Mass Transport In Plants Flashcards
Draw and label the structure of a xylem
- thin area of cell wall (pit)
- thick cell wall containing lignin
- space containing no cytoplasm
What are the adaptations of the xylem?
- Long cells end-to-end with no end walls Enables continuous water columns
- No cytoplasm/no organelles (the tissue is dead), Nothing to obstruct flow of water
- Cellulose cell walls thickened with lignin to Withstand tension (otherwise the xylem vessel would collapse as water is pulled through)
- Pits in walls, Lignin is waterproof. Pits are unlignified patches of cell wall that allow lateral (sideways) movement of water out of the xylem. This can also be important to allow water to bypass blocked xylem vessels.
Describe the cohesion tension theory of water transport in the xylem
- Water lost from leaf because of transpiration
- Lowers water potential of mesophyll
- Water pulled up xylem creating tension
- Water molecules cohere together by hydrogen bonds
- Forming continuous water column
- Adhesion of water molecules to walls of xylem
Suggest precautions students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot
- Seal joints to prevent water loss
- Cut shoot under water to stop air entering shoot to ensure continuous water columns in xylem
- Cut shoot at a slant increases surface area for water uptake
- Dry off leaves to ensure stomata are not blocked by water droplets
- Insert into apparatus under water to stop air bubbles getting in
- Ensure no air bubbles present other than the single air bubble being tracked
- Shut tap to stop air bubble moving backwards
- Note where bubble is at start to measure how far the bubble goes
What molecules does the xylem transport?
- water and dissolved ions
Describe the process of transpiration
-
What kind of process is transpiration?
- passive
What are the features of lignin in the xylem?
- support
- adhesion of H2O molecules
- Waterproof
What do the pits allow in the xylem?
- lateral movement of water if there are any blockages
Define transpiration
- loss of water vapour from mesophyll cells of a leaf
Define tension in the cohesion tension theory
- upward force of molecules due to transpiration
Define cohesion in cohesion tension theory
- water molecules being attracted to each other due to being dipolar and forming hydrogen bonds
Define adhesion
- attraction of water molecules to lignin
What does cohesion and adhesion along with transpiration cause?
- forms an upward force and tension
What happens to the water potential of the mesophyll as transpiration occurs?
- decreases
Describe how water is moved through a plant
- water transpires from leaves
- water is drawn out from the xylem
- this decreases water potential in cells
- cohesion- water molecules attracted to each other through hydrogen bonds causing continuous column as there are cohesive forces between water molecules
- water molecules also adhere to the walls of the xylem which helps pull water column upwards
- as this column of water is pulled up the xylem, it creates tension, pulling the xylem in to become narrower
- water moves up in a continuous column
What is the purpose of transpiration?
- acts as a means of cooling the plant as it absorbs heat energy from cells, reducing temperature of plant
What are the factors that increase transpiration?
- Increase in air movement
- Increase in temperature
- Decrease in humidity
- Increase in light intensity
- Time of day
How does increase in air movement increase transpiration?
- removes water molecules water molecules from near the leaf and thus causes increase in water potential gradient
- positive correlation
How does increase in temperature increase transpiration?
- increase in kinetic energy causes liquid water to water vapour
- positive correlation
How does decrease in humidity increase transpiration?
- causes decrease in water potential gradient as more water vapour in air will make water potential more positive outside the leaf
- negative correlation
How does increase in light intensity increase transpiration?
- causes more stomata to open for photosynthetic gas exchange and increase surface area for evaporation
- positive correlation
How does the time of day increase transpiration?
- plants may open or close stomata at different times of the day. Balancing act between photosynthesis gaseous exchange and transpiration
occurring
What happens to water vapour on a windy day?
- wind blows water vapour away from leaf
What happens to water vapour on a still day?
- On still day moist air is not blown away and a diffusion shell forms resulting in a boundary layer: a layer of undisturbed air on the outside of the leaf. So water vapour stays around the stomata
- an increase in air movements will blow away the boundary layer and increase diffusion gradient between air spaces in the leaves and the surrounding air
How do xerophytes prevent water loss?
