3: EXCHANGE SYSTEMS AND MASS TRANSPORT - PLANT MASS TRANSPORT Flashcards
What are the 2 types of tissue involved in plant mass transport?
- xylem tissue
- phloem tissue
Where are the xylem and phloem found?
in vascular bundles throughout the roots to the leaves
What is the function of the xylem?
transports water up the plant from the roots to the leaves
What is the function of the phloem?
transports organic substances like sugars in solution to where they’re needed in the plant, both up and down the plant
What are the adaptations of the xylem?
- long dead cells, tubes with no end walls (uninterrupted tube to allow for continuous water columns)
- no cytoplasm/no organelles (avoid obstruction and allow for easier water flow)
- thickened walls with lignin (supports and withstands tension of the water flow and prevents water loss)
- pits in walls (allows lateral movement of water)
Describe how water moves up the xylem
- water evaporates from leaves at the top of the xylem (transpiration)
- this creates tension (suction) which pulls more water into the leaf (transpiration pull)
- water molecules are cohesive so when some are pulled into the leaf others follow meaning that the whole column of water (from the leaves down to the roots) is pulled upwards
- water enters the stem through the roots
What is transpiration?
the evaporation of water from a plants surface (the leaves)
What factors affect the rate of transpiration?
- light
- temp
- humidity
- wind
How does light affect the rate of transpiration?
- the lighter it is, the faster the rate of transpiration
- this is because the stomata open when its light to let in CO2 for photosynthesis
- when its dark the stomata are usually closed so there’s little transpiration
How does temp affect the rate of transpiration?
- the higher the temp the faster the transpiration rate
- warmer water molecules have more energy so they evaporate from cells inside the leaf faster
- this increases the conc. gradient between the inside and the outside of the leaf so water diffuses out of the leaf faster
How does humidity affect the rate of transpiration?
- the lower the humidity, the faster the transpiration rate
- if air around the plant is dry the conc. gradient between the leaf and the air is increased which increases rate of transpiration
How does wind affect the rate of transpiration?
- the windier it is, the faster the rate of transpiration
- lots of air movement blows away water molecules from around the stomata
- this increases conc. gradient which increases rate of transpiration
What is a potometer?
a piece of apparatus used to estimate rate of transpiration by measuring water uptake by the plant
How can a potometer be used to measure water uptake?
- cut a shoot underwater to prevent air from entering the xylem. cut it at a slant to increase SA available for water uptake
- assemble the potometer in water and insert the shoot underwater so no air can enter
- remove the apparatus from water but keep the end of the capillary tube submerged in a beaker of water
- check the apparatus is watertight and airtight
- dry the leaves, allow time for the shoot to acclimatise and then shut the tap
- remove the end of the capillary tube from the beaker of water until 1 air bubble has formed then put the end of the tube back into the water
- record the starting position of the air bubble
- start a stopwatch and record the distance moved by the air bubble per unit time. the rate of air bubble movement is an estimate of the transpiration rate
Describe the structure of the phloem
- formed from cells arranged in tubes
- sieve tube elements are living cells that form the tube for transporting solutes
- there’s a companion cell for every sieve tube element which carry out living functions for the sieve cells (eg: providing energy needed for active transport of the solutes)
What is translcoation?
the movement of solutes to other sites in the plant where they are required via the phloem
Is translocation a passive process/an active process?
active
What are assimilates?
the solutes that are transported throughout the plant via the phloem and will then be incorporated into plant tissues
What is the source of a solute?
where the solute is made
Is the solute at a high/low conc. at the source?
high conc.
What is the sink?
the area where a solute is used up
Is the solute at a high/low conc. at the sink?
low conc.
Describe the mass flow hypothesis
- solutes are actively (by active transport) loaded into the phloem sieve tubes by companion cells
- solutes in sieve tubes lowers water potential so water enters sieve tubes from the xylem
- movement of solutes and water increases hydrostatic pressure in sieve tube at source end
- high pressure and low water potential at source, lower pressure and high water potential at sink so mass flow from source to sink
- solutes removed from phloem to be used for respiration/storage
- increase in water potential in sieve tubes so water moves from phloem back to xylem by osmosis resulting in lowered pressure inside the sieve tubes at the sink
What is the evidence for the mass flow hypothesis?
- if a ring of bark (which contains the phloem but not the xylem) is removed from a woody stem a bulge forms above the ring. the fluid from the bulge has a higher conc. of sugars than the fluid from below the ring. this is evidence that there’s a downwards flow of sugars
- a radioactive tracer (eg: C-14) can be used to track movement of organic substances in a plant
- pressure in the phloem can be investigated using aphids (they pierce the phloem then their bodies are removed leaving the mouthparts behind which allows sap to flow out). the sap flows out quicker nearer the leaves than further down the stem which is evidence of a pressure gradient
- if a metabolic inhibitor which stops (ATP production) is put into the phloem then translocation stops. this is evidence that active transport is involved
What is the evidence against the mass flow hypothesis?
- sugar travels to many different sinks, not just to the one with the highest water potential as the model would suggest
- the sieve plates would create a barrier to mass flow. a lot of pressure would be needed for solutes to get through at a reasonable rate
