Unit 2.3b - Adaptations for transport in plants Flashcards
What do plants need to transport?
Inorganic ions and water
Where do plants need to transport inorganic ions and water?
From the soil to where they’re synthesising new compounds, which is in the leaves (photosynthesis)
What’s the name of the transport system used to transport inorganic ions in plants?
The transpiration system
Transpiration
The loss of water vapour through the stomata of plants
What’s the first step of the transpiration system?
Inorganic ions are dissolved in water and carried up to the leaves to be used in the plants metabolism
Name some inorganic ions that are transported in a plant
Mg2+
NO3^2-
PO4^3-
Where is water “pulled up” in a plant?
The xylem vessels
How is water pulled up the xylem vessels?
Cohesion tension
Adhesion
Cohesion tension
Hydrogen bonding between H and O between different water molecules
Adhesion
The polar nature of water giving them an attraction to the cellulose in the walls of the xylem vessels - capillarity
What’s the name of the other transport system used in plants as opposed to the transpiration system?
Translocation
Translocation
The process of moving the products of photosynthesis from where they’re mad or stored to other parts of the plant
From where is large quantities of water lost from a plant and how?
Through the stomata
Via the transpiration stream
Why must water be replaced in the soil?
Large quantities of water are lost through the stomata via the transpiration stream
How is water replaced into a plant?
A specialised region of root - the root hair zone - absorbs water (and inorganic ions)
Which part of a plant is responsible for absorbing water from the soil?
The root hair zone - a specialised region of root
How does the root hair zone of root absorb water?
Via osmosis
How does the root hair zone of root absorb inorganic ions?
Via active transport or fascilitated diffusion
Root hair cell
Epidermal cells with the extension - the root hair zone
Root hair cells adaptations to their function
Large surface area for the absorption of water by osmosis
Thin cell walls for a short diffusion pathway
Draw and label the transverse section of the root of a plant
(See notes)
Where are root hairs on the root?
On the upper epidermis layer
What does xylem tissue do?
Transports water and minerals throughout the plant
Where is xylem tissue found?
At the centre of the root
What is xylem tissue surrounded by?
A single layer of cells - the endodermis
What does the endodermis surround?
Xylem tissue
What does the stele of a root contain?
Vascular tissue
Where is the stele of a root?
The central part of the stem
And
In the root
Draw and label a stem transection
(See notes)
Draw and label a high power view of the stele
(See notes)
Endodermis
A single layer of cells around the stele
What is the endodermis important for?
Absorbing water and inorganic ions into the plant
What does endodermis include and what does this do?
Water proof Casparian strip which stops transport via the apoplast route
What does Phloem do?
Transports the products of photosynthesis e.g - sucrose and amino acids
What does Xylem do?
Transports water and minerals
How are xylem tissues fit for their purpose?
They’re dead cells with thickened cell walls so the middle is empty for transporting water and inorganic ions
What makes up the vascular bundle?
Xylem
Cambium
Phloem
What do the xylem, cambium and phloem make up?
The vascular bundle
What chemical do two parts of the stem both have and what does this mean?
Xylem and schlerenchyma have lignin
Stain the same colour
Chemical of xylem and schlerenchyma
Lignin
What’s the schlerenchyma of the vascular bundle also known as?
Fibres
Fibres of the vascular bundle
Schlerenchyma
What do all 3 routes of transporting water and minerals in the root start with?
Water enters the root hairs on the epidermis via osmosis (water potential gradient from the soil into root hair cell)
Inorganic ions dissolved in water enter the root hair cells via facilitated diffusion or active transport
Where do the 3 routes for transport of water and minerals in the root go to and from?
From epidermis
To cortex
To the vascular tissue of the steel in the centre of the root
What are the 3 possible routes for the transport of water and minerals in the root?
Symplastic route
Apoplastic route
Vacuolar route
Symplastic route
Water and dissolved ions are absorbed into the cytoplasm of the root hair by osmosis active transport and fascilitated diffusion
Then move through the root tissues via the cytoplasm and plasmodesmata of adjacent cells
What are the symplastic, apoplastic and vacuolar routes all routs for?
