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Adaptations For Transport In Plants Flashcards

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1
Q

What is the function of vascular tissue?

A
  • vascular tissue transports materials around the body
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2
Q

Where is vascular tissue found in plants?

A
  • In plants it is xylem and phloem, found adjacent to each other in vascular bundles
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3
Q

Where is vascular tissue found in animals?

A
  • In animals, the vascular tissue is in blood
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4
Q

The distribution of vascular tissue in plants (in the roots)

What is the advantage of this?

A
  • The xylem is central and star-shaped with phloem between groups of xylem towards the centre and phloem towards the outside
  • This gives flexible support and resist bending
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5
Q

The distribution of vascular tissue in plants (in the leaves)

A
  • In leaves the vascular tissue is in the midrib and in a network of veins, giving flexible strength and residence to tearing
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6
Q

What are the main cell types in the xylem?

A

Tracheids

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7
Q

What plants do tracheids occur in? (Structure of the xylem)

Why do they not occur in Moses?

A
  • ferns
  • conifers
  • angiosperms (flowering plants)

Moses have no water-conducting tissue and are therefore poorer at transporting water and cannot grow as tall as there other plants

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8
Q

Vessels occur only in what plants ?

Why?

A
  • Vessels occur only in angiosperms
  • As lignin builds up in their cell walls, the contents die, leaving an empty space, the lumen
  • As the tissue develops, the end walls break down, leaving a long hollow tube, like a drainpipe, through which water climbs straight up the plant
  • The lignin is laid down in a characteristic spiral pattern and, unlike cellulose oh phloem cell walls, stains red so xylem is easy to identity in microscope sections
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9
Q

What are the 2 functions of the xylem?

A
  1. Transport of water and dissolved minerals

2. Providing mechanical strength and support

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10
Q

Transport in the xylem (Water uptake by the roots) P1

A
  • Terrestrial plants, like animals, risk dehydration and must conserve water
  • Water is taken up from the soil through their roots and transported to the leaves, where it maintains turgidity and is a reactant in photosynthesis
  • But much is lost through the stomata, in a process called transpiration
  • The loss must be offset by constant replacement from the soil
  • The region of greatest uptake is the root hair zone, where the surface area of the root is enormously increased by the presence of root hairs and uptake is enhanced by their thin cell walls
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11
Q

Transport in the xylem (Water uptake by the roots) P2

A
  • Soil water contains a very dilute solution of mineral salts and has a high water potential
  • The vacuole and cytoplasm of the root hair cell contain a concentrated solution of solutes and have a lower, more negative, water potential
  • Water passes into the root hair cell by osmosis, down a water potential gradient
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12
Q

Movement of water through the roots

A

Water must move into the xylem to be distributed around the plant. It can travel there, across the cells of the root cortex

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13
Q

Water can travel through the xylem across the cells of the root cortex by 3 different routes

A

1) The apoplast pathway- water moves in the cell walls. Cellulose fibres in the cell wall are separated by spaces through which the water moves
2) The symplast pathway- water moves through the cytoplasm and the plasmodesmata. Symplast is a continual pathway across the root pathway
3) The vauolar pathway- water moves from vacuole to vacuole

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14
Q

Which is the fastest pathway

A

Apoplast pathway

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15
Q

Define plasmodesmata

A

Strands of cytoplasm through pits in the cell wall joining adjacent cells

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16
Q

Why can’t water enter the xylem from the apoplast?

How can water pass into the xylem?

A
  • Lignin makes xylem walls waterproof

- Water can only pass into the xylem from the symplast or vacuolar pathways so it must leave the apoplast pathway

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17
Q

What is pericycle?

A

The vascular tissue, in the centre of the root, is surrounded by a region called pericycle

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18
Q

What is the endodermis?

A

The pericycle is surrounded by a single layer of cells, the endodermis

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19
Q

What is the Casparian strip?

