9 - Transport In Plants 🌵 Flashcards
What are the main 3 reasons why multicellular plants need transport systems?
- metabolic demands
- size
- SA:V
Why is a transport system needed for metabolic demand?
- need oxygen and glucose transported for photosynthesis
- absorb mineral ions
Why is a transport system needed for size?
to move substances across the cell from roots to leaves
Why is a transport system needed for SA:V?
- leaves have large SA:V
- whole plant has small SA:V
- means they can’t rely on diffusion alone so need system
What do dicotyledonous (dicots) plants do?
make seeds that contain 2 cotyledons
What are cotyledons?
organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates
What are herbaceous dicots?
have soft tissues and a relatively short life cycle (leaves and stem die down at end of growing season to soil)
What are arborescent (woody) dicots?
have hard lignified tissues and a long life cycle (hundreds of years)
What is the series of transport vessels running through dicots called?
vascular system
In herbaceous dicots, what is the vascular system made up of?
xylem and phloem, arranged in vascular bundles
Summarise the function of the xylem
non-living tissue used in the transport of water and mineral ions, and for support
Which way do materials flow in the xylem?
up from the roots to the shoots and leaves
What are xylem vessels made of?
hollow structures made by several columns of cells fusing together end to end
What cells pack around xylem vessels in herbaceous dicots, storing food and containing tannin deposits?
the thick-walled xylem parenchyma
What is tannin?
a bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores
What are xylem fibres?
long cells with lignified secondary walls that provide extra mechanical strength
What do xylem fibres not do?
transport water
How can lignin be laid down in the walls of xylem vessels?
- rings
- spirals
- solid tubes with small unlignified areas called bordered pits
In xylem vessels, what do bordered pits do?
where water leaves thee xylem and moves to other cells of the plant
Summarise the function of phloem
a living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis
What does the phloem supply cells with to help with cellular respiration and synthesis?
sugars and amino acids
Which way does the flow in phloem go?
both up or down
What is the main transporting vessel of the phloem?
sieve tubes
What makes up sieve tubes?
many cells joined end to end to form a long, hollow structure, unlignified
In areas between cells in phloem, what happens to the walls?
the walls become perforated to form sieve plates, which look like sieves and let the phloem contents flow through
What organelles begin to break down as the large sieve plates appear?
tonoplast and nucleus
What happens as a phloem tube matures after sieve plates are formed?
phloem becomes a tube filled with phloem sap, and the mature phloem cells have no nucleus
What forms with sieve tubes?
companion cells
How are companion cells and sieve tubes linked?
by many plasmodesmata
What are plasmodesmata?
microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells
What do plasmodesmata function as for sieve tube cells and why?
a life support system, as the sieve tubes have lost most normal cell functions
What supporting tissues do phloem cells contain?
fibres and sclereids (cells with extremely thick cell walls)
How does turgor pressure help plants?
- Provides a hydrostatic skeleton to support the stems and leaves
- drives cell expansion
What does the loss of water by evaporation help a plant to do?
Stay cool
What does water transport in plants?
Mineral ions and the products of photosynthesis
What is a root hair cell?
The exchange surface in plants where water is taken into the body
What adaptations to root hair cells have?
- large SA:V for diffusion
- thin surface layer shortens diffusion pathway
Why do root hair cells have a conc of solutes in their cytoplasm?
To maintain a water potential gradient between soil water (higher wp) and the cell (lower wp)
What is the concentration/wp of soil water of mineral ions?
Low conc = high water potential
What are the 3 pathways water can travel along to get from the root to the xylem?
Symplast
Vacuolar
Apoplast
Where does water move in symplast pathway?
Through the living spaces of the cell = cytoplasm
How does water move from cell to cell in the symplast and vacuolar pathway?
Through plasmodesmata
How is water drawn up the plant as water moves away from the roots?
Cells away from roots have a lower w.p. so water is drawn up the plant to replace it
What is the slowest pathway for water movement?
Vacuolar pathway
What makes the Vacuolar pathway different to the symplast?
Water moves into cytoplasm, then into vacuole, then into cytoplasm and to next cell by plasmodesmata
How does water move in the apoplast pathway?
