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Biology A-Level > Transport in Xylem and Phloem (Chapter 7) > Flashcards

Transport in Xylem and Phloem (Chapter 7) Flashcards

1
Q

What are the two possible pathways that water can move down the water potential gradient from cell to cell?

A

1) symplastic pathway

2) apoplastic pathway

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2
Q

Describe briefly the symplastic pathway

A

Water moves from cell to cell via the plasmodesmata

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3
Q

Describe briefly the apoplastic pathway

A

Water moves through the cell walls

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4
Q

What are 3 unusual characteristics of xylem tissue?

A

1) made from cells joined end to end to form tubes
2) the cells are dead
3) walls of the cells are thickened with lignin

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5
Q

What are the two functions of xylem?

A

Support and transport

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6
Q

What are the three types of cell in xylem tissue in flowering plants?

A

1) vessel elements and tracheids (cells involved with transport of water)
2) sclerenchyma fibres (dead, elongated cells with lignified walls that help support the plant)
3) parenchyma cells

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7
Q

What are xylem vessels made up of?

A

Many vessel elements (elongated cells) arranged end to end

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8
Q

How does a vessel element form?

A

1) each vessel element starts as a normal plant cell in whose wall lignin is laid down
2) as lignin builds up around the cell, the contents of the cell die, leaving a completely empty space (lumen) inside

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9
Q

What is lignin?

A
  • A very hard, strong substance which is impermeable to water
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10
Q

What are pits, how are they formed, and describe their characteristics?

A
  • Pits are non-lignified areas formed where no lignin is laid down in the original cell walls where groups of plasmodesmata are found
  • They are seen as gaps in the thick walls of the xylem vessels
  • They are not open pores bc they are crossed by permeable, unthickened cellulose cell wall
  • The pits in one cell link with the those in the neighbouring cells, so water can pass freely from one cell to the next
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11
Q

How is a xylem vessel formed?

A

When the end walls of neighbouring vessel elements break down completely, to form a continuous tube running through the plant

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12
Q

What is hydrostatic pressure?

A

The pressure exerted by a liquid

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13
Q

How do changes in hydrostatic pressure cause water to move up the xylem vessels?

A

1) the removal of water from xylem vessels in the leaf reduces the hydrostatic pressure at the top of the xylem vessels
2) ∴ the hydrostatic pressure at the top of the xylem vessel becomes lower than the pressure at the bottom
3) this pressure difference causes water to move up the xylem vessels in continuous columns, as the higher pressure at the bottom pushes water up the vessel
4) the lower the hydrostatic pressure, the lower the water potential ∴ hydrostatic pressure gradient is also a water potential gradient

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14
Q

Why do xylem vessels have strong, lignified walls?

A

To stop them from collapsing as a result of pressure differences as the water in the xylem vessels is under tension

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15
Q

By what process does water move up xylem vessels?

A

By mass flow (i.e. all water molecules and any dissolved solutes move together as a body of fluid)

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16
Q

How do cohesion and adhesion allow mass flow of water?

A

1) Water molecules are attracted to each other (cohesion) and to the cellulose and lignin in the walls of the xylem vessels (adhesion)
2) Cohesion and adhesion ∴ help to keep the water in a xylem vessel moving as a continuous column

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17
Q

What is the benefit of xylem vessel elements being dead cells?

A

There is no protoplasm to get in the way of transport

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18
Q

Why do xylem vessels have a small diameter?

A

To help prevent breaks in the column of water (air locks) forming due to air bubbles

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19
Q

What is the benefit of having pits in xylem vessels?

A
  • They allow water to move out into neighbouring vessels and ∴ bypass such an airlock
  • Air bubbles cannot pass through pits
  • Pits allow water to move out of xylem vessels to surrounding living cells
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20
Q

What is the effect of an airlock?

A

It prevents water from moving up the vessel as the column of water breaks and the difference in pressure between the water at the top and the water at the bottom cannot be transmitted through the vessel

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21
Q

How does the arrangement of xylem tissue in a series of roads around the centre of the stem help the plant?

