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
Q
How do the the endodermal
cells move mineral ions into
the xylem?
A
Active transport, as the solute
concentration in the cytoplasm of
the endodermal cells is relatively
dilute compared to cells in the xylem
26
Q
What increases the rate of
water moving into the xylem
by osmosis?
A
• 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
Q
What happens once water is
inside the vascular bundle?
A
It returns to the apoplast pathway to
enter the xylem itself and move up
the plant
28
Q
What is root pressure?
A
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
Q
Give 4 pieces of evidence
for the role of active
transport in root pressure
A
• 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
Q
What is photosynthesis?
A
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
Q
What is transpiration?
A
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
Q
Describe leaves
A
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
Q
How are gases exchanged
with the air through leaves?
A
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
Q
What happens when the
stomata are open?
A
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
Q
What is the transpiration
stream?
A
The movement of water through a
plant from the roots until it is lost by
evaporation from the leaves
36
Q
What is cohesion-tension
theory?
A
The best current model explaining
the movement of water through a
plant during transpiration
37
Q
Describe the stages in
cohesion-tension theory
A
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
Q
What is capillary action?
A
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
Q
What is the transpiration
pull?
A
Water is drawn up the xylem in a
continuous stream to replace the
water lost by evaporation
40
Q
Give evidence for the
cohesion-tension theory
A
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
Q
How is transpiration a
benefit for plants?
A
• 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
Q
How is transpiration a
problem for plants?
A
• 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
Q
How do guard cells control
the opening and closing of
stomata?
A
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
Q
What are the factors affecting the rate of transpiration?
A
Light • Relative humidity • Temperature • Air movement • Soil-water availability
45
Q
How does light affect the rate of transpiration?
A
• 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
Q
How does relative humidity affect the rate of transpiration?
A
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
Q
How does temperature affect the rate of transpiration?
A
• 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
Q
How does air movement affect the rate of transpiration?
A
• 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
Q
How does soil-water availability affect the rate of
transpiration?
A
If it is very dry, the plant will be
under water stress, and the rate of
transpiration will be reduced
50
Q
What is glucose used for in plants?
A
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
Q
What is translocation?
A
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
Q
What are the main sources of assimilates in a plant?
A
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
Q
What are the main sinks in a plant?
A
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
Q
Why is sucrose transported
rather than glucose?
A
Sucrose is not used in metabolism
as readily as glucose and therefore
is less likely to be metabolised
during the transport process
55
Q
How does phloem loading happen via the apoplast
route?
A
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
Q
Why is co-transport known as secondary active
transport?
A
It results from the active transport of
the H+ ions out of the cell, and
moves sucrose against its
concentration gradient
57
Q
What are the adaptations of companion cells?
A
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
Q
What happens as a result of the build up of sucrose in
the companion cell and sieve element?
A
• 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
Q
What does the loss of solutes from the phloem lead to?
A
• 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
Q
What is the evidence for the main principles of translocation?
A
• 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
Q
What is there no evidence for?
A
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
Q
What are the general adaptations of most plants
to conserve water?
A
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
Q
What are xerophytes?
A
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
Q
What are the adaptations of xerophytes?
A
- Thick waxy cuticle
- Sunken stomata
- Reduced numbers of stomata
- Reduced leaves
- Hairy leaves
- Curled leaves
- Succulents
- Leaf loss
- Root adaptations
- Avoiding the problem
65
Q
What are hydrophytes?
A
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