The transport of water and minerals in the root
Apoplastic route
Water and dissolved ions move through the root tissues via the cell walls of adjacent cells (it doesn’t actually enter the cells)
There is no restriction to flow until the endodermis is reached
Why are water and dissolved ions able to move through the root tissues via the cell walls when taking the apoplasic route?
Cellulose cell walls and permeable to water and ions
At which point in the apoplastic route is there a restriction to the flow of water and why?
The endodermis
The endodermis contains a band of waterproof tissue known as the casparian strip which prevents water from going through the apoplast route
Where is the casparian strip?
The endodermis
What’s the purpose of the casparian strip in the endodermis?
To prevent water from going through the apoplast route
Vacuolar route
Water and dissolved ions move through the tissues of the root from the vacuole of one cell to the vacuole of the neck cell down a water potential gradient
Sketch the vacuolar route
(They look like coffee beans)
Sketch the symplastic route
(Nodiadau)
Sketch the apolastic route
(Nodiadau)
Draw a diagram to represent the transport of water and minerals in the route down both the apoplastic and symplastic route
Yes
How are inorganic ions taken from the soil solution?
Active transport
Ho do inorganic ions move through the plant once they’ve been absorbed?
Move along the apoplastic pathway (carried in solution by the water) in the transpiration system
How are inorganic ions carried in the transpiration stream?
In solution by the water
What happens when minerals reach the casparian strip?
It prevents further movement via the apoplast
What’s the casparian strip formed from?
Waterproof Suberin
What happens to minerals once they’ve been restricted by the casparian strip?
They must enter the cytoplasm and are transported from cell to cell via diffusion or active transport
How does nitrate enter a plant?
As nitrate or ammonium ions
Why do plants need nitrates?
To make amino acids
How do nitrate ions enter the apoplastic pathway?
Diffuse along a concentration gradient
How do inorganic ions travel from the apoplastic to the symplastic pathway when restricted by the casparian strip in the endodermis?
By active transport against the concentration gradient, through the selectively permeable cell membrane into the cytoplasm
What must ions do at the endodermis and why?
Enter the symplastic pathway by active transport to bypass the casparian strip
Is the apoplastic pathway living or non-living?
Non-living
Is the symplastic pathway living or non-living?
Living
How does the casparian strip actually help? How?
Allows the plant to selectively take up ions
When ions have to travel from the apoplastic to the symplastic pathway, they have to travel into the cytoplasm to get into it, through the selectively permeable membrane
How does the casparian strip in the endodermis allow a plant to selectively take up ions?
Inorganic ions have to travel from the apoplastic to the symplastic pathway, and to do this they need to travel into the cell’s cytoplasm, and to do THIS they need to travel through the selectively permeable membrane
What does root pressure do?
Helps move water up the plant
(Although not to the same extent as cohesion tension)
What IS the casparian strip?
A layer of Suberin within the cell walls of the endodermal cells
How is root pressure formed? [5]
The Suberin of the casparian strip is water proof and stops water and dissolved ions form following the apoplast root through the endodermis, forcing them to cross the cel membrane and enter the symplastic route
Transport proteins in the membrane of the endodermis cells actively transport dissolved ions/salts across the endodermis and into the xylem vessels
This lowers the water potential within the xylem vessels and water moves into the xylem by osmosis from the root cortex
The movement of water into the xylem creates a hydrostatic pressure which forces the xylem contents upwards - this is root pressure
What creates hydrostatic pressure in the roots and what does this cause?
The movement of water into the xylem, which forces the xylem contents upwards (root pressure)
Where does water move into the xylem by osmosis from to form root pressure?
The root cortex
What is the ultimate cause of root pressure?
The casparian strip forcing inorganic ions to cross the cell membranes and enter the symplastic route
What’s an experiment that we can do to test for root pressure?
Cut the plant close to the soil to leave a stump
Tightly seal to an s-shaped tube half filled with mercury, half filled with water
Over time, the mercury will be pushed to the top of the tube due to the hydrostatic pressure
Measure pressure of the liquid using a manometer
How can we measure the pressure of a liquid?
Manometer
Where are the vascular bundles in stems?
Around the periphery
Why are the vascular bundles around the periphery in stems?
Gives flexible support and resistance to bending strain due to the tough xylem cells - fibres
What gives flexible support and resistance to a stem?
