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2.3b adaptations for transport in plants Flashcards

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1
Q

what are the two main types of tissue that vascular tissue in plants is made up of?

A
  • xylem
  • phloem
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2
Q

what is xylem responsible for?

A
  • the transport of water and mineral ions from the roots to the stem and leaves
  • support
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3
Q

what is phloem responsible for?

A
  • the translocation of organic solutes e.g sucrose and amino acids
  • transports the products of photosynthesis to the roots from the leaves
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4
Q

what is the arrangement of vascular tissue (xylem and phloem) in the root? why?

A
  • xylem is arranged centrally into a star shape
  • with phloem outside it
  • this helps to anchor plant into the soil, resisting pulling forces
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5
Q

what does a diagram of a root include?

A
  • endodermis
  • epidermis
  • pericycle
  • cortex
  • xylem
  • phloem
  • root hair
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6
Q

what is the arrangement of vascular tissue (xylem and phloem) in the stem? why?

A
  • arranged towards the periphery in a ring
  • which provides support to resist bending
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7
Q

what does a diagram of a stem include?

A
  • epidermis
  • collenchyma
  • medulla
  • cortex
  • vascular bundle } xylem, phloem, fibres
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8
Q

what is the arrangement of vascular tissue (xylem and phloem) in the leaf? why?

A
  • arranged in the midrib
  • giving both resistance to tearing and flexability
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9
Q

what does a diagram of a leaf include?

A
  • vascular bundle in midrib } xylem, phloem
  • vascular bundle in leaf vein
  • adaxial surface (facing the central axis of the plant)
  • abaxial surface (facing away from the centre axis of the plant)
  • collenchyma
  • compacted parenchyma
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10
Q

angiosperms definition

A

flowering plants

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11
Q

define vascular bundle

A
  • vascular system in herbaceous dicotyledonous plants
  • consists of two transport vessels, the xylem and the phloem
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12
Q

describe the structure and function of the vascular system in the roots of dicotyledons

A
  • xylem arranged in an X shape to provide resistance against force
  • phloem found as patches between the arms
  • surrounded by endodermis, aiding water passage
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13
Q

describe the structure and function of the vascular system in the stem of dicotyledons

A
  • vascular bundles organised around a central pith
  • xylem on the inside of the bundle to provide support and flexibility
  • phloem on the outside
  • cambium is found between the two
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14
Q

which structure in plants is adapted for the uptake of water and minerals?

A

root hair cells

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15
Q

how is water taken up from the soil?

A
  • root hair cells absorb minerals by active transport, reducing the water potential of the root
  • water potential of root hair cells is lower than that of the soil
  • water moves into the root by osmosis
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16
Q

outline how plant roots are adapted for the absorption of water and minerals

A
  • plant roots are composed of millions of root hair cells which have:
    • long hairs that extend from the cell body, increasing the surface area for absorption
    • many mitochondria which produce energy for the active transport of mineral ions
    • thin cellulose cell wall on root hair extension so the distance for transport is short
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17
Q

state 3 pathways by which water moves through the root:

A
  • apoplast pathway
  • symplast pathway
  • vacuolar pathway
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18
Q

describe the apoplast pathway:

A
  • water moves through intercellular spaces between cellulose molecules in the cell wall
  • it diffuses down its water potential gradient by osmosis
  • (involves water moving between spaces in the cellulose cell wall)
  • the most significant route
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19
Q

describe the symplast pathway:

A
  • water enters the cytoplasm through the plasma membrane and moves between adjacent cells via plasmodesmata
  • water diffuses down its water potential gradient by osmosis
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20
Q

describe the vacuolar pathway:

A
  • water enters the cytoplasm through the plasma membrane and moves between vacuoles of adjacent cells
  • water diffuses down its water potential gradient by osmosis
  • is a minor route
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21
Q

describe the structure and function of the endodermis:

A
  • innermost layer of the cortex of a dicot root
  • impregnated with suberin which forms the Casparian strip
  • endodermal cells actively transport mineral ions into the xylem
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22
Q

what is the function of the Casparian strip?