A

It helps to support the stem

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22
Q

How else can plants increase the hydrostatic pressure difference between the top and the bottom of the xylem vessel?

A

By increasing the pressure at the base of the vessels (roots)

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23
Q

How is hydrostatic pressure raised at the bottom of the vessels?

A

1) the cells surrounding the vessels actively pump solutes across their membranes, into the water inside the xylem vessels in the root by active transport
2) the presence of the solutes lowers the water potential of the solution in the xylem ∴ drawing in water from the surrounding root cells
3) this influx of water increases the water pressure at the base of the xylem vessel

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24
Q

Why is water transport a largely passive process?

A

The water simply moves down a continuous water potential gradient from the soil to the air, driven by transpiration in the leaves

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25
Q

Where are xylem vessels in roots?

A

In the centre

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26
Q

How is water move across the root into the xylem?

A

1) water taken up by root hairs crosses the cortex of the root and enters the xylem in the centre of the root bc the water potential inside the xylem vessels is lower than the water potential in the root hairs
2) ∴ water moves down this water potential gradient across the root

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27
Q

Describe the apoplastic pathway in roots

A

Water soaks into the cell walls of the cells of the cortex and seeps across the root from cell wall to cell wall without ever entering the cytoplasm of the cortical cells

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28
Q

Outline the 3 steps of the apoplastic pathway?

A

1) water enters the cell in the cell wall
2) water moves through the cell wall
3) water moves from cell wall to cell wall through intracellular spaces directly

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29
Q

Describe the symplastic pathway in roots

A

Water moves into the cytoplasm/vacuole of a cortical cell by osmosis and then into adjacent cells through the interconnecting plasmodesmata, adjacent CSM or cells walls

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30
Q

When does more water travel via the symplastic pathway?

A

Most of the time (when transpiration rates are low/normal)

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31
Q

When does more water travel via the apoplastic pathway?

A

When transpiration rates are especially high

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32
Q

What happens when water reaches the endodermis in roots?

A

The apoplastic pathway is abruptly blocked

1) the cells in the endodermis have a thick, waterproof, waxy band of suberin in their cell walls called the Casparian strip
2) this forms an impenetrable barrier to water in the walls of the endodermis cells ∴ the only way for water to cross the endodermis is through the cytoplasm (i.e. via symplastic pathway)
3) water can continue to pass freely through passage cells, where suberin deposits have not become more extensive like endodermal cells
4) water then continues to move down the water potential gradient across the pericycle and then into xylem vessels through pits in their walls

33
Q

What does the Casparian strip allow plants to do?

A
  • This arrangement gives plants control over what miner ions pass into xylem vessels as everything has to cross CSMs
  • It may also help with the generation of root pressure
34
Q

Describe the structure of the tip of a root

A
  • The tip is covered by a tough, protective root cap and is not permeable to water
  • Behind the tip, some of the cells in the epidermis (outer layer) are drawn out into long, thin extensions called root hairs - these reach into spaces between the soil particles, from where they absorb water
35
Q

How does water move into the root hairs from the soil?

A

By osmosis down a water potential gradient

1) soil has a relatively high water potential bc even though it contains some inorganic ions in solution, it is a relatively dilute solution
2) cytoplasm and cell sap inside the root hairs have a relatively low water potentials because they have considerable quantities of inorganic ions and organic substances e.g. proteins and sugars, dissolved in them
3) ∴ water diffuses down this water potential gradient through the partially permeable CSM and into the cytoplasm and vacuole of root hair cells

36
Q

Why are root hairs important?

A

1) the large no. of very fine root hairs provides a large SA in contact with the soil surrounding the root ∴ increasing the rate at which water can be absorbed
2) important in the absorption of mineral ions e.g. Mg2+
3) they are regularly replaced (every few days) as the root grows

37
Q

Describe the path of mineral ions in solution similar to that of water

A

Mineral ions in solution are absorbed along with water by the roots (roots hairs), cross the root by apoplastic and symplastic pathways before moving in the mass flow of xylem sap up the xylem to the rest of the plant

38
Q

How else can mineral ions move in the plant?

