plant (Bio directional)

  plant roots are responsible for uptake of water and mineral ions and have root hairs which increase surface area

  uptake of water is passive and happens via osmosis
• what is phloem 


  a tube that transports dissolved organic compounds around plants (bio directional)

  uptake of minerals is active or passive via simple diffusion or active transport
• what is apoplast pathway

  water travels via apoplast pathway when transpiration levels high ( series f species running through cellulose cell wall, dead cells and hollow tubes of xylem)

  water moves by diffusion and from cell wall to cell wall

  when water reaches endodermis , thick waxy band of Suberin blocks pathway . called casparian strip and must now go through symplast pathway .
• benefits of casparian strip

  Helps plant control which mineral ions reach xylem and helps prevent toxins entering xylem
• what is symplast pathway

  smaller volume of water travels this way and involves cytoplasm and plasmodesmata and vacuole of cells

  water moves via osmosis into cells and into vacuole and between cells through plasmodesmata

  movement is slower that apoplast pathway
• What is transpiration stream 


  Movement of water through plant xylem due to evaporation of water vapour from leaves and cohesive and adhesive properties of water molecules between roots and leaves
• what is transpiration 


  loss of water vapour due to opening of stomata via diffusion
• why can transpiration occur

  as water potential gradient between water from soil (high potential and atmosphere (low water potential)
• why is transpiration important

  provides cooling to plant via evaporation

  transpiration stream helps uptake of mineral ions

  turgor pressure of cells ( due to water as it moves up plant) provides support to leaves and stem of non woody plants enabling a increased surface area of leaf blade
• movement of water through leaves

  transpiration occurs and water vapour leaves through stomata

  this owners water-potential in air spaces in mesophyll cells

  water within mesophyll cell evaporate into air spaces resulting in transpiration pull

  pull results in water moving through mesophyll cell wall or out of mesophyll cytoplasm
• what causes transpiration stream

  movement of water through mesophyll cells results in water leaving xylem through pits and water moves up vessel to replace lost water
• what is the role of stomata

  surrounded by guard cells

  guard cells open stomata when turgid and close when lose water

  when stomata open there is greater rate of transpiration and gas exchange

  when closed transpiration and gas exchange rate decreases
• factors that affect transpiration

  wind - removes water vapour from leaf and sets up concentration gradient between leaf and increases transpiration

  humidity - humidity lowers water potential gradient so decreases transpiration gradient

  light intensity - increased light intensity open guard cells which open more stomata so rate increases

  temperature - more particles have KE so transpiration increases as evaporation rate of water increases
• How to measure factors in a potometer

  Airflow - use fan or hairdryer

  humidity-spray water in plastic bag and wrap around plant

  light intensity -change distance of light source from plant

  temperature - change temperature of room
• what is translocation 


  transport of assimilates (sucrose) from source to sink
• what is a source

  where assimilate produces / acquired

  green leaves and green stems that contain chlorophyll for respiration

  storage organs e.g tubers and tap roots unloading stores before growth period

  food stores in seeds that are germinating
• what are sinks 


  where assimilate is required

  meristems that are dividing

  roots that are growing or actively absorbing materials

  any part of plant where assimilates are being stored
• How has loading and unloading off sucrose been studied

  Is a active process can be slowed by high temp or respiratory inhibitors

  understanding came from collecting sap

  using aphids to collect sap

  using radioactivity lab led metabolites and trace during transpiration

  advancement in microscopes

  observations about mitochondria during translocation
• how are assimilates transported from source to sink

  through phloem sieve tubes
• why are carbohydrates generally transported as sucrose

  allows efficient energy transfer and increases energy storage as is a disaccharide so contains more energy

  less reactive than glucose as non reducing sugar
• process of loading assimilates

  companion cells actively pump hydrogen ions out of cytoplasm and into their cell walls using atp and creates a hydrogen ion concentration gradient between cytoplasm and companion cell

  hydrogen ions move and sucrose molecules move into cytoplasm via co transport through channel protein

  sucrose molecules move into sieve tube through diffusion via plasmodesmata
• process of loading assimilates pt2

  concentration of sucrose build up in sieve tube lowers water potential creating gradient

  water moves from xylem into sieve tubes via osmosis

  creates high pressure gradient and enables mass flow of sugars towards sink tissue

  at sink tissue sugars unloaded
• unloading of assimilates 


  occurs at sinks

  similar to loading with sucrose activity pumped out of companion cues and moving out of phloem tissue via apoplastic or symplastic pathway
• how is concentration gradient of sucrose maintained at sink tissue

  Sucrose converted to tiger storange molecules like starch . Metabolic reaction and requires enzymes e.g invertase which hydrolyses sucrose into glucose and fructose
• how are sieve tubes adapted for mass transport

  few organelles

  small volume of cytoplasm

  large vacuole present in intercellular space so sap can flow through open corridor easily as low resistance

  have thick walls to withstand pressure exerted by mass flow
• what is mass flow hypothesis

  used initially to explain movement of assimilates in phloem tissue
• mass flow hypothesis experiment

  modeled by Ernest münch in 1930 .

  two partially permeable membranes containing solutions of different concentration

  placed in chamber containing water and connected via passageway

  membranes joined via a tube

  water moved by osmosis across membrane containing concentrated solution which forced solution inward . membrane containing more dilute solution forced out due to hydrosttic pressure
• results of experiment 


  lead to discovery of pressure flow gradient
• advantage of mass flow

  Moves solutes faster than diffusion
• Why is water important in mass flow

  Presence of water in sieve elements increases hydrostatic pressure at source and as assimilates unloaded water follows out via osmosis at sing and creates low hydrostatic pressure . Gradient occurs

  pressure difference between source and sink results in mass flow of water which moves assimilates around cell
• evidence supporting mass flow hypothesis

  when phloem punctured it oozes out showing pressure

  phloem near sap has higher concentration than taken from sink

  when virus applied to leaves it can be observed moving down phloem towards roots

  when virus applied in dark it does not move. suggest that for translocation to occur photosynthesis and production of sucrose is required in source tissue
• evidence against hypothesis 


  rate of translocation different in organic substances as amino acids travel slower . hypothesis states that all travel should be same rate

  substance move in opposite direction not flowing in same direction

  some sieve tubes translocate at different times not at same times even though connected to same source
• what and ringer experiments

  used to investigate mass transport in plants

  involves removal of a ring of surface tissue from stem of plant while leaving stem core intact.

  ring removes phloem as located its located outwards but xylem intact

  plant exposed to radioactive tracer and do direction and rate of translocation observed.

  14ˆCO2 used as readily absorbed and removed via respiration .

  sucrose formed radioactive and movement due to translocation can be traced
• what does tracer and ringer experiments test

  Test mass flow hypothesis . If correct bulk movement of phloem sap happens mono directional from source to sink and occurs at same rate and time s.

  if xylem damaged during process plant will not have enough water and wilt
• what do results show of tracer and ringer experiments

  show that phloem involved in transport of sucrose . no radioactive sucrose detected pat ring point on stems where phloem removed

  phloem transports sucrose up and downwards as sucrose translocation from source tissue in leaves to sink tissue above and bellow