A
  • blocks the apoplast pathway, forcing water through the symplast route
  • enables control of the movement of water and minerals across the route and into the xylem
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23
Q

what molecule makes the Casparian strip waterproof?

A

suberin

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24
Q

relate the structure of the xylem to its function:

A
  • long, continuous columns made of dead tissue, allowing the transportation of water
  • containing bordered pits, allowing the sideways movement of water between vessels
  • walls impregnated with lignin, providing structural support
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25
Q

define transpiration

A
  • the loss of water vapour from the parts of a plant exposed to the air due to evaporation and diffusion (out of the stomata into the atmosphere)
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26
Q

what is the transpiration stream?

A
  • the flow of water from the roots to the leaves in plants, where it is lost by evaporation to the environment
  • the properties of water molecules mean that a constant stream of water molecules can be formed between the roots and the leaves. this is called the transpiration stream
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27
Q

how does water move up the stem?

A
  • root pressure
  • cohesion tension theory
  • capillarity
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28
Q

what is root pressure?

A
  • the force that drives water into and up the xylem by osmosis due to the active transport of minerals into the xylem by endodermal cells
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29
Q

explain the cohesion-tension theory:

A
  • water molecules form hydrogen bonds with each other, causing them to ‘stick’ together
  • surface tension of the water also creates this sticking effect
  • ∴ as water is lost through transpiration, more is drawn up the stem from the roots
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30
Q

define capillarity:

A
  • the tendency of water to move up the xylem, against gravity, due to adhesive forces that prevent the water column dropping back
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31
Q

state the factors that affect the rate of transpiration

A
  • light (intensity)
  • temperature
  • humidity
  • air movement
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32
Q

how does temperature affect the rate of transpiration?

A
  • a higher temperature causes the water molecules to gain KE and therefore increasing the rate of diffusion out into the atmosphere through the stomata
  • therefore increasing the rate of transpiration
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33
Q

how does light affect the rate of transpiration?

A
  • a higher light intensity increases the rate of photosynthesis, causing more stomata to open for gas exchange
  • therefore increasing the rate of transpiration
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34
Q

how does humidity affect the rate of transpiration?

A
  • high humidity means the water content of the air next to the leaf is high
  • this reduces the concentration gradient, therefore decreasing the rate of transpiration
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35
Q

how does air movement affect the rate of transpiration?

A
  • large amounts of air movement blow moist air away from the leaves, creating a steep concentration gradient
  • therefore, increases rate of transpiration
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36
Q

what is a hydrophyte?

A
  • a plant that is adapted to live and reproduce in very wet habitats e.g water lilies
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37
Q

give adaptation of hydrophytes that allow them to live in wet conditions

A
  • thin or absent waxy cuticle
  • stomata often open
  • wide, flat leaves
  • air spaces for buoyancy
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38
Q

what is a xerophyte?

A
  • a plant that is adapted to live and reproduce in dry habitats where water availability is low e.g cacti and marram grass
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39
Q

give adaptations of xerophytes that allow them to live in dry conditions

A
  • small/rolled leaves
  • densely packed mesophyll
  • thick waxy cuticle
  • stomata often closed
  • hairs to trap moist air
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40
Q

what are mesophytes?

A
  • terrestrial plants adapted to live in environments with average conditions and an adequate water supply
  • they have features that enable their survival at unfavourable times of the year
  • live in temperate regions with an adequate water supply but must survive times of the year when water is scarce or unavailable e.g water is frozen
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41
Q

relate the structure of the phloem to its function

A
  • sieve tube elements transport sugar around the plant
  • companion cells designed for active transport of sugars into tubes
  • plasmodesmata allow communication and the exchange of substances between sieve tubes and companion cells
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42
Q

what are cytoplasmic strands?