Tough xylem cells - fibres
Draw and label the vascular bundle in a stem
(See notes)
What does the phloem contain?
Living material (e.g - nucleus, cytoplasm)
Which 2 tissues in vascular bundles contain lignin and what does this lead to?
Fibres
Xylem
Both stain red
Compare lignin to cellulose
Lignin is harder
What gives a plant the texture of wood?
Xylem vessels with lignin
What does lignin give plants?
Rigidity
What are the 4 types of xylem cells?
Vessels
Tracheids
Fibres
Parenchyma
What are vessels, tracheids, fibres and parenchyma all examples of?
Xylem cells
What happens when a xylem cell matures?
Gets thicker and fills with lignin
Is lignin permeable to water?
No
Why are xylem cells dead cells?
As they mature, the cell walls get thicker and fill with lignin
Lignin is impermeable to water and stronger than cellulose
Therefore, water can’t easily get in, so the cell contents die away
How does lignin lead to the dead of a xylem cell?
Impermeable to water and stronger than cellulose - water can’t get in easily, cell contents die away
What type of cells are xylem cells?
Dead
Why are xylem cells important?
Form tubes to carry water and dissolved ions
Provide mechanical strength to support the plant
Why is it useful that xylem cells are dead?
Form tubes to carry water and dissolved ions
Xylem function
Transports water and mineral salts from the root to the leaves
Phloem function
Transports soluble produce of photosynthesis (sucrose and amino acids) from the leaves to other parts of the plant
How are the functions of the xylem and phloem different?
Xylem - transports water and mineral salts from the root to the leaves
Phloem - transports soluble products of photosynthesis (sucrose and amino acids) from the leaves to other parts of the plant
Where does the phloem carry the products of photosynthesis to and from?
From the leaves to other parts of the plant
Where does xylem transport water and mineral salts/inorganic ions to and from?
From the root to the leaves
Describe tracheid xylem tissues
Longer, slender cell with an empty lumen to transport water and inorganic ions
Thick to support the plant
Describe the fibre xylem tissues
Specialised for support
Has almost no central cavity when the cell dies at maturity
Thickened cell walls
Lots of lignin
Describe the vessel xylem tissues
Specialised for water transport
Has a wide lumen to reduce the resistance to water flow through the tissue
Shorter and wider
No end walls between vessel elements (cells) that make up a vessel
Describe the parenchyma xylem tissue
Live cells that develop to form the other types of xylem cells
Contain cytoplasm, nuclei…
Cell walls thicken as they develop
Which xylem cells are the live cells that develop to form the other types?
Parenchyma
What are fibrous xylem tissues specialised for?
Support
What are tracheid xylem tissues specialised for?
Transporting water and inorganic ions
What are vessel xylem tissues specialised for?
Water transport
What would happen eventually when testing for root pressure and why?
There would be no more upward movement of mercury due to gravity resisting this movement
Describe the steps involved in transporting water FROM the xylem in the leaf
- Water arrives in the leaf through xylem vessels
- Water leaves the xylem vessels through the pits in the walls and travel through the living spongey mesophyll cells down a water potential gradient via osmosis
- Water evaporates from the surface of mesophyll cells into the sub stomatal air chamber
- When the stomata is open (sufficient light intensity), water vapour escapes - transpiration
The water potential of the cells near the stomata has now lowered
Where does water evaporate from the surface of mesophyll cells into?
The sub stomatal air chamber
How does water leave the xylem vessels and where does it go to?
Through the pits in the walls
Travels through the living spongey mesophyll cells
How does water move from the xylem vessels through the living spongey mesophyll cells?
Down a water potential gradient via osmosis
What is the route taken when water is transported FROM the xylem in the leaf?
The same as for the transport of water and minerals in the root…
Symplastic
Apoplastic
Vacuolar
How does water travel in the xylem to the leaves?
Via the apoplast, symplastic and vacuolar pathways
What happens to most of the water as it travels from the xylem through the leaf?
Most is lost as it evaporates from the internal leaf surface and passes out as water vapour into the atmosphere
Transpiration
Water loss from the surface of leaves by the evaporation through the stomata
Why does transpiration happen at all?
