6.2 Transport systems in plants

Question 1

what is the role of a mass transport system in a plant?

Answer 1

to meet metabolic demands

to cope with size of plant

to cope with SA:vol

Question 2

what is the structure of xylem?

Answer 2

elongated walls of dead cells impregnated with lignin (lignification)

fibres

xylem parenchyma - packing cells

Question 3

what is the function of bordered pits?

Answer 3

allows lateral movement of water and ions

to keep cells turgid and cool

Question 4

what is the function of xylem?

Answer 4

to transport water from roots to the rest of the plant

Question 5

what is the function of phloem?

Answer 5

to transport organic material from source to sink

Question 6

what is the structure of phloem?

Answer 6

sieve tube elements, perforated by pores (sieve plates)

• no nucleus, few organelles, little cytoplasm
• bidirectional

companion cells, with plasmodesmata for exchange of materials with sieve tube elements

Question 7

what is the function of leaves?

Answer 7

SA for gas exchange

absorption of light energy for photosynthesis

make organic solutes

Question 8

what is the function of the stem?

Answer 8

structure/support

lifting leaves to sunlight

transport of substances in vascular tissue

Question 9

what is the function of roots?

Answer 9

anchorage

absorbing nutrients from the soil

taking up water via osmosis

storage organ

Question 10

what are the features of a monocotyledonous plant?

Answer 10

single primary leaf in seed

xylem and phloem in ring in root

vascular bundles scattered in the stem

leaf veins parallel

flowers in multiples of three

e.g. cereals/grasses

Question 11

what are the features of a dicotyledonous plant?

Answer 11

two primary leaves in seed

phloem in xylem cross in root

vascular bundles in ring around edge of stem

net veins in leaves

flowers in multiples of four and five

Question 12

what is the role of water in a plant?

Answer 12

maintaining turgor pressure

cooling

cytosol and cell sap

transportation of assimilates

key reactant in photosynthesis

Question 13

what is transpiration?

Answer 13

the movement of water through a plant and the evaporation of water through aerial parts of a plant

Question 14

why is transpiration a ‘necessary evil’?

Answer 14

water is lost through stomata, which must be open for gas exchange for photosynthesis

Question 15

how are stomata closed?

Answer 15

abscisic acid (ABA) binds to specific receptors on membranes of guard cells

Ca2+ ion channels open and Ca2+ enter

proton pumps activated

influx of H+ generates a PMF

PMF opens voltage-gated K+ channels

K+ leave via facilitated diffusion

w.p. of cytosol increases

∴ water leaves via osmosis down w.p.g.

cells become flaccid –> closed stoma

Question 16

outline the apoplast pathway

Answer 16

water moves into space outside CSM and through cell wall/extra cellular spaces

casparian strip (waxy suberin layer) prevents water moving across cell walls

Question 17

outline the symplast pathway

Answer 17

water moves through plasma membrane into cytoplasm via plasmodesmata down w.p.g.

Question 18

outline the vacuolar pathway

Answer 18

water moves vacuole to vacuole via neighbouring cells, crossing the symplast and apoplast pathways

Question 19

how does windspeed/air movement affect transpiration?

Answer 19

blows water vapour away from leaf

reduces humidity

increases w.p.g.

∴ faster rate of diffusion of water vapour

Question 20

how does light affect transpiration?

Answer 20

increases rate of PHS

∴ more stomata open

∴ water vapour can diffuse out faster

Question 21

how does temperature affect transpiration?

Answer 21

increases Ek of water vapour

∴ more evaporation

∴ more movement down w.p.g.

Question 22

how does the number of leaves affect transpiration?

Answer 22

more stomata

larger SA for gas exchange/loss of water vapour

Question 23

how does the thickness of the cuticle affect transpiration?

Answer 23

thicker = harder for water vapour to diffuse out

Question 24

what is translocation?

Answer 24

the movement of nutrients around the plant from source to sink

Question 25

give three examples of a source

Answer 25

green leaves/stems

storage organs

food stores in germinating plants

Question 26

give three examples of a sink

Answer 26

roots

dividing meristem

food stores

Question 27

how is sucrose (S) loaded into phloem - symplast?

Answer 27

S into cytoplasm of mesophyll

S into STE via diffusion through plasmodesmata

H2O follows by osmosis from xylem

hydrostatic pressure generated

S moves through phloem by mass flow

Question 28

how is sucrose (S) loaded into phloem - apoplast?

Answer 28

S into companion cells/STEs by diffusion

H+ pumped out of companion cells and return down concentration gradient via co-transpiration proteins with S

S builds up in companion cells/STEs

H2O follows by osmosis

turgor pressure increases

water + assimilates move into STEs, reducing pressure in companion cells –> movement by mass flow

Question 29

what are the adaptations of companion cells?

Answer 29

infolding membrane –> incr. SA

many mitochondria –> incr. ATP synthesis

plasmodesmata for incr. mass flow

Question 30

how are assimilates unloaded from phloem?

Answer 30

diffusion into tissues - v. rapid

loss of S from phloem –> incr. water potential in phloem

water drawn out into surrounding cells