Module 3 Section 3: Plant Transport Flashcards
Why do plants need transport systems
They are multicellular:
Increasing transport distances
Small surface area : volume ratio
Relatively high metabolic rate
Direct diffusion would be too slow to meet metabolic demands
What do transport systems do in plants
Need transport systems to move substances to and from individual cells quickly
Two types of plants
Herbaceous dicots (non woody stem)
Woody dicots (woody stem)
What are vascular system of a plant
Transport vessels that run through the root, stem, leaves arranged in vascular bundles
Herbaceous dicots have vascular systems made up of two transport vessels: phloem and xylem
Why does size of the plant cause the need for a transport system
The plant is made up of lots of cells and each one requires water, glucose and mineral ions
The roots take up water and mineral ions while the leaves produce glucose by photosynthesis
These molecules need to be transported to the other ends of the plant and this large distance means that simple diffusion cannot be used as it wouldn’t be fast enough to meet the metabolic demands
Why does the surface area to volume ratio of a plant cause the need for a transport system
They have less surface area available for substances to diffuse through, so the rate of diffusion may not be fast enough to meet its cells requirements
Large plants cannot rely on diffusion alone to supply their cells with substances such as food and oxygen and to remove waste products.
How are plants adapted to increase SA:V
Branching body shape
Leaves are flat and thin
Roots have root hairs
Why does having a high metabolic rate cause the need for a transport system in plants
Have more cells so there is a high demand for oxygen and nutrients and more waste is produced
What are the different plant transport systems
Transpiration System
The movement of water molecules and dissolved minerals ions
Xylem vessels
Passive process
Translocation system
The movement of sugars (Sucrose) & amino acids
Phloem vessel – sieve & companion cells
Active process
How are xylem and phloem arranged
Xylem and Phloem are arranged in vascular bundles in the roots, stems and leaves
There is a layer of cambium between these vessels which contain meristem cells
How is xylem and phloem arranged in the root
Xylem in the centre (cross shape) with phloem in four separate structure to provide a drill-like structure and support for the root as it pushes into the soil
How is xylem and phloem arranged in the stems
Xylem and phloem are near the outside to provide a scaffolding that reduces bending
Xylem on the inside phloem on the outside
How is xylem and phloem arranged in the leaf
Xylem and phloem make up a network of veins which supports the thin leaves
Structure of xylem vessels
Lignified cell walls with spiralised lignin
No end plates (mature)
No protoplasm (no nucleus or cytoplasm)
Pits in wall (non-lignified)
Vessels have small diameter
Function of lignified cell walls in xylem vessels
Adds strength to withstand hydrostatic pressure so vessels don’t collapse, impermeable to water
Function of having no end plates in xylem vessels
Allows the mass flow of water and dissolved solutes to be cohesive (between water molecules) and adhesive (between water and the walls)
These forces would be disrupted with end plates
Function of having no protoplasm in the xylem vessel
Doesn’t impede the mass flow of water and dissolved solutes (transpiration stream)
Function of having pits in wall for xylem vessels
Lateral movement of water
Allows flow of water even if air bubbles form in vessels
Function of xylem vessels having a small diameter
Helps prevent the water column from breaking and assists with capillary action
Structures of phloem tissue
Made up of sieve tubes and companion cells
Structure of sieve tubes
Living cells forming a tube for transportation
Joined end to end to form sieve tubes
Sieve section has holes in to allow solutes to pass through
Sieve tube elements have no nucleus, very thin cytoplasm and a few organelles
Cytoplasm of adjacent cells is connected through holes in sieve plates
Function of companion cells
Cells accompany sieve tube elements and carry out living functions for both of them
e.g. they provide energy for active transport of solutes
What is the need for water in plants
Mineral ions and sugars are transported in aqueous solution
Water is a raw materials of photosynthesis
Cooling effect (by transpiration)
Turgor pressure - hydrostatic skeleton
Adaptations of root hair cells
Very thin cellulose walls to provide a short pathway
Microscopic in size
Large SA : V ratio
Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
What are the water movement pathways
Symplastic pathway (through cytoplasm)
Vacuolar pathway (through vacuoles)
Apoplast pathway (through cell wall)
