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Biology > 9.2 Transport in the phloem of plants > Flashcards

9.2 Transport in the phloem of plants Flashcards

1
Q

What type of tissue is found throughout plants?

A

Phloem tissue

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2
Q

Where can phloem tissue be found specifically? (3)

A
  • stems
  • roots
  • leaves
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3
Q

What is the phloem composed of?

A

Sieve tubes

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4
Q

What are sieve tubes composed of?

A

columns of specialized cells called sieve tube cells

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5
Q

What are individual sieve tube cells separated by?

A

Perforated walls called sieve plates

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6
Q

What are sieve tube cells closely associated with and how?

A

Companion cells

due to the fact that the sieve tube cell and its companion cell share the same parent cell

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7
Q

What function do the companion cells perform? (2)

A
  • many of the genetic and metabolic functions of the sieve tube cell
  • maintain the viability of the sieve tube cell
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8
Q

What is the structures and functions of companion cells? (3)

A
  • Abundant mitochondria - support active transport of sucrose (figure 6)
  • infolding of the plasma membrane - increases the phloem loading capacity using the apoplastic route
  • Plasmodesmata - connect the cytoplasm of the companion cells with the sieve tube cells, have a larger diameter than plasmodesmata found in other parts of the plant to accomodate the movement of oligosaccharides and genetic elements between the two cells
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9
Q

What are the structures and functions of sieve tube cells? (3)

A
  • presence of active transport proteins or enzyme activity - produce the oligosaccharides from the accumulation of sucrose in the sieve tube element-companion cell pair
  • rigid cell walls of the sieve tube cells - allow for the establishment of the pressure necessary to achieve the flow of phloem in the sieve tube cell
  • sieve plates - seperates individual sieve tube cells with remnants of cell walls that separated the cells. The perforated walls in combination with the reduced cytoplasm means that the resistance to the flow of phloem sap will be lower
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10
Q

What does the phloem transport?

A

organic compounds throughout the plant
* products of photosynthesis (mainly sucrose sugar), in solution, from leaves to parts of the plant that do not photosynthesise, such as roots, flowers and growing shoots

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11
Q

What is translocation?

A

the transport of organic solutes (assimilates) in a plant
* Phloem sap flows through the pores in the sieve plate

Assimilates are substances made by the plant, using substances absorbed from the environment. They include sugars (mainly transported as sucrose) and amino acids

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12
Q

Why is glucose transported as surcrose?

A

Glucose is a reducing sugar so is reactive and would be used up in chemical reactions

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13
Q

What parts of the plant does the phloem link together?

A

Links parts of the plant that need a supply of sugars and other solutes such as amino acids to other parts that have a surplus

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14
Q

What is a source in a plant?

A

areas where sugars and amino acids are loaded into the phloem

A part of the plant that load sassimilates into the phloem sieve tubes

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15
Q

What is a sink in a plant?

A

where the sugars and amoino acids are unloaded and used

A part of the plant that removes assimilates from the phloem sieve tubes

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16
Q

What are the sources of plants? (4)

A

Sources
Photosynthetic tissues:
* Mature green leaves
* green stems
Storage organs that are unloading their stores:
* storage tissues in germinating seeds
* tap roots or tubers at the start of the growth season

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17
Q

What are the sinks of plants? (4)

A

Sink
Roots that are growing or absorbing mineral ions using energy from cell respiration (parts of the plant that are growing or developing food stores):
* Developing fruits
* developing seeds
* growing leaves
* developing tap roots or tubers

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18
Q

What is the ringing experiment?

A
  • two rings of bark were removed from an apple tree
  • the bark contains the phloem tissue
  • the effects on apple growth are clearly visible
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19
Q

What can sinks and sources sometimes do?

A

Sinks can become sources
Sources can become sinks

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20
Q

Since sinks can turn into sources, or vice versa, what must the tubes in phloem must be able to do?

A

must be able to transport biochemicals in either direction

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21
Q

How is the phloem and the blood system of animals different?

A

there are no valves or central pump in phloem

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22
Q

What are the similarities between transport in phloem and blood vessels?

