Plant Physiology Ch. 11 Phloem Transport | Quizlet

For review, what is the function of the xylem?

the tissue that transports water and minerals from the root system to the aerial portions of the plant

For review, what is the function of the phloem?

the tissue that translocates the products of photosynthesis -- sugars -- from mature leaves to areas of growth and storage, including the roots

The cells of the phloem that conduct sugars and other organic materials throughout the plant are called..

sieve elements

Sieve element is a comprehensive term that includes both the highly differentiated ___________ of the __________ and the relatively unspecialized __________ of ___________

sieve tube elements; angiosperm; sieve cells; gymnosperms

The phloem tissues contains what kind of cells?

companion cells and parenchyma cells

________ is translocated in the phloem sieve tube elements

sugar

Mature sieve elements are living cells specialized for...

translocation

What are some characteristics of sieve tube elements? (i.e what are they lacking)

- they lose their nuclei and tonoplast during development

- microfilaments, microtubules golgi bodies, and ribosomes are generally absent from mature cells

- modified mitochondria, plastids, and smooth ER

- nonlignified walls

How are damaged sieve elements sealed off?

- short term mechanisms involve sap proteins

- long-term mechanisms involve closing sieve plate pores with callose, a glucose polymer

The main phloem proteins involved in sealing damaged sieve elements are structural proteins called..

P-proteins

- absent in gymnosperms

What is a longer-term solution to sieve tube damage?

production of the glucose polymer, callose

Each sieve tube element is usually associated with one or more _________

companion cells

What role do companion cells play?

they play a role in the transport of photosynthetic products from producing cells in mature leaves to sieve elements in the minor veins of the leaf

- take over some of the critical metabolic functions, such as protein synthesis, that are reduced or lost during differentiation of the sieve elements

What are the 3 types of companion cells?

• ordinary companion cells

• transfer cells

• intermediary cells

all have dense cytoplasm and abundant mitochondria

___________ have chloroplasts with well-developed thylakoids and a cell wall with a smooth inner surface

ordinary companion cells

_________ are similar to ordinary companion cells, except for the development of fingerlike wall ingrowths, particularly on the cell walls that face away from source sieve elements

transfer cells

____________ appear to be well suited for taking up solutes via cytoplasmic connections.

intermediary cells

What is the most characteristic feature of intermediary cells?

the presence of many plasmodesmatal connections to surrounding cells, having many small vacuoles, and poorly developed thylakoids

Where are transfer cells found in plants?

where transport sugars enter the apoplast during the movement of sugars from mesophyll cells to sieve tube elements

What do transfer cells do?

transport sugars from the apoplast to the symplast of the sieve elements and companion cells in the source

What do intermediary cells do?

they function in symplastic transport of sugars from mesophyll cells to sieve elements

What is the function of ordinary companion cells?

they can function in either symplastic or apoplastic short-distance transport in source leaves, depending in part on plasmodesmatal frequencies

Sap is translocated from area of supply called __________ to areas of metabolism or storage called __________

sources; sinks

What do sources include?

exporting organs, typically mature leaves that are capable of producing photosynthate in excess of their own needs

- photosynthate refers to products of photosynthesis

What do sinks include?

all nonphotosynthetic organs of the plant and organs that do not produce enough photosynthetic products to support their own growth or storage needs.

ex: roots, tubers, developing fruits, and immature leaves

What is the pattern of transport in the phloem?

source-to-sink movement

What is the most abundant substance in the phloem?

water

What is the dissolved in the water in the phloem?

translocated solutes -- carbohydrates, amino acids, hormones, some inorganic ions, RNAs, proteins, secondary compounds, etc

What is the most common sugar transported in the sieve elements?

sucrose

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

as velocity or as mass transfer rate

The most widely accepted mechanism of phloem transport in angiosperms is...

the pressure-flow model

What is the pressure flow mode?

It explains phloem translocation as a flow of solution driven by an osmotically generated pressure gradient between source and sink

The movement of photosynthate (products of photosynthesis) into sieve elements is called...

phloem loading

The movement of photosynthate (products of photosynthesis) from sieve elements to sink cells is called...

phloem unloading

What establishes the pressure gradient of the pressure-flow model?

phloem loading at the source and phloem unloading at the sink

What are the 3 different mechanisms that exist to generate high concentrations of sugars in the sieve elements of the source?

- photosynthetic metabolism in the mesophyll

- conversion of photoassimilate to transport sugars in intermediary cells (polymer trapping)

- active membrane transport

In source tissues, an accumulation of sugars in the sieve elements generates a...

low negative solute potential and causes a steep drop in water potential

According to the pressure-flow model, movement in the translocation pathway is driven by..

transport of solutes and water into source sieve elements and out of sink sieve elements

There is no ________________ in single sieve tube elements, and solutes and water move at the _________ velocity

bidirectional transport; same

The energy requirement for transport through the phloem pathway is __________ in herbaceous plants

small

In the process called ______________, sugars are transported into the sieve elements and companion cells

phloem loading

In the phloem loading, once inside the sieve elements, sucrose and other solutes are translocated away from the source, a process known as..

export

Translocation through the vascular system to the sink is referred to as..

long-distance transport

Sucrose moving from producing cells in the mesophyll to cells in the vicinity of the sieve elements in the smallest veins of the leaf is referred to as..

short-distance transport

Phloem loading can occur via the..

apoplast or symplast

What is the process for apoplastic loading?

