3.3.5 - MASS TRANSPORT IN PLANTS Flashcards
describe the cohesion-tension theory of water transport in the xylem (5)
- water lost from leaf because of transpiration/evaporation of water
- lowers the water potential of mesophyll/leaf cells
- water pulled up the xylem, creating tension
- cohesion between water molecules due to hydrogen bonds
- forms a continuous water column
- adhesion of water molecules to the walls of the xylem
describe the mass flow hypothesis for the mechanism of translocation in plants (4)
- in source/leaf sugars actively transported into phloem
- by companion cells
- lowers water potential of sieve cell/tube and water enters by osmosis.
- increase in water volume, increases pressure
- increase in pressure causes mass movement towards sink/root
- sugars used/converted in root for respiration for storage
the rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.
suggest one reason why (1)
ANY ONE:
- plant has roots
- xylem cells narrow
- xylem damaged when cut
suggest and explain one way in which sieve cells are adapted for mass transport (2)
- no/few organelles/very little cytoplasm/large space
- so easier/more flow
suggest and explain one way in which companions cells are adapted for mass transport (2)
- mitochondria release energy (site of respiration)
- for active transport
OR
- ribosome produces proteins
- proteins are linked to transport - carrier proteins
one theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots.
describe how a high pressure is produced in the leaves (3)
- water potential becomes lower/becomes more negative as sugar enters the phloem
- water enters the phloem by osmosis
- increased volume of water causes increased pressure
explain why coloured water moved up the stalks (3)
- water evaporates/is transpired from leaves
- water potential gradient/lower water potential creates tension/pulls up water
- cohesion maintains column
describe the processes involved in the transport of sugars in plant stems (5)
- at source (leaves) sucrose is actively transported into the phloem
- by companion cells
- this lowers the water potential in phloem and so water enters by osmosis
- this produces a hydrostatic pressure
- which causes mass flow/transport towards sink
- at sink (roots) sugars are stored
suggest two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration (2)
- water is used for support
- water is used in photosynthesis
- water is used in hydrolysis
- water is produced during respiration
the mass flow hypothesis is used to explain the movement of substances through phloem.
explain how pressure is generated inside this phloem tube (3)
- sucrose actively transported into phloem
- lowers water potential in phloem
- water moves into phloem by osmosis from the xylem
describe the transport of carbohydrate in plants (5)
- sucrose actively transported into phloem cell
- by companion cells
- lowers WP in phloem and water enters from the xylem by osmosis
- produces high hydrostatic pressure
- mass flow to storage tissue
- unloaded by active transport
explain how sucrose-transport proteins in leaf cells enable the production of a pressure gradient in the phloem (3)
- sucrose co-transported/actively-transported into phloem
- causes decrease in WP in the phloem
- so water enters the phloem by osmosis
lignin is a polymer found in the walls of xylem vessels in plants.
lignin keeps the xylem vessel open as a continues tube.
explain the importance of the xylem being kept open as a continuous tube (3)
- allows water column
- cohesion from hydrogen bonds between water molecules OR cohesion from polar attraction between water molecules
- pulls up water which creates tension OR evaporation/transpiration creates tension