A

The cell walls walls of the endodermis is impregnated with a waxy material, Suberin forming a distinctive band on the radial and tangential walls, called the Casparian strip

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20
Q

What is the function of the casparian strip?

A

Suberin is waterproof so the Casparian strip prevents water moving further in the apoplast and drives it into the cytoplasm

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21
Q

Water moves from the root endodermis into the xylem by osmosis across the endodermal cell membranes. For this to be efficient, the water potential of the xylem must be much more negative than the water potential of the endodermal cells.
This is achieved in 2 ways:

A

1) The water potential of the endodermis cells is raised by water being driven in by the Casparian strip
2) The water potential of the xylem is decreased by active transport of mineral salts, mainly sodium ions, from the endodermis and pericycle into the xylem

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22
Q

How does water enter a plant?

A
  • Water moves into the xylem, by osmosis, down a water potential gradient
  • Water coming into the xylem generates an upwards push, the root pressure, on water already in the xylem
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23
Q

3 main mechanisms that allow the movement of water through the plant: Cohesion-tension

A
  • Water vapour evaporates from leaf cells into the air spaces and diffuses out through the stomata into the atmosphere
  • This draws water across the cells of the leaf in the apoplast, symplast and vacuolar pathways, from the xylem
  • As water molecules leaves xylem cells in the leaf, they pull up other water molecules behind them in the xylem
  • The water molecules all move because they show cohesion
  • This continuous pull produces tension in the water column
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24
Q

Destine adhesion

A

The charges on the water molecules also cause attraction to the hydrophilic lining of the vessels. This is adhesion and contributes to water movement up the xylem

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25
Q

What is cohesion theory?

A

Describes water movement up the xylem, by this combination of cohesion of water molecules and tension in the water column resulting from their cohesion

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26
Q

3 main mechanisms that allow the movement of water through the plant: Capillarity

A
  • Capillarity is the movement of water up narrow tubes, in this case the xylem, by capillarity action
  • It only operates over short distances, up to a metre
  • It may have a role in mosses, but only makes a small contribution to water movement in plants more than a few centimetres high
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27
Q

3 main mechanisms that allow the movement of water through the plant: Root pressure

A
  • Operates over short distances in living plants and is a consequence of osmotic movement of water into the xylem pushing water already there further up
  • It is caused by the osmotic movement of water down the water potential gradient across the root and into the base of the xylem
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28
Q

In the transpiration stream, water is drawn upwards by:

A
  • The cohesive forces between water molecules

- The adhesive forces between the water molecules and the hydrophilic lining of the xylem vessels

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29
Q

What is transpiration?

A
  • The continual flow of water in at the roots,up the stem to the leaves and out to the atmosphere, is the transpiration stream
30
Q

What percentage of water absorbed by the plant is lost by continual evaporation by continual evaporation from the leaves?

A

99%

31
Q

Why do plants have to balance water uptake with loss?

A
  • If they lose more than they absorb, the leaves wilt
  • If only a small volume is lost, the plant recovers when water is available
  • If an excessive volume is lost, the plant cannot regain its turgor after wilting and it dies
32
Q

What dilemma does the plant have?

A
  • Plants face a dilemma
  • The stomata must be open during the day to allow gas exchange between the leaf tissues and the atmosphere
  • But this means that the plant loses valuable water
33
Q

Factors affecting the rate of transpiration

A

1) Genetic factors as those controlling the number, distribution and size of the stomata
2) Environmental factors such as temperature, light intensity, humidity and air movement

34
Q

Factors affecting the rate of transpiration: temperature

A
  • A temperature lowers the water potential of the atmosphere
  • It increases the kinetic energy of the water molecules, accelerating their rate of evaporation from the walls of the mesophyll cells and, if the stomata are open, speeds up their rate of diffusion out into the atmosphere
  • The higher temperature causes the water molecules to diffuse away from the leaf more quickly, reducing the water potential around the leaf
35
Q

