AS Booklet 8- Transport in Plants Flashcards
What is transpiration?
The evaporation of water from a plants surface, particularly through stomata when they open to allow CO2 in for photosynthesis.
Rate of transportation affected by external factors and internal factors (plant structure).
What are the names environmental factors?
Light:
• Daylight = stomata open to allow CO2 for photosynthesis, so increases rate of transpiration as H2O evaporates out.
Temperature:
• Increase in temperature = increase in rate of transpiration as more KE for H2O molecules, so evaporate more readily.
Humidity:
• Stops sweat and water evaporating.
• Increase in humidity = decrease in rate of transpiration bc water potential gradient decreases (as wait has similar or same WP as leaves).
Air movement:
• Increase in air movement = less water vapour = Increase in WP gradient = increase in rate of transpiration.
• Decrease in air movement = more water vapour = decrease in WP gradient = decrease in rate of transpiration.
What is a xerophyte?
A plant adapted to retain as much water as possible, usually found in dryer habitats.
- Thick cuticle = long diffusion pathway = decrease of rate of evaporation.
- Hair’s on leat trap layer of still air, becomes saturated with water vapour = reduction in WP gradient = decrease in rate of transpiration.
- Rolling of leaves traps layer of still air = reduction in WP gradient = decrease in rate of transpiration (Marram grass for example).
- Reduction of SA:V = reduction in SA for water loss.
- Stomata positioned in epidermal pits of grooves beneath leaf surface = less exposure to air currents = trapped air saturated with water vapour = recursion in WP gradient = reduction of rate of transpiration.
What is the bubble potometer?
An experiment that can measure rate of transpiration.
It assumes that rate of water uptake = rate of transpiration (not entirely valid as some water is used to maintain turgidity and a small % in photosynthesis).
To calculate rate of transpiration in cm3hr-1, take these measurements:
• distance (d) in air bubble movement/cm.
• time taken for air bubble to move distance (d)
• radius (r) of lumen of capillary tubing (measure diameter, divide by 2).
• volume of water taken up calculated by pi r2d
What is the cohesion-tension theory?
How water and dissolved ions move around a plant, e.g. from roots to leaves via xylem.
• Solar heat enters causes evaporation/transpiration of H2O from leaves.
• H2O evaporates from mesophyll cells and diffusés out of stomata into air.
• WP of mesophyll cells is rescued compared to inner mesophyll cells.
• H2O moves from inner -> outer mesophyll cells by osmosis, along WP gradient.
• WP gradient extends across leaf mesophyll cells to xylem vessels. H2O drawn from xylem creating negative pressure (tension) in xylem vessels, pulls up water and dissolve ions.
• H2O cohesive with itself so stays in column due to H bonds.
• Adhesion to xylem walls lets H2O move up in columns because of H bonds.
How does water travel from root to leaf?
- H2O in soil = high WP, H2O in atmosphere = low WP. H2O moves from high to low H2O conc. via WP gradient through plant.
- Gradient is maintained by solar energy and evaporation/transpiration of H2O on surface of plant.
What is the evidence that shows movement of ions in the xylem?
- Radioactive isotopes used as tracers, can be tracked.
- Xylem and phloem are separated using wax cylinder to prevent lateral (sideways) transport.
- Roots are provided with radioactively labelled potassium ions (42K).
- Plant is left is for a few hours and amount of radioactivity in xylem and phloem tissues in region of wax cylinder is measured compared.
- Amount of radioactivity in xylem is greater than phloem indicating transport of K+ occurs in xylem.
- Small amount of radioactivity in phloem due to lateral transport.
What is the structure of the phloem?
- Living cells.
- Sieve elements are joined end to end forming sieve tubes. Modified end cell walls known as sieve plates possess pores. Mature sieve elements don’t have a nucleus and the cytoplasm has barely any organelles.
- Companion cells are next to each sieve element. They have dense cytoplasm and many mitochondria.
What is the evidence that photosynthetic products are transported in the phloem?
Ringing/girdling experiments.
• Take full ring of phloem from around stem, preventing transport trough phloem at this point.
• Swelling occurs above the ring = build-up photosynthetic products from leaves, prevented from being transported past rings
What is the evidence for the phloem used as tissue for translocation?
- Ringing/girdling
- Microautoradiography-> surcease and amino acids transported in sieve tube elements not other cells of phloem.
- Radioactive CO2 incorporated into 14C at mature leaves, which is largely restricted to phloem when autoradiographed.
How do you calculate number of stomata?
Mean number in field of view
——————————————
Area mm2
What is it about the stud tyre of the xylem that helps the movement water and mineral ions?
- Xylem tissue is dead, no cell contents = leaves hollow tubes so minimal resistance to flow of H2O and ions.
- Cell wall has been strengthened by lignin = cell walls more rigid and provides support.
How does transport in the phloem?
- Translocation: transport of photosynthetic products.
- Carbs are transported in the form of sucrose, proteins as amino acids and lipids as fatty acids and glycerol.
- These organic molecules are produced in leaves (source) during photosynthesis and then transported to sinks e.g. respiring cells, growing cells like young leaves, shoot/root tips and storage areas.
What is mass flow theory?
Main hypothesis for method of translocation in phloem.
Thought to occur along hydrostatic it turgor pressure gradient.
- Photosynthetic products produced in source.
- Sucrose actively transported into sieve tubed by transfer cells (modified companion cells).
- Lowers WP of sieve tubes, causes water to enter from xylem by osmosis.
- This creates high hydrostatic or turgor pressure in sieve tubes in leaf.
- In sinks, sugars are being used or stored as insoluble starch. Increases WP, so H2O moves out by osmosis into xylem.
- Sinks now have low hydrostatic or turgor pressure.
- Photosynthetic products transported along this pressure gradient from source to sink by mass flow.
