Transpiration and Translocation Flashcards
What is the humidity of the atmosphere usually like?
Less than that of the air spaces next to the stomata
What is created given that the humidity of the atmosphere is less than that by the stomata?
A water potential gradient from air spaces though stomata to the air
What happens due to the water potential gradient between stomata and air?
Provided stomata are open, water vapour molecules diffuse out of air spaces into air
How is water lost by diffusion from air spaces replaced?
Evaporating from the cell walls of the surrounding mesophyll cells
How do plants control their rate of transpiration?
By changing the size of the stomatal pores
How is water lost from mesophyll cells?
Evaporation from their cells walls to the air spaces in the leaf
How is water lost from mesophyll cells replaced?
Water reaching the mesophyll cells from the xylem via cells walls or the cytoplasm
How does the movement occur via the cytoplasmic route?
- Mesophyll cells lose water to air spaces by evaporation
- These cells now have a lower water potential
- Water enters by osmosis from neighbouring cells
- Water potential lowers in these cells
- So take water from their neighbours via osmosis
What is established due to the movement of water via the cytoplasmic route to replace the water in the mesophyll cells?
A water potential gradient that pulls water from the xylem, across the leaf mesophyll, and into the atmosphere
What is the main factor that is responsible for the movement of water up the xylem?
Cohesion - tension
How do water molecules have cohesion?
They from hydrogen bonds so tend to stick together
What does water form across the mesophyll cells and down the xylem?
A continuous, unbroken column
What happens as water evaporates from the mesophyll cells in terms of the column of water?
More molecules of water are drawn up behind as a result of cohesion
What is pulled up the xylem as a result of transpiration?
A column of water - called the transpiration pull
What does the transpiration pull put the xylem under?
Tension - there is a negative pressure within the xylem
(hence the name cohesion-tension)
How does the diameter of a tree trunk change between day and night?
- In the day, transpiration at its greatest, so more tension in xylem, so pulls walls inwards and causes trunk to shrink in diameter
- At night, transpiration at lowest, so less tension and diameter of trunk increases
What happens if a xylem vessel is broken and air enters?
Tree can no longer draw up water as continuous column is broken and water molecules can’t stick together
What proves that the xylem vessel is under tension?
When it is broken, air does not leak out but is drawn in
Does the transpiration pull require energy?
No - it is passive
Energy is still needed to drive the process - comes in the form of heat that evaporates water from leaves which comes from sun
Are the xylem vessels alive or dead?
Dead
How do xylem vessels form a series of continuous, unbroken tubes from roots to leaves?
Vessels have no end walls - essential to cohesion-tension theory
What is used to strengthen the xylem vessels?
Lignin - often forms rings or spirals around the vessel
What do we use for the measurement of water uptake?
A potometer
Where do we measure water uptake from?
The shoot
What different conditions can we measure water uptake under?
Various humidities, wind speeds or temperatures
How is the leafy shoot cut in the potometer experiment?
Cut under water - care is taken to not get water on leaves
What is the potometer filled with at the beginning of the experiment and what are we careful about?
Filled with water, ensuring no air bubbles
How is the leafy shoot fit into the potometer?
Using a rubber tube, and fitted under water
What happens after the leafy shoot is fitted into the potometer?
Potometer is removed from under the water and all joints are sealed with waterproof jelly
How do we start the potometer experiment after the apparatus has been set up?
An air bubbles is introduced into the capillary tube
What do we measure in the potometer experiment?
The distance moved by the air bubble in a given time
Measured a number of times to calculate a mean
What does the mean value give us in the potometer experiment?
Volume of water lost, which is plotted against time on a graph
What happens as the air bubble nears the junction between the reservoir tube and the capillary tube in the potometer experiment?
Tap on reservoir opened and syringe pushed down until bubble is pushed back to start of scale on capillary tubes
What is phloem made up of?
Sieve tube elements - long thin structures arranged end to end
What are the end walls of phloem like?
They are perforated to form sieve plates
What are associated with the sieve tube elements in the phloem?
Companion cells
From where do phloem transport sugars?
From sites of production - sources
Where do phloem transport sugars to?
Places where they are used directly or stored - sinks
Why is the translocation of molecules in either direction?
Sinks can be anywhere in a plant
Which organic molecules are transported in the phloem?
Sucrose and amino acids
Which inorganic ions are transported in the phloem?
Potassium
Chloride
Phosphate
Magnesium
What does current thinking favour for the mechanism of translocation?
Mass flow theory
What is the first stage of translocation?
Transfer of sucrose into sieve elements from photosynthesising tissue
Where is sucrose manufactured?
