Vasculature 2: phloem and phloem loading Flashcards
what is phloem
specialized in the movement of large organic compounds throughout the plant
what are photosynthates
sugar products of photosynthesis
what is phloem sap
solution of sugars, water and other organic compounds found in the phloem
what are the types of phloem cells
sieve-tube elements
fibers
parenchyma cells
what are sieve-tube element cells
highly specialized unique cells
lack nucleus, ribosomes, vacuole
what are 2 types of sieve tube elements
sieve tube members
sieve cells
how are sieve tube elements divided
along taxonomic lines
what are sieve tube members
specialized phloem cell found mainly in angiosperms
contains a sieve plate
growth supported by a companion cell
what is a sieve plate
dividing structure between adjacent sieve tube member cells
what does a sieve plate consist of
many plasodesmata enlarged to allow for the efficient transport of nutrients betweencells
how does xylem support growth
doesnt because cells are dead at amturity
how does phloem support growth
partners with companion cell
what is a companion cells role
provides most of the proteins and other metabolites that the sieve tube member needs to remain alive
characterizations of the companion cells
are specialized parenchyma cells
connect to a sieve tube member through plasmodesmata
what are sieve cells
phloem cell found mainly in gymnosperms and seedless vasc plants
what characterizes sieve cells
a lack of sieve plate
being generally longer and thinner than sieve tube members
associate with an albuminous cell
what is an albuminous cell
a modified parenchyma cell that supports the metabolic activity of the sieve cell
difference between albuminous cell and companion cell
albuminous does not arise from the same mother cell as the sieve cell
what is translocation
process of sugars moving through the plant
moves substances from source to sink in the plant
what is the main form of sugar transported through the phloem
sucrose
what are source tissues
any plant part which generates more sugars than it consumes
what do source tissue include
storage organs
young and fully expanded leaves
what are sink tissues
any plant part which is unable to meet its nutritional sugar needs
what do sink tissues include
roots storage organs older leaves leaves which are expanding flowers/fruits
what is the vegetative stage of development
- younest matur leaves send sugars toward apical stem
- oldest leaves send sugars to roots
- middle leaves send sugars in either direction
what is the reproductive stage of development
fruits require vast majority of sugars in the plant
vegetative growth slows down through lack of sugars for growth
middle leaves will send all their sugars to the developing repro structures
what drives movement of phloem
source and sink tissues
phloem sap can move up or down the plant
what drives the movement of xylem
transpiration
water can only move up the plant
what is phloem loading/unloading
movement of sugars in and out of the phloem
what is pressure-flow hypthesis
movement of sugars through the phloem
what is passive phloem loading
- movement of sugars from the mesophyll cels of the leaf to the phloem cells along a concentration gradient
- driven by a concentration gradient of sucrose
- sucrose moves through plasmodesmata of adjacent cells through a symplastic pathway driven by simply diffusion
what is active phloem loading
- movement of sugars from the mesophyll cells of the leaf to the phloem cell against a concentration gradient through the spending of ATP
- sugars travel by the symplastic pathway until the cell layer before the companion cell
what happens in active phloem loading
ATP powered protons pumps create a proton gradient
sucrose is cotransported with protons as they move down their concentration gradient
what is polymer trapping
how some plants can move sucrose agaisnt a concentration gradient without the need for membrane transporters
what happens in polymer trapping
concentrating sucrose by converting it to different, larger, sugar polymers
what is an intermediary cell
special type of parenchyma cell where sugars are concentrated
what are the steps of sugar transport in polymer trapping
- sugars move into the intermediary cell through symplastic active transport
- sucrose is actively combined with other sugars to form larger sugar polymers
- polymers too large move back into the mesophyll cell
what is symplastic
when something moves with the concentration gradient
what is apoplastic
when something moves agaisnt the concentration gradient
what is phloem unloading
the process of unloading sugars to the sink parnchyma cells from the phloem cells
when is phloem unloading passive
in young leaves, actively growing tissues, roots
when is phloem unloading active
in storage
once in the phloem cells, how does sugar move in the vasculature
with pressure flow
what is pressure flow hypothesis
a combination of osmosis, concentration gradients, water pressure and water movement in the xylem
steps of pressure flow
- phloem loading (high concentration of sucrose in the phloem)
- water pulled from xylem into phloem
- increased water pressure in cell
- water moves to area of lower pressure through bulk flow (source to sink)
- phloem unloading (reduces sucrose concentration in phloem
- water moves back to xylem through transpiration
what directions can water flow in the xylem and phloem in pressure flow hypothesis
same direction
opposite direction
only 1 direction in phloem even i it can do both
why dont we expect water to flow toward the source
water along with solutes moves down the pressure gradient and not the potential gradient
how is sucrose transported to seeds/flowers
water pressure in the source region of the phloem drives the movement of sucrose into the developing seeds or flowers
