7.8- TRANSPORT OF ORGANIC SUBSTANCES IN THE PHLOEM Flashcards
What is translocation?
process by which organic molecules + some mineral ions transported from one part of plant to another
What is the tissue that transports biological molecules in flowering plants?
phloem
What is the phloem made up of?
sieve tube elements, long thin structures arranged end to end
What are the end walls of phloem like?
perforated to form sieve plates
What are associated with the sieve tube elements of the phloem?
cells called companion cells
What is the site where sugars are produced from photosynthesis?
sources
Where does the plant transport sugars from photosynthesis from the source to?
to places where they will be used directly or stored for future use- known as sinks
As sinks can be anywhere in a plant- sometimes above or sometimes below source, what follows?
translocation of molecules in phloem can be in either direction
Examples of organic molecules to be transported by phloem?
sucrose and amino acids
What else does the phloem transport?
inorganic ions- potassium, chloride, phosphate + magnesium ions
What are materials in phloem at rate of movement too fast to be explained by?
explained by diffusion
What is the theory for the precise mechanism by which translocation achieved?
mass flow theory
What three phases can mass flow theory be achieved?
- transfer of sucrose into sieve elements from photosynthesising tissue
- mass flow of sucrose through sieve tube elements
- transfer of sucrose from sieve tube elements into storage or other sink cells
mass flow theory- transfer of sucrose into sieve elements from: where is sucrose manufactured from? #1
sucrose manufactured from products of photosynthesis in cells with chloroplasts
mass flow theory- transfer of sucrose into sieve elements from: what does sucrose diffuse down? #2
sucrose diffuses down concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
mass flow theory- transfer of sucrose into sieve elements from: what happens to hydrogen ions? #3
hydrogen ions actively transported from companion cells into spaces within cells walls using ATP
mass flow theory- transfer of sucrose into sieve elements from: what do the hydrogen ions diffuse down? #4
hydrogen ions then diffuse down concentration gradient through carrier proteins into sieve tube elements
mass flow theory- transfer of sucrose into sieve elements from: what are the sucrose molecules transported with?
sucrose molecules transported along with hydrogen ions in process known as co-transport
protein carried known as co-transport proteins
What is mass flow?
bulk movement of substance through given channel or area in specified time
mass flow theory- mass flow of sucrose through sieve tube elements: what is sucrose produced by? #1
sucrose produced by photosynthesising cells (source) actively transported into sieve tubes
mass flow theory- mass flow of sucrose through sieve tube elements: what does the sucrose being actively transported into the sieve tubes cause? #2
causes sieve tubes to have lower (more negative) water potential
mass flow theory- mass flow of sucrose through sieve tube elements: what does the water potential of the xylem cause? #3
as xylem has much higher (less negative) water potential, water moves from xylem into sieve tubes by osmosis, creating high hydrostatic pressure within them
mass flow theory- mass flow of sucrose through sieve tube elements: what happens with glucose at the respiring cells? #4
at respiring cells (sink), sucrose either used up during respiration or converted to starch for storage
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
cells have low sucrose content so sucrose actively transported into them from sieve tubes lowering their water potential
mass flow theory- mass flow of sucrose through sieve tube elements: what happens due to lowered water potential of cells? #6
due to lowered water potential, water also moves into respiring cells, from sieve tubes, by osmosis
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
hydrostatic pressure of sieve tubes in this region lowered
mass flow theory- mass flow of sucrose through sieve tube elements: what happens as the hydrostatic pressure in the sieve tubes lowered? #8
because of water entering sieve tube elements at source + leaving at sink. there’s high hydrostatic pressure at source +low one at sink
mass flow theory- mass flow of sucrose through sieve tube elements: what s there down hydrostatic gradient in sieve tubes?
so there’s mass flow of sucrose solution down hydrostatic gradient in sieve tubes
What sort of process is mass flow?
passive process
What does mass flow occur as a result of?
as a result of active transport of sugars
What is the process of mass flow described as a whole as it occurs as a result of active transport of sugars?
active
As mass flow as a whole is active, what is it affected by e.g.?
temperature + metabolic poisons
Evidence supporting mass flow hypothesis: pressure within sieve tubes
there’s pressure within sieve tubes, as shown by sap being released when they’re cut
Evidence supporting mass flow hypothesis: concentration of sucrose
concentration of sucrose higher in leaves (source) than in roots (sink)
Evidence supporting mass flow hypothesis: downward flow
downward flow in phloem occurs in daylight, but ceases when leaves shaded, or at night
Evidence supporting mass flow hypothesis: what is increase in sucrose levels in leaf followed by
increase in sucrose levels in leaf followed by similar increase in sucrose levels in phloem a little later
Evidence supporting mass flow hypothesis: what do metabolic poisons and/or lack of oxygen do?
inhibit translocation of sucrose in phloem
Evidence supporting mass flow hypothesis: what do companion cells posses and produce?
possess many mitochondria + readily produce ATP
Evidencing questioning mass flow hypothesis: function of sieve plates
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)
Evidencing questioning mass flow hypothesis: solutes speed
not all solutes move at same speed- should do so if movement is by mass flow
Evidencing questioning mass flow hypothesis: sucrose delivery rate
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
mass flow theory- transfer of sucrose from sieve tube elements into storage or other sink cells: what is sucrose actively transported by?
sucrose actively transported by companion cells, out of sieve tubes + into sink cells
