6.2 Transport systems in plants

Question 1

Explain how SA:vol restrictions are overcome in plants

Answer 1

o Whole plants generally have small SA:vol
o This is usually too small to enable the gas exchange demand to be met by simple diffusion (don’t confuse with leaves and hair cells that are adapted to have large SA:vol)
o plants need to move substances to all cells at a quick rate to meet demand
o so they need an efficient transport system

Question 2

Explain how large flowering plants such as horse chesnut trees depend on a suxccessful transport system

Answer 2

o Large multicellular plants have relatively high metabolic rate
o Need effective transport system to move substances from root tip to growing tip

Question 3

Decribe how a tranposrt system in a plant enables the metabolic demands to be met

Answer 3

o Only areas with photosynthetic pigments can produce glucose
o All cells need supply of oxygen & glucose
o All cells need to remove waste products of metabolism
o Growth plant regulators (‘hormones’) need to be transported from their site of synthesis to site of use
o Mineral ions need to be taken up & transported to all cells for protein & nucleic acid production

Question 4

Give 3 reasons a mass transport system is needed in a plant

Answer 4

to meet metabolic demands

to cope with size of plant

to cope with SA:vol

Question 5

State 6 functions of water within plants

Answer 5

• Turgor pressure (hydrostatic pressure)
  o provides hydrostatic skeleton to support stem & leaves
  o causes cell expansion -> force enables plant roots to force their way through soil
• Transpiration stream acts as cooling mechanism for plant
• Main component of cytosol
• Main component of cell sap
• Enables mineral ions and products of photosynthesis (assimilates) to be transported from where they are obtained to where they are needed (translocation)
• Water is key reactant in photosynthesis

Question 6

Describe the structure of xylem

Answer 6

 No end walls
 Elongated, dead cells
 Form uninterrupted tubes
 Thickened with lignin (woody waterproofing substance)
 Cells become more lignified as they age
 Has no protoplasm (and hence no metabolic activity)

Question 7

State the function of bordered pits in xylem tissue

Answer 7

allows lateral movement of water and ions

to keep cells turgid and cool

Question 8

State the function of xylem tissue

Answer 8

Transport water and dissolved mineral ions up the plant from the roots to the apex

Question 9

State the function of phloem tissue

Answer 9

to transport dissolved solutes and assimilates e.g. sucrose from source to sink

Question 10

Describe the structure of phloem tissue

Answer 10

Sieve tube elements:
 Lacks nucleus, RER, central vacuole, golgi apparatus - to reduce resistance to the flow of phloem sap and to provide more space for more phloem sap
 No tonoplast (and therefore no large central vacuole) and limited cytoplasm which is only located on the periphary – to reduce resistance to flow of assimilates
 Few mitochondria
 perforated by pores (sieve plates)

Companion cells,
 Normal contents i.e. nucleus & all organelles
 Linked to protoplasm of PSTE by many plasmodesmata
 Many mitochondria – to supply ATP to the PSTE

Question 11

Explain the significance on the perforated end walls in the sieve tube elements

Answer 11

they form sieve pores
which allow solutes to pass through unimpeded by mass flow

Question 12

Describe the function of leaves

Answer 12

SA for gas exchange

absorption of light energy for photosynthesis

make organic solutes

Question 13

what is the function of the stem?

Answer 13

structure/support

lifting leaves to sunlight

transport of substances in vascular tissue

Question 14

Describe the function of the roots

Answer 14

anchors the plant in the soil

absorbing mineral ions from the soil (by active transport)

taking up water via osmosis in the root hair cells

storage organ (acts as a sink)

Question 15

State 5 features of a monocotyledon (monocot plant)

Answer 15

single primary leaf (cotyledon)

xylem and phloem in ring in root

vascular bundles scattered in the stem

leaf veins parallel

flowers in multiples of three

fibrous roots (also called adventitious roots)

e.g. cereals/grasses

Question 16

State 5 features of a dicotyledon (dicot plant)

Answer 16

two primary leaves (cotyledons)

phloem in xylem cross in root

vascular bundles in ring around edge of stem

net veins in leaves

has a main tap root with side branches

flowers in multiples of four and five

Question 17

State 4 main roles of water within a plant

Answer 17

maintaining turgor pressure

cooling

cytosol and cell sap

transportation of assimilates

key reactant in photosynthesis

Question 18

Define the term transpiration stream

Answer 18

the movement of water from the roots to the leaves via mass flow and transpiration.

Note: it is also known as the cohesion-tension theory and transpiration pull.