Most of the water travelling in the apoplast, symplastic and Vacuolar pathways from the xylem throughout the leaf is lost as it evaporates from the internal leaf surface
Transpiration pull
As water molecules leave xylem cells in the leaf, they pull up other water molecules - this pulling effect is known as the transpiration pull
What is the name for the effect caused when other water molecules are pulled up when water molecules leave xylem cells in the leaf?
The transpiration pull
What are the factors that effect transpiration rate?
Temperature
Humidity
Air movement
Light intensity
Describe how temperature can effect transpiration rate
Rise in temperature = additional kinetic energy for the movement of water molecules = increased rate of evaporation from the walls of the mesophyll cells
Also, if the stomata are open, this speeds up the rate of diffusion of water vapour into the surrounding air
Also, the water potential of the air becomes lower as its temperature is raised as it can hold more moisture
What happens to the water potential of the air as the temperature is raised and what does this mean?
It become lower
Can hold more moisture
Humidity
The amount of water vapour in the air
How much water vapour is there in the air inside the leaf?
Saturated with water vapour
Which part of a leaf has a high humidity and why?
The sub-stomatal air chamber as it’s saturated with water vapour
Relationship between humidity and rate of transpiration
Greater humidity = lower rate of transpiration
Relationship between rate of transpiration and temperature
Increased temperature = increased transpiration rate
Describe how humidity effects transpiration rate
Sub-stomatal air chamber has a very high humidity as it’s saturated with water vapour, giving it a high water potential
The humidity of the air surrounding a leaf varies (rarely exceeds 70% in Britain)
Therefore, a water potential gradient is always present between the leaf and the air
When the stomata open, water vapour rapidly diffuses out of the leaf from a high to low water potential
So, the greater the humidity, the lower the rate of transpiration
Describe the rate of transpiration in still air
Water vapour accumulates around the leaf surface
This decreases the water potential gradient between the leaf and air
This decreases the rate of transpiration
Describe the rate of transpiration in moving air (wind)
Removes the layer of saturated air
Increases the water potential gradient between the leaf and air
Increases the rate of transpiration
Is it still or moving air that increases the rate of transpiration? Why?
Moving (windy conditions)
Removes the layer of saturated air, increasing the water potential gradient between the leaf and air
What’s the relationship between the rate of transpiration and light intensity?
The higher the light intensity, the higher the rate of transpiration
Describe how light intensity effects the rate of transpiration
Controls the degree of stomatal opening
Higher light intensity = greater number of open stomata
Increases the rate of transpiration
What is the primary purpose of stomata?
To allow CO2 to enter for photosynthesis (but when it opens, water vapour DOES get released)
Potometer
Apparatus used to measure the rate of uptake of water by a leafy shoot
Apparatus used to measure the rate of uptake of water by a leafy shoot
Potometer
How do we use a Potometer?
Cut stem from plant
Plug to Potometer
Plant continues to transpire
Air bubble moves to indicate the volume of water taken up by he shoot in a certain period of time
How does a Potometer actually measure the rate of uptake of water by a leafy shoot?
Air bubble moves to indicate the volume of water taken up by the shoot in a certain period of time
Draw the set up of a Potometer
(See notes)
How must we cut the shoot of the plant to attach to a Potometer and why?
Cut it under water
To prevent air bubbles forming in the xylem. If air gets in, it’ll break the water column and break the cohesion tension
What are 4 precautions to take when setting up a Potometer?
Cut the shoot under water
Keep the leaves dry
Set up apparatus under water
Ensure all joints are airtight
Why is it important to keep the leaves dry when using a Potometer?
If there’s water vapour on the leaves, it’ll prevent water from leaving via transpiration
How do we setup the apparatus of a Potometer and why?
Under water to prevent air bubbles
How could we ensure that all joints are airtight on a Potometer and why is this important?
Use Vaseline to seal joints
Prevents air bubbles
What does using a Potometer NOT measure and why?
The rate of transpiration
Not all the water taken up by the plant and drawn over the leaf goes through the stomata - the purpose of water being there is to photosynthesise, so some is used for this
As well as measuring the rate of uptake of water by a leafy shoot, what else can we use a Potometer to measure? Give examples
Measure the factors that effect transpiration rate
e.g - lamp (light intensity)
Fan (air movement)
What are the different types of plants depending on their different environments?
