3 Substance exchange- Mass transport (plants) Flashcards
What is the xylem’s structure and function?
-Plant vessels, responsible for transporting water and mineral ions
-Long, tubes of cells, run up stems of plants
-Vessel elements (xylem cells) are dead, stacked on top of one another, no cell wall
-Continuous tube for water to flow through
-Walls lined with lignin (waterproof polymer), reinforces walls; gives structural support
What is the cohesion-tension theory?
Water and inorganic ions travel up xylem through cohesion and tension
What are the steps in the cohesion-tension theory?
Transpiration- most of water in leaves evaporates, some used in photosynthesis
Tension- formation of hydrogen bonds between water molecules and sides of xylem vessel elements, created by loss of water from leaves. Water in xylem pulled upwards by tension towards leaves
Cohesion- Individual water molecules form hydrogen bonds with each other. As water molecules are pulled up xylem, others are also pulled up due to cohesion. A combo of cohesion and tension continuously pull water upwards to replace water lost by transpiration
As water is pulled up stem water potential at plant bottom decreases
Water diffuses into roots via osmosis down concentration gradient
How is transpiration rate investigated?
Using a potometer
What is a potometer?
Piece of equipment used to estimate volume of water taken up by a plant in a given time
How is a potometer used to measure transpiration?
- Potometer filled with water, cutting of shoot placed inside- must be cut and placed into potometer while underwater to ensure no air enters xylem
- Remove end of capillary tube from water beaker, wait for air bubble to form in tube and place it back in water. Air bubble used to record volume of water used by shoot
- Mark starting position of air bubble, use stopwatch to record distance moved by air bubble in given time period
- Calculate rate of movement of bubble per hour- equal to transpiration rate
- Change variable, repeat
What is the structure of the phloem?
-Plant vessels, transport sugars
-Sieve tube elements (cells making up phloem vessel), run up stem on top of one another, living cells; cytoplasm but no nucleus. Walls made of cellulose
-Companion cells, connected to sieve tube elements; have nucleus
-Sieve plates; either end of sieve tube elements. Large pores allow sap to move through elements Allow sugars to be transported through phloem
What is translocation?
Process where sugars made in photosynthesis in leaves (source)
Describe the mass flow hypothesis steps?
-Sucrose made in leaves (source) during photosynthesis
-Sucrose actively transported (w/ energy from mitochondria) into companion cells in phloem
-H+ ions actively transported out of companion cells and then diffuses back with sucrose (co-transport)
-Loading sucrose into phloem causes sucrose concentration to increase and it diffuses from phloem into sieve tube elements
-Increase of sucrose concentration= phloem water potential decrease, water potential gradient between outside and inside. Water diffuses into phloem by osmosis
-Bottom of phloem-> sucrose used is used in cells (sink), so concentration is low. Sucrose diffuses out of phloem, into sink cells- lowers their water potential.
-Water diffuses down water potential gradient out of phloem by osmosis. Diffusion of water into phloem at source and out of phloem at sink creates hydrostatic pressure gradient; allowing mass transport of sucrose from source to sink (mass flow)
What is the evidence for mass flow?
Tracing and ringing experiments; show a link with phloem and sucrose transport
What are ringing experiments?
-Used to investigate mass transport in phloem
-Phloem vessels are outside the xylem vessels in stem of plant; cutting a ring outside of a stem halts transport in phloem but allows transport in xylem
-Tissue above ring will begin to swell, growth stops below ring as sucrose can’t be transported
What are tracer experiments?
-Sucrose made in photosynthesis can be tracked by exposing leaves to carbon dioxide with radioactive carbon
-Plant frozen very quickly w/ liquid nitrogen, placed onto photographic film
-Pathway travelled by radioactive sucrose down phloem can be traced
What is the evidence against mass flow?
Direction of flow
-Sap can move up/down phloem vessels. Hydrostatic pressure gradient by water entering source and leaving sink doesn’t explain how sap moves in both directions
Sieve plates
-Pores between sieve tube elements. Increased hydrostatic pressure required for water to flow through pores; so sieve plates should create a problem for mass flow
Living phloem cells
-Phloem cells= living, Xylem cells= dead. Purpose of phloem cells being living is unclear
