[7.8] transport of organic substances in the phloem

Question 1

what is translocation?

Answer 1

the process by which organic mole ules and some mineral ions are transported from source to sinks

Question 2

what are examples of organic molecules that can be transported in the phloem?

Answer 2

• sucrose
• amino acids

Question 3

what are examples of inorganic molecules that can be transported in the phloem?

Answer 3

• potassium ions
• chloride ions
• phosphate ions
• magnesium ions

Question 4

what is a source?

Answer 4

• where solute (sugars produced from photosynthesis) are made
• there is a high concentration of that solute here

Question 5

what is a sink?

Answer 5

where a solute is required to due low concentration of that source

Question 6

what 2 types cells phloem tubes contain?

Answer 6

• phloem sieve tubes
• companion cells

Question 7

what are phloem sieve tubes?

Answer 7

• long, tubuluar shaped cells
• contain cytoplasm but not organelles
• organic molecules (eg. sucrose, AAs) will move through these cells by translocation

Question 8

what are companion cells?

Answer 8

• adjacent to the sieve tube cells
• contain nuclei and organelles
• role is to load (at source) sucrose / AAs into sieve tube cells or remove (at sink) sucrose / AAs from sieve tubes

Question 9

what are the 3 phases of the mass flow theory mechanism of translocation?

Answer 9

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 10

what happens in the 1st stage of mass flow mechanism?

Answer 10

• sucrose is made in photosynthesising cells in ‘source’ regions of the plant with chloroplasts
• sucrose diffuses down a concentration gradient by faciliated diffusion from photosynthesising cells into companion cells
• H+ ions are actively transported from companion cells into spaces within cell walls using ATP
• H+ ions diffuse down a concentration gradient through carrier proteins (co-transport proteins) in sieve tube elemts
• sucrose molecules are transported along with H+ ions in co-transport

Question 11

what happens in the 2nd stage of mass flow mechanism?

Answer 11

• due to the active transport of sucrose from source to sieve tubes, sieve tubes have a more negative water potential
• xylem has less negative water potential so water moves from xylem into sieve tubes by osmosis, creating a high hydrostatic pressure within them
• at respiring cells (sink), sucrose is either used up during respiration or converted to starch for storage
• these cells therefore have a low sucrose content so sucrose is actively transported into them from sieve tubes, lowering their water potential
• due to this lowered water potential, water also moves into these respiring cells, from sieve tubes, by osmosis
• hydrostatic pressure of the sieve tubes in this region are therefore lowered
• as a result of water entering the sieve tube elements at the source and leaving at the sink, there is a high hydrostatic pressure at the source and a low one at the sink
• there is therefore a mass flow of sucrose solution down this hydrostatic gradient of sieve tubes

Question 12

what happens in the 3rd stage of mass flow mechanism?

Answer 12

sucrose is actively transported by companion cells out of the sieve tubes and into sink cells

Question 13

evidence supporting the mass flow hypothesis

Answer 13

• there is pressure within sieve tubes, as shown by sap being released when they are cut
• conc of sucrose is higher in leaves (source) thank in roots (sink)
• downward flow in phloem occurs in daylight, but ceases when leaves are shaded, or at night
• increases in sucrose levels in leaf are followed by similar increases in sucrose levels in phloem a bit later
• metabolic poisons and / or lack of oxygen inhibits translocation of sucrose in the phloem
• companion cells possess many mitochrondria and readily produce ATP

Question 14

evidence questioning the mass flow hypothesis

Answer 14

• function of sieve plates is unclear, as they would seem to hinder mass flow
• not all solutes move at the same speed, they should do so if movement is by mass flow
• sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory would suggest