plants Flashcards
Why do plants need water?
Photosynthesis
To transport minerals
Mantain structural rigity
Regulate temperature
Why do plants need nitrate ions?
Used to make proteins for healthy growth
Describe how the structure of xylem tissue adapts to its function
Cells joined with no end wall forming a continuous tube-Water flows as a continuous column
Cells contain no cyctoplasm/nucleus- No obstructions
Thick cell walls ligned with lignin- Provides support/withstands tension/ prevents water loss
Pits in side walls- Allows lateral water movement
What is the purpose of lignin in the xylem?
Provides xylem with thick walls, this makes it strong and prevents the tube collapsing. The lignin is also waterproof to prevent water from adhereing too much too much to the surface
Describe the mass transport hypothesis in plants
The source releases high conc of sucrose ionto sieve tubes by active loading, This decreases the water potential so that water moves in by osmosis form a xylem increase in volume and increase in turgor pressure. The sink removes sucrose from phloem as there low sucrose conc as sucrose moves out of sieve tubes down a conc gradient as cells use it or convert it to glucose/starch. This increases the water potential in sieve tubes so water moves back to xylem by osmosis reducing volume and turgor pressure. Turgor pressure created between source and sink causes solutes to be pushed in that direction.Conc gradient of sucrose and sink created as sucrose is constantly being used for respiration or converted for storage
Explain the cohesion tension theory of water transport in the xylem
Water in the leaves evaporates from mesophyll into airspaces where water vapour diffuses out through open stomata.The water potential in mesophyll tissue decreases so water from xylem drawn out down conc gradient. This creates tension ‘negative pull’ The hydrogen bonds in the water result in cohesion of the water molecules so there is continuous column of water up the xylem and water also adheres to the walls of the xylem so water enter the root via osmosis
Decribe how to set up a potometer
Cut a shoot underwater at a slant →
prevent air entering xylem
2. Assemble potometer with capillary tube
end submerged in a beaker of water
3. Insert shoot underwater
4. Ensure apparatus is watertight / airtight
5. Dry leaves and allow time for shoot to
acclimatise
6. Shut tap to reservoir
7. Form an air bubble - quickly remove end
of capillary tube from water
Describe how a potometer can be used to measure the rate of transpiration
Record position of the airbubble and record the distance moved over certain period of time. Calculate the volume of water uptake in given time using the capillary tube to calculate cross sectional area of water (πr2) x distance moved by bubble.
Calculate rate of water uptake by dividing by time taken
Suggest limitations in using a potometer to measure rate of transpiration
Rate of water uptake might not be same as rate of transpiration as water is used for turgity/photosynthesis/ produced in respiration
The rate of movement through the shoot in potometer may not be the same as rate of movement through shoot ofthe whole plant as shoot in potometer has no roots and xylem cells are very narrow
Suggest how 4 different enivronmental variables increasing affects transpiration
Light intensity- Increases- Stomata opens in light to let in CO2 for photosynthesis allowing more water to evaporate faster
Temperature- Increases- Water molecules gain kinetic energy as temperature increases so water evaporates faster
Wind intensity-Increases-Wind blows away water molecules from around stomata, decreases water potentail of air around stomata increasing water potential gradient so water evaporates faster
Humidity-Decreases- More water in air so has higher water potential, decreaes water potential gradient from leaf to air, water evaporates slower
Describe the function of the phloem tissue
Transports organic substances e.g sucrose in plants
Explain how phloem is adapted for its function
-Made of sieve tube elements which have no nucleus and few organelled maximising space for organic substances. The end walls of sieve plates allow solutes to move from one cell to the next
-Has companion cells which have many mirochondria, so there is high rate of respiration to make ATP for active transport of solutes
What is translocation?
Movement of assimilates/solutes such as sucrtose from source cells (ehre made e.g leaves) to sink cells (where stored/used e.g roots) by mass flow
Explain the mass flow hypothesis for translocation in plants
At the source, sucrose is actively transported into pholem sieve tubes by companion cells. This lowers water potential in sieve tubes so water enters from xylem by osmosis. This increases the hydrostatic pressure in sieve tubes at the source and creates a hydrostatic pressure gradient so mass flow occurs (movement from source to sink). At the sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs
Describe the use of tracer experiments to investigate transport in plants
Leaf is supplied with radioative tracer e.g CO2 containing radioactive isotope C14. Radioactive carbon incorporated into organic substances during photosynthesis. These plants move around by translocation. Movement tracked using autoradiography or geiger counter
