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Biology IB HL > 9.1 Xylem Transport > Flashcards

9.1 Xylem Transport Flashcards

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1
Q

What is transpiration?

A

Transpiration is the loss of water vapour from the stems and leaves of plants

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2
Q
  1. What role does light energy play in transpiration?
A

Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata

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3
Q
  1. What does the evaporation of water cause? (transpiration)
A

New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)

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4
Q
  1. What is the transpiration stream and what role does it play? transpiration
A

Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)

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5
Q

What are stomata and what are their role?

A

Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)

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6
Q

What requires stomata to be open and how does this affect transpiration?

A

As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis

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7
Q

What is transpiration a consequence of?

A

Hence, transpiration is an inevitable consequence of gas exchange in the leaf

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8
Q

How is water lost from the plant?

A

Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata

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9
Q
  1. Where is the first place that transpiration occurs and what happens?
A

Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll

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10
Q
  1. Where does the water vapour go? What is created?

evaporation

A

This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf

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11
Q
  1. What does this negative pressure create? What is its use?

evaporation

A

This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)

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12
Q
  1. What causes the tension?

evaporation

A

The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls

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13
Q

How is water loss regulated?

A

The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata

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14
Q

What roles do guard cells play in the regulation of water loss? (specifically decreasing)

A

Guard cells flank the stomata and can obstruct the opening by becoming increasingly flaccid in response to cellular signals

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15
Q

What is released when plants wilt?

A

When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)

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16
Q

What is the role of abscisic acid?

A

Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)

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17
Q

What does loss of turgor do to the stomatal pore?

A

A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening

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18
Q

When will transpiration rates be higher?

A

Transpiration rates will be higher when stomatal pores are open than when they are closed

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19
Q

What will affect levels of transpiration? (direct link, not just a factor)

A

Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration

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20
Q

What factors will affect transpiration?

A

humidity
temperature
light intensity
wind

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21
Q

What is the transpiration stream?

A

The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream

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22
Q

What two key properties of water help it travel through the xylem?

A

Water rises through xylem vessels due to two key properties of water – cohesion and adhesion

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23
Q

What is cohesion?

A

Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)

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24
Q

How do water molecules exhibit cohesion?

A

Water molecules are polar and can form a type of intermolecular association called a hydrogen bond

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25
Q

How does cohesion help the water travel up the xylem?

A

This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream

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26
Q

What is adhesion?

A

Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)

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27
Q

What is water attracted to in the xylem (adhesion) and why?

A

The xylem wall is also polar (specifically lignin) and hence can form intermolecular associations with water molecules

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28
Q

How does adhesion help water travel up the xylem?

A

As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension

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29
Q

What is the xylem?

A

The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant

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30
Q

What are 4 key structural properties of the xylem?

A
  1. composed of dead cells
  2. dead cells are hollow
  3. cells wall contains pits
  4. walls have thickened cellulose and are reinforced by lignin
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31
Q

Why is it important the xylem is composed of dead and HOLLOW cells?

A

It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of water

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32
Q

In which direction does water move during transpiration and to which structural property is this related to?

A

Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only

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33
Q

Why is it important that the xylem has pits?

A

The cell wall contains numerous pores (called pits), which enables water to be transferred between cells

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34
Q

Why is it important that the xylem has thickened cellulose walls and is reinforced by lignin?

A

Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension

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35
Q

What is the xylem composed of?

A
  • tracheids (all plants)

- vessel elements (angiosperms)

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36
Q

What are tracheids?

A

Tracheids are tapered cells that exchange water solely via pits,

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37
Q

How do tracheids affect the rate of water transport?

A

lead to a slower rate of water transfer (water only transported through pits)

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38
Q

What are vessel elements? (structure)

A

In vessel elements, the end walls have become fused to form a continuous tube

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39
Q

How do vessel elements affect the rate of water transfer?

A

result in a faster rate of water transfer

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40
Q

What are all xylem vessels reinforced by?

A

lignin

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41
Q

In what 2 ways can lignin be deposited?

A
  1. in annular vessels

2. in spiral vessels

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42
Q

How is lignin deposited in annular vessels?

A

In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other

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43
Q

How is lignin deposited in spiral vessels?

A

In spiral vessels, the lignin is present in the form of a helix or coil

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44
Q

How do plants take up water and minerals?

A

Plants take up water and mineral ions from the soil via their roots

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45
Q

What do plants need to ensure their roots have to optimise mineral and water uptake?

A

maximal surface area to optimise this uptake

46
Q

In what two ways (simple) do plants maximise the SA of their roots?

A

Some plants have a fibrous, highly branching root system

Other plants have a main tap root with lateral branches

47
Q

How does a fibrous, highly branching root system help increase SA?

