7.8- TRANSPORT OF ORGANIC SUBSTANCES IN THE PHLOEM

Question 1

What is translocation?

Answer 1

process by which organic molecules + some mineral ions transported from one part of plant to another

Question 2

What is the tissue that transports biological molecules in flowering plants?

Answer 2

phloem

Question 3

What is the phloem made up of?

Answer 3

sieve tube elements, long thin structures arranged end to end

Question 4

What are the end walls of phloem like?

Answer 4

perforated to form sieve plates

Question 5

What are associated with the sieve tube elements of the phloem?

Answer 5

cells called companion cells

Question 6

What is the site where sugars are produced from photosynthesis?

Answer 6

sources

Question 7

Where does the plant transport sugars from photosynthesis from the source to?

Answer 7

to places where they will be used directly or stored for future use- known as sinks

Question 8

As sinks can be anywhere in a plant- sometimes above or sometimes below source, what follows?

Answer 8

translocation of molecules in phloem can be in either direction

Question 9

Examples of organic molecules to be transported by phloem?

Answer 9

sucrose and amino acids

Question 10

What else does the phloem transport?

Answer 10

inorganic ions- potassium, chloride, phosphate + magnesium ions

Question 11

What are materials in phloem at rate of movement too fast to be explained by?

Answer 11

explained by diffusion

Question 12

What is the theory for the precise mechanism by which translocation achieved?

Answer 12

mass flow theory

Question 13

What three phases can mass flow theory be achieved?

Answer 13

1. transfer of sucrose into sieve elements from photosynthesising tissue
2. mass flow of sucrose through sieve tube elements
3. transfer of sucrose from sieve tube elements into storage or other sink cells

Question 14

mass flow theory- transfer of sucrose into sieve elements from: where is sucrose manufactured from? #1

Answer 14

sucrose manufactured from products of photosynthesis in cells with chloroplasts

Question 15

mass flow theory- transfer of sucrose into sieve elements from: what does sucrose diffuse down? #2

Answer 15

sucrose diffuses down concentration gradient by facilitated diffusion from photosynthesising cells into companion cells

Question 16

mass flow theory- transfer of sucrose into sieve elements from: what happens to hydrogen ions? #3

Answer 16

hydrogen ions actively transported from companion cells into spaces within cells walls using ATP

Question 17

mass flow theory- transfer of sucrose into sieve elements from: what do the hydrogen ions diffuse down? #4

Answer 17

hydrogen ions then diffuse down concentration gradient through carrier proteins into sieve tube elements

Question 18

mass flow theory- transfer of sucrose into sieve elements from: what are the sucrose molecules transported with?

Answer 18

sucrose molecules transported along with hydrogen ions in process known as co-transport
protein carried known as co-transport proteins

Question 19

What is mass flow?

Answer 19

bulk movement of substance through given channel or area in specified time

Question 20

mass flow theory- mass flow of sucrose through sieve tube elements: what is sucrose produced by? #1

Answer 20

sucrose produced by photosynthesising cells (source) actively transported into sieve tubes

Question 21

mass flow theory- mass flow of sucrose through sieve tube elements: what does the sucrose being actively transported into the sieve tubes cause? #2

Answer 21

causes sieve tubes to have lower (more negative) water potential

Question 22

mass flow theory- mass flow of sucrose through sieve tube elements: what does the water potential of the xylem cause? #3

Answer 22

as xylem has much higher (less negative) water potential, water moves from xylem into sieve tubes by osmosis, creating high hydrostatic pressure within them

Question 23

mass flow theory- mass flow of sucrose through sieve tube elements: what happens with glucose at the respiring cells? #4

Answer 23

at respiring cells (sink), sucrose either used up during respiration or converted to starch for storage

Question 24

mass flow theory- mass flow of sucrose through sieve tube elements: as respiring cells either use up sucrose in respiration or converted to starch for stage, what is the sucrose level like in them and so what happens? #5

Answer 24

cells have low sucrose content so sucrose actively transported into them from sieve tubes lowering their water potential

Question 25

mass flow theory- mass flow of sucrose through sieve tube elements: what happens due to lowered water potential of cells? #6

Answer 25

due to lowered water potential, water also moves into respiring cells, from sieve tubes, by osmosis

Question 26

mass flow theory- mass flow of sucrose through sieve tube elements: hydrostatic pressure of sieve tubes where water moves into respiring cells from sieve tubes? #7

Answer 26

hydrostatic pressure of sieve tubes in this region lowered

Question 27

mass flow theory- mass flow of sucrose through sieve tube elements: what happens as the hydrostatic pressure in the sieve tubes lowered? #8

Answer 27

because of water entering sieve tube elements at source + leaving at sink. there’s high hydrostatic pressure at source +low one at sink

Question 28

mass flow theory- mass flow of sucrose through sieve tube elements: what s there down hydrostatic gradient in sieve tubes?

Answer 28

so there’s mass flow of sucrose solution down hydrostatic gradient in sieve tubes

Question 29

What sort of process is mass flow?

Answer 29

passive process

Question 30

What does mass flow occur as a result of?

Answer 30

as a result of active transport of sugars

Question 31

What is the process of mass flow described as a whole as it occurs as a result of active transport of sugars?

Answer 31

active

Question 32

As mass flow as a whole is active, what is it affected by e.g.?

Answer 32

temperature + metabolic poisons

Question 33

Evidence supporting mass flow hypothesis: pressure within sieve tubes

Answer 33

there’s pressure within sieve tubes, as shown by sap being released when they’re cut

Question 34

Evidence supporting mass flow hypothesis: concentration of sucrose

Answer 34

concentration of sucrose higher in leaves (source) than in roots (sink)

Question 35

Evidence supporting mass flow hypothesis: downward flow

Answer 35

downward flow in phloem occurs in daylight, but ceases when leaves shaded, or at night

Question 36

Evidence supporting mass flow hypothesis: what is increase in sucrose levels in leaf followed by

Answer 36

increase in sucrose levels in leaf followed by similar increase in sucrose levels in phloem a little later

Question 37

Evidence supporting mass flow hypothesis: what do metabolic poisons and/or lack of oxygen do?

Answer 37

inhibit translocation of sucrose in phloem

Question 38

Evidence supporting mass flow hypothesis: what do companion cells posses and produce?

Answer 38

possess many mitochondria + readily produce ATP

Question 39

Evidencing questioning mass flow hypothesis: function of sieve plates

Answer 39

function of sieve plates unclear, as they would seem to hinger mass flow (has been suggested they may have structural function, helping to prevent tubes from bursting under pressure)

Question 40

Evidencing questioning mass flow hypothesis: solutes speed

Answer 40

not all solutes move at same speed- should do so if movement is by mass flow

Question 41

Evidencing questioning mass flow hypothesis: sucrose delivery rate

Answer 41

sucrose delivered at more or less same rate to all regions, rather than going more quickly to ones with lowest sucrose concentration, which mass flow theory would suggest

Question 42

mass flow theory- transfer of sucrose from sieve tube elements into storage or other sink cells: what is sucrose actively transported by?

Answer 42

sucrose actively transported by companion cells, out of sieve tubes + into sink cells