**4.7 Transport in plants** Flashcards
What is a vascular bundle?
- Groups of xylem, phloem + support tissues found in the stems + roots of plants.
What is transported in the xylem tissue?
- Water + mineral ions.
What is transported in the phloem tissue?
- Sugars (photosynthesis) + AAs.
Lignin.
- Strengthen cell wall.
Why do woody stems of plants form annual rings?
- New large vessels formed every spring to restore water transport.
What is the purpose of the structural adaptation of xylem having dead empty xylem vessels?
- Creates wide lumen for unrestricted water flow.
What is the purpose of the structural adaptation of xylem having end walls of vessels break down?
- Long continuous tube for water transport.
What is the purpose of the structural adaptation of xylem having cell walls of vessels lignified?
- Prevents vessels collapsing when contents are under tension.
What is the purpose of the structural adaptation of xylem having cell walls lignified with rings, spirals + in a reticulate manner?
- Allows vessels to be flexible, preventing breakage as the stem moves.
Why does xylem tissue have different patterns of lignin thickening?
- Allow them to withstand pressure + be flexible.
What are pits in xylem vessels?
- Areas of cell wall that lack lignin + so allow lateral transport.
What is the only type of living tissue in the xylem?
- Xylem parenchyma.
Xylem tracheids.
- Similar to vessels –> narrower + shorter.
- Found in less-advanced species as main water-carrying tissue.
Xylem parenchyma cells.
- Plant cells w/ no thickening.
- Found among xylem + tracheids and remain as living tissue.
Xylem fibres.
- Narrow, highly thickened dead cells with only small gap (lumen) in centre.
- Cannot transport water but are used for support.
Xylem vessels die when their walls become thickened, why?
- They cannot get water due to waterproofing by lignin.
Sieve plates.
- Perforated end walls of phloem sieve tubes.
Companion cells.
- Plant cells w dense cytoplasm connected to sieve tubes in phloem tissue.
Plasmodesmata
- Gaps in cell wall w plasma membrane.
What organelles can be found in a companion cell?
- Nucleus, mitochondria, ER + golgi.
What is the purpose of the structural adaptation of sieve tubes having limited peripheral cytoplasm + organelles?
- Creates space for sugar transport through the cell.
What is the purpose of the structural adaptation of companion cells + sieve tubes being connected by plasmodesmata?
- Enables sieve tube to stay alive wo/ a nucleus + w/ very limited cytoplasm.
What are the 3 types of water movement through a plant?
- Mass flow.
- Diffusion.
- Osmosis.
Which pathway does mass flow happen through?
- Apoplast pathway.
Apoplast pathway.
- Passes through fibrous cell walls + dead xylem cells.
Which pathway does mass flow happen through?
- Symplast pathway.
Symplast pathway.
- Passes flow cell to cell through cytoplasm + plasmodesmata (living contents of the cell).
- Slower as cytoplasm of cells is packed with organelles.
Which pathway does mass flow happen through?
- Vacuolar pathway.
Which model suggests a way in which water can move up a plant?
- Cohesion-tension model.
What is transipration?
- Loss of water vapour from aerial parts of plant.
What is cohesion?
- The water molecules are attracted to each other due to the hydrogen bonds in the water molecules.
What type of strength does cohesion cause?
- Great tensile strength.
What is adhesion?
- The force of attraction between the water molecules and the vessel wall.
What are 4 pieces of evidence for the cohesion-tension model?
- Xylem vessel punctured, air enters (water under tension not pressure).
- Xylem have lignin to stop collapsing under tension.
- Rates of water uptake linked to rate of transpiration.
- Fine columns of water under tension show sufficient tensile strength to account for transport to highest trees.
What are the 4 factors affecting rate of water movement?
- Temperature.
- Light.
- Humidity.
- Air movements.
How does raising temperature affect the rate of water movement?
- Water molecules have more kinetic energy.
- More energy - more molecules evaporate into air spaces + diffuse out faster ∴ higher rate of transpiration.
How does decreasing light intensity affect the rate of water movement?
- Guard cells loose turgidity + flatten against each other.
- Closes stomatal pores ∴ diffusion of water vapour severely restricted.
How does increasing humidity affect the rate of water movement?
- Number of water molecules increases.
- Diffusion grad. compared to in leaf is reduced ∴ slower rate of diffusion.
How decreasing air movement affect the rate of water movement?
- Water vapour tends to build up close to surface of leaf.
- Reduces diffusion grad. + slows transpiration.
What is the casparian strip?
- Wax strip in endodermis cells near root hair cells.
What is the consequence of having a casparian strip?
- Water has to pass through cytoplasm as apoplast pathway is temporarily blocked.
Translocation?
- Transport of manufactured solutes e.g. sucrose + AAs in phloem.
Mass flow?
- Transport in which pressure difference are used to move fluid to carry substances in one direction.
What is the mass-flow hypothesis?
- Pressure differences drive fluid movement.
- Leaf - source area - sugars made ∴ low water potential, water enters ∴ high hydrostatic pressure.
- Other parts - sink area - sugar converted into insoluble starch ∴ high water potential, water out of cell ∴ low hydrostatic pressure.
- Source - phloem - sink - xylem - source - etc.
What are 3 strengths of the mass flow hypothesis model?
- Can measure gradients + show they are present.
- Pierced by insect mouth, contents of sieve tubes flow out, showing them to be under pressure.
- Links xylem + phloem systems in plausible way.
What are 4 weaknesses of the mass-flow hypothesis model?
- Organic solutes move around the plant in different directions, not just to lowest sink pressures.
- Sieve + companion are alive + do not work if killed, model doesn’t explain why.
- Starch found in many plant cells not just sinks.
- Model suggests entirely passive process, but phloem has higher metabolic rate than most other plant tissues.
What are 3 other features that the mass-flow hypothesis model does not explain?
- Sugars need to be loaded into sieve tubes at the source, this is not fully explained.
- Why do all sieve tubes contain phloem protein strands?
- What is a purpose of sieve plates? They seem to hinder mass flow not help it.
