Transport in Plants

Question 1

Potometer

Answer 1

used to estimate the transpiration rate
actually measures water uptake by the plant
assumed that uptake is directly related to water loss

Question 2

Radioactive tracer

Answer 2

i.e. C14

tracks the movement of organic substances in a plant

Question 3

Ringers

Answer 3

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

Question 4

Aphids

Answer 4

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

Question 5

Metabolic inhibitor

Answer 5

if put into phloem, translocation stops therefore active transport is involved

Question 6

OBJECTIONS

Answer 6

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

Question 7

Phloem tissue

Answer 7

adapted for transporting solutes
sieve tube elements - living cells that form tube
supported by companion cell, providing the energy required for active transporting solutes

Question 8

Translocation

Answer 8

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)

Question 9

Mass Flow Hypothesis

Answer 9

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

Question 10

Mass Flow Hypothesis (2)

Answer 10

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

Question 11

Mass Flow Hypothesis (3)

Answer 11

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

Question 12

Xylem

Answer 12

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

Question 13

Cohesion Tension theory

Answer 13

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

Question 14

Transpiration

Answer 14

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

Question 15

Light and Transpiration

Answer 15

lighter = faster transpiration rate

stomata open when light to get more CO2 for photosynthesis. stomata close in the dark

Question 16

Temperature and transpiration

Answer 16

higher temperature = faster transpiration rate
water molecules have more energy so evaporate from cells faster, increasing conc. grad from leaf to surroundings
diffusion increses

Question 17

Humidity

Answer 17

lower humidity = faster transpiration rate

dry air = steep conc. grad = higher transpiration rate

Question 18

Wind

Answer 18

windier = faster transpiration rate
air blows water molecules away from around stomata
=increased conc. grad