3.4.2 Mass transport in plants Flashcards
What 2 types of tissues are involved in transport in plants
- Xylem
- Phloem
What does the xylem transport
Water and dissolved mineral ions
What is the xylem
The tissue that transports water and dissolved mineral ions from the roots, up the stem, to the leaves
What does the phloem transport
- Organic substances - like sucrose
- Amino acids
What is the phloem
The tissue that transports organic substances (e.g. sucrose) from the source to the sinks in plants
What is the structure of the xylem
- Long, hollow tube
- Formed from dead cells, joined together
- No end walls between cells
- Walls contain lignin
In the xylem is the flow one way or both ways
One way
What is the function of lignin
Provides mechanical strength - helps provide structural support for the plant
Define transpiration
The evaporation of water from a plant’s surface, especially the leaves
What are the 4 main factors that effect the rate of transpiration
- Light intensity
- Humidity
- Temperature
- Wind
How does light intensity effect the rate of transpiration in plants, and why does it effect it this way
The greater the light intensity, the faster the rate of transpiration because the stomata open to allow more carbon dioxide to diffuse into the leaf for photosynthesis- this then increases the rate of diffusion of water vapour out of the leaf which therefore increases the rate of transpiration
If the light intensity increases what happens to the rate of transpiration
Increases
How does humidity effect the rate of transpiration, and why does it effect it this way
The greater the humidity the slower the rate of transpiration because there is a lower concentration gradient of water vapour between inside and outside of the leaf - so the rate of diffusion of water vapour out of the leaf decreases
Does a greater humidity increase or decrease the rate of transpiration of plants
Decreases the rate sine the rate if diffusion of water vapour is slower
How does the temperature effect the rate of transpiration and why
The higher the temperature the faster the rate of transpiration because water molecules have more kinetic energy so more water molecules evaporate which creates a higher concentration gradient of water vapour between inside and outside of the leaf which increases the rate of diffusion of water vapour out of the leaf
Does an increases temperature increase or decrease the rate of transpiration
Increases the rate since evaporation happens faster due to steeper concentration gradient of water vapour
How does wind effect the rate of transpiration and why
The more wind the faster the rate of transpiration because wind removes water molecules from outside the stomata maintaining a favourable concentration gradient for water vapour to diffuse out of the leaf
What is the role of the waxy cuticle on plants
Its impermeable to water so reduces water loss by evaporation
What is the role of the upper epidermis
- Its transparent to allow light to pass through
- It protects against mechanical damage
What is the role of the palisade mesophyll layer
Its the main site of photosynthesis since is has the highest density of chloroplasts
What is the role of the spongy mesophyll layer
- Site of gas exchange
- Contains the vascular bundle (xylem and phloem)
What is the role of the lower epidermis
Protect leaf tissue against mechanical damage
What is the role to the stoma
Allows entry and exit of gases
What is the role of the guard cells in plants
- To open and close the stomata
- These are the only lower epidermal cells that contain chloroplasts
What is a xerophyte
A plant that is adapted to living in dry conditions/ areas what water is in short supply
What are the main 5 adaptations of xerophytes
- Thick, waxy cuticle
- Leaf curling / rolling
- Hairs of the leaf
- Sunken stomata
- A reduced SA:Vol of the leaves
Why is having a thick waxy cuticle an adaptation to xerophytes
- It forms a waterproof barrier, reducing the water loss by evaporation from the epidermal cells
Why is leaf curling/ rolling an adaptation to xerophytes
It traps the water vapour which decreases the water potential gradient of water vapour therefore the rate of transpiration is reduced
Why are hairy leaves an adaptations to xerophytes
- Traps moist air next to the leaves surface
- The water potential gradient between the inside and the outside of the leaf is reduced
- So less water is lost by transpiration
Why are sunken stomata an adaptation to xerophytes
- Traps moist air next to the leaf
- Reducing water potential gradient
- Reducing the rate of transpiration
Why is having a reduces SA:Vol of the leaves an adaptation of xerophytes
- Leaves that are small and circular cross section rather than broad and flat leaves reduces the water loss
- There are also fewer stomata
- Both of these reduce the rate of transpiration
Do plants have a circulatory system
No, because they don’t have a pump
What are angiosperms
Flowering plants
Are the xylem and phloem tissues or organs
Tissues
What is the role of packing cells
To fill space
What is lignin
A substance that gets lade down in cells walls