Through cell walls and intracellular spaces
Why is the apoplast pathway the fastest?
Water doesn’t have to go through membranes
What pulls water through the cell walls in the apoplast pathway?
Cohesive and tension forces
Water moves through the pathways until it reaches what?
The endodermis - the layer of cells surrounding vascular tissue of the roots
Why is the endodermis particularly noticeable in the roots?
The effect of the Casparian strip
What is the Casparian strip?
A band of waxy material called Suberin that runs around each of the endoderm’s cells forming a waterproof layer
What pathway does the Casparian strip force water into and why?
Into the symplast pathway to regulate water entering xylem
The Casparian strip is important to force water into cytoplasm because it forces water to pass through what?
Selectively permeable plasma membrane - this excludes any potentially toxic solutes in soil water from reaching living tissues
What is the solute concentration in the cytoplasm of endodermal cells compared to xylem?
Relatively dilute = water potential is higher than xylem so increases rate of water moving into xylem
What does root pressure do?
Gives water a push up the xylem to produce movement
Evidence for the role of active transport in root pressure
What does cyanide do generally?
Affects the mitochondria and prevents the production of ATP
Evidence for the role of active transport in root pressure
What does cyanide do if applied to root cells?
The root pressure disappears as there is no energy supply
Evidence for the role of active transport in root pressure
What suggests chemical reactions are involved with root pressure?
Root pressure increases with a rise in temperature and falls with a fall in temperature
Evidence for the role of active transport in root pressure
What happens if oxygen/respiratory substances decrease?
Root pressure falls
Evidence for the role of active transport in root pressure
What is guttation?
When xylem sap is forced out of special pores at the ends of leaves in some conditions from cut ends
What is translocation?
When plants transport organic compounds in the phloem from sources to sinks
Translocation
What are sinks?
Tissues that need the transported substance
Translocation
Is it active or passive?
Active
Translocation
What are the products of photosynthesis that are transported known as?
Assimilates
Translocation
What is the main assimilate transported around plants?
Sucrose
Translocation
What is the sucrose content of most cell sap?
0.5%
Translocation
What is the sucrose content in phloem sap?
20-30%
Translocation
What are the main sources of assimilates in plants?
- green leaves and stems
- storage 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
Translocation
What are the main sinks in a plant?
- roots that are growing/actively absorbing mineral ions
- meristems that are actively dividing
- any part of the plant that’s laying down food stores (developing seeds/fruits/storage organs)
Translocation
What is phloem loading?
When plants load assimilates into the phloem for transport
Translocation
Phloem Loading
Is the symplast route active or passive?
Largely passive
Translocation
Phloem loading
In some species of plants, how is the symplast route used?
The source moves through the cytoplasm of the mesophyll cells and on into the sieve tubes by diffusion
Translocation
Phloem loading
How does sucrose move through the apoplast pathway?
Through cell walls to the companion cells and sieve elements
Translocation
Phloem loading
In companion cells, how is sucrose moved into the cytoplasm?
Across the cell membrane in an active process
Translocation
Phloem loading
What happens to H+ ions in companion cells during the apoplast route?
Actively pumped out into the surrounding tissue using ATP
They then return to the companion cell down a conc grad via a co-transport protein
Translocation
Phloem loading
What molecule is co-transported?
Sucrose
Translocation
Phloem loading
What does sucrose being co-transported do?
Increase sucrose concentration in the companion cells and in the sieve elements through the many plasmodesmata
Translocation
Phloem Loading
What does the many infolding companion cells have do?
Increases SA for active transport of sucrose into cell cytoplasm
Translocation
Phloem loading
What does the many mitochondria do in companion cells?
Supplies the ATP needed for the transport pumps
Translocation
Phloem loading
What happens as a result of sucrose building up in the companion cells and sieve tube element?
Water moves in by osmosis = turgor pressure = water carrying assimilates moves into sieve tubes = reduces pressure in companion cells = moves up or down plant by mass flow to sinks
Translocation
Phloem loading
What does solute accumulation do in source phloem?
Leads to an increase in turgor pressure that forces sap to regions of lower pressures in the sinks
What is the pressure in phloem and how does this help?