Process of water through the apoplast pathway
Water enters and moves through the cell wall
Moves by diffusion (not crossing a partially permeable membrane
Water may move from cell wall to cell wall, across the intercellular spaces or it may move directly from cell wall to cell wall
This is faster than the symplastic pathway
Process of water through the symplastic pathway
Water enters the cytoplasm across the partially permeable plasma membrane
Water may move from cell to cell through the plasmodesmata
Water may move from cell to cell through adjacent plasma membranes and cell walls
Process of water through the vacuolar pathway
Water enters the cytoplasm across the partially permeable plasma membrane
Water can move into the sap in the vacuole, through the Tonoplast
Water may move from cell to cell through the plasmodesmata
Water may move from cell to cell through adjacent plasma membranes and cell walls
Similar to symplast pathway
Slowest route
What is the casparian strip and where is it found
Found in the endodermis
The caspian strip is an impermeable layer of suburin - a waxy material
It forces all the water in the apoplast pathway into the symplastic pathways
What is the endodermis
This is a continuous cylinder of endodermal cells which surrounds the central vascular tissue (xylem and phloem)
Process when water reaches the casparian strip
When water reaches the endodermis of the root, it’s path is blocked.
The endodermis has a waterproof, impenetrable layer called the Casparian strip in its walls.
This is because of the waxy layer of suberin in the walls of endodermal cells.
To cross the endodermis, water in the apoplast pathway moves into the symplast pathway
In this way the selectively permeable plasma membrane of the cells can control what enters the xylem tissue.
This is important as the cell surface membrane can remove any toxic solutes from the soil, and only allow necessary water molecules and mineral ions to enter.
Function of the casparian strip
Helps to control which substances reach the xylem vessels
Plays a part in increasing root pressure
How does active transport allow water to enter the endodermis
Active transport reduces the water potential of endodermal cells
Water moves by osmosis, down a water potential gradient
How does water leave the leaf once it has been transported through the xylem
Xylem vessels transport the water all round the plant
At the leaves, water leaves the xylem and moves into the cells mainly by the apoplast pathway
Water evaporates from the cell walls into the spaces between cells in the leaf
When the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the leaf (down the water potential gradient) into the surrounding air
The loss of water from a plant’s surface is called transpiration
How is water transported by cohesion
Water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow (due to hydrogen bonds)
This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards
Water enters the stem through the root cortex cells
How is water transported by adhesion
Water molecules are attracted to the walls of the xylem vessels as they are polar
This helps water to rise up through the xylem vessels
This is done through capillary action
What is translocation
Translocation is the movement of assimilates within phloem sieve tubes (e.g. sucrose/amino acids, hormones etc) from where they are made (source) to where they are required (sink)
It is an active process
Features of translocation
Translocation occurs in phloem vessels.
Requires ATP energy to create a pressure difference.
Movement is bidirectional (from source to sink).
Liquid being transported is called ‘phloem sap’
Glucose is transported as sucrose in the phloem sap (20-30%)
What is a source
This is the site where sucrose/assimilates are made and loaded into the phloem
This has a high concentration
Examples of sources in plants
Green leaves and green stem
Storage organs e.g. tubers and tap roots
Food stores in seeds (which are germinating)
How is glucose transported in a plant and why is this
Glucose is transported as sucrose
Sucrose has less of an osmotic effect
What is a sink
Site where sucrose/assimilates are unloaded from the phloem for use or storage
Examples of sinks in plants
Meristems (apical or lateral) that are actively dividing
Roots that are growing and/ or actively absorbing mineral ions
Parts of the plant where the assimilates are being stored (e.g. developing seeds, fruits or storage organs)
3 stages or translocation
Active loading at the source into the phloem sieve tube
Mass flow of sucrose through the sieve tube elements (involves water from xylem)
Active unloading of sucrose at the sink
Pathways of loading of assimilates in translocation
Symplastic pathway (through the cytoplasm and plasmodesmata) which is a passive process as the sucrose molecules move by diffusion.