A

1) diffusion

2) active transport and facilitated diffusion - allow cells to control what ions enter/leave cells

39
Q

What is translocation?

A

Moving from one place to another

40
Q

What is translocation used to describe?

A

The transport of soluble organic substances (assimilates) within a plant

41
Q

What are assimilates?

A

Substances that the plant has made itself e.g. sucrose and amino acids

42
Q

Where are assimilates transported

A

In sieve elements

43
Q

What is found in phloem tissue?

A

Sieve tubes (made up of sieve elements), fibres, parenchyma and companion cells

44
Q

What is a sieve tube?

A

A unique tube-like structure made up of many elongated sieve (tube) elements, joined end to end vertically to form a continuous tube

45
Q

What is a sieve element

A

A living cell (but special)

  • Contains: cellulose cell wall, CSM, cytoplasm, ER and mitochondria
  • Does not contain: nucleus, ribosomes
46
Q

What is different about the cytoplasm in a sieve element?

A

It is very cell and only forms a thin layer lining the inside of the inside wall of the cell

47
Q

What happens where the end walls of 2 sieve elements meet?

A

A sieve plate is formed

48
Q

What is a sieve plate made up of?

A

The walls of both sieve elements, perforated by large pores

49
Q

Describe the pores in sieve plates in living phloem

A

They are open, presenting little barrier to the free flow of liquids through them

50
Q

What does each sieve element have?

A
  • At least one companion cell lying close beside it

- A companion cell and sieve element are very closely associated and regarded as a single functional unit

51
Q

Describe the characteristics of companion cells

A
  • They have the structure of a normal plant cell BUT the number of mitochondria and ribosomes is large than normal and the cells are very metabolically active
  • Numerous plasmodesmata pass through their cell walls, making direct contact between the cytoplasm of the companion cell and that of the sieve element
  • They have many mitochondria to produce ATP required for active transport
  • They carry out the metabolic processes needed by sieve tube elements
52
Q

What is phloem sap?

A

The liquid inside phloem sieve tubes

53
Q

What are the contents of sieve tube under?

A

Very high turgor pressure because of its high solute content

54
Q

What happens when a sieve tube is cut?

A

1) the release of pressure inside the tube causes a surge of its contents towards the cut
2) when the contents come up against the sieve plate, the plate may block the contents, helping to prevent escape of the contents of the sieve tube
3) within minutes, clotting occurs - the sieve plate is properly sealed with a carbohydrate called callose, preventing leakage and loss of valuable substances e.g. sucrose, which the plants cannot afford to lose in large quantities - these would leak out rapidly if the holes in the sieve plate were not quickly sealed due to high turgor pressure
4) this clotting also helps to prevent the entry of microorganisms which might feed on nutritious sap or cause disease

55
Q

How does phloem sap move?

A

Mass flow (much faster than diffusion)

56
Q

What does the plant need to do to create the pressure differences needed for mass flow created in the phloem?

A

The plant has to use energy ∴ phloem transport is an active process (vs passive in xylem)

57
Q

What is the pressure difference produced by in phloem?

A

Active loading of sucrose into sieve elements at the place from which sucrose is to be transported

58
Q

What is a source?

A

Any area of a plant in which assimilates are loaded into the phloem e.g. leaf/storage organ

59
Q

What is a sink?

A

Any area where assimilates are taken out of the phloem e.g. buds, flowers, fruits, roots and storage organs (normally growing or storage points)

60
Q

How is a pressure difference created in the phloem?

A

1) loading a high conc of sucrose into a sieve element greatly decreases the water potential of the sap inside of it ∴ water enters the sieve element by osmosis, down the water potential gradient
2) this causes a correspondingly high build up of hydrostatic/turgor pressure in the sieve element
3) ∴ a pressure difference is created between the source and sink, causing mass flow of water and dissolved solutes trough the sieve tubes from the high pressure area to the low pressure area (source to sink)

61
Q

What happens at the sink?

