9.2 Transport in the phloem of plants

Question 1

Diagram showing transport and exchange in plants

Answer 1

Question 2

What portion of plant cells perform photosynthesis?

Answer 2

A small portion

Question 3

How do the plant cells that perform photosynthesis get energy?

Answer 3

The cells that do, usually the mesophyll cells of the leaf, produce large enough quantities to supply their own needs and the needs of the rest of the plant.

Question 4

What is phloem?

Answer 4

A vascular tissue that has evolved to efficiently conduct sugar molecules from a source where they are present in abundance to a sink where they are needed because they are being used or stored.

Question 5

Phloem consists of columns of living cells called ___

Answer 5

Sieve tubes

Question 6

Each cell in a sieve tube is called a ___

Answer 6

Sieve element (or sieve tube element).

Question 7

What do sieve elements contain and how does this affect their needs?

Answer 7

-Limited cytoplasm with a few mitochondria and other organelles, but the nucleus, vacuole, cytoskeleton, and many other structures are broken down.

-Thus, sieve elements require ‘life support’ from companion cells that run alongside each element.

Question 8

How are sieve elements and companion cells connected?

Answer 8

By pores called plasmodesmata

Question 9

Diagram showing a sieve tube cell and companion cell connected by plasmodesma

Answer 9

Question 10

What are sieve elements largely filled with?

Answer 10

-Fluid called phloem sap, which is a combination of water, dissolved sucrose and other carbohydrates, amino acids, proteins, some types of minerals, and plant hormones.

-The sap is contained in a single plasma membrane that extends throughout the sieve tube, from one cell to the next.

Question 11

Describe the structure of sieve elements (other than organelles)

Answer 11

-Sieve elements are largely filled with a fluid called phloem sap

-They have perforated end walls called sieve plates that allow sap to flow like a river from cell to cell.

Question 12

Diagram of phloem in a longitudinal section

Answer 12

.

Question 13

Sieve elements, sieve tubes, and sieve plates

Answer 13

-Sieve elements or sieve tube elements are elongated living cells.

-Many sieve elements are connected end to end to form a sieve tube.

-The walls connecting sieve elements become perforated during development to form sieve plates.

Question 14

What does the movement of sugars and organic molecules in phloem require?

Answer 14

-Unlike the movement of water in the xylem, the movement of sugars and other organic molecules in the phloem requires active transport.

-This requires phloem cells to be living, unlike xylem vessels.

-Like the xylem, the structure of the phloem is closely related to its function.

Question 15

What are the structures in a phloem?

Answer 15

-Reduced organelles in sieve elements

-Companion cells

-Plasmodesmata

-Sieve plate

-Cell membrane

Question 16

What is the function of reduced organelles in sieve elements?

Answer 16

Absence of cell structures (including nucleus, cytoskeleton, Golgi, ribosomes and vacuole) frees the lumen to conduct a large volume of sap

Question 17

What is the function of companion cells?

Answer 17

Metabolic support cells (containing all the standard organelles) provide biomolecules (e.g. enzymes) necessary to maintain life functions in the sieve elements

Question 18

What is the function of plasmodesmata?

Answer 18

Openings between the sieve elements and companion cells allow communication and support from companion cells

Question 19

What is the function of sieve plates?

Answer 19

Pores through the horizontal cells that join sieve elements allow sap to flow freely

Question 20

What is the function of the cell membrane of the phloem?

Answer 20

Presence of a fully functional cell membrane in sieve elements that contains specialised protein pumps provides the structures needed to control the composition of sap

Question 21

What is phloem well-adapted to do?

Answer 21

To move sugar-rich sap through its sieve tubes

Question 22

What does sugar-rich sap contain?

Answer 22

The sap is actively loaded with carbohydrates, primarily sucrose, at a source (a tissue with a high concentration of dissolved sugars).

Question 23

Give an overview of what happens to sap during translocation

Answer 23

The sap flows toward a sink (a tissue that requires sugars for storage or use in respiration) in a process called translocation.

