Phloem Flashcards
What is the difference between active and passive transport?
passive transport: spontaneous movement of molecules down a chemical potential gradient
at equilibrium, no further movement occurs without input energy
active transport: movement of substances against chemical potential gradient, not spontaneous requires energy
most transport processes are energized by one primary active system coupled to ATP hydrolysis by generating ion gradients
What is a chemical potential gradient?
How are proton gradients coupled to the formation of ATP?
membrane potential of plant cells -200 to -100 mV, provided by plasma electrogenic membrane H+-ATPase, energy comes from hydrolysis of ATP
cyanide poisons mitochondria and depletes ATP, so membrane potential decreases to match the diffusion potential = H+ not pumped/no active transport = passive transport of all ions halted
What is the difference between channels, carriers, and pumps?
channels: selective pores that extend completely across membranes and enhance diffusion
carriers: do not have pores that extend across membranes, but bind and transport specific molecules
pumps: require energy
primary active transport: coupled directly to ATP hydrolysis (ex. H+-ATPase)
secondary active transport: uses proton motive force (stored energy created by H+ gradient), symport (same direction) or antiport (opposite direction) (ex. sulfur moved into cell with H+)
What is the difference between symports and antiports?
symport (same direction) or antiport (opposite direction)
Where are the phloem and xylem located within a plant?
phloem generally found between spokes of central xylem on the outer side in vascular bundles
in plants with secondary growth, phloem is in the inner bark
What are the major cell types in phloem and what are their identifiable anatomical features?
sieve elements conduct sugars and organic compounds
sieve tube elements in angiosperms (highly differentiated) compared to sieve cells in gymnosperms (relatively unspecialized)
lack nuclei, vacuoles, Golgi, ribosomes, microfilaments, microtubules
companion (because no RNA left in sieve element for metabolic functions) and parenchyma cells
no lignin, unlike xylem
sieve areas (gymno)/sieve plates (angio) contain pores that interconnect cells, in gymnos pores are obstructed by ER, angio if damage occurs: initially sealed by P-proteins, then deposition of callose (Sugar polymer) for long term fix
purpose of sieve plates: maintain pressure, create resistance to dissipation of pressure
What are plasmodesmata?
What is the difference between companion, intermediary, and transfer cells?
companion cells: associated with each sieve element by highly branching plasmodesmata, take over critical functions of sieve elements (protein synthesis and ATP supply), ordinary CC have chloroplasts and smooth inner cell wall, transfer cells have finger-like wall ingrowths for efficient solute transfer with sieve elements, intermediary cells have many connections to surrounding cells as opposed to just one like ordinary and transfer
What is the relationship between sources and sinks in terms of phloem transport?
defines direction of phloem transport (bidirectional)
sources: include exporting organs, mature leaves, storage roots (ie. excess solute)
sinks: include organs that do not produce or store enough photosynthetic product to meet their own needs (roots, tubers, developing fruits, immature leaves)
source and sink status of the same organ can change due to growth/ development
can use radioactive labelling to show sinks (Darker) that contain more labelled sucrose
physiological/anatomical changes in leaves from sink to source transition: plasmodesmatal closure, fewer plasmodesmata, reduced symplastic continuity
What is the pressure-flow model and how can it be explained using the water potential equation?
passive mechanism, bulk flow of phloem sap is driven by osmotically generated pressure gradient between source and sink
energy is required in sources and sinks for the synthesis and consumption of photosynthate, which is required for active phloem loading and unloading
energy is also required to maintain necessary cellular and anatomical structures
phloem loading at source and unloading at sink establish water potential difference
sieve plate pores must be totally unobstructed, limited ATP supply does not immediately stop phloem transport but some energy is required over longer periods (low temp has little effect), pressure gradient must be greater than resistance in sieve elements for bulk flow to occur
What processes are involved in phloem loading and unloading?
3 mechanisms generate high sugar concentration in sieve elements of source: photosynthesis in mesophyll, conversion of photoassimilate to sugars in intermediary cells, active membrane transport
pressure potential = water potential - solute potential
accumulation of sugars in sieve elements generates negative solute potential, causing decreased water potential, so water enters sieve elements and increases pressure potential
at sink: phloem unloading lowers sugar concentrations to generate positive pressure potential, as water potential rises above the water potential of xylem, water leaves the phloem in response to reduced turgor in sieve elements of the sink
phloem loading: sucrose moves from mesophyll to vicinity of sieve elements, sugars transported into sieve elements and companion cells, can occur via symplast, apoplast, or transverse
phloem unloading: predominated by symplastic because low sucrose concentration in sink cells, some apoplastic in fruits, seeds, storage organs (required due to lack of plasmodesmata between maternal and embryonic cells), no need for symplastic unloading with tubers because sucrose turned into starch which is insoluble
What is the role of the sucrose symporter in phloem transport?
more negative solute potential in sieve elements and companion cells than in mesophyll due to phloem loading of sucrose by active transport
ATP hydrolysis drives H+ against concentration gradient, then passive movement of H+ back to lower concentration brings sucrose with it (sucrose-H+ symporter)
sucrose concentration correlated with number of sucrose H+ symporter proteins in leaf
some symplastic pathways have been found in squash and melon that transport raffinose and stachyose in addition to sucrose
What is the polymer-trapping model?
how can diffusion-dependent symplastic loading account for selectivity and accumulation of sugars against a concentration gradient?
sucrose is converted to raffinose and stachyose in intermediary cells, which diffuse into sieve elements
larger size compared to sucrose prevents sugars from diffusing back into mesophyll cells
What is allocation and partitioning in terms of phloem transport?
allocation: regulation of distribution of fixed carbon into different metabolic processes, synthesis of storage compounds, metabolic utilization, synthesis of transport compounds
partitioning: differential distribution of photosynthates within a plant, turgor pressure could be means of communication between sources/sinks, hormones, mineral nutrients, sugars are messengers, important for maximizing crop yield