A
  • small extensions of the cytoplasm between adjacent sieve tube elements and companion cells
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43
Q

describe the function of cytoplasmic strands:

A
  • allow communication and the exchange of materials between sieve tube elements and companion cells
  • hold the nucleus in place
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44
Q

define translocation:

A
  • the movement of organic compounds in phloem, from sources to sinks
  • the products of photosynthesis are transported in the phloem as sucrose from where they are produced (the source) to where they are used or stored as insoluble food reserves e.g starch (the sink)
  • the phloem also transports amino acids
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45
Q

summarise the mass-flow hypothesis of translocation:

A
  • sugar loaded into sieve tubes via active transport
  • lowers water potential, causing water to move in from the xylem
  • hydrostatic pressure causes sugars to move towards the sink
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46
Q

give evidence for the mass-flow hypothesis:

A
  • sap is released when the stem is cut ∴ must be pressure in phloem
  • sap exuding from the stylet (mouthpart) of an aphid inserted into sieve tubes provides evidence that sugars are carried in the phloem
  • there is a higher sucrose concentration in the leaves than the roots
  • autoradiographs produced using carbon dioxide labelled with radioactive carbon provide evidence for translocation in the phloem
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47
Q

what is autoradiography?

A

a technique used to record the distribution of radioactive material within a specimen

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48
Q

what is a potometer?

A

an apparatus used to measure water uptake from a cut shoot

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49
Q

tracheids are present in what?
vessels are present in what?

A

• tracheids - present in flowering plants (angiosperms), ferns and conifers
• vessels - only present in flowering plants

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50
Q

water is conducted through vessels and tracheids, which are dead cells due to lignin deposition in the walls

A

fibres provide support, and xylem parenchyma acts a packing tissue

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51
Q

why does water enter the root hair cell by osmosis?

A

because the soil solution has a higher water potential than the vacuole of the hair cell which contains ions and sugars

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52
Q

what does the presence of lignin in the cell walls of the xylem vessels do?

A
  • waterproofs them
  • which will also prevent water from entering the xylem via the apoplast pathway
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53
Q

in the root, what is the pericycle surrounded by?

A
  • a single layer of cells called the endodermis
  • which forms a ring surrounding the vascular tissue in the centre of the root
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54
Q

the cell walls of the endodermis are impregnated with what? what does it form?

A
  • suberin
  • forming an impermeable band known as the Casparian strip that drives water from the apoplast pathway into the cytoplasm
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55
Q

what does the endodermis help to regulate?

A

the movement of water, ions and hormones into and out of the xylem

56
Q

by what and why is the water potential of endodermal cells raised?

A
  • by water being forces into them by the Casparian strip and the active transport of sodium ions into the xylem
  • this lowers the water potential of fluid in the xylem, forcing water into the xylem by osmosis

THIS IS KNOWN AS ROOT PRESSURE

57
Q

minerals including nitrates and phosphates are _______ into the root hair cells?

A

actively transported

  • against their concentration gradient

(they can also pass along the apoplast pathway in solution)

58
Q

describe the uptake of minerals into the plant:

A
  • minerals are actively transported into the root hair cell
  • they can also pass along the apoplast pathway in solution
  • once they reach the Casparian strip they enter the cytoplasm via active transport and then pass via diffusion or active transport into the xylem
59
Q

what are adhesive forces created between?

A

are created between the charges on the water molecules and their attraction with the hydrophilic lining of the vessels

60
Q

what are cohesive forces created by?

A

are created by the attractive forces between water molecules due to their dipolar charges forming hydrogen bonds

61
Q

what does the cohesion-tension theory explain?

A

how water moves up the xylem

62
Q

what is the main mechanism that pulls water up the stem?

A

transpiration

63
Q

is transpiration a passive or an active process?

A

passive

64
Q

what does transpiration pull rely on?

A
  • adhesive forces between water molecules and xylem
  • cohesive forces between water molecules
  • root pressure
  • capillarity

(root pressure and capillarity alone would not be sufficient to raise water up the xylem to any significant height)

65
Q

how is transpiration pull created?

A
  • as water evaporates from the leaf air space through the stomata (although there is some diffusion through the cuticle)
  • water is drawn from inside the cells lining the space by osmosis
  • these cells now have a lower water potential and so draw water from adjacent cells by osmosis, and this continues across the leaf until water is drawn from the adjacent xylem vessels
  • as water is drawn out of the xylem, water molecules are ‘pulled’ up to replace those lost due to cohesive forces that exist between water molecules
  • water molecules enter the xylem to replace those moving up by osmosis from the endodermal cells
  • and water crosses the cortex from the root hair by the same method as in the leaf
66
Q

is water loss inevitable?