A

In both systems
* a fluid flows inside the tubes because of pressure gradients
* Energy is needed to generate the pressures
* So the flow of blood and the movement of phloem sap are both active processes

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23
Q

What is the movement through xylem and phloem by?

A

mass transport
* all particles move together in the same direction, such as in the transpiration stream up a xylem

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24
Q

What is diffusion?

A

Diffusion is the result of the random motion of particles - particles move down their ceoncentration gradient and different particles may move in different directions

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25
Q

Why is the movement through xylem and phloem by mass transport rather than diffusion?

A

Mass flow is faster than diffusion and can move large amounts of fluid long distances, quickly

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26
Q

What are the features of sieve tube elements?

A
  • no nucleus
  • very little cytoplasm
  • very thin cell walls
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27
Q

How do sieve tube elements of phloem compare to the vascular elements of xylem?

A
  • sieve tube elements are living unlike vascular elements of xylem
  • but sieve tube elements have reduced quantities of cytoplasm and no nucleus
28
Q

Why do sieve cells need to be living?

A

they depend on the membrane to help maintain the sucrose and organic molecule concentration that has been established by active transport

29
Q

What are sieve tube elments?

A

the cells that make up phloem vessels

30
Q

What do sieve plates do?

A
  • keep the sieve tubes open
  • block movement of sap through phloem if plant is infected
31
Q

What is sap?

A

dissolved sucrose and amino acid in water

32
Q

What is the most prevalent solute in phloem sap?

A

Sucrose

33
Q

How is sucrose a good transport form of carbohydrate?

A
  • it is not as readily available for plant tissues to metabolize directly in respiration
  • it will not be metabolized during transport
34
Q

What are the features of companion cells?

A
  • Large nucleus
  • many mitochondria to provide ATP for active transport
  • Plasmodesmata connect the cytoplasm of sive tube elements and companion cells
35
Q

Why does phloem not have walls impregnated with lignin?

A
  • Lignin would kill the phloem
  • However the sieve tube elements need to be alive in order to have a partially permeable membrane that allow active loading of assimulates (sucrose and amino acid) into the phloem
36
Q

What are the differences of xylem and phloem (6)

A
37
Q

What is phloem loading?

A

the mechanism by which the plants bring sugars into the phloem

38
Q

Does phloem loading differ for plants?

A

Yes

39
Q

What are the two types of route for phloem loading?

A
  • apoplast route
  • symplast route
40
Q

What is the apoplast route in phloem loading?

A
  • significant amount of sugar travels through cell walls from mesophyll cells to the cell walls of companion cells
  • sometimes sieve cells, where a sucrose transport protein then actively transports the sugar in
41
Q

How is an concentration gradient of sucrose established?

A

by active transport

42
Q

What is the process of the movement of sucrose across a sieve tube membrane

A
  • H+ ions are actively trnasported out of the companion cell from surrounding tissues using ATP as an energy source
  • this results in the movement of sucrose molecules by facilitated diffusion and diffusion
  • the build-up of H+ then flows down its concentration gradient into the companion cells through a co-transport protein (only allow movement of the H+ ions into the cell if they are accompanied by sucrose molecules)
  • the energy released is used to carry sucrose into the companion cell-sieve tube complex
  • sucrose diffuses into sieve tube element through the plasmodesmata
43
Q

What happens as the concentration of sucrose in the companion cell sincreases?

A

surcrose can diffuse through the plasmodesmata into the sieve tube element

44
Q

What is the symplast route of phloem loading?

A
  • much of the sucrose travels between cells thorugh connections between cells called plasmodesmata
  • once the sucrose reaches the companion cell it is converted to an oligosaccharide to maintain the sucrose concentration gradient
45
Q

What is surcrose converted to maintain the sucrose concentration gradient?

A

Once it reaches the companion cells, it is converted to an oligosaccharide

46
Q

What is mass flow caused by?

A

A difference in hydrostatic pressure between the two ends of the phloem, which creates a pressure gradient

47
Q

What does sucrose entering the phloem at the source do to the water potential in the phloem?

