Transport in plants Flashcards
(12 cards)
State what is meant by the terms source and sink.
source= where sucrose loads into phloem
sink= where sucrose unloads from phloem
Explain the significance of the relationship between rate of diffusion and the surface area to volume ratio for large plants (2)
(large plants) have a, small / low, SA : VOL ratio ;
idea of diffusion too slow (to supply requirements) ;
idea of need transport system (for water / minerals /
assimilates) ;
idea of need (special) surface area for, gaseous exchange
/ uptake of minerals ;
State the function of this spiral band of lignin and explain why it is important that the xylem vessel becomes lignified in this way.
- lignin strengthens/thickens the xylem wall to prevent collapse of xylem
- lignin waterproofs walls
- improves adhesion of water molecules
- reduced water loss
why is water movement very important in plants?
- important for hydrostatic pressure (turgor pressure), provides support in stems/leaves.
- Loss/evaporation keeps plants cool
- Reactant in photosynthesis
4.Mineral ions transported in aqueous solution
how are roots well adapted as a gas exchange surface?
Their microscopic= can penetrate easily between soil particles
Have a large SA:V= more efficient diffusion
Thin layers ( cell wall and cell-surface membrane) through which diffusion and osmosis can take place quickly
The vacuole of root hairs contains sap/sugars/minerals so maintains water potential gradient for osmosis.
how does water move into the xylem through the apoplast/symplast pathway?
Water moves across the root in the apoplast/symplast pathways until it reaches the endodermis.
When the water reaches the endodermis the apoplast route is blocked by the Casparian strip (waxy material called suberin which is waterproof). So, the water here joins the symplast route.
The cell membrane is selectively permeable, so it can control the entry of substances e.g. it allows water but excludes toxins (no carrier proteins to transport them in)
what is the transpiration stream also known as?
the mass flow of water
what forces contribute to the transpiration stream?
Adhesion= attraction between unlike molecules e.g. water and cell wall (water adheres to the walls of xylem vessels, holding the column of water inside the vessel.
Cohesion= attraction between like molecules e.g. water. There are strong cohesive forces between water molecules, this prevents the column of water from breaking
Tension= the column of water formed gives a high tensile strength (unlikely to break). Evaporation at the surface causes tension all the way through the plant (a pull force on the water).
describe cohesion- tension theory.
Shows how water moves from the soil in a continuous stream up the xylem and across the leaf. first, water evaporates from the surface of the leaf (mesophyll). so water diffuses out the leaf through the stomata down a concentration gradient because then outside of the leaf has a low water potential and the inside has a high water potential. this creates tension throughout the leaf, due to the low water potential. water moves from the roots into the xylem either through the symplast or apoplast pathway. then water is drawn out the xylem to replace water in the cell walls. it moves up the xylem due to cohesion and adhesion, creating a transpiration stream and lateral flow of water up the xylem.
define transpiration
loss of water vapour by evaporation through stomata
explain why the plant shoot must be cut underwater
due to cohesion- tension theory creating a negative pressure in the xylem, if it was cut in the air it would draw air into the xylem tube. this would break up the continous water column and prevent transpiration. therefore, by cutting underwater only water is drawn up the xylem.
