3.5 Mass Transport in Plants Flashcards
(11 cards)
What does the xylem do?
Transports water and mineral ions through the stem, up the plant to the leaves
How is xylem tissue structured?
• Cells joined with no end walls, ensuring a continuous column of water
• Cells contain no cytoplasm, ensuring water flow with no obstructions
• Thick cell walls with lignin, providing structural support
• Pits in side walls, allowing lateral water movements
What does cohesion-tension theory suggest?
• Water is lost from leaf through stomata by transpiration
• Reducing water potential of mesophyll cells
• So water drawn out xylem down water potential gradient
• Creating tension in xylem
• Hydrogen bonds result in cohesion between water molecules so water pulled up as continuous column
• Water also adheres to walls of xylem
• Water enters roots via osmosis
How can rate of transpiration be estimated?
• Cut shoot underwater at slant to prevent air entering xylem
• Assemble potometer with capillary tube end submerged in water
• Insert shoot underwater
• Ensure apparatus is airtight
• Dry leaves and allow for shoot to acclimatise
• Shut tap to reservoir
• Quickly remove end of capillary tube from water to form air bubble
• Rate of water uptake = (CSA of capillary tube x distance moved by bubble) / time
What are some limitations of using a potometer?
• Rate of water uptake may not be same as transpiration as water used in photosynthesis / produced during respiration
• Rate of movement through shoot in potometer may not be same as through shoot of whole plant as xylem cells are very narrow / shoot in potometer has no roots
What variables affect rate of transpiration?
• Light intensity
• Temperature
• Wind speed
• Humidity
What does the phloem do?
Transports organic substances (eg. Sucrose) in plants
How is phloem tissue structured?
• Sieve tube elements with no nucleus to allow easier flow of organic substances
• Sieve plates
• Companion cells with many mitochondria
What does the mass flow hypothesis suggest?
• At source sucrose is actively transported into phloem by companion cells
• This lowers water potential in sieve tubes so water enters from xylem by osmosis
• This increases hydrostatic pressure in sieve tubes
• So mass flow occurs (movement of solutes from source to sink)
• At sink, sucrose is removed by active transport to be used by respiring cells or stored
How are tracer experiments used to investigate translocation?
• Leaf supplied with radioactive tracer
• Radioactive carbon incorporated into organic substances during photosynthesis
• These move around plant by translocation
• Movement tracked using autoradiography
How are ringing experiments used to investigate translocation?
• Remove phloem / ring of bark
• Bulge forms on source side of ring
• Fluid from bulge has a higher concentration of solutes than below
• Tissues below ring die as cannot get organic substances
