13. Mass Transport In Plants. Flashcards
What are the properties that make water important?
- polar molecule which forms Hydrogen bonds making it cohesive allowing it to move up the plant xylem.
- high heat capacity so buffers temp changes in cells.
- large latent heat vaporisation providing a cooling effect
- solvent so substances dissolve in them for transport around body.
- metabolite so involved in reactions like condensation/hydrolysis.
- cohesion so support columns of water.
Inorganic ions and there functions?
- Iron ion - in haemoglobin and bind with O2
- sodium ion- involved in cotransport of glucose and amino acids in small intestine.
- phosphate ion - components of ATP, phospholipid, DNA and RNA.
- Hydrogen ion - lower pH of blood, causes bohr shift.
Describe the cohesion-tension theory of water transport in the xylem?
Water is lost by transpiration through stomata.
Lowering the water potential gradient in the mesophyll cells.
H2O drawn out xylem down a water potential gradient by osmosis.
H2O is cohesive so pulled up xylem as a continuous colemn of water.
Adhesion of water molecules to walls of xylem prevent movement back down.
How is the xylem cell adapted? And what is role of xylem?
Transport water in stem and leaves of plants.
- thick walls - withstand high pressure due to adhesion/cohesion.
- narrow tube- move in water column.
- hollow/no cells - reduce resistance to flow of water.
Why does the diameter of a tree reduce slightly on hot days? What factors affect transpiration?
Due to rapid transpiration as (more heat energy), reduce diameter making xylem narrower.
Factors:
- Light intensity (as stomata open longer with more light so more co22 diffuse in, increase transpiration),
- humidity (increase humidity decreases water potential gradient between air outside stomata and spongy mesophyll, decreases transpiration.)
- air movement ( increase movement around stomata increases water potential gradient between air previously saturated with water vapour and spongy mesophyll increasing rate.)
- temp (increase, increases rate as more evaporation of water through stomata, creating a higher tension in xylem)
How do you use a potometer?
- cut shoot underwater - prevent air entering water column
- measure distance bubbles moved and repeat 9increase reliability) by opening tap on reservoir - which returns bubble to the start point and cal mean length.
- add layer oil to water in beaker - ensure joint is air tight.
- measure diameter and divide by 2 to get radius - use pir^2 to find volume of H2O taken up.
How do you calculate rate of water uptake?
Distance moved by air bubble in mm / time taken for air bubble to move that distance (s)
Potometer measures the rate of water uptake rather than rate of transpiration. Why does it not truly measure the rate of transpiration?
Water is used for structural support in the plant.
Water is used in photosynthesis and hydrolysis reactions (to break/make a bond)
Water is produced during photosynthesis.
Apparatus may not be sealed.
Whats the role of the phloem?
Transports organic substances ( e.g. sucrose and amino acid) in plants from the source to the sink.
It has reduced cytoplasm, is living and has a 2 way flow.
What is the mass flow hypothesis for the mechanism of Translocation (transport of sugars)?
At a source, sucrose is actively transported using ATP into sieve tube element of phloem by companion cells.
This lowers water potential in the sieve tube element of phloem, and water enters by osmosis, increasing the volume causing high hydrostatic pressure.
The increase in pressure causes mass movement, transporting sucrose towards roots and other storage tissues down a pressure gradient.
At the sink, sucrose removed from the phloem where its unloaded and can be used in respiration to produce ATP or stored.
Water moves back in to the xylem by osmosis to maintain a pressure gradient
What is ringing experiment and how does it show mass flow hypothesis?
Cutting out of the phloem from stem forming a ring. After time, swelling above the ring and reduced growth below and the leaves are unaffected. This shows phloem removed so sucrose cant travel down, lowering water potential above ringing as accumulates. Water also moves down water potential gradient by osmosis causing swelling.
How are companion cells adapted?
Many mitochodria to produce ATP to actively transport sucrose into the sieve tube element of the phloem.
Why is autoradiography using radioactive tracers used to show mass flow hypothesis?
Used to determine which tissue carries sucrose.
Leaf exposed to 14C radioactive isotope. This is taken up as 14CO2 used in photosynthesis to produce glucose then sucrose. Using photographic film can detect where the radioactive component travels showing the movement of sucrose into a plant overtime.
What does puncturing phloem and xylem show?
Puncturing phloem, sap oozes out as high pressure in the phloem.
Puncturing xylem, sap sucked in as low pressure in the phloem.
Shows that water pulled up xylem and sap pushed down phloem.
Why does temperature affect the phloem but not the xylem?
Phloem is a living cell so is damaged by high temp but xylem is dead so isn’t damaged so can still transport substances.
