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Biology IB HL > 9.2 Phloem Transport > Flashcards

9.2 Phloem Transport Flashcards

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1
Q

What is translocation?

A

Translocation is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks

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2
Q

What is the source?

A

The source is where the organic compounds are synthesised – this is the photosynthetic tissues (leaves)

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3
Q

What is the sink?

A

The sink is where the compounds are delivered to for use or storage – this includes roots, fruits and seeds

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4
Q

Where does translocation occur?

A

Organic compounds are transported from sources to sinks via a vascular tube system called the phloem

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5
Q

How are sugars transported?

A

Sugars are principally transported as sucrose (disaccharide), because it is soluble but metabolically inert

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6
Q

What liquid is found in the phloem?

A

The nutrient-rich, viscous fluid of the phloem is called plant sap

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7
Q

What are the phloem sieve tubes mainly composed of?

A

Phloem sieve tubes are primarily composed of two main types of cells
– sieve element cells and companion cells

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8
Q

What other cells apart from the main 2 may be found in the phloem and for what purpose?

A

The phloem also contains schlerenchymal and parenchymal cells which fill additional spaces and provide support

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9
Q

What are sieve elements?

A

Sieve elements are long and narrow cells that are connected together to form the sieve tube

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10
Q

How are sieve elements connected?

A

Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells

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11
Q

Do sieve elements have nuclei?

A

NO

Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials

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12
Q

Describe the cell walls in sieve elements

A

The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow

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13
Q

What are companion cells?

A

Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink

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14
Q

How do companion cells increase SA:VOL ratio?

A

Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange

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15
Q

What type of organelles do companion cells have a large amount of?

A

Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink

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16
Q

What type of proteins do companion cells have many of?

A

Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube

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17
Q

Can sieve elements “survive” without companion cells? Why/not?

A

NO
Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell

This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)

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18
Q

How do companion cells deliver necessary substances to the sieve elements?

A

Plasmodesmata exist between sieve elements and companion cells in relatively large numbers

These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites

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19
Q

How are the xylem and phloem grouped?

A

Xylem and phloem vessels are grouped into bundles that extend from the roots to the shoots in vascular plants

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20
Q

What does the arrangement of vascular bundles depend on?

A

Differences in distribution and arrangement exist between plant types (e.g. monocotyledons vs dicotyledons)

  • whether in root or stem
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21
Q

How can the xylem and phloem usually be differentiated?

A

Xylem and phloem vessels can usually be differentiated by the diameter of their cavity (xylem have larger cavities)

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22
Q

How does the stele look like in monocotyledons? in the roots

A

In monocotyledons, the stele is large and vessels will form a radiating circle around the central pith

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23
Q

Where will the xylem and phloem be located in the roots of monocotyledons?

A

Xylem vessels will be located more internally and phloem vessels will be located more externally

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24
Q

How does the stele look like in dicotyledons in the root?

A

In dicotyledons, the stele is very small

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25
Q

Where will the xylem and phloem be located in the roots of dicotyledons?

A

the xylem is located centrally with the phloem surrounding it

Xylem vessels may form a cross-like shape (‘X’ for xylem), while the phloem is situated in the surrounding gaps

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26
Q

How are the vascular bundles arranged in monocotyledons?

A

In monocotyledons, the vascular bundles are found in a scattered arrangement throughout the stem

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27
Q

How are phloem vessels located in the stem of monocotyeldons?

A

Phloem vessels will be positioned externally (towards outside of stem) – remember: phloem = outside

28
Q

How are the vascular bundles arranged in dicotyledons?

A

In dicotyledons, the vascular bundles are arranged in a circle around the centre of the stem (pith)

29
Q

How are vessels located in the stem of dicotyledons?

A

Phloem and xylem vessels will be separated by the cambium (xylem on inside ; phloem on outside)

30
Q

What mechanism is used to transport organic compounds into the phloem sieve tubes?

A

Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells

31
Q

In what two ways can organic materials be transported into the phloem sieve tubes?

A

symplastic loading

apoplastic loading

32
Q

What does symplastic loading involve?