Apoplastic pathway (through the cell walls) which is an active process.
Process of active loading at the source
- Hydrogen ions (H+) are actively pumped out of the cytoplasm of companion cells via a proton pumps into their cell walls (involves the hydrolysis of ATP – active process).
- This increases the hydrogen ion concentration in the cell walls of the companion cells compared to the inside. Creating a concentration gradient.
- Hydrogen ions, re-enter the cytoplasm of the companion cell, down their concentration gradient via a cotransporter protein.
- While transporting the hydrogen ions this co-transporter protein also carries sucrose molecules (at a different binding site) into the companion cell against the concentration gradient for sucrose (by facilitated diffusion).
- The sucrose molecules then diffuse into the phloem sieve tubes via the plasmodesmata from the companion cells.
Adaptations of companion cells for active loading
They have infoldings in their cell surface membrane which increases the available surface area for the active transport of solutes
Many mitochondria to provide the energy for the proton pump
This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the mesophyll.
Process of mass flow through phloem sieve tubes
Sugars/sucrose/assimilates enter the sieve tube element (at the source), this lowers the water potential in the sieve tube.
Water enters the sieve tube by osmosis from the xylem.
This raises the hydrostatic pressure at the source.
When assimilates leave the sieve tube at the sink, this increases the water potential inside the sieve tube.
Water leaves the sieve tube by osmosis, down a water potential gradient and lowers the hydrostatic pressure (at the sink).
Water moves down the hydrostatic pressure gradient (from high to low) towards the sink, also moving sucrose (and other assimilates) along the phloem.
This is called mass flow.
Active unloading of sucrose at the sink
Sucrose is actively transported out of the companion cells and then moves out of the phloem sieve tubes into the sinks via the apoplastic or symplastic pathways.
In the sink sucrose is converted into other molecules e.g. starch. This helps to maintain a concentration gradient.
When sucrose diffuses out of the sieve tubes, this increases the water potential of the tube.
Water therefore moves out of the sieve tube (back into the xylem vessels) by osmosis.
This creates a low hydrostatic pressure at the sink, compared to the higher hydrostatic pressure at the source.
Uses of glucose in plants
Raw material for growth, repair and replacement of damaged parts
Used to release energy in respiration - energy then used make amino acids then proteins
Used to make cellulose
Energy stored as starch
Energy stored as sucrose in fruit
Makes fats and oils
Evidence for mass flow
Translocation can be slowed down or even stopped at high temperatures or by respiratory inhibitors
Collecting and studying the sap from plants with ‘clotting’ sap (eg. castor oil plants).
Using radioactively labelled metabolites (eg. Carbon-14 labelled sugars) which can be traced during translocation.
Advances in microscopes enabling the adaptations of companion cells to be seen.
Observations about the importance of mitochondria to the process of translocation
Explain the cohesion tension theory
Water vapour diffuses/evaporates out of the leaf via the stomata (transpiration) from an area of high ψ to an area of low ψ.
This loss of water vapour creates a low hydrostatic pressure at the top of the xylem (i.e. in the leaf).
Water is drawn into the xylem in the root (higher hydrostatic pressure). Pressure gradient is created.
This creates a tension (suction) in the xylem which pulls up water in a continuous column.
