Transport in plants (xylem and phloem) Flashcards
What is transpiration?
The evaporation of water from the leaves of the plants (closely related to how water is moved throughout the plant)
Describe how water moves out of a plant?
- In spongy mesophyll cells, the liquid water evaporates from a higher water potential in the cell to the air spaces in the leaf
- This water vapour moves out of the leaf by diffusion (down a water vapour potential gradient)
What are 3 features of the xylem?
- walls lined with lignin
- no end walls
- vessel elements
What piece of apparatus could be used to investigate transpiration?
potometer
What word describes how the H atoms in water interact with the sides of the xylem vessel?
adhesion
Why do xerophytes have smaller leaves?
it reduces the surface area for water to evaporate off of the leaves
Outline the cohesion-tension theory
Tension occurs between the water molecules.
Water molecules can form hydrogen bonds, they are cohesive
therefore, they are pulled up due to the cohesion
What makes up the phloem vessels?
companion cells and sieve tube elements
What is translocation?
an energy requiring process which transports sugars from the source cells to the sink cells
What are plasmodesmata?
gaps between cell walls which allow flow of substances between cells
Describe how sucrose enters companion cells
- H+ ions are actively transported into the source cell, by a H+ ion pump, they then move back into the companion cell in co-transport proteins, carrying with them sucrose
How does sucrose enter the sieve tube element?
- simple diffusion of sucrose molecules through the plasmodesmata
How does increased concentration of sucrose impact the sieve tube elements?
- decreases water potential
- water moves from the xylem into the sieve tube element by osmosis
- this increases hydrostatic pressure
What happens at sink cells?
- sucrose will move into companion cells and sink cells by diffusion or active transport
- this increases water potential, and causes water to move back into the xylem by osmosis
- here it is either stored, or converted back into glucose for respiration
How does the release of sap when the phloem is cut provide evidence for mass transport?
it shows that the phloem has high hydrostatic pressure
What evidence is there against mass transport?
- sieve tube elements; no clear purpose, could hinder
- not all solutes move at same speed
- sucrose is delivered at similar speeds to all regions
What is ringing?
the bark and phloem of a tree are removed leaving just the xylem in the centre.
Overtime the tissues above the missing ring swell with sucrose solution and the tissue below dies
What is tracing?
sucrose from plants grown in an environment with radioactive CO2 can be observed moving through the plant
Describe the location of xylem and phloem in the tree trunk
Phloem just under the bark, xylem inside this
Describe results of a ringing experiment
A bulge containing sugars appears above the ring
Describe the conclusions from the ringing experiment
Sugars are transported in the phloem (when phloem is removed sugars accumulate above, tissues below the ring die)
Describe how radioactive tracers can be used to locate sugars in plants
Plant grown in radioactive carbon, incorporated into sugars produced in photosynthesis, radiography can locate the position of the sugars in the plant