Phloem Flashcards
Define translocation
process by which organic molecules and some mineral ions are transported from one part of a plant to another
What is a source?
site of production eg leaf
What is a sink?
site of use or storage eg root fruit or baby leaf
Name 2 organic substances and 2 inorganic substances
organic substances = amino acids and glucose
inorganic substances = ions Mg, K, Cl, P
Why is mass flow accepted?
the rate is too fast for diffusion
Describe the structure of the sieve tube elements
no nucleus/organelles
little cytoplasm
cross walls w/ pores to allow sap to flow ( sieve plates)
Describe the structure of the companion cell
large nucleus
dense cytoplasm
lots of mitochondria
small vacuole
What is plasmodesmata?
channel of cytoplasm in plant cells to allow the exchange of substances.
Define mass flow
Mass flow is the bulk movement of substances through a given channel or area in a specified time
Name the three steps in mass flow
1= transfer of sucrose from photosynthesising tissue into STE
2= mass flow of sucrose through STE
3=transfer of sucrose from STE into sink cells
Describe the first step of mass flow
hydrolysis fo ATP moves H+ ions out of companion cell via AT
causes a deficit in companion cell so H+ diffuses back in with sucrose in co-transport
the sucrose then facilitated diffuses into phloem STE
Describe the second step of mass flow
sieve tube has a lower WP than Xylem so water moves in via osmosis
this increases the hydrostatic pressure so water moves down the gradient to sink cell when wp is lower as is sucrose.
mass flow of sucrose solution down the hydrostatic gradient into the sieve tubes
Describe the third step of mass flow
sucrose is AT by companion cells put of the tube and into the sink cells
List 6 pieces of evidence that supports the theory
there is pressure in ST ( as sap is released when cut )
con. of sucrose is higher in source than sink
downward flow in phloem only occurs in the day
increase in sucrose levels in leaf are followed by similar in phloem
metabolic position and lack of o2 inhibits translocation of sucrose in the phloem
companion cell= many mitochondria to produce ATP
List 3 pieces of evidence that questions the theory
function of sieve plates is unclear as it hinders mass flow not all solutes move at the same speed sucrose delivered at more or less the same rate to all regions