Within the xylem vessels the columns of water are held together by cohesion and by adhesion
Column (of water) is pulled up by tension
Evidence for the cohesion tension theory
Changes in tree diameter – at high transpiration rates (during the day) diameter decreases due to the tension. At night, during low transpiration rate diameter increases
Cut flowers – often they draw air in rather than leaking water out, as water continues to move up the cut stem
Broken xylems – broken or cut xylems stops drawing up water as the air drawn in breaks the transpiration stream – cohesion between water molecules
How do the stomata open and close in the leaf
When guard cells are turgid: stomata open
When guard cells are flaccid: stomata closed
How do guard cells open the stomata
Guard cells are turgid – Stomata Open
Water moves into the vacuoles by osmosis.
Outer wall is more flexible than the inner wall, so to cell bends and opens the stoma
How do guard cells close the stomata
Guard cells are flaccid – Stomata closed
Water moves out of the vacuoles by osmosis.
Outer wall is more flexible than the inner wall, so to cell bends back and closes the stoma
What conditions are stomata open
Low CO2 concentration inside the leaf
High light intensity
What conditions are stomata closed
High CO2 concentration inside the leaf
Low light intensity (e.g. darkness)
What is transpiration
Transpiration is the loss of water vapour (by evaporation and diffusion) from the surface of leaves and stems of a plant
How does the transpiration stream work
Roots take up water from the soil
Water is drawn up the stem to the leaves
Veins carry water into the leaves
Water evaporates from the leaves
How does transpiration relate to gas exchange in the leaf
Happens as a consequence of gas exchange
Stomata need to be open already to allow CO2 in and O2 out
So water moves out of the leaf from an area of high WP to low WP
How is the rate of transpiration controlled so not too much water is lost
Waxy cuticle (waterproof layer)
Guard cells can open or close stomata
Very few stomata on the upper surface of the leaf
Difference between transpiration and the transpiration stream
Transpiration: the loss of water vapour/evaporation of water from the aerial parts of a plant (leaves, stem, stomata)
Transpiration stream: the flow of water (in continuous columns), up the xylem vessels from roots to leaves
Function of stomata
When stomata are open they allow gas exchange between the leaf and the outside environment.
Carbon dioxide can enter the leaf through the stomata and oxygen and water vapour can diffuse out of the stomata.
Transpiration is mainly controlled by the opening and closing of the stomata.
Conditions for stomata opening
Open during the day and close at night
High water potential outside stomata
Low concentrations of CO2 inside the leaf cause stomata to open
High CO2 in leaf causes stomata to close.
Adaptations of guard cells
Unevenly thickened (cell) wall – wall beside the pore is thicker.
Able to change shape/bend
Transport proteins/ion channels in the plasma membrane. Absorption of K+ ions by the guard cells.
K+ ions decrease the water potential hence water enters by osmosis and guard cells can become turgid.
Presence of chloroplasts & mitochondria to provide ATP energy
What is the rate of transpiration
The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period
What factors affect the rate of transpiration
Temperature
Light intensity
Humidity
Wind
How does temperature affect transpiration
Temperature increase causes an increase in kinetic energy of molecules
This means that water molecules will have more kinetic energy and will move down a concentration gradient at a faster rate
If the temperature gets too high the stomata close to prevent excess water loss, this decreases the rate of transpiration
How does light intensity affect the rate of transpiration
Stomata close in the dark
This greatly reduces the rate of transpiration
When light is sufficient enough for the stomata to open, the rate of transpiration increases
Once the stomata are open, any increase in light intensity has no effect on the rate of transpiration
Stomata will remain open at relatively low light intensities
Why do stomata open at high light intensity
At high light intensity, the rate of photosynthesis increases.
As photosynthesis increases, the amount of stored glucose in the guard cells
increases.
This lowers the water potential of the leaf (i.e. the contents of the leaf are less dilute).
As the water potential decreases, more water enters the guard cells making them more turgid.
