E&T: Transport in Plants - Xylem and Phloem Flashcards
Xylem tissue transports what?
Water + mineral ions in solution that move up the plant from roots to leaves.
Phloem tissue transports what?
Organic substances in solution both up and down the plant.
Xylem vessels
Long, tube-like structures formed from dead cells joined end to end with no end walls.
What helps water move up plants against the force of gravity?
Cohesion
Tension
Explain how tension helps water move up a plant:
- Transpiration occurs from leaves
- Creates tension
- Pulls more water into the leaf
Explain how cohesion helps water move up a plant:
- Cohesion means water molecules ‘stick’ together
- When some are pulled into the leaf, others follow.
Transpiration
The evaporation of water from a plant’s surface, especially the leaves.
Briefly describe transpiration:
- Water evaporates from the moist cell walls
- Accumulates in spaces between cells in the leaf
- When stomata open, water moves out of leaf + down concentration gradient.
What 4 main factors affect transpiration rate?
Light
Temperature
Humidity
Wind
How does light affect transpiration?
- Lighter = faster the transpiration rate
- Because stomata open in light to let in CO2.
How does temperature affect transpiration?
- Higher temperature = faster transpiration rate
- Water H2O molecules have more energy = evaporate faster
How does humidity affect transpiration?
- Lower humidity = faster transpiration rate
- Dry air increases concentration gradient between leaf + air
How does wind affect transpiration?
- High wind = faster transpiration rate
- Air movement blows away water molecules around stomata = increasing concentration gradient
What instrument can be used to estimate transpiration rate?
Potometer
Describe how to use a potometer to estimate transpiration rates:
- Cut shoot underwater at a slant - prevents air escaping + increases SA
- Assemble potometer in water + insert shoot underwater
- Remove apparatus from water but keep end of capillary tube submerged.
- Check apparatus is air+watertight
- Dry leaves, allow for shoot to acclimatise + shit tap
- Remove end of capillary tube until one airbubble forms - put end back in
- Record starting position of air bubble
- Stopwatch + record distance moved per unit time, eg. per hour.
What are two important cell types in phoem tissue and why?
Sieve tube elements = form tube for transporting solutes
Companion cells = carry out living functions for sieve cells, eg. providing energy.
Why do sieve tube elements need companion cells?
Sieve tube elements have no nucleus + few organelles
Translocation
Movement of solutes from sources to sinks.
What role do enzymes play in translocation?
Maintain a concentrtion gradient from source to sink by changing the solutes at the sink to make sure they are lower concentration.
What is the best supported theory for transport in the phloem?
Mass flow hypothesis
Describe the mass flow hypothesis:
- AT actively loads solutes from companion cells to sieve tubes at source
- Lowers WP inside sieve tubes, so water enters by osmosis.
- Creates high pressure inside sieve tubes at source end
- At sink end, solutes are removed
- Increases WP inside sieve tubes so water leaves by osmosis
- Lowers pressure in sieve tubes
- Creates pressure gradient between source + sink
- Pushes solutes along sieve tubes towards sink.
What is the evidence for the mass flow hypothesis?
- Ring of bark (phloem, not xylem) removed from woody stem = bulge forms with higher conc. sugars than fluid lower down = evidence of downward flow of sugars
- Radioactive tracers can track movement of organic substances
Describe how radioactive tracers can be used to demonstrate translocation of solutes:
- Radioactively-labelled CO2supplied to a single leaf
- Incorporated into organic substances produced by the leaf, which will be transported via translocation
- Plant is killed by freezing in liquid nitrogen
- Placed on photographic film.
- Where film turns black = radioactive substance present.