2 MPa - helps move stuff rapidly
Translocation
Phloem unloading
When is sucrose unloaded from the phloem?
At any point the cell needs it
Translocation
Phloem unloading
What’s the main mechanism of phloem you loading?
Seems to be by diffusion of the sucrose from the phloem into the surrounding cells
Translocation
Phloem unloading
Where does the sucrose go after it’s diffused out of the phloem?
Rapidly moves on into other cells / converted into another substance so that a conc grad of sucrose is mainatined
Translocation
Phloem unloading
What does the loss of the solutes from the phloem lead to?
A rise in the water potential of the phloem as water moves out into the surrounding cells by osmosis
Translocation
Evidence
What allows us to see the adaptations of the companion cells for active transport?
Advances in microscopy
Translocation
Evidence
What happens if the mitochondria of companion cells are poisoned?
Translocation stops
Translocation
Evidence
What suggests that it’s an active process driving the mass flow?
The fact that the flow of sugars in the phloem is 10,000 times faster than if it was diffusion alone
Translocation
Evidence
How can aphids be used to demonstrate the translocation of organic solutes in the phloem?
Has been shown there is a positive pressure in the phloem that forces the sap out through the stylet
What are xerophytes?
Plants in dry habitats that have evolved a wide range of adaptations that enable them to live and reproduce where water availability is low
What’s an example of a xerophyte?
Conifers / cacti
What is marram grass (xerophyte)?
A plant found widely on sand dunes and coastal areas in dry salty conditions
Can plants in cold conditions be described as xerophytes?
Yes
Xerophytes Conserving Water
How does a thick waxy cuticle help?
Minimise water loss by transpiration
Xerophytes Conserving Water
How do sunken stomata located in pits help?
Reduces air movement = produces microclimate of still, humid air = reduces water vapour potential gradient = reduces transpiration
Xerophytes Conserving Water
How does a reduced number of stoma help?
Reduces water loss by transpiration and reduces their gas exchange capabiltiies
Xerophytes Conserving Water
How does reducing the amount of leaves help?
Reduces SA:V, minimising area of water loss by transpiration
Xerophytes Conserving Water
How do hairy leaves help?
Reduces water vapour potential gradient = minimises loss of water by transpiration from surface of leaf
Xerophytes Conserving Water
How do curled leaves help?
Confines stomata within microclimate = reduces diffusion of water vapour
Xerophytes Conserving Water
How do succulents help?
Succulent plants store water in specialised parenchyma tissue in their stems and roots so it’s plentiful in times of need
Xerophytes Conserving Water
How does leaf loss help?
Prevents any water loss through leaves by simply losing leaves when water isn’t available
Xerophytes Conserving Water
How do deep roots help?
Can access water from deep in the ground
Xerophytes Conserving Water
How do lots of shallow roots help?
Wide surface area which can absorb water straight after rainfall
Xerophytes Conserving Water
How can daffodils survive without water?
Survive as storage organs such as bulbs
What are hydrophytes?
Plants that live in water
Examples of hydrophytes?
Water crews / water lilies
Why is it important in surface water plants that the leaves float?
So they are near the surface over light needed for photosynthesis
Why is water logging a major problem for hydrophytes?
The air spaces of the plant need to be full of air, not water, for the plant to survive
Adaptations of hydrophytes
Why do they have very thin / no waxy cuticles?
They don’t need to conserve water
Adaptations of hydrophytes
Why do they have many always-open stomata on the upper surfaces?
There is no risk to the plant of loss of turgor, so open stomata maximises gaseous exchange
Adaptations of hydrophytes
Why do they have a reduced structure to the plant?
The water supports the leaves and flowers so there is no need for strong supporting structures
Adaptations of hydrophytes
Why do they have wide, flat leaves?
Spread across the water to capture as much light as possible
Adaptations of hydrophytes
Why do they have small roots?
As water can diffuse directly into stem and leaf tissue so less need for uptake by roots
Adaptations of hydrophytes
Why do they have large SA of stems and roots under water?
Maximises area for photosynthesis
Adaptations of hydrophytes
Why do they have air sacs?
to enable the leaves / flowers to float to the surface of the water
Adaptations of hydrophytes
What do aerenchyma have?
