transport in plants Flashcards

1
Q

Give 3 reasons why
multicellular plants need
transport systems?

A
Metabolic demands 
• O2 and glucose need to be 
transported around the plant, and 
the waste products of cell 
metabolism removed 
• Hormones need to be transported 
• Mineral ions absorbed by the roots 
need to be transported to all cells 
to make proteins and enzymes 
required for cell structure 
Size 
• Plants continue to grow 
throughout their lives
• Perennial plants are large 
• An effective transport system is 
needed to move substances both 
up and down from the tip of the 
roots to the very topmost leaves 
and stems
SA:V 
• Relatively small SA:V ratio so they 
can’t rely on diffusion alone to 
supply their cells with everything 
they need
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2
Q

What are dicotyledonous

plants (dicots)?

A
Plants that produce seeds 
containing two cotyledons, which 
act as food stores for the developing 
embryo and form the first leaves 
when the seed germinates
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3
Q

Describe the two types of

dicots

A
Herbaceous dicots 
• Soft tissues, relatively short life 
cycle 
Woody (arborescent) dicots
• Hard lignified tissues, long life 
cycle
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4
Q

What is are vascular

bundles?

A

The vascular system of herbaceous
dicots, made up of xylem and
phloem tissue

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5
Q

Vascular bundles in the stem

A

Around the edge to give strength

and support

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6
Q

Vascular bundles in the roots

A

In the middle to help the plant
withstand the tugging strains that
result as the stems and leaves are
blown in the wind

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7
Q

Vascular bundles in leaves

A
• The midrib of a dicot leaf is the 
main vein carrying the vascular 
tissue through the organ 
• Helps to support the structure of 
the leaf 
• Many small, branching veins 
spread through the leaf, 
functioning in both transport and 
support
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8
Q

Describe the structure of the

xylem

A
• Xylem is a largely non-living tissue 
• Made up of several types of cells, 
most of which are dead 
• Xylem vessels are the main 
structures: long, hollow structures 
made by several columns of cells 
fusing together end to end 
• They have no cytoplasm or 
organelles present 
• Thick-walled xylem parenchyma 
packs around the xylem vessels 
storing food and containing tannin 
deposits (bitter tasting chemical 
that protects plant tissues from 
herbivore attacks)
• Xylem fibres are long cells with 
lignified secondary walls that 
provide extra mechanical strength 
(and waterproofing) but do not 
transport water
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9
Q

How can lignin be laid down
in the walls of xylem
vessels?

A
• Can form rings, spirals or relatively 
solid tubes with lots of small 
unlignified areas called bordered 
pits 
• This is where water leaves the 
xylem and moves into other cells 
of the plant
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10
Q

What is the function of the

xylem?

A
• The transport of water and mineral 
ions
• Support 
• The flow of materials in the xylem 
is up from the roots to the shoots 
and leaves
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11
Q

What is the phloem?

A

A living tissue that transports food in
the form of organic solutes around
the plant from the leaves where they
are made by photosynthesis

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12
Q

Describe the structure of the

phloem

A
Main transporting vessels are the 
sieve tube elements 
• Sieve tubes are made up of many 
cells joined end to end to form a 
long, hollow structure 
• Phloem tubes are not lignified 
• In the areas between the cells, 
walls become perforated to form 
sieve plates, which let the phloem 
contents flow through 
• Tonoplast, nucleus and some of 
the other organelles break down 
• Mature phloem cells have no 
nucleus 
• Companion cells are linked to the 
sieve tube elements by many 
plasmodesmata. They are active 
cells with a nucleus and all their 
organelles 
• Phloem tissue also contains 
supporting tissues including fibres 
and sclereids (cells with extremely 
thick cell walls)
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13
Q

How is water important for

the structure of plants?

A
Turgor pressure (or hydrostatic 
pressure) as a result of osmosis in 
plant cells provides a hydrostatic 
skeleton to support the stems and 
leaves 
• Turgor pressure in leaf cells is 100 
times greater than human systolic 
blood pressure 
• Turgor also drives expansion - it is 
the force that enables plant roots 
to force their way through tarmac 
and concrete
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14
Q

How is water important for

the metabolism of plants?

A
• The loss of water by evaporation 
helps to keep plants cool 
• Mineral ions and the products of 
photosynthesis are transported in 
aqueous solutions 
• Water is a raw material for 
photosynthesis
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15
Q

What are root hair cells?

A
Cells found just behind the growing 
tip of a plant root that have long 
hair-like extensions that greatly 
increase the surface area available 
for the absorption of water and 
mineral ions from the soil
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16
Q

What is a root hair?

A

A long, thin extension from a root
hair cell, a specialised epidermal cell
found near the growing root tip

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17
Q

How are root hairs adapted

as exchange surfaces?