A

Materials can pass into the sieve tube via interconnecting plasmodesmata

33
Q

What does apoplastic loading involve?

A

Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)

34
Q

What does apoplastic loading require?

A

Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure

35
Q
  1. What protein is used to help with the transport?

apoplastic loading

A

Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)

36
Q
  1. What type of gradient is created?

apoplastic loading

A

The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient

37
Q
  1. How is the sucrose then transported?

apoplastic loading

A

Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement

38
Q
  1. What does apoplastic loading result in?
A

This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source

39
Q
  1. At the source, what does the loading of sucrose do to the sap solution?

mass flow

A

The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic

40
Q
  1. What does the hypertonic sap cause?

mass flow at source

A

This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)

41
Q
  1. What increases due to the flow of water into the phloem?

mass flow at source

A

Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase

42
Q
  1. What does the increase in hydrostatic pressure cause?

mass flow at source

A

This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)

43
Q

5 What do all these steps result in?

mass flow at source

A

Hence, the phloem transports solutes away from the source (and consequently towards the sink)

44
Q
  1. At the sink, what mechanism is used to transport organic molecules into the cells?

phloem u

A

The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)

45
Q
  1. What does the unloading cause (change to solution)?

phloem unloading at sink

A

This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)

46
Q
  1. What happens to the water?

phloem unloading at sink

A

Consequently, water is drawn out of the phloem and back into the xylem by osmosis

47
Q
  1. Why is the movement of water necessary?

phloem unloading at sink

A

This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source

48
Q
  1. What does the whole process result in?

phloem unloading at sink

A

Hence, phloem sap will always move from the source towards the sink

(but can occur in 2 ways i.e up and down plant)

49
Q

What is done to the organic molecules once they are transported into the sink?

A

When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles

50
Q

1 Why were aphids used to test translocation rate?

A

Aphids are a group of insects, belonging to the order Hemiptera, which feed primarily on sap extracted from phloem

51
Q

2 How do aphids extract sap?

translocation rate

A

Aphids possess a protruding mouthpiece (called a stylet), which pierces the plant’s sieve tube to allow sap to be extracted

52
Q

3 What helps the acids extract the sap?

translocation rate

A

The penetration of the stylet into the sieve tube is aided by digestive enzymes that soften the intervening tissue layers

53
Q

4 What will happen if the aphid’s stylet is severed?

translocation rate

A

If the stylet is severed, sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube

54
Q

4 How can aphids be used to measure translocation rate?

translocation rate

A

Aphids can be used to collect sap at various sites along a plant’s length and thus provide a measure of phloem transport rates

55
Q

5 In what conditions is the plant (that will be used for the experiment) grown in?

translocation rate

A

A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide

56
Q

6 What will the leaves do to the CO2?

translocation rate

A

The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem

57
Q

7 Where are the aphids placed on the plant?

translocation rate

A

Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap

58
Q

8 What is done once the aphids start feeding?

translocation rate

A

Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions

59
Q

8 What is done with the sap?

translocation rate

A

The sap is then analysed for the presence of radioactively-labelled sugars

60
Q

8 How is translocation rate calculated?

translocation rate

A

The rate of phloem transport (translocation rate) can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length

61
Q

What is the main factor affecting the translocation rate?

A

The rate of phloem transport will principally be determined by the concentration of dissolved sugars in the phloem

62
Q

What can the concentration of dissolved sugars be affected by? (4 simple)

A
  1. rate of photosynthesis
  2. Rate of cellular respiration
  3. rate of transpiration
  4. diameter of sieve tubes
63
Q

How can the rate of photosynthesis be affected?

A

is affected by light intensity, CO2 concentration, temperature, etc

64
Q

How can the rate of cellular be affected?

A

this may be affected by any factor which physically stresses the plant

65
Q

How will the rate of transpiration affect the concentration of dissolved sugars?

A

this will potentially determine how much water enters the phloem

66
Q

How will the diameter of the sieve tubes affect the concentration of dissolved sugars?

A

will affect the hydrostatic pressure and may differ between plant species

Biology IB HL (56 decks)

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