- The presence of hairs trap damp air close to the stomata reducing the amount of air movement and reducing transpiration.
- prevents water potential gradient
What increases the impact of adhesion?
- narrower xylem
What is root pressure?
- as water moves into roots by osmosis, it increases volume of liquid inside the root and therefore the pressure inside the root increases
What does root pressure cause?
- forces water above it upwards (positive pressure)
What does a potometer measure?
- rate of the uptake of water from a plant to indicate rate of transpiration
What can potometers be used for?
- effect of a named environmental variable on the rate of transpiration
Describe how to use a potometer
- Cut sample of plant diagonally underwater to prevent air from entering xylem and breaking water column
- Potometer is filled with water and all air bubbles removed
- The cut leafy plant is attached to potometer using rubber seals and petroleum jelly to make equipment air tight
- One air bubble is introduced into the equipment and distance this air bubble has moved towards the plant is recorded
How to convert potometer results to estimate of transpiration rate?
- the distance the bubble moved can be used to work out the volume of water in the tube that evaporated
- volume is divided by the time it took to lose that volume of water to get a rate
How to reset a potometer?
- open the tap and redo the experiment
What may be used to replicate sunlight?
- lamp
What may be used to replicate wind?
- fan
What may be used to increase humidity?
- plastic bag over plant
Explain why the apparatus must be set up and plant shoot must be cut underwater
- due to cohesion tension creating negative pressure in the xylem, it if was cut in air it would draw air into the xylem tube
- this would break the continuous water column and prevent transpiration
- by cutting underwater only water is drawn into the xylem
Why must all joints in the apparatus be covered in petroleum jelly?
- petroleum jelly is waterproof so prevents any water leaking out and ensures all water can only leave by evaporation out of the stomata on the plant
What variables would have to be controlled if you were to perform this experiment on two different plant species?
- surface area of leaves (number and size of leaves)
Define translocation
- is the transport of soluble organic substances,
sucrose and amino acids within a plant. Movement is bidirectional at the same time and at different rates
Draw and label a phloem
- sieve element (sieve plate, sieve pore, mitochondrion, cytoplasm)
- companion cell (cell wall, mitochondrion, rough endoplasmic reticulum, plasma membrane, plasmodesmata, vacuole, middle lamella, nucleus, cytoplasm)
Which features are only found in companion cells?
- nucleus
- ribosomes
- vacuole
- tonoplast
Adaptation of sieve tube element
- few organelles, cytoplasm at the edge allows more
flow within the sieve element
Adaptation of companion cell
- many mitochondria for production of ATP. ATP when hydrolysed will release energy required in active transport
What are the features common to both sieve elements and companion cells?
- cell wall
- plasma membrane
- cytoplasm
- mitochondria
- endoplasmic reticulum
Describe the process of translocation
- At source (e.g. photosynthesising leaf)
companion cells actively transport sucrose
into the sieve tube elements (“loading” of
sucrose); - This lowers the water potential in the sieve
tube elements and water enters them from
the xylem by osmosis; - This produces a high hydrostatic pressure in
the sieve tube; - The resulting pressure gradient leads to the mass flow of fluid towards the sink;
- At sink sucrose is removed (“unloaded”) by active transport to the sink (e.g. storage tissue/respiring cells);
- The water potential in the sieve tube is now
higher and water moves by osmosis back to
the xylem. This lowers the hydrostatic
pressure in the sieve tube
Where in the plant is sucrose converted to starch?
- bulbs and tubes
Describe the transport of carbohydrate in
plants.
- Sucrose actively transported into phloem (cell)
O R
Sucrose is co-transported/moved with H+ into phloem (cell); - (By) companion/transfer cells;
- Lowers water potential (in phloem) and water
enters (from xylem) by osmosis; - ((Produces) higher) (hydrostatic) pressure;
O R
(Produces hydrostatic) pressure gradient; - Mass flow to respiring cells
O R - Mass flow to storage tissue/organ;
Unloaded/removed (from phloem) by active
transport;
What are the sources of evidence that transport of organic compounds occurs in the phloem?
