3.2.8 Mass Transport Systems in Plants Flashcards Preview

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Flashcards in 3.2.8 Mass Transport Systems in Plants Deck (67)
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What is the function of the xylem?

  • Xylem tissues transports water and minerals in solution
    • Allows substances to move up the plant from roots to leaves


What are xylem vessels apart of?

Part of xylem tissue that transports water and ions


Describe the structure of xylem vessels

  • They're long, tube-like structures formed from dead cells joined end to end
    • There's no end walls on these cells


Why are there no end walls on the cells that make up xylem vessels?

Makes uninterrupted tube = allows water to pass up through middle easily


Describe how water gets from the soil to the roots

  1. Water enters root hair cells by osmosis
  2. ∵ active uptake of mineral ions has created a Ψw gradient (conc. of solutes in soil is lower than in roots)


What happens to the water once it has entered the roots?

Water moves through the cortext by osmosis down a Ψw gradient


Name the 2 possible pathways across the cortex to the endodermis

  • Apoplastic pathway 
  • Symplastic pathway 


Describe how water travels through the apoplastic pathway 


  • Water soaks into cellulose walls of cells in cortex
  • Water seeps towards xylem


Where and why is the water in the apoplastic pathway stopped & what does this mean?

  • Pathway stopped at endodermis as there's the Casparian strip
  • Which is impermeable
  • Water is forced into cytoplasm, enters cell by osmosis


Describe how water travels through the symplastic pathway

  • Cells in cortex joined by plasmodesmata (small pores)
  • Water moves by osmosis through cytoplasm + vacuoles of each cells and through plasmodesmata between cells


Name the mechanisms that help the movement of water up the xylem vessels

  1. Mass flow 
  2. Cohesion tension theory
  3. Adhesion


Transport through Xylem Vessels

Describe mass flow

  • Whole body of water moving together 
  • Pressure from water moving into roots = high pressure at base of xylem
  • Pressure is higher than top ∴ water forced upwards

e.g. similar to water moving up straw


Transport through Xylem Vessels

Describe cohesion tension theory

  • Water molecules are held together by hydrogen bonds (weak bonds)
  • ∴ if one molecule moves, it drags another with it
  • Attraction between them = cohesion


Transport through Xylem Vessels

Describe adhesion

  • Same bond between water molecules will bind molecules to side of xylem vessels
  • ∴ water almost "crawls" up side of vessels


Describe how water in the roots moves up the stem (5)

  1. Water evaporates from the leaves, creates Ψw gradient (Ψw ↓ in leaves)
  2. Water drawn out of xylem by osmosis
  3. Creates tension on water in xylem
  4. Water molecules cohesive, so column of water in xylem move upwards
  5. Due to h-bonding, column doesn't break ∵ of adhesion with xylem walls


What is transpiration?

Evaporation of water from plant's surface -- leaves


Describe Transpiration

  1. Water evaporates from moist cell walls and accumulates air spaces in leaf
  2. When stomata open, water vapour diffuses out leaf down concentration gradient
  3. (Higher concentration of water vapour inside leaf than outside leaf)


Describe the transpiration pull/stream 

  1. Water evaporates from cell walls of mesophyll
  2. Water from xylem vessels replaces this
  3. Water moving out of xylem reduces pressure = water at higher pressure so it can move up the xylem vessels


Name 4 factors that affect the rate of transpiration

  • Light Intensity 
  • Temperature 
  • Humidity 
  • Wind


Describe how light intensity affects the rate of transpiration

  • Lighter = faster transpiration rate
  • ∵ stomata open when there's light to let in CO2 for photosynthesis


Describe how temperature affects the rate of transpiration

  • Higher temp. = faster transpiration rate
  • Water molecules have more kinetic energy = evaporate from cells inside leaf faster
  • Increases water potential gradient = water diffuse out leaf faster


Describe how humidity affects the rate of transpiration (3)

  • As humidity increases = transpiration rate decreases
  • ∵ higher humidity = reduced water potential gradient
  • Less evaporation 


Describe how wind affects the rate of transpiration

  • Windier = faster transpiration rate
  • Lots of air movement blows away water molecules around stomata
  • Increases water potential gradient


What does a potometer essentially do?

estimate transpiration rates


How does a potometer estimate transpiration rates?

Measures water uptake by plant but assumes water uptake by plant is directly related to water loss by leaves


Describe how you would investigate the rate of water uptake of plant using a potometer

  1. Cut shoot underwater
    • Cut it at a slant to increase SA available for water uptake
  2. Assemble potometer in water and insert shoot underwater = no air can enter
  3. Remove apparatus from water but keep end of capillary tube submerged in beaker of water
  4. Check apparatus if watertight and airtight
  5. Dry leaves, allow time for shoot to acclimatise and shut tap
  6. Remove end of capillary tube from beaker of water until one air bubble forms & put tube back in water
  7. Record starting position of air bubble
  8. Start stopwatch and record distance moved by bubble per unit time
  9. Rate of air bubble movement = estimate of transpiration rate


Potometer Pratical 

Name the 2 formulas you need for calculations 

  • Rate = Distance moved (mm) / Time (s)
  • Volume of water = Distance moved x Area of circle


Potometer Pratical 

Is it a direct measurement of water lost from the stomata? Explain your answer.

No, water is used for photosynthesis & to maintain turgor pressure


Potometer Pratical 

Suggest how reservoir allows repeat measurements to be made

Allows bubble to be returned to start


Potometer Pratical 

Why should you use only a healthy shoot?

To ensure stomata are open and transpiration can happen

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