What does lignin replace in plants cells
Cellulose
Why do cells that get surrounded by lignin die
Because lignin is impermeable to everything so the cell can’t exchange substances- so the cell looses all of the organelles
When lignification happens, what happens to the end walls of the elongated stacked cells
The end walls begin to disintegrate
What are the 3 functions of lignin
- Acts the same way cartilage does in the trachea - give strength to the tube when water is being pulled up
- Being completely impermeable to water so water can’t escape outside of the tube
- Has a hydrophilic lining which aids the transport of water
What type of lining does lignin have
Hydrophilic
What is the function of xylem vessel elements
Transport water and mineral ions
Is the xylem vessel elements lignified
Yes, there are non-living
How do tracheids differ from normal xylem vessel elements
They have tapered ends
Do xylem vessel elements and tracheids perform the same function
Yes, they both transport water and mineral ions
What is the function of fibres in the xylem
For mechanical support for tissue
Are the fibres in the xylem lignified
Yes, they are non-living
What is the function of parenchyma cells in the xylem
To act as packing cells between vessel elements
Are the parenchyma cells in the xylem lignified (what do they have in their cell wall instead)
No, they have cellulose in their cell wall
What is the scientific word for drying out
Desiccating
What is transpiration
It’s both evaporation and diffusion of water
What % of water is lost via transpiration
99%
Why is the majority of water lost via transpiration
Due to the cooling effect (plant’s sweating basically) and to bring up mineral ions through the roots
How does water move from cell to cell in a plant, and why
Via osmosis since the mesophyll cells lose water due to evaporation into the air spaces which lowers the water potential gradient so water diffuses in by osmosis from neighbouring cells
What is a transpiration stream
A single continuous column of water from roots to the leaves
Other than transportation, what is the other function of the vascular bundle
To support the stem and roots hence why where the vascular bundle is located in the roots and the stem is different
Where is the vascular bundle located in the stem, and why
The periphery (towards the edges) because the stem is above ground so needs support from winds etc.
Where is the vascular bundle located in the roots, and why
In the middle of the root, because the roots are subject to the pull of gravity
What are the names of the 3 theories of water movement up stem
- Capillary action
- Root pressure theory
- Cohesion-tension theory
Explain what capillary action is, in term of the theory of the movement of water up a stem
The ability of liquid to move through a narrow space usually against external forces like gravity. It occurs because of the intermolecular forces between molecules and forces between water and external environment.
Why is capillary action not 100% the correct explanation to how water travels up the stem of a plant
Because it has been calculated that capillary action could only move water up about 1 metre and the majority of tress are taller, so it can’t be capillary action alone
Explain what the root pressure theory is, in terms of how water moves up the stem of a plant
As the mineral ions are being actively transported into the roots, the water potential lowers so more water moves in. This then increases the hydrostatic pressure inside the root, so the water is pushed along the roots and up the xylem.
Why is root pressure alone not a valid explanation to how water moves up the stem of a plant
Since root pressure can reach a maximum of 0.5 kPa and for a 100 metre tree it would require around 4000 kPa therefore it can’t be root pressure alone
What is cohesion
Attracting/ bonding between the same molecules
What is adhesion
Attraction/ bonding between different molecules
What is the cohesion in the xylem
Water molecules bonded to water molecules
What is the adhesion in the xylem
Water molecules attracting to the hydrophilic lining of the lignin
What creates the tension in the cohesion-tension theory
The transpiration of water that pulls the column of water up
What is the role of nitrate ions in plants
To synthesis amino acids to make proteins
What is the role of magnesium ions in plants
For chlorophyll production
What is the role phosphate ions in plants
To create phospholipids and other essential components of cells
What 2 transportation processes used when a plant uptakes mineral ions
- Facilitated diffusion
- Active transport
What evidence is there that active transport is involved in the uptake of mineral ions in a plant (2 pieces)
- The ions travel against a concentration gradient, since the concentration is much greater inside the plant than in the soil
- When respiratory inhibitors are added the rate of uptake since ATP can’t be produced so active transport cannot be happening therefore it must be a process plants use to uptake mineral ions
What are the 4 adaptations of a root hair cells
- Large surface area
- Thin cell wall
- Lots of mitochondria