Mesophytes
Hydrophytes
Xerophytes
What does the availability of for plants change depending on where they live?
The availability for water
Mesophytes
Plants that live in habitats where there is sufficient water available for their survival
Plants that live in habitats where there is sufficient water available for their survival
Mesophytes
Which type of plants don’t really have any special adaptations to avoid water loss and why?
Mesophytes - they live in habitats where there is sufficient water available for their survival
What are the general adaptations of mesophytes to avoid water loss?
Closing stomata (+ most on the base for slower water evaporation)
Waxy cuticle to prevent water from evaporating from the surface
Why is it important that Mesophytes have a waxy cuticle?
They would otherwise lose a lot of water due to their large surface area
What type of plants are most of those around us?
Mesophytes
Hydrophytes
Plants that live in or on water and have a plentiful supply of water at all times
Plants that live in or on water and have a plentiful supply of water at all times
Hydrophytes
Which types of plants don’t have any specific adaptations to avoid water loss and why?
Hydrophytes - they have a plentiful supply of water at all times
Xerophyte
Plants that live in areas of low water availability (e.g - deserts, sand dunes, tundra (frozen soil))
Examples of areas of low water availability
Deserts, sad dunes, tundra (frozen soil)
What type of plant is a lily pad an example of?
Hydrophyte
Hydrophyte example
lily pad
Why do sand dunes have low water availability?
Sand dries up quickly
Sea water is salty = drier air
Draw and label a hydrophyte transection
(See notes)
Which tissue do we observe when looking at a hydrophyte transection and where is this?
Specialised tissue - aerenchyma
Underneath the palisade layer
What are the adaptations of a hydrophyte?
Large air spaces
Stomata on the upper epidermis
Thin or absent cuticle
Lack of supporting tissues
Less organised vascular tissue
Increased surface area
Roots usually reduced in size
Why do hydrophytes have large air spaces?
Gives leaves buoyancy to float on the water surface
Reservoirs of O2 and CO2
Why is the stomata on the upper epidermis of a hydrophyte?
Allows gas exchange with the air above from the floating leaf
Why does the hydrophyte have a thin or absent cuticle?
Do not need to reduce water loss
Why do hydrophytes have a lack of supporting tissues?
High density of water gives support to the submerged leaves and stems
Name some supporting tissues that a hydrophyte lacks
Fibres
Collenchyma
Sclerenchyma
What is the vascular tissue less organised in hydrophytes compared to?
Compared to the midrib in mesophytes
Why is the vascular tissue of hydrophytes less organised than the midrib of mesophytes?
Don’t need a good supply of water
Why do hydrophytes have a large surface area?
Increases the surface area for gas exchange and photosynthesis
Why are the roots of hydrophytes usually reduced in size?
They act mainly to anchor the plant as water absorption can take place over the whole surface of the plant
What type of specialised roots have some hydrophytes developed and why?
Roots that extend into the air and can absorb oxygen
(If anchored in mud at the bottom of water and they don’t receive sufficient oxygen)
Pneumaphores
Specialised roots in hydrophytes that extend into the air to absorb oxygen as the roots would otherwise be anchored in mud at the bottom of water with insufficient oxygen
Specialised roots in hydrophytes that extend into the air to absorb oxygen as the roots would otherwise be anchored in mud at the bottom of water with insufficient oxygen
Pneumaphores
Draw and label a xerophute transection
(See notes)
Name a xerophyte
Ammophila leaf
What is an ammophila leaf an example of?
A xerophyte
What are the adaptations of xerophytes?
Rolled leaves
Hairs
Thick waxy cuticle
Sunken stomata in pits
Hinge cells
Succulent (thick) leaves
White leaves/spines
Reduced number of stomata
CAM photosynthesis
What causes the leaves of xerophytes to roll up?
Large, thin walled epidermal cells at the bases of the grooves shrink when they lose water from excessive transpiration, causing the leaf to roll inwards
Why are xerophyte leaves rolled up?
It reduced the leaf area exposed to air, and so reduces transpiration
Name for the hairs of xerophytes
Trichomes
Trichomes
Hairs of xerophytes
Why do xerophytes have Trichomes (hairs)?
Stiff, interlocking hairs trap water vapour and reduce the water potential gradient, thus reducing the rate of transpiration
Why do xerophytes have a thick waxy cuticle?
