3.5 Mass Transport in Plants Flashcards
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
