3.3. Transport in Plants

• Based on what principle does phloem unloading work?

  Diffusion of sucrose- by rapidly moving into surrounding cells or by converting glucose, decreasing sucrose concentration within cells hence maintaining sucrose concentration difference between cells in sink and sieve tube
• Describe two adaptations of roots that could help a plant survive arid conditions?

  Roots could be very long to reach water deep in soil; roots could be very widespread to absorb water from a large area when it does rain.
• How do bordered pits form and what is their purpose?

  They form where lignification is incomplete, leaving gaps in the cell wall; the bordered pits in 2 adjacent vessels are aligned to allow water to leave one vessel and pass into the next, as well as leave the xylem
• WHat is the Casparian strip?

  A band of waterproof, waxy suberin around each endodermis (on cell wall)
• Describe how water is lost via the stomata (refer to gradients)?

  Water vapour in higher concentration in air spaces than in space outside leaf; water diffuses down water vapour potential gradient
• Plant transport systems consist of two major specialised vascular tissues. Name these, state their transport material and the direction of the transport


  Xylem- water, soluble mineral ions (upwards)

  Phloem- assimilates (up or down)
• State two precautions that should be taken to ensure no air bubbles are in the potometer setup

  Set up under water; cut stem under water to prevent air entering the xylem
• What two components of phloem tissue are concerned with transport?

  Sieve-tube elements and companion cells
• Explain the significance of the Casparian strip

  It is waterproof hence locks the apoplast pathway between the cortex and the medulla, forcing water and dissolved mineral ions to pass through the selectively permeable membrane into the cytoplasm, filtering out toxic chemicals
• State two sinks (for translocation) in a plant.

  Roots growing or active uptaking mineral ions; actively dividing meristematic tissue; part plants that are laying down food stores (e.g. developing seeds, fruits etc)
• Define the term translocation.

  Transport of assimilates from source to sink (tissue that needs them)
• Describe the role of hydrogen ions in active loading.

  The hydrogen ions are pumped out of the companion cells, creating a hydrogen ion concentration gradient across the cell membrane. The hydrogen ions can diffuse back into the companion cells through special transport proteins- but they can only move if sucrose is carried in with them (cotransport)
• How does sucrose move from the companion cells into the sieve tube elements?

  Increasing sucrose concentration causes water to also move into companion cells, which builds up turgor pressure (and water potential). The water carrying assimilates (sucrose) then enters sieve tubes down the pressure/ WP gradient through plasmodesmata
• Cacti are succulents. What does this mean?

  They store water in their stems which become fleshy and swollen
• What is the purpose of parenchyma cells in xylem tissue?

  To separate and support the vessels (act as packing tissue)
• What allows water to move through cells via the symplast pathway, and by which mechanism of movement?

  Different water potential of cytoplasm of adjacent cells; water moves by osmosis
• Why does the presence of starch in the endodermis suggest that an active process is involved?

  Starch acts as a store of sugars, which can be released and respired to release energy for active processes
• Describe how water moves up the stem via the transpiration pull.

  Loss of water by evaporation at the top of the plant must be replaced by water from the xylem. This puts water at the top of the xylem under tension. Tension pulls the column of water up the xylem as water molecules are cohesive.
• Why do sieve tube elements contain no nucleus and very little cytoplasm?

  Allows space for mass flow of sap to occur
• Describe 3 adaptations of marram grass (xerophyte) and explain their importance?

  Leaf rolled longitudinally trapping air inside (air becomes humid and reduces water loss from the leaf)

  Thick waxy cuticle on the upper epidermis (reduces evaporation)

  Stomata on lower epidermis inside the rolled leaf (protected by enclosed air space); stomata are in pits in the lower epidermis which is folded and covered by hairs (reduces air movement and hence water loss)

  Spongy mesophyll is very dense with few air spaces (less surface area for evaporation of water)
• What is the purpose of lignin in xylem vessels?

  Strengthens vessel wall and prevents collapse
• Based on what principle does mass flow work in sieve tubes?

  Turgor pressure difference- sieve tubes at source has higher turgor pressure due to 'intake' of sucrose and water, whereas sieve tubes at sinks have lower turgor pressure due to the cells removing the sucrose from the sieve tubes
• What property of water causes cohesion?

  The polarity of the water molecule, which produces hydrogen bonds between the molecules
• Describe the effects of light intensity, humidity and wind speed on the rate of transpiration?

  Light intensity: Increases transpiration rate as stomata are more open Humidity: Decreases transpiration rate as there is a lower water vapour potential gradient Wind speed: Increases transpiration rate as maintains a high water vapour potential gradient
• Why is sucrose transported in phloem and not glucose?

  Sucrose is more stable hence less likely to be metabolised in the transport process
• Describe two major pathways taken by water to move between cells

  Apoplast- through spaces in cell walls and between cells (mass flow, not osmosis)

  Symplast- moves through cytoplasm and between cells via plasmodesmata
• describe three adaptations of xylem vessels that relate structure to function.

  Dead cells aligned end to end to form a continuous column

  Tubes are narrow so water column doesn't break easily and capillary action can be effective

  Bordered pits allow sideways movement of water between vessels

  Lignin allows stretching of xylem (spiral, annular or reticulate patterns) as plant grows
• How are companion cells adapted for active loading?

  Many mitochondria to produce ATP
• What tissue is found in between the xylem and phloem? What is it's function?

  Meristematic tissue (the source of stem cells)
• Why is it advantageous for some xerophytes to have a low water potential inside their leaf cells?

  Reduces evaporation of water from the cell surfaces as the water potential gradients between the cells and the leaf air spaces is reduced
• What allows water to move continuously via the apoplast pathway?

  Cohesion, by diffusion
• What is the difference between active loading and active transport?

  Active transport is the movement of particles against their concentration gradient using metabolic energy (ATP). Active loading is a more extensive process which involves active transport at some stage. In this case, active loading uses active transport to pump hydrogen ions out of the companion cells. This results in the movement of sucrose molecules by facilitated diffusion and diffusion.
• How do you set up a potometer?

  • cut stem under water

  • have apparatus under water

  • insert stem under water

  • joint(s) must be, sealed / tight

  • so no air can enter, stem / shoot / xylem /

  apparatus

  • air / bubble, could block xylem

  • obtain a continuous column of water
• How do you use a potometer?

  • do not allow the bubble to move too far

  • use a syringe to move the bubble

  • so air bubble does not enter, xylem/stem

  • so the same air bubble can be re-used

  • place open end in water

  • so no, air / (new) bubble, introduced

  • keep shoot, still / supported

  • to avoid breaking, seal/water column

  • to measure transpiration accurately

  • ensure validity