Mass transport in plants

Question 1

the xylem transports _______

the phloem transports _______

Answer 1

= water

= organic substances

Question 2

where us the xylem and phloem found found?

Answer 2

in the stem, vascular bundle of parenchyma cells

Question 3

Structure of xylem:

1. no ___ walls
2. no cell ______
3. strengthen by rings of ______

Answer 3

xylem:
1. end
2. contents
3. lignin

Question 4

define transpiration

Answer 4

evap. of H2O from leaves. Passive process, energy supplied from sun

Question 5

How does H2O move out of stomata?

Answer 5

1. humid near stomata = H2O gradient formed
2. H2O diffuses out stomata
3. H2O lost from diffu. replaced by H2O evap. from cell walls of mesophyll cells
4. change size of stomata = control rate of trans.

Question 6

name the 2 ways H2O lost from mesophyll cells is replaced (the 2 routes)

Answer 6

from xylem via cell walls or cytoplasm

Question 7

describe the cytoplasmic route of H2O across a leaf

Answer 7

1. mesophyll cells lose H2O to air spaces by evap.
2. cells lower H2O potential = H2O enters via osmosis from neighbouring cells
3. loss of H2O in neighbouring cells = lower potential = H2O taken up from neighbouring cells

Question 8

how is H2O moved up xylem? and what is this called?

Answer 8

• H2O molecules form hydrogen bonds (-O) (H+) , forms a continuous column called transpiration pull.
• H2O drawn up by cohesion, xylem under negative pressure = cohesion-tension theory

Question 9

changes of the diameter of tree trunks gives evidence for cohesion-tension theory. Why?

Answer 9

according to rate of transpiration:

• day: transpiration greatest, more tension, more negative pressure = xylem pulled inward and diameter reduced
• night is the opposite

Question 10

name 2 pieces of evidence (except changes in diameter of trunks) for cohesion-tension theory

Answer 10

• if xylem broken: H2O not leak which would be the case if under pressure instead air drawn in
• if xylem broken air drawn in = continuous column of H2O broken

Question 11

a potometer does not measure ________ but measure the amount of ____ taken up in a ____ time. Assumption is that all the H2O being taken up is being ________.

Answer 11

1. transpiration
2. water
3. given
4. transpired

Question 12

when using a potometer don’t get any water on the ______ and make sure there is no __ ______.

Answer 12

1. leaves

2. air bubbles

Question 13

when using a potometer how do you put the air bubble back at the start of the scale?

Answer 13

syringe pushed down until air bubble is back at the start

Question 14

when doing the potometer experiment what 2 things are your control variables?

Answer 14

1. keep SA of leaves the same

2. same diameter/radius of shoot

Question 15

Phloem: companion cells

• small cells between _____ tubes
• large nucleus, small vacuole, dense cytoplasm and lots of ________ to provide ___ to load sucrose into ______ tube elements.
• cytoplasm linked to ____ tube elements by _________.

Answer 15

1. sieve
2. mitochondria
3. ATP
4. sieve
5. sieve
6. tube
7. plasmodesmata

Question 16

what is the difference between source and sink and give examples

Answer 16

source: sites of production of sugar i.e. leaves
sink: place where sugar is stored/used up i.e. roots and respiring cells

Question 17

name the 2 organic ions the phloem transports

Answer 17

glucose/sucrose, amino acids

Question 18

Describe the transfer of sucrose into sieve tube elements from photosynthesising tissue

Answer 18

1. H+ ions pumped out companion cells into palisade (produce ATP, active transport
2. Facilitated diffu. of H+ and sucrose thru cotransport protein symports into companion cells
3. sucrose diffu. down conc. gradient into sieve tube elements, in phloem

Question 19

describe the movement of sucrose thru phloem

Answer 19

1. phloem, low H2O potential
2. H2O moves via osmosis to phloem down H2O potential gradient (high to low hydrostatic pressure)
3. hydrostatic pressure in phloem increases

Question 20

describe what happens at the sink

Answer 20

1. respiring cells = use up sucrose, low conc.
2. active transport of sucrose into sink cells = lowers H2O potential, H2O enters them
3. hydrostatic pressure of sieve tube elements in area lowered

Question 21

there is a low hydrostatic pressure at the source and a high one at the sink t or f

Answer 21

f = high at source low at sink

Question 22

name the 6 pieces of evidence that supports mass flow theory

Answer 22

1. pressure in sieve tube elements: when cut, sap released
2. conc. of sucrose higher in leaves than roots
3. downward flow in phloem in day, stops at night
4. increase sucrose in leaves:similar increase in phloem later
5. metabolic poisons/lack of O2 inhibit translocation
6. companion cells: lots of mitochon. produce ATP

Question 23

name the 3 pieces of evidence to question mass flow theory

Answer 23

1. function of sieve plates unclear: hinder flow. Structural?
2. not all solutes moved at same speed, should do if mass flow
3. sucrose is delivered at same rate to all regions. If mass flow theory: sucrose go rapidly to areas of low conc.

Question 24

Name the 3 methods to investigate transport in plants

Answer 24

ringing, tracer and use of aphids

Question 25

briefly explain ringing experiments

Answer 25

1. section of outer layer of woody stem cut around circumference of tree (xylem intact)
2. after a while: region above missing ring = swells (sugars of phloem)
3. samples contain sugars and organic substances
4. above ring: tissues continue to grow but non photosynthetic tissues die below it
   suggests that phloem not xylem for translocation

Question 26

briefly explain tracer experiments

Answer 26

using radioactive isotopes i.e. isotope 14^C

1. used to make radioactively labelled CO2 (14^CO2)
2. 14^CO2 in sugars by photosynthesis
3. can be traced using autoradiography = thin cross sections of stem on X-Ray film
4. X-Ray blackened if exposed to 14^C

Question 27

briefly explain the use of aphids

Answer 27

1. stylet of aphid penetrates phloem: extracts contents of sieve tube elements
2. take off stylet = sugars continue to flow out stylet
3. sucrose conc. in leaves mirrored later in phloem

Question 28

Structure of Phloem:

1. has ____ tube ______:
2. no nucleus or _____
3. has cross ___ and ____ called _____ plates

Answer 28

1. sieve tube elements
2. ribosomes
3. walls and pores called sieve plates