Reduced water loss by evaporation from the epidermal tissue as they’re waterproof
Why do xerophytes have sunken stomata in pits?
A more humid microenvironment is created, as they allow water vapour to accumulate above the stomatal pore
+ wind can’t blow the water vapour away (rolled leaf) - this decreases the water potential gradient between the inside of the leaf and the gas chamber, reducing transpiration rate
What do hinge cells in xerophytes do and why is this important?
Absorb water from the surroundings to become turgid and open when it’s not too dry to increase the surface area for photosynthesis
Why do xerophytes have succulent (thick) leaves?
To store water
Why do xerophytes have white leaves/spines?
Light colours reflect light and heat, thereby cooling the plant
Why do xerophytes have a reduced number of stomata?
Fewer gaps for water to evaporate out through
What is CAM photosynthesis and why is it used by xerophytes?
Stomata open when it’s cooler at night
CO2 is fixed so that it can be used during the day for photosynthesis without having to open the stomata
Draw and label the transection of a pine leaf
(See notes)
Give an example of a xerophyte
Pine leaf
What is a pine leaf an example of?
A xerophyte
How do we improve the accuracy of the experiment with the Potometer?
Use a capillary tube with smaller graduations
Time over a greater distance
What implies more reliable data with range bars?
If they do not overlap
If they’re smaller
Translocation
The transport of the products of photosynthesis from source to sink in the plant
Source for translocation
The site of photosynthesis in the leaves
Sink in translocation
Areas that use the materials of photosynthesis for growth, respiration, storage and other metabolic processes
Products of photosynthesis
Soluble organic materials, sucrose and amino acids
Are the products of photosynthesis transported with inorganic ions and water?
No, they’re transported separately in the phloem
Where are the products of photosynthesis carried to and from in the phloem?
From the source to the sink
How is sucrose a product of photosynthesis?
Glucose (made during photosynthesis) + fructose —> sucrose
What are the types of cell in phloem tissue?
Sieve tubes
Companion cells
Phloem fibres
Phloem parenchyma
What is a sieve tube made up of?
Sieve cells/ sieve element
Function of sieve tubes
Transport organic materials such as sucrose and amino acids (the products of photosynthesis)
What are the cells of sieve tubes called?
Sieve elements
Where are the sieve elements in sieve tubes?
End to end
Sieve plates
The ends of the walls of each sieve cell do not break down, but are perforated by pores at either side, called sieve plates
What’s the name for the pores at the ends of the walls of sieve cells?
Sieve plates
Draw and label a sieve tube and it’s surroundings
(See notes)
Describe sieve cells
Long, columnar
Not completely empty like xylem - contain living material such as a thin cytoplasm and a few organelles
Adaptations of sieve cells/elements for their function
Sieve plates containing pores allow bidirectional flow form element to element throughout the plant
Thin cytoplasm with no large organelles, which allows the products of photosynthesis to flow without obstruction
Plasmodesmata are present in the walls, which allow the transport of ATP and other molecules from the companion cell to the sieve tube cell/element
No nucleus, and most of the other cell organelles disintegrate during sieve tube development
Cytoplasmic filaments contains phloem protein extend from one sieve cell to the next through the pores in the sieve plate
Which part of sieve tubes contain pores and why?
Sieve plates
Allow bidirectional flow from element to element throughout the plant
Describe and explain the cytoplasm of sieve cells/elements
Thin cytoplasm with no large organelles
Allow the products of photosynthesis to flow without obstruction
Where are plasmodesmata present in sieve tubes and why?
In the walls of sieve tube cells/elements
Allow the transport of ATP and other molecules from the companion cell into the sieve tube cell/element
Why do sieve cells/elements not have most of their organelles?
They disintegrate during sieve tube development
What do cytoplasmic filaments contain in sieve tubes?
Phloem Protein
What contain phloem protein in sieve tubes?
Cytoplasmic filaments
How do cytoplasmic filaments extend from one sieve cell to the next?
Through the pores in the sieve plate
Describe companion cells
Dense cytoplasm
Centrally placed large nuclei
Many mitochondria
Rough endoplasmic reticulum
Golgi body
Connected to the sieve tube elements by plasmodesmata
Make proteins and ATP for the sieve tube cells/elements
What do companion cells make and for what?