A

yes - plants have to balance water loss by transpiration with the need to get water and mineral ions to the leaves themselves

67
Q

plants exposed to a bright, hot, dry, windy day will show (lower/higher) rates of respiration?

A

higher

68
Q

what is the equation used to calculate the volume of water taken in by a plant using a potometer?

A

volume = 𝛑r^2d
r = radius of capillary tube
d = distance bubble travels

69
Q

what does a potometer actually measure?

A

the rate of water uptake

  • some water will be used in photosynthesis, but if cells are turgid, this uptake rate is approximated as the transpiration rate
70
Q

when setting up a potometer, what is important to do?

A
  • cut the stems and fit it to the potometer underwater, as this prevents the formation of any air bubbles in the xylem vessels
  • seal all joints with vaseline to prevent air entry
  • blot leaves dry as any water on the leaf surface could create a humid layer
  • a layer of oil on the surface of the water prevents evaporation therefore ensuring that any change in mass is during to water uptake by the shoot
71
Q

potometer experiment - comparing rates of transpiration :

A
  • set potometer up
  • introduce an air bubble at the end of the capillary tube
  • measure the distance it travels in a set period e.g one minute
  • the volume can be calculated easily if the diameter of the capillary tube is known
  • repeat
72
Q

does the cuticle prevent water loss fully?

A

no it only reduces water loss

73
Q

how do mesophytes conserve water?

A
  • closing stomata if water is scarce, as they cannot maintain turgor in guard cells
  • shedding leaves and becoming dormant during winter
  • overwintering beneath the ground as bulbs or corms
  • annual plants producing seeds that can overwinter
74
Q

what are some adaptations of xerophytes?

A
  • sunken stomata, which trap humid air, reducing the water potential gradient between air spaces inside the leaf and the outside air
  • hairs around stomata, which trap water vapour, reducing the water potential gradient between the leaf and the air
  • rolled leaves, which reduce the surface area over which transpiration occurs. some plants take this to extremes by reducing leaves to spines and using the stem to photosynthesis e.g cacti
  • thick cuticle, which further reduces water loss from leaf surface
75
Q

what have hydrophytes needed to adapt to?

A
  • ensure they receive adequate light and carbon dioxide for photosynthesis
  • as they grow partially or fully submerged in water, a lack of water is never a problem
76
Q

how have water lillies adapted? (hydrophytes)

A
  • having stomata on the upper leaf surface, which is in contact with the air
  • stems and leaves have large air spaces providing buoyancy and a reservoir of oxygen and carbon dioxide
  • having poorly developed xylem tissue as there is no need to transport water as it’s all around
  • leaves have little or no cuticle as water loss is not a problem
  • support tissue is not needed as water is a supportive medium
  • increased leaf surface area for gas exchange
77
Q

what are the 3 main types of cells in phloem?

A
  • sieve tubes
  • companion cells
  • phloem parenchyma
78
Q

sieve tubes:

A
  • walls perforated with pores to produce longitudinal tubes that contain cytoplasm but no nucleus
  • and most organelles disintegrate during their development
  • the end walls do not break down but instead become perforated by pores, forming the end plates
  • cell wall made of cellulose
  • contain little cytoplasm and few or no organelles
79
Q

companion cells :

A
  • dense cytoplasm with nucleus and many mitochondria
  • connected to each sieve tube by plasmodesmata
  • metabolically active
  • accompany sieve tube cells and provide the support needed to keep them alive
  • also involved in transport of substances into and out of sieve tubes
80
Q

phloem parenchyma :

A
  • acting as a packaging tissue
81
Q

how are autoradiographs used as evidence that phloem is the vessel used in translocation

A
  • by radioactively labelling carbon dioxide using 14-C, products and their paths can be traced by exposing the plants to x-ray film
  • these are called autoradiographs
  • developed film emulsion is fogged by the presence of radioactivity in the phloem
82
Q

how are ringing experiments used as evidence that phloem is the vessel used in translocation

A
  • the outer ring of stem is cut to remove the phloem whilst leaving the xylem behind
  • a bulge forms above the ring suggesting that sugar moves down the stem in the phloem
83
Q

what was the early experiment to provide evidence that phloem is the vessel involved in translocation?