A
• Microscopic size means they can 
penetrate easily between soil 
particles 
• Each microscopic hair has a large 
SA:V ratio, and there are 
thousands on each growing tip 
• Each hair has a thin surface layer 
(just cell wall and cell-surface 
membrane) through which 
diffusion and osmosis can take 
place quickly 
• Concentration of solutes in the 
cytoplasm of root hair cells 
maintains a water potential 
gradient between the soil water 
and the cell
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18
Q

Why does water move into

root hair cells by osmosis?

A
• Soil water has a very low 
concentration of dissolved 
minerals so it has a very high 
water potential 
• Cytoplasm and vacuolar sap of the 
the root hair cell (and the other 
root cells) contain many different 
solvents including sugars, mineral 
ions, and amino acids so the water 
potential in the cell is lower 
• As a result water moves into the 
root hair cells by osmosis
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19
Q

What are the 2 pathways
that water can move across
the root to the xylem
through?

A
Symplast pathway - Movement of 
water through the symplast 
(continuous cytoplasm of living plant 
cells that is connected through 
plasmodesmata)
Apoplast pathway - Movement of 
water through the apoplast (the cell 
walls and the intercellular spaces)
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20
Q

Describe the symplast

pathway

A
1. Water moves through the 
symplast by osmosis 
2. Root hair cell has a higher water 
potential than the next cell along 
3. This is the result of the water 
diffusing in from the soil which 
makes the cytoplasm more dilute 
4. So water moves from the root 
hair cell into the next door cell by 
osmosis 
5. This process continues from cell 
to cell across the root until the 
xylem is reached 
6. As water leaves the root hair cell 
by osmosis, the water potential 
(Ψ) of the cytoplasm falls again, 
maintaining a steep Ψ gradient 
to ensure that as much water as 
possible continues to move into 
the cell from the soil
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21
Q

Describe the apoplast

pathway

A
1. Water fills the spaces between 
the loose, open network of fibres 
in the cellulose cell wall 
2. As water molecules move into 
the xylem, more water molecules 
are pulled through the apoplast 
behind them due to the cohesive 
forces between the water 
molecules 
3. The pull from water moving into 
the xylem and up the plant along 
with the cohesive forces 
between the water molecules 
creates a tension that means 
there is a continuous flow of 
water through the open structure 
of the cellulose wall, which offers 
little or no resistance
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22
Q

How far does water move

across the root?

A
Water moves across the root in the 
apoplast and symplast until it 
reaches the endodermis (layer of 
cells surrounding the vascular tissue 
of the roots)
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23
Q

What is the Casparian strip?

A

A band of waxy material called
Suberin that runs around each of the
endodermal cells, forming a
waterproof layer

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24
Q

What happens when water
in the apoplast pathway
meets the casparian strip?