- Lots of carrier proteins
What is the name of the equipment used to estimate transpiration rates
Potometer
What are the key features when setting up a potometer to avoid interfering with results (4 points)
- Cut the shoot underwater
- Place the cut shoot in rubbered end of the potometer underwater
- Keep the leaf dry
- Keep joint air tight by using vaseline
When using a potometer why must you set it up underwater
Because liquid water may block the stomata hindering the diffusion of water vapour out of the leaf
How does a potometer work
As the leaf transpires, it draws up water from the potometer, this moves an air bubble, the distance the air bubble moves in a certain period of time can be used to calculate the rate of water uptake
What is the equation of rate of water uptake for a potometer
Speed of air bubble movement (mm/s) x cross-sectional area of capillary tube(mm^2) =rate of water uptake (mm^3/s)
What are sources in plants
Site where photosynthesis occurrs
What are sinks in plants
Where the products of photosynthesis are used or stored
Does the phloem transport organic substances from source to sink or from sink to sources
From sources to sink
What is the function of sieve tube elements in the phloem
To transport sucrose and amino acids (organic solutes)
Describe the structure of sieve tube elements in the phloem (4 features)
- Elongated cells that are stacked to form a series of tubes
- Perforated end walls called sieve plates
- Sieve plates have no nucleus and no organelles
- Sieve plates allow form cytoplasmic connection (so large molecules can pass through)
What are the functions of companion cells of the phloem (3 functions)
- Provide sieve tube elements with ATP
- Synthesise proteins for sieve tube elements
- Involved in the loading and unloading the sucrose from the sieve tube element
Describe the structure of a companion cell in the phloem
- Adjacent to every single sieve tube element
- Larger nucleus and have lots of organelles
- Must have plasmadesmata- allowing for the cytoplasm of the companion cell to connect to the cytoplasm of the sieve tube element
What is translocation
The movement of organic molecules from parts of the plants where the molecules are made (sources) to parts where they are needed or stored (sinks)
Is transport in the phloem unidirectional or bidirectional
Bidirectional, it’s always from source to sink but sometimes the sink is above the source
When can sinks become sources, and why
In winter when the leaves and flowers have dropped off, so the actively developing leaves are now in need of the organic solutes for the before sinks now provide the solutes since they were a storage organ during the summer
What are the names of the 3 theories behind translocation up a plants
- Diffusion
- Cytoplasmic streaming
- Munch’s mass flow theory
Why is diffusion not the accepted reason behind translocation in plants
The rate of diffusion is too slow for the rate of translocation we can observe
What is cytoplasmic streaming, in terms of the theory behind translocation
When the organic solutes within the cytoplasm move around and when the plasmadesmata are open the organic solutes move from one cell to the next.
What is the widely most accepted theory behind translocation
Munch’s mass flow theory
What are the 2 main criticisms of Munch’s mass flow theory
- Sucrose and amino acids have been observed to translocate at different rates in the same phloem, not possible in his theory
- Sucrose and amino acids have been observed to translocate in different directions in the same phloem tissue, again not possible with his theory
What 3 pieces of evidence are there that translocation isn’t passive
- Companion cells have a high number of mitochondria and companions are there to provide sieve tube elements with ATP, so a passive process
- There is a high level of oxygen consumption in the phloem tissue which suggests high rate of respiration, so again not passive
- The rate of translocation is reduced when respiratory inhibitors are added to the phloem tissue
What 2 suggestions are made by Munch’s mass flow theory
- That there is a passive flow of sucrose from source to sink
- That the sap moves down a hydrostatic pressure gradient
What is the 1st step out of the 4 steps that the sucrose is actively transported from the companion cells into the phloem vessel, add the detail
- Hydrogen ions are actively pumped out of the companion cells into the source cell using ATP through a carrier protein, which sets up a hydrogen ion gradient
What is the 2nd step out of the 4 steps that the sucrose is actively transported from the companion cells into the phloem vessel, add the detail
- Hydrogen ions passively diffusion back into the companion cell via facilitated diffusion through a co-transport protein called a symport. Sucrose is also taken in down the hydrogen ion gradient in the co-transport protein, so the sucrose is using indirect active transport
What is the 3rd step out of the 4 steps that the sucrose is actively transported from the companion cells into the phloem vessel, add the detail
- The sucrose now sets up a concentration gradient from the companion cell to the sieve tube element