A
  • aphids were allowed to feed on plants
  • then were anaesthetised before removing the head, leaving the feeding stylet in place
  • analysis of the liquid extruding from the stylet showed that it was sucrose
84
Q

what is the most accepted theory to explain how organic solutes are transported?

A

the mass flow theory

85
Q

even tho its the most accepted theory, what does the mass flow hypothesis still fail to explain?

A

how sucrose and amino acids are transported at different rates in opposite direction in the same phloem vessel, or how transport occurs thousands of times faster than is possible by diffusion

86
Q

what is the mass flow theory:

A
  1. photosynthesising cells (source cells) produce glucose, which is converted into sucrose, which lowers the water potential of the cell. as water enters the cell by osmosis, hydrostatic pressure forces sucrose into the phloem sieve tube
  2. by increasing the level of solutes in the phloem, the water potential ψ is lowered and water moves in from the adjacent cells and xylem, by osmosis, down a water potential gradient. this raises the hydrostatic pressure in the phloem so that it has a higher pressure
  3. sucrose and dissolved solutes move by mass flow from a high to a low hydrostatic pressure, down a pressure gradient
  4. at the roots/growing points (sink cells) the sucrose diffuses into the cells down a concentration gradient, so is ∴ removed from sieve tubes. in the sieve tubes, it is converted to starch for storage or converted to glucose to be respired. the loss of sucrose from phloem raises the water potential higher than in the xylem and adjacent cells
  5. water enters the xylem by osmosis
  6. water also moves from phloem to xylem down a water potential gradient, causing a reduction in the hydrostatic pressure
  7. water moves up the xylem by transpiration

other theories involving the use of protein filaments and cytoplasmic streaming, which could account for bidirectional transport, have been suggested

87
Q

what is the function of the epidermis in roots?

A
  • presence of root hairs for the uptake of water and mineral ions
  • epidermal cells protect roots as they grow through the soil
88
Q

what is the function of the cortex parenchyma in roots?

A
  • can act as a storage organ
  • intercellular spaces allow movement of water and ions
89
Q

what is the function of the endodermis in roots?

A
  • has a waterproof layer that forces water and ions into the cytoplasm of the endodermal cells and controls transport into the xylem
90
Q

what is the function of the pericycle in roots?

A
  • has a role in controlling transport into the xylem
  • site of lateral root growth
91
Q

what is the function of the cambium in roots?

A

a meristematic tissue that can undergo mitosis to produce more xylem and phloem

  • the cells can keep on dividing by mitosis
  • the cells then differentiate to form the other cells found in the vascular bundle
92
Q

what is the function of the cuticle in stem tissues?

A
  • reduces water loss through evaporation
  • transparent to allow light to pass though for photosynthesis (in green stems)

(reducing EVAPORATION from surface of leaf)

93
Q

what is the function of the epidermis in stem tissues?

A
  • protection of the stem
  • may have hairs to detect insects/animals from eating them
94
Q

what is the function of the collenchyma in stem tissues?

A

cell walls thickened with cellulose to strengthen the stems while remaining flexible

95
Q

what is the function of the cortex parenchyma in stem tissues?

A
  • can act as a storage organ
  • intercellular spaces allow movement of water and ions and gases
96
Q

what is the function of the pith parenchyma in stem tissues?

A
  • thin-walled cells that act as packing tissue - often breaks down in older stems
97
Q

what is the function of the sclerenchyma in stem tissues?

A
  • lignified cells that provide strength and support to the stem
98
Q

what is the function of the cambium in stem tissues?

A

a meristematic tissue that can undergo mitosis to produce more xylem and phloem

  • the cells can keep on dividing by mitosis
  • the cells then differentiate to form the other cells found in the vascular bundle
99
Q

what are the number of different cell types that are found in xylem? and what do they do?