Factors affecting the rate of transpiration: Humidity

A
  • The air inside the leaf is saturated with water vapour, so its relative humidity is 100%
  • The humidity of the atmosphere surrounding a leaf varies, but is never greater than 10
  • There is a water potential between leaf and the atmosphere and when the stomata are open, water vapour diffuses out of the leaf, down the water potential gradient
  • Transpiration is still air results in the accumulation of a layer of saturated air at the surfaces of the leaves
  • The water vapour gradually diffuses away, leaving concentric rings of decreasing humidity the further away fromt the leaf you go
  • The higher the humidity, the higher the water potential
  • Water vapour diffuses down this gradient of relative humidity, which is also a gradient of water potential, away from the leaf
36
Q

Factors affecting the rate of transpiration: Air movement

A
  • Movement of the surrounding air blows away the layer of humid air at the leaf surface
  • The water potential gradient between the inside and outside of the leaf consequently increases and water vapour diffuses out through the stomata more quickly
  • The faster the air is moving,t the faster the concentric shells of water vapour get blown away, the faster transpiration occurs
37
Q

Factors affecting the rate of transpiration: Light intensity

A
  • Light intensity affects transpiration by controlling the degree of the stomatal opening
  • In most plants, the stomata open wider as light intensity increases, increasing the rate of transpiration
  • So stomata tend to open widest in the middle of the day, less widely in the morning and evening to be closed at night
  • These factors affecting transpiration do not act independently, but interact with each other
  • More water is lost on a dry, windy day than a humid still day
  • this is because the walls or the spongy mesophyll cells are saturated with water which evaporates and moves down a gradient of water potential, which has a low humidity, the wind having reduced the thickness of the layer of saturated air at the leaf surface
38
Q

What is another name for a photometer?

A

A transpirometer

39
Q

What does it measure?

What does it not measure?

A
  • It does not primarily measure transpiration
  • it actually measures water uptake
  • but since most of the water taken up by a leafy shoot I a lost through transpiration, the rate of uptake is almost the same as the rate of transpiration
40
Q

What can a photometer be used for?

A

To measure water uptake by the same shoot under different conditions or can be used to compare the uptake by leafy shoots of different species under the same conditions

41
Q

Steps for using a potometer:

A

1) Cut a leafy shoot under water, so no air enters the xylem
2) Under water, fill the potometer with water, ensuring there are no air bubbles
3) fit the leafy shoot to the potometer with rubber tubing under water, to prevent air bubbles forming In the apparatus or the xylem
4) Remove the potometer and shoot from the water, seal joints with Vaseline and dry carefully
5) Introduce an air bubble or meniscus into the capillary tube
6) Measure the distance the air bubble moves in a given time
7) Use the water reservoir to bring the air bubble or meniscus back to the starting point. Repeat the measurement a number of times and calculate a mean distance
8) The experiment may be repeated to compare the rates of water uptake under different conditions, for example altered light intensity or air movement

42
Q

Plants can be classified depending on the prevailing water supply: what are the 3 types

A
  • mesophytes
  • xerophytes
  • hydrophytes
43
Q

Characteristics of Mesophytes

A
  • most land plants growing in temperate regions are mesophytes
  • They have an adequate water supply and, although they lose a lot of water, it is readily replaced by uptake from the soil, so they do not require any special means of conserving it
  • If such a plant loses too much water, it wilts and the leaves droop
  • The stomata close and leaf surface area available for absorbing light is reduced so photosynthesis because less efficient
44
Q

Conditions in which mesophytes grow

A

Most crop plants are mesophytes

  • They are adapted to grow best in well-drained soils and moderately dry air
  • Water uptake during the night replaces the water lost during the day
  • Excessive water loss is prevented because stomata generally close at night, when it is dark
45
Q

Mesophytes must survive unfavorable times of year, particularly when the ground is frozen and liquid water is not available:

