2 - mass transport in plants Flashcards
(20 cards)
Factors affecting rate of transpiration
Light (positive correlation)
-more evaporation, more stomata open so larger SA
Temperature (positive correlation)
-more evaporation, particles have more KE
Humidity (negative correlation)
-more water vapour, decreases water potential gradient
air flow (positive correlation)
-saturated air removed, maintains steep water potential gradient
Transpiration
The loss of water vapour through evaporation from a plant’s surface.
Cohesion
Cohesion
-Water molecules attracted to each other because of hydrogen bonds, creating a column of water
Phloem
bidirectional (both directions)
transports dissolved substances, such as sucrose and amino acids from parts of the plant where they are made (sources) to the parts of the plant where they are used (sinks)
Made up of two types of cells:
1) Sieve Tube
-Living
-No nucleus and few organelles so more space for solutes to be transported
2) Companion cells
-Provide ATP
apoplast pathway
moves water through cell walls
diffusion
quick
more common
when water reaches Casparian strip, must take the symplast pathway
symplast pathway
moves water through cytoplasm
plasmodenta
slower
plant viruses
responsible for losses in crop production and quality of produce
range of symptoms:
yellowing of leaves,
leaf distortion
abnormalities
aphids
have sucking mouthparts called stylets
feed by inserting the stylets into phloem vessels and the sap, which is under high pressure, is forced into the aphid’s gut
Some species produce winged females which migrate to start colonies on a new host plant
mass flow hypothesis
Sucrose at source actively transported to phloem cells by companion cells
Lowers water potential of sieve cell and water enters through osmosis
Increase in hydrostatic pressure near source causes mass movement towards sink
In sink, sugar removed from phloem and used for aerobic respiration or storage
movement of water to xylem
enters root hair cells through osmosis
travels through cortex via apoplastic or symplastic pathways
at endodermis, Casparian strip forces water to travel via symplastic pathway because casparian strip is impermeable
transpiration reduces hydrostatic pressure in xylem so water drawn in
root pressure
Ions actively transported into xylem (by endodermal cells);
Lowers water potential (in xylem)
Water moves into xylem by diffusion/osmosis (forcing water up
stem)
movement of water up xylem
water transpires from leaves
reduces water potential in cell
water is drawn out of xylem
creates tension (negative pressure)
cohesive forces between water molecules so water pulled up as a column
why potometer needs to be cut under water
prevent air from entering xylem
disrupt continuous column of water
prevent transpiration
what variable need to be controlled for potometer to compare results
surface area of leaves
Xylem
dead, hollow cells with no end cell walls
one continuous tube
no organelles or cytoplasm
contains lignin, which strengthens the xylem vessel and provides structure and support to the plant
guard cells
turgid - stomata open
flaccid - stomata closed
Adhesion
-attraction of water to non-water molecules, such as molecules that make up xylem wall
-narrower xylem, greater adhesion effect so easier to transport water
Xylem adaptations
Tubes with no end walls so water flows continuously
No organelles so flow is not restricted
Lignin to withstand tension
Xerophytes adaptations
Sunken stomata reduce air movement so decrease concentration gradient
Rolled leaves reduce surface area
Waxy cuticle reduces evaporation
rate of transpiration equation
volume of water (3.14 x diameter x length bubble moved)
divided by
time taken