A
  • xylem vessels = transport water and minerals from the roots to other parts of a plant; the end walls of xylem vessels break down to form long tubes that extend from the roots to the stems and leaves
  • tracheids = provide strength to the tissue
  • xylem parenchyma = thin walled, living cells that act as packing tissue between the xylem vessels
100
Q

tracheids and xylem vessels are (alive/dead) cells and (have/dont have) any cell content

A

dead cells
have no cell content

101
Q

what are the walls of tracheids and xylem vessels made of?

A

lignin
- which is impermeable to water

102
Q

what does water enter and leave the xylem vessels through?

A

holes in the walls called pits

103
Q

do lignin fibres form in different ways in young xylem (protoxylem) and older xylem (metaxylem)?

A

yes - lignin forms in rings in protoxylem and a network in metaxylem

104
Q

are all cells in phloem alive?

A

yes

105
Q
  • the end walls of phloem sieve tube cells (also called phloem sieve tube elements) are perforated by small pores
  • the end walls are called ____?
A

sieve plates

  • strands of cytoplasm pass though these pores from one phloem sieve tube element to the next
106
Q

what do vascular bundles contain?

A

both xylem and phloem together with cambium and other cells

107
Q

key difference between xylem and phloem:
- cells in tissue

A

• xylem = xylem vessels, tracheids
• phloem = sieve tube elements, companion cells

108
Q

key difference between xylem and phloem:
- nature of transport cells

A
  • xylem = dead
  • phloem = alive
109
Q

key difference between xylem and phloem:
- cell wall

A
  • xylem = thick, lignin-rigid, impermeable
  • phloem = thin, cellulose-flexible, permeable
110
Q

key difference between xylem and phloem:
- cytoplasm

A
  • xylem = none
  • phloem = cytoplasmic strands
111
Q

key difference between xylem and phloem:
- transports

A
  • xylem = water and mineral ions
  • phloem = products of photosynthesis e.g sucrose and amino acids
112
Q

key difference between xylem and phloem:
- direction of transport

A
  • xylem = from roots upwards
  • phloem = to and from sites of photosynthesis/storage (source) to growing regions and sites of storage (sink)
113
Q

the apoplast route is (passive/active) and the water moves by _____

A

passive
cohesion

114
Q

which route, apoplast, symplast and vacuolar is the fastest?

A

apoplast

there is less resistance as there are no membranes to cross and the cellulose cell wall is fully permeable to water

115
Q

which route, apoplast, symplast and vacuolar is the slowest?

A

vacuolar route
- each time water had to cross membranes, there is resistance to movement and the speed of transport decreases

116
Q

most ions enter plant cells at the root hair cells via the ____ route although some will move passively with water along the apoplast route

A

symplast

117
Q

how are ions absorbed?

A

by a combination of active transport and co-transport

118
Q

in the apoplast pathway, water and ions _____ through the cellulose cell walls

A

diffuse

119
Q

in the symplast pathway, water enters the ___ of cells by osmosis

A

cytoplasm

120
Q

the Casparian strip is _____ to water and ions and stops both the apoplast and vacuolar? pathways

A

inpermeable

121
Q

the slowest route for the movement of water across a root is the vacuolar pathway as water has to cross cell membranes and enter the vacuoles by passing through the ____?

A

tonoplast

122
Q

ions enter the cytoplasm of cells by ____?

A

active transport

123
Q

the absorption of ions by plant cells requires a source of ___ as it needs a supply of ATP produced mainly through aerobic respiration

A

oxygen

124
Q

ions accumulate in the endodermis and pericycle. this increased concentration of ions makes the solute potential of the tissues more _____?

A

negative

which lowers the water potential

125
Q

what is root pressure caused by?

