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
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
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
4
Q
What is are vascular
bundles?
A
The vascular system of herbaceous
dicots, made up of xylem and
phloem tissue
5
Q
Vascular bundles in the stem
A
Around the edge to give strength
and support
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
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
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
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
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
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
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)
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
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
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
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
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
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
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)
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
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
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)
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
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
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
```
