Transport in Plants Flashcards
Potometer
used to estimate the transpiration rate
actually measures water uptake by the plant
assumed that uptake is directly related to water loss
Radioactive tracer
i.e. C14
tracks the movement of organic substances in a plant
Ringers
if ring of bark (includes phloem, not xylem) is removed from a wood stem, bulge formsabove the ring
fluid from bulge has higher sugar conc. than fluid below - evidence of downward flow of sugars
Aphids
aphids used to pierce the phloem to allow sap to flow out
sap flows out quicker nearer leaves than further down stem - evidence of pressure gradient
Metabolic inhibitor
if put into phloem, translocation stops therefore active transport is involved
OBJECTIONS
sugar travels to many sinks, not just to one with highest water potential
sieve plates would create a barrier to mass flow.
lots of pressure required for solutes to get through at a reasonable rate
Phloem tissue
adapted for transporting solutes
sieve tube elements - living cells that form tube
supported by companion cell, providing the energy required for active transporting solutes
Translocation
movement of solutes
requires ATP
moves solutes from source to sink (where it’s made to where it’s required)
enzymes maintain conc. grad from source to sink by breaking down solutes constantly (always a conc. grad)
Mass Flow Hypothesis
solutes actively transported from companion cells into sieve tubes of phloem at source
water potential lowered
water enters phloem via osmosis from xylem and companion cells
high pressure in sieve tubes at source end of phloem
Mass Flow Hypothesis (2)
solutes at sink end used up
water potential less negative inside sieve tubes so water leaves sieve tubes via osmosis
pressure at sink end of phloem decreased
Mass Flow Hypothesis (3)
all results in pressure gradient from source to sink end
gradient pushes solutes along sieve tubes towards sink
when they reach sink they’ll be used for respiration or stored i.e. as starch
Xylem
long tube structure formed from dead cells (vessel elements) joined from end to end
no end walls meaning uninterrupted tube so water passes through easily
Cohesion Tension theory
water evaporates from leaves at ‘top’ of xylem
creates tension, pulling more water into leaf
cohesion of water molecules means column of water in xylem from leaves to roots moves upwards
water enters the stem through the roots
Transpiration
Evaporation of water from a plants surface (especially the leaves)
water evaporates from moist cell walls
accumulates in spaces between cells in leaf
stomata open, water moves out of leaf down conc. grad
Light and Transpiration
lighter = faster transpiration rate
stomata open when light to get more CO2 for photosynthesis. stomata close in the dark