A
  • active transport of ions into the stele lowers the water potential within the stele
  • water passively flows into the roots, pushing the water up
  • gravity acts against root pressure, so in tall plants very little root pressure is generated due to the weight of the water pushing down on the contents of the xylem
126
Q

cohesion-tension theory:

A
  • stomata open during the day to let CO2 in
  • water vapour leaves the air spaces of the plant via the stomata
  • this lowers the water potential in the leaves and creates a water potential gradient between the roots, the leaves and the air outside the plant
    • high water potential in the soil, very low water potential in the air
  • water lost from the leaves is replaced by evaporation of the thin layer of water that clings to the spongy mesophyll cells
  • because water has strong adhesive and cohesive properties, as the water evaporates, it is replaced by water clinging to the inside of the air spaces
  • this creates a tension (pulling) on the water in the xylem and gently pulls the water towards the direction of water loss
  • the cohesion of water is strong enough to transmit this pulling force all the way down to the roots
  • adhesion of water to the lignin cell walls of the xylem also aids in resisting gravity
127
Q

not all the water absorbed through the roots is lost though the leaves. where else is it used?

A
  • used during photosynthesis
  • some water is retained in cells for support - it maintains turgor pressure

(-some water is produced through respiration)
- this means that the volume of water taken up in the roots is not exactly equal to the volume of water lost from the leaves through transpiration

128
Q

if water loss exceeds water uptake, plants will ___ as the leaves and stems lose turgor pressure?

A

wilt

129
Q

what is a test to prove that water is lost form the leaves?

A
  • anhydrous cobalt paper can be used
  • when anhydrous (without water), cobalt chloride paper is dark blue but turns pink when it absorbs water
130
Q

what is the reservoir used for in a potometer experiment?

A
  • it is used to let water into the capillary tube and force the bubble backwards
131
Q

what are the 2 ways of estimating the area of the leaf you use in a potometer experiment?

A

(if you determine the area of the leaves on the shoot, you can also express the rate of water uptake in terms of leaf area)

  1. place a leaf/leaves on graph paper, draw around each leaf and estimate how many cm^2 are covered
  2. cut out and weigh 1cm^2 of leaf. weigh all the leaves from the shoot used in the experiment. divide the mass of the leaves by the mass of 1cm^2 of leaf
132
Q

what are some alternative theories to explain translocation?

A
  1. electro-osmosis = suggests that the sieve plates become charged due to the movement of water and ions across them. this sets up a charge gradient that will attract/repel substances and move them at different rates and in different directions depending on the charge
  2. cytoplasmic streaming = phloem sieve tubes contain very few organelles but do contain strands of cytoplasm that are continuous between cells, passing holes in the sieve plates. cytoplasm has been observed to move within cells and in different directions in different parts of the same cell
  3. protein contraction/peristalsis = there are many protein microtubules (cytoplasmic filaments) in the cytoplasm of sieve tubes. these are continuous from one cell to the next and can contract and push cytoplasm along in different directions and at different rates within the same sieve tube vessel

all if these alternative theories propose active processes and support the fact that ATP is used along the whole length of a phloem sieve tube and not just at the loading point

133
Q

explain how the use of radio-isotopes can be used to prove that it is the phloem that carry the products of photosynthesis

A
  • exposing leaves to radioactive CO2 containing the radioactive 14-C isotope of carbon as their only external source of CO2
  • sugars and other substances made as a result of photosynthesis would then be radioactive
  • when photographic or x-ray film is exposed to radiation, it becomes exposed
  • when sections of the stem are examined using chemicals that become black under exposure to radiation, black regions appear in the phloem
  • the only source of radioactivity was from the 14-CO2, therefore the radioactive carbon must now be incorporated into organic chemicals produced via photosynthesis and carried in the phloem
134
Q

what are conclusions from the use of radioisotopes in investigating translocation?

A
  • transport of carbon compounds from the leaves is bi-directional —> radioactivity detected both above and below the site of photosynthesis using 14-CO2
  • the products of photosynthesis must be transported in the phloem —> radioactive compounds only detected in phloem not xylem
135
Q

how do aphids feed?

A
  • by inserting their mouthparts - called a stylet - through the outer tissues of a plant stem into a phloem sieve tube
  • the stylet is hollow and is used to feed on the sugar-rich contents of the phloem
  • pressure in the phloem is higher than in the aphid so the fluid inside the phloem is forced out

as level biology (13 decks)

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