Question 19

Define the term transpiration

Answer 19

the loss of water vapour (by diffusion) from the underside of the leaf (aerial part of the plant) via the open stomata

Question 20

State the 3 forces that play a role in transpiration and explain thier importance

Answer 20

 Cohesion: water molecules are attracted to each other via hydrogen bonds (as water molecules are polar & have dipoles) hence they are cohesive.
 Tension: the evaporation of water molecules from the walls of the PMC to the mesophyll air spaces generates a suction (tension) that draws more water into the leaf
 Adhesion: the water molecules are attracted to the cellulose walls of the xylem vessels and the hydrophilic parts of lignin which aids the mass flow of water up the xylem (do not just say adhesion between water and the cell wall or adhesion between water and lignin)

Question 21

Describe the events that occur at the surface of the leaf that play a role in transpiration

Answer 21

• Water evaporates from the cellulose cell walls of the mesophyll cells into mesophyll air spaces
• Water vapour then diffuses through the mesophyll air spaces to the sub-stomatal air spaces
• Water vapour then diffuses down the water potential gradient through the open stomata to the surrounding external air

Question 22

Describe how water moves through the xylem during transpiration

Answer 22

• Xylem vessels transport water up the stem of the plant from roots to the leaves due to a difference in water potential at the top of the stem compared to the bottom
• Water is drawn up the xylem by capillary action due to hydrogen bonds between the adjacent water molecules (which are polar i.e. possess dipoles)
• Water moves up the xylem in a continuous column due to cohesion forces
• At the leaves the water moves out of the xylem into the cells by the apoplast pathway
• this is described as the cohesion-tension theory
• i.e. the transpiration pull

Question 23

Suggest the meaning behind the statement “transpiration is the necessary evil in a plant”

Answer 23

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

Question 24

Describe how stomata are closed

Answer 24

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 25

Outline the apoplast pathway

Answer 25

o Water goes through cellulose cell walls
o Walls are very absorbent so water diffuses along the cellulose fibres as well as moving by osmosis through the spaces between the fibres
o As cell walls are freely permeable this route offers little resistance and hence most of the water travels via this route

NB when water reaches the endodermis the pathway is blocked by the Casparian strip (continuous band of cells that have waxy suberin layer in their cell walls). At this point the water has to cross the endodermis via the symplast pathway – this is useful as the csm is partially permeable and allows the cell to control which substances in the water pass through to the xylem vessels via the parenchyma cells.

Question 26

Outline the symplast pathway

Answer 26

o Water travels through the csm to the protoplasm of the cell via osmosis from the RHC through the cells of the root cortex
o water moves down the w.p.g.
o cytoplasm of neighbouring cells are connected via plasmodesmata and water also moves between these intercellular connections

Question 27

Outline the vacuolar pathway

Answer 27

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

Question 28

Explain why traspiration occurs

Answer 28

• Main reason = as a consequence of gas exchange
• Plants must open their stomata to enable the uptake of CO2 for PHS and for the release of O2 to the atmosphere.
• Insufficient CO2 will result in the rate of PHS reducing
• Hence this reduces glucose production which is potentially fatal to theplant
• However, it also helps regulate the temperature of the plant and ensures that mineral ions are transported through the plant (as a consequence of mass flow of the water which carries dissolved ions within in it).

Question 29

Explain the effect of windspeed/air movement on the rate of transpiration

Answer 29

blows water vapour away from leaf

reduces humidity

increases w.p.g.

∴ faster rate of diffusion of water vapour

Question 30

Explain the effect of light intensity on the rate of transpiration

Answer 30

increases rate of PHS

∴ more stomata open

∴ water vapour can diffuse out faster

Question 31

Explain the effect of temperature on the rate of transpiration

Answer 31

Higher temp = water molecules possess more kinetic energy = greater evaporation from the spongy mesophyll cells into air spaces inside the leaf = increases the diffusion gradient between the air inside of the leaf and the external atmosphere = faster rate of diffusion of water molecules from sub-stomatal air space to external atmosphere

ALSO
Increase in temperature increases the concentration of water vapour that can be held by the external air before it becomes saturated (so decreases its humidity and water potential maintaining the diffusion gradient for longer)

Question 32

Explain the effect of varying number of leaves has on the rate of transpiration

Answer 32

more stomata

larger SA for gas exchange/loss of water vapour

Question 33

Explain the effect of varying cuticle thickness has on the rate of transpiration

Answer 33

thicker = harder for water vapour to diffuse out

Question 34

Define translocation

Answer 34

the movement of nutrients around the plant from source to sink

Question 35

State 3 examples of a source

Answer 35

green leaves/stems

storage organs

food stores in germinating plants

Question 36

State 3 examples of a sink

Answer 36

roots

dividing meristem

food stores

Question 37

Describe how is sucrose loaded into phloem via the symplast route

Answer 37

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 38

Describe how is sucrose loaded into phloem via the apoplast route

Answer 38

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 39

Describe how companion cells are adapted for their function

Answer 39

infolding membrane –> incr. SA

many mitochondria –> incr. ATP synthesis

plasmodesmata for incr. mass flow

Question 40

Describe how assimilates are unloaded from phloem

Answer 40

diffusion into tissues - v. rapid

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

water drawn out into surrounding cells

Question 41

Describe how water moves throug hthe xylem during transpiration

Answer 41

• Xylem vessels transport water up the stem of the plant from roots to the leaves due to a difference in water potential at the top of the stem compared to the bottom
• Water is drawn up the xylem by capillary action due to hydrogen bonds between the adjacent water molecules (which are polar i.e. possess dipoles)
• Water moves up the xylem in a continuous column due to cohesion forces
• At the leaves the water moves out of the xylem into the cells by the apoplast pathway
• i.e. by cohesion-tension