A

Sucrose may be removed and used, causing the water to follow by osmosis ∴ maintaining the pressure gradient

62
Q

Where can sinks be in the plant and what does this mean for direction of flow of phloem sap?

A
  • Anywhere in the plant, both above and below photosynthesising leaves
  • ∴ phloem sap flows both upwards and downwards in the phloem (unlike xylem which is always upwards)
  • Within any vascular bundle, phloem sap may be flowing upwards in some sieve tubes and downwards in others, but it can only flow one way in any particular sieve tube at any one time
63
Q

By what pathways is sucrose loaded into phloem tissue?

A

Sucrose produced in photosynthesis in leaf mesophyll cells moves across the leaf in solution to the phloem tissue by both the symplastic and apoplastic pathways

64
Q

Why what transport method is sucrose loaded into a companion cell/sieve element from a leaf?

A

Active transport

65
Q

Explain how sucrose is loaded into the phloem

A

1) H+ are pumped out of the companion cell into its cell wall, using ATP as an energy source
2) this creates a large excess of H+ in the apoplastic pathway outside the companion cell
3) the H+ can move back into the cell down their conc gradient, through a co-transporter protein
4) the sucrose molecules are carried through the co-transporter molecule into the companion cell, against the conc gradient for sucrose
5) the sucrose molecules can then move from the companion cell into the sieve tube, through the plasmodesmata which connect them (symplastic pathway)

66
Q

What is a co-transporter protein?

A

A protein which acts as a carrier for both H+ and sucrose at the same time

67
Q

Where does unloading of sucrose from phloem occur?

A

Into any tissue which requires sucrose

68
Q

What happens when sucrose is unloaded from the phloem?

A
  • Phloem unloading requires energy and similar methods as loading are used
  • Once in the tissue, the sucrose is converted into something else by enzymes ∴ decreasing its conc and maintaining a conc gradient e.g. invertase hydrolyses sucrose to glucose and fructose
69
Q

How do assimilates move between sources and sinks?

A

In phloem sieve tubes

70
Q

What is the similarity between sieve tubes and xylem vessels?

A

Liquid moves by mass flow down a pressure gradient, through tubes formed by cells stacked end to end

71
Q

What is the difference between transport in phloem and xylem?

A
  • Unlike water transport through xylem, which occurs through dead xylem vessels, translocation through phloem sieve tubes involves active loading of sucrose at sources and unloading at sinks ∴ requiring living cells
  • Sucrose would leak out of xylem vessels, bc they have no membranes which control entry/loss of solutes
72
Q

What is the difference between xylem vessel elements and sieve elements?

A
  • Vessel elements have lignified cell walls - advantage bc cells can be dead and ∴ entirely empty so water can flow unimpeded and dead xylem vessels with strong walls help to support the plant
  • Sieve tubes do not have lignified cell walls but reduce resistance to flow by having only a thin layer of cytoplasm and no nuclei
73
Q

What is the difference between the ends of xylem vessel elements and the ends of sieve plates?

A
  • The end walls of phloem sieve elements form sieve plates - advantage bc without the resistance of these sieve plates along the pathway, the steep positive pressure gradient inside the sieve tubes would quickly be lost with the different pressures at source and sink quickly equilibrating
  • The end walls of xylem elements disappear completely ∴ xylem has to withstand high negative pressure (tension) inside its tubes and buckling is prevented by its lignified walls
74
Q

What is the function of lignin?

A

1) to provide strength and support
2) to withstand cohesion tension from water
3) to prevent inwards collapse
4) to allow unimpeded flow of water without friction

75
Q

What is the mechanism of pulling water up the stem called?

A

Cohesion-tension mechanism

76
Q

What is the driving force for the movement of water up the xylem?

A

Transpiration in the leaves

77
Q

What two features of xylem vessels adapt them for water transport?

A

1) low resistance

2) hollow - no living material

78
Q

What is mass flow?

A

The movement of fluids containing dissolved/suspended substances e.g. glucose over a long distance in living organisms, caused by a pressure gradient

Biology A-Level (44 decks)

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