Question 24

Describe the speed of translocation

Answer 24

-Translocation in angiosperms may move sugars at a rate of 10 to 200 centimeters per hour.

-Though translocation rate is influenced by a variety of factors, it can generally supply sugars to a sink hundreds of times faster than diffusion would.

Question 25

Table showing sinks and sources in translocation (make into separate flashcards?)

Answer 25

Question 26

What are sources?

Answer 26

Photosynthesising tissues and storage organs that are exporting sugars to other parts of the plant.

Question 27

What are sinks?

Answer 27

Organs that cannot produce (sufficient) sugars and need them for respiration or storage.

Question 28

The same organ may be a ___ at some stages and a ___ at others

Answer 28

Sink Source

Question 29

Give an example of how the same organ may be a sink at some stages and a source at others

Answer 29

-For example, while an onion is developing and growing it is a sink because it is receiving sugars for storage. -Later, when the onion begins to sprout, it will release its stored carbohydrates to nourish the embryonic leaves, and thus will be a source. -As a result, phloem movement can be bidirectional, meaning the sap can flow in different directions. -This is in contrast to the xylem, where water flow is unidirectional, always moving from roots to shoots.

Question 30

Diagram showing translocation in the phloem

Answer 30

Question 31

Steps 1-4 of translocation

Answer 31

1) Sucrose produced by a source (in photosynthesis or released from storage organs) is actively loaded using ATP into phloem tubes. (Technically, companion cells are actively loaded and sucrose then diffuses through plasmodesmata into the sap of the sieve tube elements.) 2) The active loading of sucrose increases the solute concentration in the sieve tubes. 3) Water then moves from xylem vessels to the sieve tubes by osmosis, moving from an area of lower solute concentration to higher solute concentration. 4) Water is essentially incompressible, meaning its volume has almost no change at different pressures. As water enters sieve elements and pushes on the phloem cell walls, it causes increased internal pressure. The pressure exerted by water is called hydrostatic pressure.

Question 32

Steps 5-9 of translocation

Answer 32

5) The high hydrostatic pressure at the source squeezes the sap through the pores of the sieve plates, away from the source, and towards the sink. This movement is called mass flow, or bulk flow because the water and solutes are moving together. 6) At the sink, companion cells unload sugars from the sieve tube. This may be active or passive transport, depending on the relative sugar concentrations. As sugars leave the sieve tube, the concentration of solutes decreases. 7) Decreasing solute concentration allows water to return to xylem vessels. 8) The decrease in water in the sieve tube causes lower hydrostatic pressure near the sink . This allows sap to flow from the areas of high hydrostatic pressure by the source to areas of low hydrostatic pressure by the sinks. 9) As phloem sap flows from source to sink, it flows down hydrostatic pressure gradients. This process is also referred to as the pressure-flow mechanism.

Question 33

Diagram of mass flow from source to sink in phloem

Answer 33

Question 34

What is phloem loading and what does it require?

Answer 34

-The process by which soluble carbohydrates (sugars) enter the phloem. -This loading requires active transport. Sucrose is the primary, but not the only, sugar transported by phloem.

Question 35

Describe the presence of organic molecules other than carbohydrates in the phloem

Answer 35

-Other organic molecules in the phloem, like amino acids and plant hormones, are found in small concentrations compared to sugars. -They do not establish movement from source to sink, but rather are carried along in the mass flow of sap in the direction established by the sugars.

Question 36

Sap will flow from an area of ___ to an area of ___

Answer 36

High hydrostatic pressure Low hydrostatic pressure

Question 37

Explain how water from the xylem enters the phloem and what this leads to

Answer 37

-Water from the xylem enters the phloem by the source due to areas of high solute concentration. -The entry of water creates the high hydrostatic pressure used to drive translocation of sap towards the sink.

Question 38

The force that causes sucrose to enter a companion cell is called ___

Answer 38

Active transport

Question 39

Which type of cell is primarily responsible for the transport of sugars over long distances?