Has many large air spaces which seem to be formed by apoptosis in normal parenchyma
Adaptations of hydrophytes
What are the functions of aerenchyma?
- making the leaves/stems more buoyant
- forming a low resistance internal pathway for the movement of substances such as oxygen to tissues below the water
Adaptations of hydrophytes
How does aerenchyma forming low resistance internal pathway help?
helps the plant to cope with anoxic (extreme low oxygen conditions) conditions in the mud, by transporting oxygen to tissues
What is a major problem with aerenchyma?
They provide low resistance pathway by which methane produced by the rice plants can be vented into the atmosphere
Transpiration
Why does the surface of the leaf have a waxy cuticle?
Makes them waterproof to prevent water loss from leaves by evaporation
Transpiration
How do stomata open and close?
By guard cells
Transpiration
How does CO2 / O2 move into / out of the leaf?
By diffusion down a conc grad through stomata
Define transpiration
The loss of water vapour from the leaves and stems of plants by evaporation
Transpiration
Why do (only) some stomata need to be open all the time?
In the day, plant needs to take in CO2 for photosynthesis. At night, plant needs to take in O2 for cellular respiration
Transpiration
What is a transpiration steam?
- pull of water up xylem
- evaporates from cell walls of mesophyll cells
- into the air spaces in leaves
- water vapour moves out through stomata
Transpiration Stream
What does the loss of water from a mesophyll cell do?
Lowers water potential of the cells, so water moves into the cell from adjacent cells
Transpiration Stream
What is adhesion?
Water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels
Transpiration Stream
What is cohesion?
Water molecules form hydrogen bonds with each other and so tend to stick together
Transpiration Stream
What is the transpiration pull?
Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation
Transpiration
What is the cohesion-tension theory?
The model of water moving from the soil in a continuous stream up the xylem and across the lead
Transpiration
Evidence for the cohesion-tension theory
Why does the tree’s diameter change?
Day - tension in xylem vessels is high, so the tree shrinks in diameter
Night - tension in xylem vessels is low, so tree increases in diameter
Transpiration
Evidence for the cohesion-tension theory
What happens when a xylem vessel is broken (eg when you cut stems)?
Air is drawn in the xylem rather than water leaking iut
Transpiration
Evidence for the cohesion-tension theory
What happens when xylem vessel is broken and air is pulled in?
The plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces is broken
Transpiration
What happens when turgor is low in the stomata?
The asymmetric configuration of the guard cell walls closes the pores
Transpiration
How do guard cells increase their turgor?
When the environmental conditions are favourable, guard cells pump in solutes by active transport
Transpiration
What prevents guard cells from swelling in width, only lengthways?
Cellulose hoops
Transpiration
Why do guard cells become bean-shaped?
Because the inner wall is less flexible
Transpiration
What happens to guard cells when water becomes scarce?
Hormonal signals from the roots can trigger turgor loss from the guard cells, which can close the stomatal pore and so conserve water
Factors affecting Transpiration
How does light intensity affect it?
Increasing light intensity increases open of stomata, increasing rate of water vapour diffusing out, increasing evaportation
Factors affecting Transpiration
What does a relatively high humidity do?
Lowers transpiration rate because of the reduced water vapour potential gradient between inside and outside of lead
Factors affecting Transpiration
What does very very dry air do?
Increases rate of transpiration
Factors affecting Transpiration
How does an increase in temperature affect KE?
Increases KE of water = increases rate of evaporation from spongy mesophyll cells into the air spaces of leaf
Factors affecting Transpiration
How does an increase in temp affect concentration of water vapour?
Increases conc of water vapour that’s he external air can hold before it becomes saturated
= decreases relative humidity & water potential
Factors affecting Transpiration
What does reducing air movement do?
Water vapour that diffuses out of lead accumulates in cells = so water vapour potential around stomata increases = reduces diffusion gradient l
Factors affecting Transpiration
Does air movement or wind increase rate of transpiration?
Yes
Factors affecting Transpiration
How does soil-water availability affect it?
If it is very dry the plant will be under water stress and the rate of transpiration will reduce