From photosynthesis in cells with chloroplasts
How does sucrose move from photosynthesising cells to companion cells?
Down a concentration gradient via facilitated diffusion
What happens to hydrogen ions in the process of translocation?
They are actively transported from companion cells into the spaces within cells walls using ATP
What happens to the hydrogen ions once they are actively transported into the cell walls?
They diffuse down a concentration gradient through carrier proteins into sieve tube elements
How do hydrogen ions aid sucrose molecules?
They are transported along with the hydrogen ions in a process known as co-transport
What is the second stage of translocation?
Mass flow of sucrose though sieve tube elements
What is caused by the fact that sucrose is actively trasported into the sieve tubes?
The sieve tubes have a lower water potential
What happens when the xylem has a much higher water potential than the sieve tubes?
Water moves from the xylem into the sieve tubes by osmosis
What does the movement of water into the sieve tubes from the xylem create inside them?
A high hydrostatic pressure
What happens at the repsiring cells (sinks) to the sucrose?
It is used for respiration or converted to starch for storage
What is the sucrose content like in respiring cells?
Low
What happens given that the sucrose content in repsiring cells are low?
Sucrose is actively tranported into them from the sieve tubes lowering their water potential
What happens given that respiring cells have a low water potential?
Water moves into these cells from the sieve tubes by osmosis
What happens other to the hydrostatic pressure in the sieve tubes in the region by the respiring cells?
It is lowered
What happens as a result of the high hydrostatic pressure at the source and the low one at the sinks?
There is a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes
Is mass flow a passive process?
Yes
What does mass flow occur as a result of even though it is passive?
The active transport of sugars
What is the mass flow process as a whole?
Active
What can the mass flow process be affected by?
Temperature, metabolic poisons, etc.
What is the third stage of translocation?
Transfer of sucrose from the sieve tube elements into storgae or other sink cells
How is sucrose transported to sink cells?
It is actively transported by companion cells out of the sieve tubes and into the sink cells
What is evidence for the mass flow theory?
Concentration of sucrose is higher in leaves than in roots
When does downward flow in the phloem occur and when does it cease?
It occurs in daylight but ceased when leaves are shaded or at night
What are increases in sucrose levels in the leaves followed by?
Similar increase in sucrose levels in the phloem a little later
What inhibits translocation of sucrose in the phloem?
Metabolic poisons and/or lack of oxygen
What do companion cells have a lot of?
Mitochondria and they readily produce ATP
What is the evidence against the mass flow theory in terms of sieve plates?
Their function is unclear as they would seem to hinder mass flow
What is the evidence against the mass flow theory in terms of speed of solutes?
Not all solutes move at the same speed (they would do so if movement is by mass flow)
What is the evidence against the mass flow theory in terms of delivery of sucrose to different regions?
Sucrose is delivered at more of less the same rate to all regions - mass flow would suggest it is delivered more quickly to ones with lowest sucrose concentration
What do woody stems have on the inside and outside?
- An outer protective layer of bark
- Then a layer of phloem
- Then the xylem
- All circular and inside of each other
What happens at the start of a ringing experiment?
A section of the outer layers is removed around the complete circumference of a woody stem
What does the removing of the phloem around the stem lead to in the ringing experiment above the ring?
An accumulation of sugars (so swelling)
What does the removing of the phloem around the stem lead to in the ringing experiment below the ring?
Interruption of flow of sugars so death of tissues in this region
What is the conclusion of the ringing experiment?
Phloem, not xylem, is responsible for translocating sugars in plants
What would happen if xylem was responsible for translocating sugars in the ringing experiment?
Xylem is not removed, so continuity is not broken and so sugars would not accumulate above the ring and tissues below wouldn’t die
What is used for the tracing of movement of substances in plants?
Radioactive isotopes
What can C14 be used to make?
Radioactively labelled CO2
What happens if a plant is grown in an atmosphere containing 14CO2?
The C14 will be incorporated into the sucrose produced during photosynthesis
What can we do given that C14 is in the sucrose in the plant?
The radioactive sugars can be traced within the plant using autoradiography
What happens when phloem is cut?
A solution of organic molecules flow out
What can plants with radioactive CO2 be shown to have after a short amount of time?
Radioactively labelled carbon in phloem
Which insect can be used to extract the content of the sieve tubes?
Aphids
How can aphids be used to extract the content of the sieve tubes?
They have needle like mouthparts that penetrate the phloem
From aphids, what do we discover about sucrose content levels in different parts of the plant?
- Show daily variation in sucrose content of leaves
- This is mirrored a little later by identical changes in sucrose content of phloem