What is the 4th step out of the 4 steps that the sucrose is actively transported from the companion cells into the phloem vessel, add the detail
- Sucrose moves into the sieve tube element via facilitated diffusion
What is the name of the co-transport protein that transports the hydrogen ions and the sucrose into the companion cell
Symport
What type of transport does the sucrose use when co-transporting into the companion cell going down the hydrogen ion concentration gradient
Indirect active transport
Is the process of transporting sucrose into the sieve tube element from the companion cell active or passive, and explain why
Active, because ATP had to be used to set up the hydrogen ion gradient and without this sucrose wouldn’t be moved in
What are the 7 steps explaining Munch’s mass flow theory
- The sucrose is actively transported into the phloem vessel from the source
- This lowers the water potential so water moves from the neighbouring xylem by osmosis which increases the hydrostatic pressure
- The hydrostatic pressure is lower at the sink cell so the sap moves down a hydrostatic pressure gradient
- Organic solutes are actively transported into the sink cell, increases the water potential
- There’s a lower water potential in the neighbouring xylem due to the presence of mineral ions
- So water travels down the water potential gradient
- Water is then pulled up the xylem by cohesion-tension theory
What 3 experiments are used to explain translocation
- Aphid experiment
- Ringing
- Tracers
What do scientists do for the ringing experiment to take place
They cut a section of the outer layers including the phloem while leaving the xylem intact
What happens to the stem after the ringing experiment, above and below the cut, and explain why
- Above- Stem begins the swell due to sap pooling/ accumulating
- Below- narrowers because growth hasn’t occurred since sap hasn’t been able to get there so respiration couldn’t take place so energy couldn’t be provided
How does the tracer experiment help scientists to investigate translocation
Because you can track all of the compounds that contain the radioactive carbon
How is the radioactive carbon taken into the plant for the tracer experiment to happen
Via photosynthesis
What compound containing the radioactive carbon in the tracer experiment do scientists trace in translocation
Sucrose
What is the name of the picture the x-ray produces after scanning the radioactive carbon containing plant
An autoradiograph
In an autoradiograph, what colour indicates the presence of the radioactive carbon compounds
Black, the blackened areas are found to correspond to where the phloem is
What gives plant cell walls their strength
Cellulose
Name 2 ways in which plants use water
- Maintain turgidity
- Photosynthesis
Name the carbohydrate that is transported in the phloem
Sucrose
Name 2 examples of sink cells
- Tubers
- Bulbs
Describe 3 difference between the structure of the phloem tissue and the structure of xylem vessels
- Phloem tissue has companion cells which has organelles, whereas xylem vessel has no organelles
- Sieve tube elements have perforated end walls whereas xylem vessel doesn’t have end walls
- Xylem vessels are non-living whereas phloem has living tissue
Explain the difference between cohesion and adhesion
Cohesion is when molecules are attracted to each other (water molecule to water molecule)
Adhesion is when molecules are attracted to surrounding molecules (water molecules to xylem molecules)
Describe the 4 precautions that should be taken when setting up a potometer
- Don’t get the leaves wet
- Cut the shoot underwater
- Place shoot into the potometer under water (set up underwater)
- Make joints air tight
What type of molecule is sucrose
Disaccharide
The mass flow hypothesis is used to explain the movement of substances through phloem, explain how pressure is generated inside the phloem tube
- Sucrose is actively transported into phloem
- Lowering water potential
- Water moves into phloem by osmosis from xylem
Why does the phloem pressure reduced during the hottest part of the day, use understanding of transpiration and mass flow
- High rate of transpiration
- Water lost through stomata
- Causes less water movement from xylem to phloem
Describe the cohesion-tension theory of water transport in the xylem (5 marks)
- Water lost from leaf because transpiration
- Lower water potential of leaf cells
- Water pulled up xylem
- Water molecules cohere together by hydrogen bonds
- Forming continuous column of water
- Adhesion of water molecules to walls of xylem
Describe the mass flow hypothesis for the mechanism of translocation in plants (4 marks)
- In source/ leaf sugars actively transported into phloem
- By companion cells
- Lowers water potential of sieve cell and water enters by osmosis
- Increases in pressure causes mass movement
- Sugars used/ converted in root for respiration for storage
Explain the importance of the xylem being kept open as a continuous tube
- Allows unbroken water column
- Cohesion from hydrogen bonds between water molecules
- Transpiration creates tension