The sugars enter the apoplast near the sieve element - companion cell complex. Sugars are then actively transported from the apoplast into the sieve elements and companion cells by an energy-driven, selective transport located in the plasma membranes of these cells.

Apoplastic phloem loading leads to three basic predictions:

1. transported sugars should be found in the apoplast

2. in experiments in which sugars are supplied to the apoplast, the exogenously supplied sugars should accumulate in sieve elements and companion cells

3. inhibition of sugar efflux from the phloem parenchyma or of uptake from the apoplast should result in inhibition of export from the leaf

Sucrose uptake in the apoplastic pathway requires...

metabolic energy

The fact that sucrose is at a higher concentration in the sieve element-companion cell complex than in surrounding cells indicates that...

sucrose is actively transported against its chemical-potential gradient

Phloem loading in the apoplastic pathway involves a..

sucrose-H+ symporter

A __________ pathway has become evident in species that transport raffinose and stachyose, in addition to sucrose

symplastic

What is the polymer-trapping model?

the model states that the sucrose synthesized in the mesophyll diffuses from the bundle sheath cells into the intermediary cells through the abundant plasmodesmata that connect the two cell types

In the intermediary cells, raffinose is sythesized from sucrose and galactinol, thus maintaining the diffusion gradient for sucrose. Because of its larger size, raffinose is not able to diffuse back into the mesophyll

Raffinose is able to diffuse into the sieve elements. As a result, the concentration of transport sugar rises in the intermediary cells and the sieve elements.

The polymer-trapping model makes three predictions:

1. Sucrose should be more concentrated in the mesophyll than in the intermediary cells

2. The enzymes for raffinose and stachyose synthesis should be preferentially located in the intermediary cells

3. The plasmodesmata linking the bundle sheath cells and the intermediary cells should exclude molecules larger than sucrose. Plasmodesmata between the intermediary cells and sieve elements must be wider to allow passage of raffinose and stachyose

What are the steps that are involved in the import of sugars into sink cells?

1. phloem unloading

2. short-distance transport

3. storage and metabolism

In ______ organs, sugars move from the sieve elements to the cells that store or metabolize them

sink

A transition from sink to source status occurs later in development, when the leaf is approximately ______% expanded, and it is usually completed when the leaf is ______ to ______% expanded.

25; 40 to 50

What does Figure 11.19 represent?

Autoradiographs of a leaf of summer squash.

A) The entire leaf is a sink, importing sugar from the source leaf

B-D) The base is still a sink. As the tip of the leaf loses the ability to unload and stops importing sugar (as shown by the loss of black accumulations) it gains the ability to load and to export sugar

What is an apoplast"?

collective cell wall sapce

What is symplast?

the collective cytoplasm

A ___________ pathway has become evident in species that transport _______ and _________

symplastic; raffinose; stachyose

The driving force for long-distance PHLOEM transport is

Pressure created by accumulated solutes and osmosis

Briefly explain apoplastic phloem loading (loading from the cell-wall space; pictures rarely hurt)

a. phloem loading from the cell wall space

b. sucrose - H+ symporters use PMF to accumulate sucrose into the companion cells

c. accumulating sucrose in the companion cell and needs energy

d. sucrose goes against its concentration gradient, and H+ goes down the concentration gradient

Briefly explain symplastic phloem loading

- sucrose cannot escape

- plasmodesmata connect mesophyll all the way to the sieve elements

- accumulation of sugar is by converting it into larger saccharides that can't diffuse back through the plasmodesmata

- does not require ATPase

What is true of phloem sieve tubes

They are membrane lined

Which of the following is common to both cellular respiration in mitochondria and the light reactions of photosynthesis in plastids?

oxygen is one of the byproducts

Classify the following as either a source tissue or a sink tissue (use the words "source" or "sink only, without quotation marks).

Cotyledon in a developing seed

Cotyledon in a germinating seed

Mature leaf

Immature leaf

Mature flower

Immature flower

- sink

- source

- source

- sink

- source

- sink

A tomato fruit growing on a vine will receive most of its nutrients and water from

The mature leaves closest to it

What is true of phloem companion cells

- They are cytoplasmically dense

- they are closely associated with sieve elements

If you analyzed the phloem sap of a plant and found high concentrations of ________________, the plant most likely phloem loads through the symplasm

raffinose family oligosaccharides

If I had a compound that specifically inhibited the activity of plasma membrane ATPases, and if I observed that this compound resulted in an immediate cessation of phloem transport when applied to a plant, what type phloem loading is most likely being employed?

apoplastic phloem loading

Phloem transport of sucrose is expected to remain fairly constant during diurnal cycles (day/night cycles) because

Sucrose and starch are produced during the day, and starch is converted to sucrose at night

What is the driving force for phloem transport?

Pressure created by accumlated solutes and osmosis

What are examples of source?

- cotyledon in germinating seed

- mature leaf

- mature flower

What are examples of sink?

- cotyledon in developing seed

- immature leaf

- immature flower

Fruit and young leaves get nutrients from the ________ and go in all directioins

phloem