A
  • Many shed their leaves before winter, so that they do not lose water by transpiration, when liquid water may be scarce
  • The aerial parts of many no-woody plants die off in winter so they are not exposed to frost or cold winds, but their underground organs, such as bulbs and corms survive
  • Most annual mesophytes (plants, that flower, produce seed and die in the same year) over-winter as dormant seeds, with such a low metabolic rate that almost no water is required
46
Q

Characteristics of xerophytes

A
  • Xerophytes are plants with xeromorphic characteristics
  • They have adapted to living with low water availability and have modified structures which prevent excessive water loss
  • They may live in hot, dry desert regions where the soil water is frozen for much of the year or exposed, windy locations
47
Q

Give an example of a xerophyte

A

Marram grass

Which colonises sand dunes

48
Q

Describe the conditions in sand dunes

A
  • no soil
  • rainwater drains away rapidly
  • high wind speeds
  • salt spray
  • lack of shade from the sun
49
Q

Name 2 modifications that marram grass has

A

1) ROLLED LEAVES- large thin-walled epidermal cells, called hinge cells, at the bases of the grooves become plasmolysed when they loose water from excessive transpiration, and the leaf rolls with its upper (adaxial) surface inwards. This reduces the leaf area exposed to the air and so reduces Transpiration
2) SUNKEN STOMATA- stomata occur in grooves on the adaxial surface, but not the outer (abaxial) surface, of the leaf. They are in pits or depressions and humid air is trapped in the pit, outside the stomata.This reduces the water potential gradient between the inside of the leaf and the outside and so reduces the rate of osmosis of water out through the stomata

50
Q

Name 3 modifications that marram grass has

A

3) HAIRS- Stiff, interlocking hairs trap water vapour and reduce the water potential gradient between inside of the leaf and outside
4) THICK CUTICLE- The cuticle is a waxy covering over the outer (abaxial) leaf surfaces. Wax is waterproof and so reduces water loss. The thicker this cuticle, the lower the rate of transpiration through the cuticle
5) FIBRES- Fibres of sclerenchyma are stiff so the leaf shape is maintained even when the cells become flaccid

51
Q

Characteristics of hydrophytes

A

-Hydrophytes grow partially or wholly submerged in water, e.g. The water lily, which is rooted to the mud at the bottom of a pond and has leaves floating on the water surface

52
Q

Give two examples of hydrophytes

A

1) Water is a supportive medium so they have little or no lignified support tissues
2) Surrounded by water, there is little need for transport tissue, so xylem is poorly developed

53
Q

Give 3 examples of hydrophytes

A

1) Leaves have little or no cuticle, because there is no need to prevent water loss
2) Stomata are on the upper surface of floating leaves, because the lower surface is in the water
3) Stems and leaves have large air spaces, continuous down to their roots, forming a reservoir of oxygen and carbon dioxide, which provide buoyancy

54
Q

What is translocation?

A
  • The transport of soluble organic materials, such as sucrose and amino acids in plants, is called translocation
  • These products of photosynthesis are translocated in the phloem, away from the site of photosynthesis in the leaves
55
Q

What is the ‘source?’

What is the ‘sink?’

A
  • The source is the area with a high concentration of the products
  • The sinks are ares that used the products for growth or storage
56
Q

State one way (regarding translocation) in which the xylem is different from the phloem

A

The phloem can translocate up, down and sideways, to wherever the products of photosynthesis are needed

57
Q

What are the two main type of cells that make up the phloem?

A
  • Sieve tubes

- Companion cells

58
Q

Which of the two tubes that make up the phloem are adapted for the flow of material p?

A

Sieve tubes

59
Q

What do sieve tubes comprise?

A

Sieve tube elements

60
Q

The end walls of the phloem do not brake down like in the phloem, what do they do instead?

