Translocation into the Phloem Flashcards
Phloem function
is the tissue that translocates the products of photosynthesis from mature leaves to areas of growth and storage such as the roots. Also transmits signals in the form of regulatory molecules and redistributes water throughout the plant
What is phloem translocation defined by
NOT by gravity
sap is translocated from source to sink
What sources are included in the translocation of sap
exporting organs, mature leaves that produce excess photosynthate, and storage organs during exporting phase of development
What sinks are included in the translocation of sap
all non-photosynthetic organs and organs that do not produce enough photosynthetic products to support their own growth and storage
phloem loading
how photosynthate enters the phloem, it is a general term for a variety of mechanisms for sugar uptake in the phloem
phloem unloading
how photosynthate exits the phloem after arriving at the sinks
Examples of sources and sinks
sources = potato
sink = corn and potato?
Where is the phloem found
on the outer side of both the primary and secondary vascular tissues. the phloem makes up the inner bark in secondary growth
Define sieve elements function
cells of the phloem that conduct sugars and other organic materials throughout the plant
In addition to sieve elements, what others cells does the phloem contain
- companion cells
- parenchyma cells
what is the small veins of leaves and primary vascular bundles in stems often surrounded by
bundle sheath
What are sieve elements made up of
are living cells specialized for translocation, that lack a nucleus and tonoplast and other organelles during development. They retain a modified mitochondria, plastids, and smooth endoplasmic reticulum. Walls are nonlignified although some can be thickened
What is the prominent feature of sieve elements
the large pores in their cell walls where pores interconnect the conducting cells. sieve pores can differentiate into sieve plates that are found on the end walls of sieve tube elements
P-proteins
structural proteins that seal off sieve elements by plugging the sieve plate pores
What is a longer term solution to sieve tube damage
the production of callose, a glucose polymer which is synthesized in response to damage and other stresses. callose efficiently seals off the damaged elements. sealing off plays a key role to resistance of damage from feeding insects
Define companion cells function
play a role in transport of photosynthetic products from producing cells in mature leaves to the sieve elements in the minor veins of leaves. there is 1+ in each sieve tube
In the sieve tubes there is a close functional relationship between what cells
numerous plasmodesmata and companion cells
What is the most abundant substance in the phloem
water
what is dissolved in the water in the phloem sap and what is the most commonly transported substance
carbohydrates, AAs, hormones, inorganic ions, RNAs, proteins, and secondary compounds
sucrose is the most commonly transported carbohydrate
What form are sugars translocated in
non-reducing form (less reactive)
nitrogen is found in the phloem largely in ____
Amino Acids
_____ is thought to occur at least partly in the phloem
long distance transport of plant hormones
_____ are sequestered in vacuoles of cells and have no direct access of the phloem
hexoses (glucose and fructose)
sap includes proteins which may have what related function
functions related to stress and defense responses
The rate of movement of materials in the sieve elements can be expressed in what two ways:
- velocity = linear distance travelled per unit time
OR - mass transfer rate = quantity of material passing through a given cross section of phloem or sieve elements per unit of time
pressure-flow model
widely accepted mechanism of phloem translocation in angiosperms
= a flow of solution driven by an osmotically generated pressure gradient between source and sink. the phloem loading and unloading establishes the pressure gradient for passive long distance bulk flow
Explain the pressure gradient in and through the phloem in source tissues
in source tissues, accumulation of sugars in the sieve generates a negative solute potential and causes a drop in water potential too. In response water enters the sieve elements and causes turgor pressure to increase.
Explain pressure gradient in sink tissues/unloading
leads to a lower sugar concentration in sieve elements generating a less negative solute potential in the sieve elements, water flows out and a lower turgor pressure
How does phloem sap move
moves by mass flow and not by osmosis, so no membranes are crossed during transport from one sieve element to another and solutes move at the same rate as water
What are the 3 transport steps involved in the movement of photosynthates from the site of photosynthesis to the phloem
- triose phosphates are transported from the chloroplast to the cytosol (where converted to sucrose)
- sucrose moves from producing cells, to cells closer to the sieve elements in small veins
- sugars are loaded into companion cells (sieve-element-companion cell complex) for export
What kind of transport occurs between the initial short-distance between mesophyll cells
And loading into the sieve element companion cells complex can be what kind of transport
symplastic
apoplastic or symplastic
sugars enter the apoplast near the ____ and are _____ transported from apoplast to sieve elements and companion cells by energy driven ____ transporter
sieve element companion cell complex
actively
selective
What are the special kind of companion cells called in some plants
intermediary cells
with many plasmodesmata leading into minor veins
Function of intermediary cells
synthesize (polymers) raffinose and stachyose from the transported sucrose and galactinol
they can only diffuse back into sieve elements bc of larger plasmodesmata but not back into bundle sheaths
What are the 3 reverse events that take place in sink tissues and not sources
- phloem unloading = sugars leave sieve elements of sink tissues
- short distance transport
- storage and metabolism in sink cells
unloading and short distance pathways are usually what kind of transport
symplastic
Import into sinks is dependent on what
energy
what are transport sugars used for
substrates for respiration and are metabolized into compounds needed for growth, where they move from high concentration in sieve elements to low in sink cells therefore require energy in the sink organs for respiration and biosynthesis
leaves begin their development as ___ organs and at what point transition
sink
transition from sink to source when 25% expanded
the ending of import and initiation of export are _____ events with what 3 necessary conditions
independent
1. leaf is synthesizing photosynthates in sufficient quantity that some is available for export
2. minor veins responsible for loading have matured
3. sucrose-H+ symporter is expressed in the plasma membrane of sieve element companion cell complex
Define allocation
regulation of the distribution of fixed carbon into various metabolic pathways. the balance among processes determines how much photosynthate is available for export to other parts
Define partitioning
differential distribution of photosynthates within the plant (to various sinks). affected by availability of photosynthates from allocation
what are the 3 possible uses for the carbon fixed in a source cell
- storage = synthesis of storage = starch synthesized in chloroplasts is the primary storage form in the night
- metabolic utilization = fixed carbon can be used to meet energy needs of the cell or making building blocks
- transport = synthesis of transport = fixed carbon can be incorporated into transport sugars
partitioning determines _____ which must be a balance between ____ and ____ so the plant can respond to challenges of a _____
patterns of growth
shott growth and root growth
variable environment
Define harvest index and how has it changed over the years
ratio of economical yield (edible grain) to total above ground biomass
increased due to efforts of plant breeders
sources leaves regulate allocation between what 3 things
- regeneration of intermediates of calvin benson cycle
- starch synthesis
- sucrose synthesis (and distribution of sucrose)
Sink strength depends on what 2 factors
- sink size
- sink activity
how does short term vs long term changes affect sinks in response to altered conditions
short term = alter distribution of photosynthate among different sinks
long term = changes in source metabolism and alter amount of photosynthate available for transport
what does carbohydrate depletion enhance vs abundant carbon
depletion = expression of genes for photosynthesis, reserve mobilization, and export processes
abundant = resources favour expression of genes for storage and utilization
what is the role of plant hormones in regulating the source sink relationships
affect photosynthate portioning in part by controlling sink growth, leaf senescence, and other developmental processes