A

increases the surface area available for absorption

48
Q

How does a main tap root with lateral branches help uptake water and minerals?

A

can penetrate the soil to access deeper reservoirs of water

49
Q

How may the epidermis of roots be adapted to increase SA?

A

The epidermis of roots may have cellular extensions called root hairs, which further increase the surface area for absorption

50
Q
  1. Where do materials move once absorbed by the root epidermis?
A

Materials absorbed by the root epidermis diffuse across the cortex towards a central stele, where the xylem is located

51
Q
  1. What blocks the passage of water in the roots?
A

The stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions (Casparian strip)

52
Q
  1. How does water and minerals pass through the Casparian strip?
A

Water and minerals are pumped across this barrier by specialised cells, allowing the rate of uptake to be controlled

53
Q

What does fertile soil type usually contain?

A

Fertile soil typically contains negatively charged clay particles to which positively charged mineral ions (cations) may attach

54
Q

What 4 main minerals need to be absorbed by the plant?

A

Minerals that need to be taken up from the soil include Mg2+ (for chlorophyll), nitrates (for amino acids), Na+, K+ and PO43–

55
Q

In what two ways may mineral ions be transported into the roots?

A

Mineral ions may passively diffuse into the roots, but will more commonly be actively uploaded by indirect active transport

56
Q
  1. What do root cells contain which is the main component of active transport? (indirect)

AT

A

Root cells contain proton pumps that actively expel H+ ions (stored in the vacuole of root cells) into the surrounding soil

57
Q
  1. How do the H+ ions affect the positively charged mineral ions?

AT

A

The H+ ions displace the positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient

58
Q
  1. How do the H+ ions affect the negatively charged mineral ions?

AT

A

Negatively charged mineral ions (anions) may bind to the H+ ions and be reabsorbed along with the proton

59
Q

How is water taken into the root? What process?

A

Water will follow the mineral ions into the root via osmosis – moving towards the region with a higher solute concentration

60
Q

What will regulate the rate of water uptake?

A

The rate of water uptake will be regulated by specialised water channels (aquaporins) on the root cell membrane

61
Q

In what two ways can water move towards the xylem?

A

Once inside the root, water will move towards the xylem either via the cytoplasm (symplastic) or via the cell wall (apoplastic)

62
Q

How does water move in the symplastic pathway?

A

In the symplastic pathway, water moves continuously through the cytoplasm of cells (connected via plasmodesmata)

63
Q

How does water move in the apoplastic pathway?

A

In the apoplastic pathway, water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis

64
Q

What are desert plants called?xe

A

xerophytes

65
Q

What are plants that grow in high salinity called?

A

halophytes

66
Q

Why must xerophytes have adaptations for water conservation?

A

Xerophytes will have high rates of transpiration due to the high temperatures and low humidity of desert environments

67
Q

Why must halophytes have adaptations for water conservation?

A

Halophytes will lose water as the high intake of salt from the surrounding soils will draw water from plant tissue via osmosis

68
Q

What are xerophytes? (definition)

A

Xerophytes are plants that can tolerate dry conditions (such as deserts) due to the presence of a number of adaptations

69
Q

What 6 (simple) adaptations can xerophytes have?

A
  1. reduced leaves
  2. rolled leaves
  3. thick, waxy cuticle
  4. stomata in pits
  5. low growth
  6. CAM Physiology
70
Q

How do reduced leaves help reduce water loss?

A

reducing the total number and size of leaves will reduce the surface area available for water loss

71
Q

How do rolled leaves help reduce water loss?

A

reducing the total number and size of leaves will reduce the surface area available for water loss

72
Q

How does a thick waxy cuticle help reduce water loss?

A

having leaves covered by a thickened cuticle prevents water loss from the leaf surface

73
Q

How do stomata in pits help reduce water loss?

A

having stomata in pits, surrounded by hairs, traps water vapour and hence reduces transpiration

74
Q

How does low growth help reduce water loss?

A

low growing plants are less exposed to wind and more likely to be shaded, reducing water loss

75
Q

How does CAM physiology help reduce water loss?

A

plants with CAM physiology open their stomata at night, reducing water loss via evaporation

76
Q

What are halophytes?

A

Halophytes are plants that can tolerate salty conditions (such as marshlands) due to the presence of a number of adaptations:

77
Q

What 5 (simple) adaptations can halophytes have?

A
  1. cellular sequestration
  2. tissue partitioning
  3. root level exclusion
  4. salt excretion
  5. altered flowering schedule
78
Q

How does cellular sequestriation help reduce water loss?

