Lecture 6: Transport in Plants

Question 1

the vascular system plants has two components:

Answer 1

xylem
phloem
-Both occur in vascular bundles which ramify throughout the plant

Question 2

xylem is for

Answer 2

the unidirectional trnaposrt of water and nutrients from the root to the shoot

Question 3

phloem is for the

Answer 3

bidirectional transport of solutes between sources and sinks.

Question 4

structure of xylem:

Answer 4

essentially open lignified tubes

Question 5

water and solute flow into xylem =

Answer 5

into epidermis
-through plasmodesmata
-through cortex
(so far been through cytoplasm and cell wall)
-into & through endodermis (cytoplasm only as casparian strip forces it into cytoplasm)
-into pericycle & through to
-into xylem

Question 6

casparian strip blocks =

Answer 6

the apoplectic route at root endodermis

Question 7

to enter the symplasm, water must cross ___

Answer 7

membranes

Question 8

what facilitate water transport across membranes

Answer 8

Water transport proteins, aquaporins

Question 9

Forces involved in transpiration =

Answer 9

• transpiration
• adhesion
• cohesion
• tension

Question 10

transpiration =

Answer 10

pulls water into the plant through the roots and the xylem tissue

Question 11

adhesion =

Answer 11

attractive force between water molecules and other polar molecules, such as cellulose

Question 12

cohesion =

Answer 12

water has high cohesive force (equivalent to that of a steel wire) due to hydrogen bonding

Question 13

tension =

Answer 13

is created by the surface tension which develops in the leaf’s air spaces. Water columns endure this tension but may break, undergoing embolism (formation of air bubbles)

Question 14

embolism causes =

Answer 14

transpirational pull to fail.

-Drought-induced hydraulic failure is a major cause of tree mortality

Question 15

collecting xylem sap =

Answer 15

• Xylem sap can be collected as a root exudate

- or the roots can be pressurised

Question 16

collecting xylem sap in nature

Answer 16

• The leaf hopper Philaenus spumarius inserts its stylet into the xylem and can extract sap against large negative pressures (tensions) in the xylem.
• The nymphs of P. spumarius expel excess xylem sap which froths (cuckoo spit). This can be collected and xylem analysed. The adult leaf hopper excretes sap, without frothing, as discrete droplets.

Question 17

in a pea plant nitrogen is fixed into amino N in ___

Answer 17

root nodules

Question 18

in a sunflower some nitrate is reduced in the

Answer 18

roots

Question 19

in a cocklebur most nitrate is reduced in

Answer 19

the shoots

Question 20

sources =

Answer 20

• produce assimilates

- e.g. mature leaves, other mature photosynthetic parts of the plant

Question 21

sinks =

Answer 21

• utilise assimilates

- e.g. developing leaves, roots, flowers, fruits, seeds, tubers

Question 22

mechanism of phloem transport - mass flow

Answer 22

• SOURCE: solutes transported into sieve elements, water flows because of low water potential
• SINK: solutes are transported out of sieve elements, raising water potential, and water leaves sieve elements.
• Transport in the phloem can be bidirectional, although individual sieve elements can only transport in one direction at any one time.
• Transport is rapid (30-150 cm.h-1).

Question 23

whats more packed than phloem / xylem

Answer 23

Phloem is vastly more densely packed than xylem, particularly the very active
companion cells

Question 24

what happens if phloem loading is prevented?

Answer 24

• Potato plants with anti‐sense to sucrose transporter, SUT1, which loads sucrose into phloem.
• Transformants accumulate carbohydrate in the leaves and senesce.
• Starch accumulation in leaves of the transformants is shown by iodine staining (blue‐black).

Question 25

collecting phloem sap = aphid

Answer 25

Aphid inserts stylet into the phloem. Aphid is severed from stylet using a laser.
Sap exuded from the severed stylet can be analysed.