A
• The water can go no further and is 
forced into the cytoplasm of the 
cell, joining the water in the 
symplast pathway 
• To get to the cytoplasm, water 
must pass through the selectively 
permeable cell surface 
membranes, stopping any 
potentially-toxic solutes in the soil 
water from reaching living tissues, 
as the membranes would have no 
carrier proteins to admit them
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25
How do the the endodermal cells move mineral ions into the xylem?
Active transport, as the solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to cells in the xylem
26
What increases the rate of water moving into the xylem by osmosis?
``` • Endodermal cells move mineral ions into the xylem by active transport • As a result, the water potential of the xylem cells is much lower than that of the endodermal cells • This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway ```
27
What happens once water is | inside the vascular bundle?
It returns to the apoplast pathway to enter the xylem itself and move up the plant
28
What is root pressure?
``` The active pumping of minerals into the xylem by root cells that produces a movement of water into the xylem by osmosis • Independent of any effects of transpiration • Gives water a push up the xylem, but under most circumstances it is not the major factor in the movement of water up from the roots to the leaves ```
29
Give 4 pieces of evidence for the role of active transport in root pressure
``` • Some poisons, e.g. cyanide, affect the mitochondria and prevent the production of ATP. If cyanide is applied to root cells so there is no energy supply, the root pressure disappears • Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved • If levels of oxygen or respiratory substrates fall, root pressure falls • Xylem sap may exude from the cut end of stems. In the natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions e.g. overnight, when transpiration is low. This is known as guttation ```
30
What is photosynthesis?
``` Synthesis of complex organic molecules using light. • CO2 diffuses into the leaf cells down a concentration gradient from the air spaces within the leaf • Due to gas exchange, O2 also moves out of the leaf cells into the air spaces by diffusion down a concentration gradient • Water evaporates from surfaces of the leaf cells into the air spaces ```
31
What is transpiration?
``` The loss of water vapour from the stems and leaves of a plant as a result of evaporation from cell surfaces inside the leaf and diffusion down a concentration gradient out through the stomata ```
32
Describe leaves
``` Have a very large surface area for capturing sunlight and carrying out photosynthesis • Surfaces are covered with a waxy cuticle that makes them waterproof - this is an important adaptation that prevent leaf cells losing water rapidly and constantly by evaporation from their surfaces ```
33
How are gases exchanged | with the air through leaves?
``` CO2 moves from the external air into the leaf, and O2 moves out of the leaf by diffusion down a concentration gradient through stomata.The stomata can be opened and closed by guard cells ```
34
What happens when the | stomata are open?
``` Stomata open and close to control the amount of water lost by a plant, but during the day, a plant needs to take in CO2 for photosynthesis, and at night when no O2 is being produced by photosynthesis, it needs to take in O2 for cellular respiration ```
35
What is the transpiration | stream?
The movement of water through a plant from the roots until it is lost by evaporation from the leaves
36
What is cohesion-tension | theory?
The best current model explaining the movement of water through a plant during transpiration
37
Describe the stages in | cohesion-tension theory
``` 1. Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf, and move out of stomata into the surrounding air by diffusion down a concentration gradient 2. The loss of water by evaporation from a mesophyll cell lowers the Ψ of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways 3. This is repeated across the leaf to the xylem. Water moves out of the xylem by osmosis into the cells of the leaf 4. Capillary action allows water to rise up a narrow tube against gravity. Water is drawn up by the transpiration pull 5. Transpiration pull results in a tension in xylem, helping move water across roots from the soil ```
38
What is capillary action?
``` Capillary action = Adhesion + Cohesion • Adhesion - water molecules form hydrogen bonds with the carbohydrates in the walls of narrow xylem vessels • Cohesion - water molecules form hydrogen bonds with each other so tend to stick together. ```
39
What is the transpiration | pull?
Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation
40
Give evidence for the | cohesion-tension theory
``` Changes in the diameter of trees • During day, transpiration and tension in the xylem vessels is at its highest, and diameter of tree decreases • At night, transpiration and tension in the xylem vessels is at lowest, and diameter of the tree increases • Measuring circumference of a tree at different times of the day When a xylem vessel is broken, e.g. when you cut flower stems to put them in water • Air is drawn in to the xylem rather than water leaking out • Plant can no longer move water up the stem as continuous stream of water molecules held by cohesive forces has been broken by air ```
41
How is transpiration a | benefit for plants?
``` • Delivers water, and the mineral ions dissolved in that water, to the cells where they are needed • Evaporation of water from the leaf cell surfaces helps to cool the leaf down and prevent heat damage ```
42
How is transpiration a | problem for plants?
``` • The amount of water available is often limited • High intensity sunlight = rapid photosynthesis = high rate of gas exchange, the stomata will all be open, and the plant may lose so much water though transpiration that the supply cannot meet the demand ```
43
How do guard cells control the opening and closing of stomata?
``` Opening • Favourable environmental conditions • Guard cells pump in solutes by active transport, increasing their turgor • Cellulose hoops prevent the cells from swelling in width, so they extend lengthways • Inner wall of guard cell is less flexible than outer walls so cells become bean-shaped and open the pore Closing • Water becomes scarce • Hormonal signals from roots trigger turgor loss from guard cells • Asymmetric configuration of the guard cells close the stomatal pore, and so conserve water ```
44
What are the factors affecting the rate of transpiration?
``` Light • Relative humidity • Temperature • Air movement • Soil-water availability ```
45
How does light affect the rate of transpiration?
``` • Increasing light intensity increases rate of transpiration • Light is required for photosynthesis; in the light the stomata open for gas exchange needed, and in the dark, most of the stomata close • Increasing light intensity, increases number of open stomata, increasing rate of water vapour diffusing out, increasing the evaporation from the surfaces of the leaf ```