A
  • Instead, the end walls, and sometimes parts of the side walls as well, are perforated, in areas called sieve plates
61
Q

What extends from one side of the sieve tube element of the next

A

Cytoplasmic filaments containing phloem protein extend from one sieve tube element to the next through pores in the sieve plate

62
Q

Sieve tube elements

A
  • Sieve tube elements loose their nucleus and most of their other organelles during their development, allowing space for transporting materials
  • Their metabolism is controlled by at least one neighbouring companion cell
63
Q

Companion cells:

A
  • the metabolism of sieve tube elements are controlled by at least one companion cells
  • Companion cells are biochemically active, as indicated by the large nucleus, dense cytoplasm containing much RER and many mitochondria
64
Q

How are companion cells connected to sieve tube elements?

A

By plasmodesmata

65
Q

Experimental evidence shows that organic substances are translocated through the phloem. Several different techniques have been used:

ringing experiments

A

RINGING EXPERIMENTS:

  • Early evidence was obtained from ringing experiments where cylinders of outer bark tissue were removed from all the way around woody stem, in a ring
  • This removed the phloem
  • After leaving the plant some time, while it photosynthesised, the phloem contents above and below the ring were analysed
  • Above the ring, there was a lot of sucrose, suggesting that it had been translocated in the phloem
  • Below the ring there was no sucrose, suggesting that it had been used by the plant tissues but not replaced, because the ring prevented it from being moved downwards
66
Q

Experimental evidence shows that organic substances are translocated through the phloem. Several different techniques have been used:

Radioactive tracers and autoradiography

A
  • A plant photosynthesises in the presence of a radioactive isotope such as carbon 14 in CO2
  • A stem section is placed on photographic film, which is exposed if there is a radiation source, producing an autoradiograph
  • The position of exposure, and therefore the radioactivity, coincides with the position of the phloem, indicating that it is the phloem that translocates the sucrose made form 14CO2 in photosynthesis
67
Q

Experimental evidence shows that organic substances are translocated through the phloem. Several different techniques have been used:

Aphid experiments:

A
  • An aphid is a hollow, needle-like mouthpart called a stylet
  • This is inserted into a sieve tube and the phloem contents, the sap, exude under pressure into the aphids stylet
  • In some experiments the aphid was anaesthetised and removed
  • It’s styled remained embedded in the phloem
  • As the sap in the phloem is under pressure it exuded from the stylet and was collected and analysis showed the presence of sucrose
68
Q

Experimental evidence shows that organic substances are translocated through the phloem. Several different techniques have been used

Aphids and radioactive tracers:

A
  • The aphid experiments were extended to plants which have been photosynthesising within 14CO2
  • These showed that the radioactivity, and therefore, the sucrose made in photosynthesis with 14CO2
  • These showed that the radioactivity,and therefore, the sucrose made in photosynthesis, moved at speed 0.5-1 meters per hour
  • This is much faster than the rate of diffusion alone so some additional mechanism had to be considered
69
Q

Theories of translocation: What theory was proposed to explain translocation

A

Mass flow theory
- It suggests that there is a passive mass flow of sugars from the phloem of the leaf, where there is the highest concentration (the source), to the other areas, such as growing tissues, where there is a lower concentration (the sink)

70
Q

Limitations of mass flow theory

A

It does not explain several aspects of translocation

1) The rate of movement of the phloem is about 1000 times faster than if the substances were moving by diffusion
2) It does not take into account the sieve plates
3) Phloem has a relatively high oxygen consumption, and translocation is slowed or stopped at low temperatures or if respiratory poisons, such as potassium cyanide are applied
4) The companion cells are biochemically very active, but the mass flow hypothesis does not suggest a role for them

71
Q

Other theories (than mass flow) suggest:

A
  • An ACTIVE PROCESS may be involved. Cyanide and low temperature inhibit translocation, indicating that energy generated by respiration is used
  • PROTEIN FILAMENTS pass through the sieve pores so perhaps different solutes are carried along different routes through the same sieve tube element
  • CYTOPLASMIC STREAMING could be responsible for movement in different directions in individual sieve tube elements, providing there was some mechanism to transport solute through the sieve plates

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