Mass transport in plants ppt

• Mass transportdefinition


  Movement of large volumes of fluid (water) carrying dissolved molecules/ions all moving at same rate in same direction, movement caused by pressure gradient
• Why a plant needs water

  • Fills vacuole, makes cells turgid so plant is supported
  • Reactant in Photosynthesis
  • Hydrolysis reactions
  • Transport medium - ions from roots to leaves, solvent
• Xylem


  Transports water (& dissolved ions) up the stem from roots to leaves
• Phloem


  Transports sucrose (& amino acids) from the source (site of production of sucrose - leaves) to the sink
• Transport in Xylem vessels

  • Dead tissue, end walls broken down (forming hollow "pipes" to carry water)
  • Transport water in a continuous, unbroken column from the roots to the shoots
  • Vertical cell walls are thickened with LIGNIN to withstand tension (prevents the wall collapsing in on itself)
  • Lignin is waterproof to keep water inside
• Cohesion-tension theory


  1. Transpiration - water evaporates from leaf mesophyll cells & water vapour diffuses into the air via stomata

  2. Water is drawn from xylem vessels, to replace that lost from transpiration

  3. Water enters the xylem at the roots
• Cut flowers often wilt when brought home & put into fresh water- Why?


  As the plant is not cut underwater meaning there will be air bubbles in the xylem- inhibiting the flow of water and therefore rate of transpiration
• Tree trunk diameter is smaller at mid day than at midnight


  Suggests reasons for these observations
• Water rises a small distance up thin tubes


  Suggests reasons for these observations
• Describe how to set the potometer up (measuring rate of water uptake in a plant cutting) req practical

  1. Fill the potometer underwater to prevent air bubbles forming

  2. Cut stem underwater to prevent air bubbles in the xylem

  3. Ensure the stem and tube is fully watertight

  4. Ensure leaves are dry (transpiration won't occur if wet)
• Measuring rate of water uptake and how to take repeat measurements

  • Measure the distance the air bubble moves in a given time (Or time taken to move a certain distance)
  • Use the water in the syringe to return the air bubble to the start position to allow repeats
  • Rate of uptake = distance/time, units (e.g. mm min^-1)
  • (If including volume in rate, need lumen diameter/radius of tube) Volume of water = π r^2 x length (distance travelled), units (e.g. mm^3 min^-1)
• Factors affecting rate of transpiration

  • Air currents
  • Temperature
  • Light intensity
  • Air humidity
• Transport in Phloem vessels- Why a plant needs sucrose

  • In growing areas-shoots & roots (where mitosis is happening) - hydrolysed to glucose so can be used in respiration or converted to cellulose
  • In storage organs - condensed to starch for storage
  • Transports better than glucose - less reactive so less likely to be used up along the way
• Phloem vessels-features


  • Living cells
  • Sieve tube cells & Companion cells
  • Sieve tube cells - little cytoplasm & few organelles
  • Companion cells - carry out active transport (as they have hardly any organelles)
• Translocation

  1. Sucrose made after photosynthesis - glucose + fructose condense

  2. FD into companion cell

  3. AT into sieve tube cell

  4. Sucrose lowers WP

  5. Water enters sieve cell from xylem (by osmosis)

  6. Creates high HP in sieve cell near the source

  7. Mass flow to sink occurs - down a pressure gradient

  8. FD of sucrose into companion cell

  9. AT of sucrose into sink cell - sucrose used

  10. Loss of water from sink region lowers HP in sieve tube - maintaining pressure difference
• Ring-barking (evidence that sucrose is transported from the source to the sink)

  
  

  Cutting a ring of bark around the tree to cut phloem

  Bulge forms above the cut, fluid contains a higher sucrose concentration than below the cut
• Aphids pierce the phloem with mouthparts(provides evidence for a pressure gradient)


  They pierce the phloem with their mouthparts

  Sap comes out under pressure, more so at leaves than further down stem so, must be a pressure gradient
• Radioactive tracers and how they work (evidence that the movement is from the source to the sink-leaves to the roots)

  (can use CO2 or sucrose for this)