Proteins and ATP for the sieve tube cells/elements
How are companion cells connected to the sieve tube elements?
By plasmodesmata
Where in sieve tubes is where metabolic activity takes place?
Companion cell
What takes place in companion cells?
Metabolic activity
How is a companion cell different to a sieve cell/element?
Contain a lot more material and a relatively small vacuole
Wha are the 3 experiments for evidence for translocation?
Ringing experiments
Phloem sampling
Radioactive labelling
What do ringing experiments support the idea of?
Translocation in the phloem
How are ringing experiments done to provide evidence for translocation?
Cylinders of outer bark tissue are removed from all the way around a woody stem, in a ring
The phloem is removed when doing this (located towards the periphery), leaving the xylem (further in the stem)
What are the results from the ringing experiment to provide evidence for translocation? Explain this
Sucrose (a product of photosynthesis) accumulated above the cut ring
Lowers the tissue’s water potential
Water moves into the cell via osmosis
Swells as water enters the cells
What does the ringing experiment provide evidence for?
That sucrose was transported to this region of the stem by translocation in the phloem
Why are too many aphids bad for a plant?
Take the nutrients from a plant
What’s the name of the mouthpart of an aphid? Describe this
Stylet
Hollow and needle-like
What does an aphid do with its stylet?
Inserts it directly into the sieve tube, allowing the aphid to feed on the sugary sap of the phloem
Which part of a plant does an aphid insert its stylet and why?
The sieve tube of the phloem
To feed on the sugary sap
What happens during phloem sampling?
The stylet of aphids is cut off using a laser, leaving it attached to the plant, forming a useful micro pipette
Sap exudes from the stylet after being cut - the contents of the phloem must be under pressure for it to be pushed up like this
Upon collection and analysis of this sap, it contains high levels of sucrose and amino acids - the products of photosynthesis
How do we extract sap from the phloem of a plant?
Cut of the stylet of an aphid to use it as a micropipette
How does phloem sampling give evidence for translation?
Upon collection and analysis of the sap from the stylet, it contains high levels of sucrose and amino acids - the products of photosynthesis
What additional concept is proved by phloem sampling and why?
That the contents of the phloem are under pressure, a sap exudes from the stylet after being cut
What happens during radioactive labelling to provide evidence for transpiration?
Carbon dioxide labelled with a radioactive carbon isotope (14C) is supplied to an illuminated plant leaf
Which carbon isotope is supplies to a plant leaf dung radioactive labelling and why?
14C
Radioactive
What’s the method of using radioactive labelling to provide evidence or translocation using aphids?
Aphids feed on the contents of phloem
Upon cutting the stylet, we can analyse the sap that exudes for radioactivity
How is the radioactive carbon isotope transported to other parts of the plant to make radioactive labelling possible?
The radioactive carbon becomes fixed in the sucrose produced by photosynthesis and is translocated to other parts of the plant
What’s the alternative method to using aphids to using radioactive labelling to provide evidence for translocation? Describe this
Trace the radioactive carbon in the sucrose using autoradiography
Source leaf and sink tissues are places firmly on photographic film in the dark for 24 hours
When the film is developed, the presence of radioactivity in parts of the plant tissue show up as fogging of the negatives
What has radioactive labelling showed specifically about translocation in the phloem?
That sucrose is transported both upwards and downwards
How did radioactive labelling prove that sucrose is transported both upwards and downwards during translocation ?
Radioactivity was observed both where products have been used for growth and in the roots, as most plants store carbohydrates in the form of starch in the roots
What’s the most widely accepted theory for translocation?
The mass-flow hypothesis
The mass-flow hypothesis
The most widely accepted theory for translocation
When was the mass flow hypothesis proposed?
1937
Summarise what the mass-flow hypothesis suggests
There’s a passive mass flow of sugars from the phloem of the source leaf, which has the highest concentration of sugar, to other areas of the plant, such as growing tissues, which have a lower sugar concentration
Where does translocation occur to and from according to the mass-flow hypothesis?