Answer 39

Sieve tube elements

Question 40

Would a newly-developing leaf bud be a source or a sink?

Answer 40

Sink A newly-developing leaf bud would not yet have the surface area and mature tissues needed to photosynthesize, so it cannot be a source. In fact, it will require an input of carbohydrates to power its development until it can begin photosynthesis. Since it is a place where more sugar is used than produced, it is a sink.

Question 41

Why is the rate of translocation difficult to analyze?

Answer 41

Because sap flows rather slowly and phloem is sensitive to disruption, responding quickly to close off any severed sieve tubes.

Question 42

How do scientists study translocation?

Answer 42

-Scientists use aphids, small sap-sucking insects, to study translocation. -Aphids have evolved to pierce a single phloem tube with their long stylets (piercing mouthparts) without triggering defensive responses in the plant.

Question 43

Diagram of an aphid penetrating a plant to suck phloem sap

Answer 43

Question 44

What happens if the stylet is surgically separated from the aphid?

Answer 44

-The phloem sap will continue to flow out of the stylet. -This kills the aphid, and therefore the use of this method is discouraged in the IB due to ethical considerations.

Question 45

Diagram showing the aphid's stylet is being severed but phloem sap continues to flow from the stylet

Answer 45

Question 46

Why is the phloem sap pushed out even after the aphid is removed?

Answer 46

Because of the high hydrostatic pressure in the sieve tubes.

Question 47

What can the unaltered sap that is pushed out even after the aphid is removed be used for?

Answer 47

It can be analysed for its chemical composition, and in combination with radioactive labelling, can determine the direction and rate of phloem transport.

Question 48

What does phloem sap contain?

Answer 48

Amino acids, proteins, carbohydrates and other molecules.

Question 49

Explain how phloem can be used to determine the direction and rate of phloem transport

Answer 49

-If a plant is grown in the presence of radioactive 14CO2 then 14C will be used during photosynthesis, producing radioactive sugar and other organic compounds. -Phloem from aphid stylets can be tested for radioactivity to determine if the products of photosynthesis have been translocated that far. -If the phloem from the stylets is continually sampled and analysed for 14C, a rate of translocation can be determined by dividing the distance travelled by 14C-labelled sugars (cm) by the time it took to travel that distance (hours).

Question 50

Diagram of phloem sampling from aphid stylets at different distances can determine the rate of translocation

Answer 50

.

Question 51

What is the formula for the rate of translocation?

Answer 51

Distance travelled / time

Question 52

What factor would increase the rate of translocation in an experiment to measure this?

Answer 52

-Brighter lights, creating a steeper concentration gradient due to increased sugar production in the leaves (source).

Question 53

Phloem-feeding aphids stick their hollow, syringe-like stylet directly into phloem cells. Aphids do not suck the sap; rather, the phloem contents are forced into the aphid and will continue to emerge from the stylet even when the aphid is removed. Why is this?

Answer 53

Pressure within the phloem is high High hydrostatic pressure caused by water entering the phloem in areas of high solute concentration by the source causes phloem sap to be pushed through the sieve tubes.

Question 54

A photosynthesizing plant is exposed to 14CO2 at time 0 (zero). 1 hour after time 0, a radioactive sugar is detected in the phloem emerging from an aphid stylet (S1) close to the leaves. 2.5 hours after time 0, a radioactive sugar is detected in the phloem from an aphid stylet 30 cm further from the leaves than S1. What is the rate of translocation?

Answer 54

20 cm/hr The rate of translocation = distance / time = 30 cm / (2.5 h – 1 h) = 30 cm/1.5 h = 20 cm/h. It took 1.5 hours for the phloem to travel from the first to the second stylet, a distance of 30 cm. The phloem is therefore moving at a rate of 20 cm/h.

Question 55

If the aphid shown in the figure below had penetrated the xylem rather than the phloem with its stylet, what would you be most likely to find at the stylet after the aphid is removed?

Answer 55

No droplets emerging The suction of transpiration pull could make water evaporate from the surface of the stylet but it is unlikely that there would be enough pressure to squeeze droplets out of the stylet.