46
How does relative humidity affect the rate of transpiration?
``` Relative humidity is a measure of the amount of water vapour in the air (humidity) compared to the total concentration of water the air can hold • High relative humidity lowers rate of transpiration because of reduced water vapour potential gradient between the inside of the leaf and the outside air • Very dry air has the opposing effect and increases the rate of transpiration ```
47
How does temperature affect the rate of transpiration?
``` • Increases in temperature increase the KE of the water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf • Increases in temperature increases the concentration that the external air can hold before it becomes saturated (decreases its relative humidity and its water potential) • Both: Increases the diffusion gradient between air inside and outside the leaf, increases rate of transpiration ```
48
How does air movement affect the rate of transpiration?
``` • Each leaf has a layer of still air trapped around it due to the shape of the leaf, and features such as hair on the surface of the leaf decrease air movement close to the leaf • Water vapour that diffuses out of the leaf accumulates here • Water vapour potential around the stomata increases, in turn reducing the diffusion gradient • Air movement or wind will increases the rate of transpiration • A long period of still air will reduce transpiration ```
49
How does soil-water availability affect the rate of | transpiration?
If it is very dry, the plant will be under water stress, and the rate of transpiration will be reduced
50
What is glucose used for in plants?
``` Converted to sucrose for transport, then when it reaches the cells where it’s needed, it is converted to …. • Glucose for respiration • Starch for storage • Used to produce the amino acids and other compounds needed within the cell ```
51
What is translocation?
``` The movement of organic solutes around a plant in the phloem • Active process that requires energy to take place • Substances can be transported up or down the plant • The main products of photosynthesis that are transported are known as assimilates • From sources to sinks ```
52
What are the main sources of assimilates in a plant?
``` Green leaves and green stems • Stage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period • Food stores in seeds when they germinate ```
53
What are the main sinks in a plant?
``` Roots that are growing and/or actively absorbing mineral ions • Meristems that are actively diving • Any part of the plant that are laying down food stores, e.g. developing seeds, fruits or storage organs ```
54
Why is sucrose transported | rather than glucose?
Sucrose is not used in metabolism as readily as glucose and therefore is less likely to be metabolised during the transport process
55
How does phloem loading happen via the apoplast | route?
``` 1. ATP is used to actively transport H+ ions out of the companion cells 2. This increases their concentration outside the cells and decreases their concentration inside the companion cells, creating a concentration gradient 3. H+ ions diffuse back into the companion cells through cotransporter proteins that only allow the movement of H+ ions into the cell if they are accompanied by sucrose molecules 4. As the concentration of sucrose in the companion cell increases, it diffuses through plasmodesmata into the sieve tube ```
56
Why is co-transport known as secondary active | transport?
It results from the active transport of the H+ ions out of the cell, and moves sucrose against its concentration gradient
57
What are the adaptations of companion cells?
``` Many infoldings in their cell membranes to give an increased surface area for the active transport of sucrose into the cell cytoplasm • Many mitochondria to supply the ATP needed for the transport pumps ```
58
What happens as a result of the build up of sucrose in | the companion cell and sieve element?
``` • Water also moves in by osmosis • Leads to a build-up of turgor pressure due to the rigid cell walls • The water carrying the assimilates moves into the tubes of the sieve elements, reducing the pressure in the companion cells, and moves up or down the plant by mass flow to areas of lower pressure (the sinks) Phloem unloading • Diffusion of the sucrose from the phloem into the surrounding cells • Sucrose rapidly moves on into other cells by diffusion or is converted into another substance, so that a concentration gradient of sucrose is maintained between contents of the phloem and the surrounding cells ```
59
What does the loss of solutes from the phloem lead to?
``` • A rise in the water potential of the phloem • Water moves out into the surrounding cells by osmosis • Some of the water that carried the solute to the sink is drawn into the transpiration stream in the xylem ```
60
What is the evidence for the main principles of translocation?
``` • Microscopy allows us to see the adaptations of the companion cells for active transport • If the mitochondria of the companion cells are poisoned, translocation stops • Flow of sugars in the phloem is 10 000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow • Aphids can be used to demonstrate the translocation of organic solutes in the phloem - aphid studies show there is a positive pressure in the phloem that forces the sap out through the stylet. The pressure and therefore the flow rate in the phloem is lower closer to the sink than it is near the source. The concentration of sucrose in the phloem sap is also higher near to the source than near the sink ```
61
What is there no evidence for?
``` Not all solutes in the phloem move at the same rate (but sucrose always moves at the same rate regardless of the concentration in the sink) • The role of sieve plates in the process is unclear ```
62
What are the general adaptations of most plants | to conserve water?
``` Waxy cuticle to reduce transpiration from leaf surfaces • Stomata found mainly on the underside of the leaf that can be closed to prevent the loss of water vapour • Roots that down down to the water in the soil ```
63
What are xerophytes?
``` Plants with adaptations that enable them to survive in dry habitats or habitats where water is in short supply in the environment • Conifers • Marram grass • Plants that survive in very cold and icy conditions • Cacti ```
64
What are the adaptations of xerophytes?
* Thick waxy cuticle * Sunken stomata * Reduced numbers of stomata * Reduced leaves * Hairy leaves * Curled leaves * Succulents * Leaf loss * Root adaptations * Avoiding the problem
65
What are hydrophytes?
``` Plants with adaptations that enable them to survive in very wet habitats or submerged or at the surface of water • Water lilies • Water cress • Duckweeds • Marginals e.g.bulrushes ```