From source to sink
Source in the mass-flow hypothesis
Site of photosynthesis in the leaves
Sink in the mass flow hypothesis
Roots and regions of growth
Draw a diagram to represent the mass flow hypothesis
(See notes)
Describe, in detail, the steps of the mass-flow hypothesis
- When sugar is made at the source, the water potential becomes more negative and water passes into the metabolically active source cells by osmosis
- As water enters the source cells, hydrostatic pressure increases, forcing sugars and other products of photosynthesis into the sieve tubes via active transport - phloem sieve tubes are loaded
- Mass flow occurs along the sieve tubes to the sink, the products if photosynthesis are forces along by the flow of water form a high to a low hydrostatic pressure
- Hydrostatic pressure will be lower at the sink because sugars are stored as starch or are used for respiration - this reduces the water potential
- Water passes form the sink cells to the xylem to be returned to the source
Why is the hydrostatic pressure lower at the sink (mass flow hypothesis)?
Sugars are stored as starch or are used for respiration
How are the products of photosynthesis forced along during mass flow-hypothesis along the sieve tubes to the sink?
The flow of water from a high to a low hydrostatic pressure
Why does water move from the xylem to the phloem i the mass-flow hypothesis?
When the phloem sieve tubes are loaded with sugars and other products of photosynthesis, water moves via osmosis from a high to a low water potential
What causes an increase in the phloem’s hydrostatic pressure?
Water moving from the xylem to the phloem via osmosis due to the sugars and products of photosynthesis in the phloem giving it a low water potential
What explains the rising sap in the aphid experiment?
The increase in the phloem’s hydrostatic pressure when water moves form the xylem to the phloem via osmosis
What creates a pressure gradient in the phloem?
The movement of water from the xylem to the phloem
Why is the pressure gradient in the phloem important?
Contents of phloem move down a pressure gradient from source to sink
Why does water move from the phloem to the xylem?
At the same time, sucrose is taken out of the phloem into sink cells, increasing the water potential in the phloem
Water moves from the phloem back into the xylem due to the water potential gradient
What are the different arguments against the mass-flow theory?
- The rate of translocation is 10,000x faster than it would be if the substances were moving along by diffusion
- Sieve plates with tiny pores act as a barrier impeding flow
- Sucrose and amino acids move at different rates and in different directions in the same phloem tissue
- Phloem tissue has a high rate of oxygen consumption, and translocation is stopped when a respiratory position such as potassium cyanide enters the phloem
- Companion cells contain numerous mitochondria and produce ATP, but the mass flow hypothesis fails to suggest a role for the companion cells
Describe the rate of translocation compared to substances moving by diffusion
10,000x faster
How do we know the rate of translocation?
Can use radio labelling experiments to measure the rate of flow
Why is the fact that sieve plates have tiny pores that act as a barrier impeding flow an argument against the mass-flow theory?
Would be expected that plants would evolve away from them - there may be an unexplained reason for them
Why is glucose converted into sucrose in plants?
Too reactive for transport
Bidirectional movement or organic molecules in a plant
Transported both upwards and downwards
Name for the fact that organic molecules in a plant are tranported both upwards and downwards
Bidirectional
What does xylem show a similar pattern of support material to?
That seen in an insects trachae
What shows a similar pattern of support material to that seen in an insects trachae?
Xylem
What do the Xylem vessels NOT transport?
Nutrients
How many directions does mass-flow occur in?
1
What other hypothesis could be used instead of the mass flow hypothesis if flow is bidirectional?
Active transport or diffusion
If flow is bidirectional, what is it NOT and why?
Mass-flow, as this is unidirectional
When could active transport or diffusion be used as a hypothesis for the movement of materials as opposed to mass-flow and why?
When it’s bidirectional, as mass flow is unidirectional
What’s the importance of lignin in the xylem?
Mechanical support to the xylem and prevents it from collapsing
What do all three of the apoplastic, symplastic and vacuolar routes rely on?
Osmosis
Why is having the stomata sunken in pits beneficial to xerophytes?
Less air movement
What’s closest to the stele in a root? The endodermis or pericycle?
Pericycle
The transport of what are we talking about when discussing the apoplastic and symplastic routes with the casparian strip?
Mineral ions
What do we always need to refine when discussing translocation?
Sources and sinks
What is the main leaf tissue where 14C is incorporated into organic molecules? (Radioactive labelling as evidence for translocation)
Palisade mesophyll