- 14^C used to label and trace organic compounds
- Tree ringing experiments
- Sap sucking insects
Describe The use of radioisotope labelling to show that translocation is a two way
process (bi–directional)
- Plant given CO2 that contains radioactive carbon (14 C)
- During photosynthesis the plant will use 14 C and this will be fixed in the
glucose produced. - The sucrose will be moved around the plant using translocation
- To test the location of the sugars the ‘source’ leaf and ‘sink ‘tissues are placed on a photographic film for 24 hrs. The film is then developed. Radioactivity is detected by ‘fogging’ areas
- after development of film an autoradiograph is produced (‘fogging’ occurs where 14 C is detected)
- Radioactivity detected in both aerial tissues of the plant and the sinks tissues showing that sugar is transported in both upward and downward
direction (bi-directional)
Treatment D is a control. Explain how the measurement obtained from this control
is used by the scientist
- Used to compare effect of other treatments
- Measures effect of substance
The mass flow hypothesis is used to explain the movement of substances through
phloem Evaluate whether the information from this investigation supports this
In support of mass flow hypothesis
1. (F shows) phloem is involved;
2. (G shows) respiration / active transport is involved (in flow / movement);
3. Because 4 °C / cooling reduces / slows / stops flow / movement;
4. The agar block is the source;
5. Roots are the sink;
Against the mass flow hypothesis
6. No bulge above ringing (in F);
7. No (role for) osmosis / hydrostatic pressure / water movement;
Accept no turgor pressure
8. Movement could be due to gravity;
9. Roots still grow without (intact/functioning) phloem;
10. No leaves / sugars / photosynthesis to act as a source;
Give evidence supporting mass flow hypothesis
- there is a pressure within sieve tubes, as shown by sap being released when they are cut.
- the concentration of sucrose is higher in leaves (source) than in roots (sink).
- downward flow in the phloem occurs in daylight, but ceases when leaves are shaded, or at night.
(Dark= no sucrose production) - increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later.
- metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem. (Less aerobic respiration so less ATP)
• companion cells possess many mitochondria and readily produce ATP.
Give evidence questioning mass flow hypothesis
- the function of the sieve plates is unclear, as they would seem to hinder mass flow (it has been suggested that they may have a structural function, helping to prevent the tubes from bursting under pressure).
- not all solutes move at the same speed - they should do so if movement is by mass flow.
- sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory would suggest.
What does the sap sucking insects suggest?
- When aphid pierce the stem of the plant and body of aphid is removed.
- the mouth part (stylet) stays, the Sap is under pressure as it exudes from the stem
- This shows that translocation is a rapid process, too rapid to occur by diffusion
Describe the tree ringing experiments
- the bark of a tree contains the active phloem. Ringing involves removing a ring of bark. Downward movement of organic compounds stops where bark is removed;
Describe the process of treatment ringing experiment
- Remove outer bark (ring) removing phloem
- so sucrose collects above ring, acts as sink
- As translocation can’t occur
Draw and label the mass flow 2 model
- Cell A = source
- Cell B = sink
- Tube D = xylem
- Tube C = phloem
- Concentration solution of sucrose (low water potential) in Cell A
- Dilute solution of sucrose or starch suspension (high water potential) in Cell B
- Water enters Cell A by osmosis (water forced out of Cell B due to pressure created in Cell A)
- Increase in hydrostatic pressure forces content of Cell A to Cell B
Compare the tissues the xylem and phloem are composed of
- xylem is composed of xylem vessels whereas phloem are composed of sieve tube elements and companion cells
Compare the cell wall material of xylem and phloem
- xylem is made up of lignin and cellulose whereas phloem is made up of only cellulose
Compare the permeability of xylem and phloem
- xylem is permeable in areas with cellulose but not in lignin whereas phloem is permeable
Does the xylem contain cytoplasm?
- no
Does the phloem contain cytoplasm?
- yes
Does the xylem contain organelles?
- no
Does the phloem contain organelles?
- yes (only some in sieve tube)
Compare the direction of flow in the xylem and phloem
- in xylem flow is unidirectional whereas in phloem flow is bidirectional
Compare what the xylem and phloem transport
- xylem transports water and mineral ions whereas phloem transports sucrose and amino acids
