2) Phloem transport

passive vs active transport

passive: spontaneous movement of molecules down a chemical potential gradient, at eqlm, no further movement occurs without an input of energy

active: movement of substances against a chemical potential gradient, not spontaneous, requires that energy is applied, couple transport to the hydrolysis of ATP

inside of cell is usually?

negative

channels vs carriers vs pumps

channels: selective pores that remain open, no energy involved, just simple diffusion

carriers: solute specific, not dependent on energy, not simple diffusion
allows binding on one side, creating a conformational change in protein, pushing molecules through, then returning to original shape

pumps: primary active transport, against conc gradient, from low to high, require energy, coupled directly to ATP hydrolysis

true or false: both channels and carriers move along conc gradients

true

carriers require solute to bind, but is neither simple diffusion nor active transport (it’s passive transport)

details on H+ ATPase

H+ conc is higher outside cell

there is continuous pumping of protons out via H+ATPase pumps

in cell wall, higher conc of protons, making it more acidic

primary active transport

what is secondary active transport

as protons move back in along their conc gradient (opposite of primary active transport, this is just diffusion), the secondary transporter uses that energy to transport a secondary molecule against its conc gradient

symporter vs antiporter

symporter transports secondary molecule the same direction as the one moving along conc gradient

antiporter transports secondary molecule the opposite direction as the one moving along conc gradient

both move secondary molecule against conc gradient, just difference in the direction relative to the primary molecule

what is phloem

found on outer side of xylem in vascular bundles

in plants with secondary growth, phloem is inner bark

phloem cells are alive at functional maturity, unlike xylem cells (dead, no cytoplasm)

what are sieve elements

conduct sugars and organic compounds

lack nuclei and vacuoles, golgi bodies, ribosomes, microfilaments, and microtubules
these organelles would obstruct the flow of solutes and transport of other things

sieve elements in different plants

angiosperms = sieve tube elements (highly differentiated), end of sieve elements are sieve plates, allow unobstructed transport between elements, regulate as well
sieve plates contain pores that interconnect cells
damaged sieve tubes are sealed by p-proteins

gymnosperms = sieve cells (relatively unspecialized)
pores are obstructed by ER

what are companion cells

one or more associated with each sieve element

take over critical functions from sieve elements:
protein synthesis and ATP supply

assist with loading of solutes into the phloem

have chloroplasts and smooth inner cell wall

sources and sinks

sources produce excess sugar, export to other parts of plant
mature leaves, storage roots (carrot)

sinks do not produce enough photosynthetic product to support their own growth or storage needs, must import carbon or sugar from other parts of the plant
roots, tubers, developing fruits, immature leaves

potato could become source if it gets planted and starts growing new plant
developing leaves become sources as they produce more carbon than they ned to grow

what are the materials being translocated in xylem vs phloem

xylem: water and any solutes that cam from water and made it through casparian strip

phloem: mostly sugars, amino acids, organic acids, protein, potassium (mainly water and sucrose)

what is the pressure flow model for the phloem

passive mechanism

bulk flow of phloem sap driven by an osmotically generated pressure gradient between source and sink

energy is required in sources and sinks for synthesis and consumption of photosynthate (required for active phloem loading and unloading

energy is also required to maintain necessary cellular and anatomical structures

difference between transport in xylem and phloem

transport in xylem is tension/suction driven by transpiration at the sink

transport in phloem is driven by pressure, squeezing at the source

addition of solute causes lowering of water potential, but an increase in pressure causes increase in water potential

how does sucrose move in sieve elements?

conc of sucrose in sieve elements is higher than surrounding cells
won’t move by itself, sucrose has to be pumped in actively to the sieve cells

as WP in sieve element drops into negative range, the WP in the surrounding tissues gets higher
water moves into sieve cells, driven by constant loading of sucrose into sieve elements and water, increasing CP value, like squeezing a tube from one end

what happens at the sink

active unloading of sucrose, doesn’t necessarily have to be active because sink cells are designed to store sugars

combination of passive and active at the sink (at the source is almost exclusively active, to get the sucrose up to high conc in the sieve elements)

if there’s too much solute at the sink, it increases WP, which would then cause the push of water towards the source, not good

as sucrose moves out of sieve elements, there’s higher CP
as WP rises above WP in the xylem, water leaves phloem in response to the WP reducing turgor pressure in sieve elements of the sink
cross walls (sieve plates) of sieve elements maintain WP, because otherwise it would dissipate in open tubes