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Plant Phys > Translocation into the Phloem > Flashcards

Translocation into the Phloem Flashcards

1
Q

Phloem function

A

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

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2
Q

What is phloem translocation defined by

A

NOT by gravity
sap is translocated from source to sink

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3
Q

What sources are included in the translocation of sap

A

exporting organs, mature leaves that produce excess photosynthate, and storage organs during exporting phase of development

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4
Q

What sinks are included in the translocation of sap

A

all non-photosynthetic organs and organs that do not produce enough photosynthetic products to support their own growth and storage

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5
Q

phloem loading

A

how photosynthate enters the phloem, it is a general term for a variety of mechanisms for sugar uptake in the phloem

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6
Q

phloem unloading

A

how photosynthate exits the phloem after arriving at the sinks

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7
Q

Examples of sources and sinks

A

sources = potato
sink = corn and potato?

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8
Q

Where is the phloem found

A

on the outer side of both the primary and secondary vascular tissues. the phloem makes up the inner bark in secondary growth

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9
Q

Define sieve elements function

A

cells of the phloem that conduct sugars and other organic materials throughout the plant

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10
Q

In addition to sieve elements, what others cells does the phloem contain

A
  1. companion cells
  2. parenchyma cells
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11
Q

what is the small veins of leaves and primary vascular bundles in stems often surrounded by

A

bundle sheath

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12
Q

What are sieve elements made up of

A

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

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13
Q

What is the prominent feature of sieve elements

A

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

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14
Q

P-proteins

A

structural proteins that seal off sieve elements by plugging the sieve plate pores

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15
Q

What is a longer term solution to sieve tube damage

A

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

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16
Q

Define companion cells function

A

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

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17
Q

In the sieve tubes there is a close functional relationship between what cells

A

numerous plasmodesmata and companion cells

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18
Q

What is the most abundant substance in the phloem

A

water

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19
Q

what is dissolved in the water in the phloem sap and what is the most commonly transported substance

A

carbohydrates, AAs, hormones, inorganic ions, RNAs, proteins, and secondary compounds
sucrose is the most commonly transported carbohydrate

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20
Q

What form are sugars translocated in

A

non-reducing form (less reactive)

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21
Q

nitrogen is found in the phloem largely in ____

A

Amino Acids

22
Q

_____ is thought to occur at least partly in the phloem

A

long distance transport of plant hormones

23
Q

_____ are sequestered in vacuoles of cells and have no direct access of the phloem

A

hexoses (glucose and fructose)

24
Q

sap includes proteins which may have what related function

A

functions related to stress and defense responses

25
Q

The rate of movement of materials in the sieve elements can be expressed in what two ways:

A
  1. velocity = linear distance travelled per unit time
    OR
  2. mass transfer rate = quantity of material passing through a given cross section of phloem or sieve elements per unit of time
26
Q

pressure-flow model

A

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

27
Q

Explain the pressure gradient in and through the phloem in source tissues

A

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.

28
Q

Explain pressure gradient in sink tissues/unloading

A

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

29
Q

How does phloem sap move

A

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

30
Q

What are the 3 transport steps involved in the movement of photosynthates from the site of photosynthesis to the phloem

A
  1. triose phosphates are transported from the chloroplast to the cytosol (where converted to sucrose)
  2. sucrose moves from producing cells, to cells closer to the sieve elements in small veins
  3. sugars are loaded into companion cells (sieve-element-companion cell complex) for export
31
Q

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

A

symplastic
apoplastic or symplastic

32
Q

sugars enter the apoplast near the ____ and are _____ transported from apoplast to sieve elements and companion cells by energy driven ____ transporter

A

sieve element companion cell complex
actively
selective

33
Q

What are the special kind of companion cells called in some plants

A

intermediary cells
with many plasmodesmata leading into minor veins

34
Q

Function of intermediary cells

A

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

35
Q

What are the 3 reverse events that take place in sink tissues and not sources

A
  1. phloem unloading = sugars leave sieve elements of sink tissues
  2. short distance transport
  3. storage and metabolism in sink cells
36
Q

unloading and short distance pathways are usually what kind of transport

A

symplastic

37
Q

Import into sinks is dependent on what

A

energy

38
Q

what are transport sugars used for

A

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

39
Q

leaves begin their development as ___ organs and at what point transition

A

sink
transition from sink to source when 25% expanded

40
Q

the ending of import and initiation of export are _____ events with what 3 necessary conditions

A

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

41
Q

Define allocation

A

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

42
Q

Define partitioning

A

differential distribution of photosynthates within the plant (to various sinks). affected by availability of photosynthates from allocation

43
Q

what are the 3 possible uses for the carbon fixed in a source cell

A
  1. storage = synthesis of storage = starch synthesized in chloroplasts is the primary storage form in the night
  2. metabolic utilization = fixed carbon can be used to meet energy needs of the cell or making building blocks
  3. transport = synthesis of transport = fixed carbon can be incorporated into transport sugars
44
Q

partitioning determines _____ which must be a balance between ____ and ____ so the plant can respond to challenges of a _____

A

patterns of growth
shott growth and root growth
variable environment

45
Q

Define harvest index and how has it changed over the years

A

ratio of economical yield (edible grain) to total above ground biomass
increased due to efforts of plant breeders

46
Q

sources leaves regulate allocation between what 3 things

A
  1. regeneration of intermediates of calvin benson cycle
  2. starch synthesis
  3. sucrose synthesis (and distribution of sucrose)
47
Q

Sink strength depends on what 2 factors

A
  1. sink size
  2. sink activity
48
Q

how does short term vs long term changes affect sinks in response to altered conditions

A

short term = alter distribution of photosynthate among different sinks
long term = changes in source metabolism and alter amount of photosynthate available for transport

49
Q

what does carbohydrate depletion enhance vs abundant carbon

A

depletion = expression of genes for photosynthesis, reserve mobilization, and export processes
abundant = resources favour expression of genes for storage and utilization

50
Q

what is the role of plant hormones in regulating the source sink relationships

A

affect photosynthate portioning in part by controlling sink growth, leaf senescence, and other developmental processes

Plant Phys (15 decks)

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