A

halophytes can sequester toxic ions and salts within the cell wall or vacuoles

79
Q

How does tissue partitioning help reduce water loss?

A

plants may concentrate salts in particular leaves, which then drop off (abscission)

80
Q

How does root level exclusion help reduce water loss?

A

plant roots may be structured to exclude ~95% of the salt in soil solutions

81
Q

How does salt excretion help reduce water loss?

A

certain parts of the plant (e.g. stem) may contain salt glands which actively eliminate salt

82
Q

How does an altered flowering schedule help reduce water loss?

A

halophytes may flower at specific times (e.g. rainy seasons) to minimise salt exposure

83
Q

What apparatus can be used to model the movement of water up the xylem?

A

These include capillary tubing, filter or blotting paper and porous pots

84
Q

How does water flow up capillary tubing?

A

Water has the capacity to flow along narrow spaces in opposition to external forces like gravity (capillary action)

85
Q

How is water able to move against gravity?

A

This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of the tube surface

86
Q

how does the diameter of a tube affect capillary action?

A

The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are thin: 20 – 200 µm)

87
Q

What does filter paper do in relation to water?

A

Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties

88
Q

What will happen when filter paper is placed perpendicularly to a water source?

A

When placed perpendicular to a water source, the water will hence rise up along the length of the paper

89
Q

What can the movement of water up filter paper be compared to?

A

This is comparable to the movement of water up a xylem (the paper and the xylem wall are both composed of cellulose)

90
Q

What are porous pots?

A

Porous pots are semi-permeable containers that allow for the free passage of certain small materials through pores

91
Q

What is the loss of water from a porous pot similar to?

A

The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves of plants

92
Q

What can be done to a porous pot to represent the movement of water up the xylem?

A

If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative pressure that draws more liquid

93
Q
  1. What is a potometer?
A

A potometer is a device that is used to estimate transpiration rates by measuring the rate of water loss / uptake

94
Q
  1. How can a potometer be used to measure transpiration?
A

When a plant is affixed to the potometer, transpiration can be indirectly identified by the movement of water towards the plant

95
Q
  1. How can the water change in a potometer be assessed?
A

This water movement can be assessed as a change in meniscus level or by the movement of an air bubble towards the plant

96
Q
  1. How can the initial starting point of the meniscus/air bubble in a potometer be altered?
A

The initial starting position of the meniscus or air bubble can be adjusted by introducing additional water from a reservoir

97
Q

What is important to remember when using a potometer to measure transpiration?

A

When measuring transpiration rates with a potometer, it is important to remember that not all water is lost to transpiration

98
Q

In what ways can water be lost apart from transpiration?

A

A small amount of water (~2%) is used in photosynthesis and to maintain the viable turgidity of plant cells

99
Q

What variables can affect transpiration?

A

temperature
humidity
light intensity
wind exposure

100
Q

In what way does temperature affect transpiration?

A

Increasing the ambient temperature is predicted to cause an increase in the rate of transpiration

101
Q

How does temperature change transpiration rates?

A

Higher temperatures lead to an increase in the rate of water vaporisation within the mesophyll, leading to more evaporation

102
Q

How can the effect of temperature variation on transpiration be tested?

use of a potometer

A

The effect of temperature variation can be tested experimentally by using heaters or submerging in heated water baths

103
Q

In what way does humidity affect transpiration?

A

Increasing the humidity is predicted to cause a decrease in the rate of transpiration

104
Q

How does humidity change transpiration rates?

A

Humidity is the amount of water vapour in the air – less vapour will diffuse from the leaf if there is more vapour in the air

105
Q

How can the effect of humidity on transpiration be tested?

use of a potometer

A

The effect of humidity can be tested experimentally by encasing the plant in a plastic bag with variable levels of vapour

106
Q

In what way does light intensity affect transpiration?

A

Increasing the light intensity to which a plant is exposed is predicted to cause an increase in the rate of transpiration

107
Q

How does light intensity change transpiration rates?

A

Increasing light exposure will cause more stomata to open in order to facilitate photosynthetic gas exchange

108
Q

How can the effect of light intensity on transpiration be tested?

use of a potometer

A

The effect of light intensity can be tested experimentally by placing the plant at variable distances from a lamp

109
Q

In what way does wind exposure affect transpiration?

A

Increasing the level of wind exposure is predicted to cause an increase in the rate of transpiration

110
Q

How does wind exposure change transpiration rates?

A

Wind / air circulation will function to remove water vapour from near the leaf, effectively reducing proximal humidity

111
Q

How can the effect of wind exposure on transpiration be tested?

use of a potometer

A

The effect of wind can be tested experimentally by using fans to circulate the air around a plant

Biology IB HL (56 decks)

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