A

It lowers the water potential

48
Q

As a result of sucrose entering the phloem at the source and lowering the water potential, where do the water molecules move?

A

water molecules move into the sieve tube element by osmosis from the surrounding tissues

this increases the hydryostatic pressure in the sieve tube at the source

49
Q

Where does the sap moves along the phloem?

A

from higher hydrostatic pressure at the source to a lower hydrostatic pressure at the sink

50
Q

What does the removal of sucrose from the sap increase, and so what?

A

increases the water potential so that water moves out of the sieve tube into the surrounding cells by osmosis

this lowers the hydrostatic pressure in the phloem at the sink

51
Q

Describe the process of mass flow in the phloem

A
  1. sucrose enter the phloem at the source
  2. lowers the water potential in the phloem
  3. water molecules move into the sieve tube element by osmosis from surrounding tissues
  4. this increases the hydrostatic pressure in the sieve tube at the source
  5. sap moves along the phloem from higher hydrostatic pressure at the source to a lower hydrostatic pressure at the sink
  6. the removal of sucrose from the sap increases the water potential
  7. so that water moves out of the sieve tube into the surrounding cells by osmosis
  8. this lowers the hydrostatic pressure in the phloem at the sink
52
Q

How is water drawn into the companion cell through osmosis?

A

the build-up of sucrose and other carbohydrates

53
Q

What 2 things contribute to a build-up of pressure in companion cells?

A

rigid cell walls combined with the incompressibility of water result in a build-up of pressure

54
Q

Where do water flow in terms of pressure?

A

form area of high pressure to an area of low pressure

55
Q

What happens to sucrose and water at the sink end?

A

sucrose is withdrawn from the phloem and the loss of solute causes a reduction in osmotic pressure and the water that carried the solute to the sink is then drawn back in to the transpiration stream in the xylem

56
Q

What is the sucrose withdrawn from the phloem used as?

A

either as an energy source (i.e. growth) or converted to starch

57
Q

What are the 3 functions of phloem?

A
  • loading of carbohydrates
  • transport of the carbohydrates (sometimes over long distances)
  • unloading of the carbohydrates at sinks
58
Q

What are two ways in which we can measure phloem transport rates?

A
  • aphid stylets
  • radioactively-labelled carbon dioxide
59
Q

What animals consume phloem sap as part of their diet?

A

insects from a group called the Hemiptera which includes aphids, whitefly, mealybugs and psyllids

60
Q

How do Aphids consume phloem sap?

A

Aphids penetrate plant tissues to reach the phloem using mouth parts called stylets

61
Q

How can phloem transport rates be measured using aphids?

A
  • if the aphid is anaesthetized and the stylet severed while they are feeding
  • the phloem will continue to flow out of the stylet
  • both the rate of flow and the composition of the sap can be analysed
  • the closer the stylet is to the sink, the slower the rate at which the phloem sap will come out.
62
Q

What is an radioactive isotope of carbon?

A

Carbon-14

63
Q

How can radioactively-labelled carbon dioxide we used to measure phloem transport rates?

A
  1. Radioactively-labelled carbon within carbon dioxide can be fixed by plants during photosynthesis
  2. it will release radiation that can be detected either using film or radiation detectors
  3. as the carbon is metabolized, it will be found in different molecules within the plant
  4. both the formation and movement of radioactive molecules can be traced
  5. A Geiger counter device can be used to measure the radiation levels in a crop of sunflowers
63
Q

How can radioactively-labelled carbon dioxide we used to measure phloem transport rates?

A
  1. Radioactively-labelled carbon within carbon dioxide can be fixed by plants during photosynthesis
  2. it will release radiation that can be detected either using film or radiation detectors
  3. as the carbon is metabolized, it will be found in different molecules within the plant
  4. both the formation and movement of radioactive molecules can be traced
  5. A Geiger counter device can be used to measure the radiation levels in a crop of sunflowers
64
Q

What are 2 things to look out for when comparing xylem and phloem in light micrographs?

A
  • xylem cells are generally larger than phloem cells
  • wihitin one vascular bundle, phloem cells tend to be closer to the outside of the plant in stems and roots

Biology (57 decks)

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