The turgor pressure of the guard cells leads to an opening up of stomata resulting in transpiration
How does air movement affect transpiration
Usually a lower concentration of water molecules in the air outside the leaf
When the air is still water molecules accumulate near the leaf surface
This creates a small area of high humidity which lowers the concentration gradient and the rate of transpiration
Air currents can sweep water molecules away from the lead surface, maintaining the concentration gradient and increasing the rate of transpiration
How does humidity affect transpiration
High humidity means there’s a large concentration of water molecules in the air surrounding the leaf surface
This reduces the concentration gradient between inside the lead and the outside air which causes the rate of transpiration to decrease
At a certain level of humidity, an equilibrium is reached; there is no concentration gradient and so there is no net loss of water vapour from the leaves
What is a potometer
Instrument used to measure transpiration rates
Measures the rate of water uptake by a plant
Assumed that water uptake by the plant is directly related to water loss by the leaves
What can potomer be used to measure
Used to measure rate of transpiration
Can use it to estimate how different factors effect the rate of transpiration
How to use a potometer to measure rate of transpiration
Cut a shoot at a slant under water to prevent air from entering the xylem
Slanted cut increases surface area for water uptake
Assemble potometer under water and inset shoot underwater so no air can enter
Remove aparatus from the water but keep the end of the capillary tubing submerged in water
Check apparatus is water tight and airtight (can use Vaseline at joints to achieve this)
Dry leaves and allow time for shoot to acclimatise before shutting the tap
Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put end of the tube back in the water
Record starting position of air bubble
Start a stopwatch and record the distance move by the bubble per unit of time (e.g. per hour)
Rate of air bubble movement is an estimate for the rate of transpiration
Only change one variable at a time (e.g. temperature)
All other conditions (e.g, light and humidity) must be kept constant
What are the different types of plants and water
Mesophytes
Hydrophytes
Xerophytes
What are mesophytes
Plants that are able to take up sufficient water to replace transpiration (most plants)
Where are hydrophytes found
Plants that live either partially or completely submerged in water
This causes problems with O2 uptake
Where are xerophytes found
Plants that live in areas where water lost via transpiration is greater than taken up by roots
What are xerophytes
Xerophytes are plants with structural and physiological adaptations that enable them to survive in hot, dry conditions
What are the differences in xerophytes transpiration rates depending on conditions
When water is abundant, their rate of transpiration is about the same as other plants.
However, in prolonged drought, they have several adaptations, which make them successful
What are the adaptations of xerophytes
Smaller leaves which reduce the surface area for water loss.
Densely packed mesophyll and thick waxy cuticles prevent water loss via evaporation
Xerophytes respond to low water availability by closing the stomata (which are found in pits to shelter from wind) to prevent water loss
Contain hairs and pits which serve as a means of trapping moist air above the leaf thus reducing the water vapour potential.
Stomata may be found only on upper epidermis so they can open into the humid space created by the hairs
Xerophytes also roll the leaves when flaccid to reduce the exposure of lower epidermis to the atmosphere, thus trapping air e.g. Maram grass
Leaves reduced to scales, spines or needles to reduce surface area for transpiration
How do xerophytes reduce their surface area : volume ratio
Have thick leaves rather than thin, broad leaves
Leaves are reduced to spines
Spines can help protect plant from animals which would otherwise remove water from the plant
What are hydrophytes
Plants that live in water
E.g. water lilies
Adaptations of hydrophytes
Very thin or no waxy cuticle as they don’t need to conserve water
Many constantly open stomata are found on the upper surfaces of leaves to maximise gas exchange
Wide, flat leaves give a large surface area for light absorption
Air sacs are found in some hydrophytes to enable leaves to stay afloat
Reduced root system as they extract nutrients from the water through their tissues
Reduced veins in leaves as xylem is not needed to transport water through the plant
Where else can water be lost from a leaf apart from stomata
Waxy cuticle or epidermis
How to reset potometer
Use syringe to suck the bubble back to the starting place or open reservoir
Bubble is reused
Bubble must not reach stem
Labelled potometer with how each component contributes to measuring rate of transpiration
Translocation diagram