Question 56

What are the xylem and phloem?

Answer 56

The transport tissues in vascular plants

Question 57

Movement process in the phloem

Answer 57

Translocation

Question 58

Movement process in xylem

Answer 58

Transpiration

Question 59

What causes movement in the phloem?

Answer 59

-Hydrostatic pressure gradients in phloem -ATP for loading sugar into phloem at source, water follows by osmosis

Question 60

What causes movement in the xylem?

Answer 60

-Evaporation and cohesion-tension creating transpiration pull -ATP for loading ions into roots, water follows by osmosis

Question 61

What are the materials transported in the phloem?

Answer 61

Sucrose and other organics (other sugars, hormones, amino acids, proteins)

Question 62

What are the materials transported in the xylem?

Answer 62

Water and dissolved minerals

Question 63

What is the direction of movement in the phloem?

Answer 63

Source to sink (bidirectional)

Question 64

What is the direction of movement in the xylem?

Answer 64

Roots to shoots especially leaves (upwards only)

Question 65

Horizontal end walls in the phloem

Answer 65

Perforated walls called sieve plates allow the continuous flow of sap

Question 66

Horizontal end walls in the xylem

Answer 66

Continuous hollow tube with removed end walls allows an unbroken column of water

Question 67

Special features of the phloem

Answer 67

Connected by plasmodesmata to companion cells that support metabolic functions

Question 68

Special features of the xylem

Answer 68

Thickened cell walls consisting of lignin making strong, woody tissue

Question 69

What does the arrangement of the xylem and phloem in a plant depend on?

Answer 69

The part of the plant and its classification.

Question 70

___ are the largest group of flowering plants.

Answer 70

Dicots

Question 71

What is the other main group of flowering plants (other than dicots)?

Answer 71

Monocots

Question 72

What is the difference between dicots and monocots?

Answer 72

-Dicots and monocots are distinguished by a variety of features, including the arrangement of vascular tissue. -In a dicot stem, the phloem and xylem are next to each other, arranged in a ring of vascular bundles around the outside of the shoot. -The phloem is closer to the surface of the stem. The phloem tubes are narrower than the xylem.

Question 73

Diagram of a transverse section of a young dicot stem (shoot)

Answer 73

Question 74

How are vascular bundles arranged in a young vs. old dicot stem?

Answer 74

-In the cross-section of a young dicot stem, vascular bundles are arranged in a ring towards the edge of the stem. -In an older stem, the vascular bundles grow together to create a ring of xylem surrounded by a ring of phloem.

Question 75

Light micrograph of a transverse section of the root of a buttercup The vascular tissue is at the center with the xylem forming an ‘X’ shape and the phloem filling the spaces around it.

Answer 75

Question 76

How is the arrangement of xylem and phloem slightly different in monocot stems and roots?

Answer 76

-In monocot roots, the xylem is not an 'X' shape but rather forms a ring slightly inside the phloem ring. -In monocots, vascular bundles are scattered throughout the stem rather than forming a ring, although the phloem still always faces the outside.

Question 77

Electron micrograph of a transverse section of monocot root

Answer 77

Question 78

Key structural and positioning differences between the phloem and xylem

Answer 78

-Xylem vessels usually have a wider lumen than phloem. -Phloem is always located closer to the surface of the plant than xylem.

Question 79

Identifying vascular tissues in the exam

Answer 79

-If you are asked to identify the vascular tissues in a stem or root it will almost certainly be a dicot. -However, you should be aware that the arrangements are different between monocots and dicots, and be prepared to identify either.

Question 80

Which letter indicates phloem on this micrograph?

Answer 80

D

Question 81

What does this diagram show?

Answer 81

A monocot stem The vascular bundles are found in a scattered arrangement throughout the stem.

Question 82

Phloem loading- active transport mechanism

Answer 82

Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure - Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP) - The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient - Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement - This results in a build-up of sucrose within the phloem sieve tube for subsequent transport from the source