  They are used to track the movement of molecules over time

  It shows the movement from the source to the sink-leaves to roots
• Evidence for mass flow in phloem

  • Sucrose reaches roots faster than by diffusion alone
  • Pressure in phloem vessels (sap is pushed out when cut)
  • Sucrose concentration is higher in leaves than roots
  • Sap moves down when light but stops in the dark
  • Sucrose concentration rises in phloem shortly after sucrose conc. has risen in leaves
  • Respiratory poisons/lack of oxygen inhibits translocation (this provides evidence that Active transport is involved)
• Evidence against mass flow in phloem

  • Sieve plates create a barrier to mass flow
  • Dissolved molecules/ions don't all move at the same rate
  • Sucrose reaches all parts of the plant at the same rate - should go more quickly to areas with the lowest concentrations
• What does mass transport need in order to be effective?

  • Closed vessels-xylem and phloem
  • Pressure differences
• Features of xylem and phloem vessels-

  Xylem-

  Outer cells are not living

  One-way only for movement

  No end walls between cells

  Transports water and minerals

  Phloem-

  Has organic molecules

  Has end walls (sieve plates)

  Two-way movement

  Cells are living but need support
• What is cohesion?

  Water molecules 'held together' by many weak hydrogen bonds

  (giving cohesion between the water molecules)

  This allows water to move up in a continuous column
• Cohesion-tension theory

  • The transpiration 'pull' stretches the column of water molecules, it is under tension (negative pressure)
  • Lignin withstands the tension- vessels become narrower, but walls do not collapse
  • Adhesion of water molecules to walls helps the column to remain unbroken and rise upwards

  (forces of attraction between between water molecules and the walls of the xylem)
• Why do plants need water?

  • Fills vacuole, makes the cell turgid so the plant is supported
  • A reactant in photosynthesis
  • Hydrolysis reactions
  • Transport medium- ions from roots to leaves
  • Solvent
• What is the source & the sink of a plant?

  Source= The site of production of sucrose (leaves/palisade mesophyll)

  Sink= The site of the use of sucrose (growing/storage areas e.g roots and shoots)
• source= where something is being made- sucrose photosynthesised in the leaves

  Sink=Where something is being transported to-site of the use of sucrose (growing/storage areas e.g root/shoot tips)
• Xylem

  • Direction is from the roots to the leaves up the stem
  • Water transports dissolved ions
  • Tension created by transpiration pull
  • Dead tissue
  • No ATP required for transport
• Phloem

  • Direction is from the source to the sink
  • Water transports sucrose
  • Pressure (at the source) created by the entry of sucrose into phloem followed by the entry of water
  • Living tissue
  • ATP required for transport
• Xylem= cohesion-tension

  Phloem=Translocation
• Why can't the potometer measure the rate of transpiration?

  Not all of the water is used in transpiration (some is used in photosynthesis, hydrolysis, making cells turgid)
• What would you have to know/ work out to be able to standardise the rate of water uptake into units of mm^3/ mm^-2/mm^-1 (mm^3 water taken up per mm^2 leaf area per minute?)

  Surface area of the leaf

  Distance travelled and the diameter of the lumen *time taken to travel

  pi(r^2)x length= volume
• Describe how you would calculate the rate of uptake using the units mm^3/mm^-2/mm^-1?

  Vol/SA=Answer/Time= rate per.... mm^3/mm^-2/mm^-1
• Describe a method to find the area of the leaves

  Draw around the leaves on graph paper
• Why might you need to standardise units of water uptake?

  To allow for comparison
• Why should you get rid of anomalous results when calculating a mean?

  To acquire a more accurate/reliable mean
• Suggest reasons why the results may not be accurate when using a potometer-

  May not have been airtight/watertight-leaks possible

  Air bubbles may have formed in the xylem/stem

  Leaves may be wet
• What was the smallest scale division of your potometer? and therefore what is the uncertainty in each measurement?

  1mm so the uncertainty was 1 + or - 0.5mm
• Equation for percentage uncertainty?

  %uncertainty=uncertainty(x2)/measured distance x100
• Stomata practical aim: investigate if there's a SD in the mean number of stomata on the lower epidermis of 2 different species of plant