Mass transport in plants Flashcards
Define transpiration
The evaporation of water through the stomata typically through the leaves
How does water move out of stomata during transpiration
-Humidity of the atmosphere is usually less than near the air spaces by the stomata
- This creates a water potential gradient from the air spaces through the stomata to the air
- When stomata is open, water molecules diffuse out of air spaces into surrounding air
- This water lost by diffusion from air spaces is replaced by water evaporating from the cell walls of surrounding mesophyll cells
How does water move across the cells of a leaf during transpiration
Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
-These cells now have a lower water potential so water moves into these cells from neighbouring cells by osmosis
- The loss of water from these neighbouring cells lowers their water potential, in turn they take water by osmosis from other cells
This establishes a water potential gradient where water is forced from the xylem across leaft mesophyll then into the atmosphere
How does water move up the stem during transpiration
- Through the xylem using cohesion tension
-Water evaporates from mesophyll cells due to heat from the sun leading to transpiration
-Water molecules form hydrogen bonds between eachother and stick together in cohesion - Water then forms a long and continous collumn cross mesophyll and down the xylem with more molecules being drawn up behind it as a result of this cohesion
- This causes a transpiration pull putting tension on the xylem
Give evidence which supports cohesion tension theory up the xylem
-Change in diameter of tree trunks according to rate of transpiration, during the day when transpiration is at its greatest there is more tension in the xylem causing negative pressure in the xylem causing an inwards dent in the trunk, at night tension decreases so diameter increases
-If the xylem vessel is broken and air enters, the continuous stream is broken so water molecules can no longer stick together
-When xylem is broke water doesnt leak out and instead air moves in supporting tension
How does xylem move water up stem ( not cohesion)
Xylem walls have no end walls meaning the xylem forms a series of continuous unbroken tubes from root to leaves
- Made up of dead cells so energy is provided for this from the sun
Explain what a ringing experiment is and what it helps show
Rings of bark and phloem were peeled from the tree trunk
The result of this was that the trunk swelled above the removed section
- This liquid swelling was shown to contain sugar accumulated
-The interruption of the flow of sugars to the region below the ring and the death of tissues in this region
- This shows that when phloem is removed sugars could not be transported indicating the phloem is needed to transport sugars
Explain how plant tracing works and what can be concluded from this
- Radioactively labelling carbon
- Plants were provided with only radioactively labelled carbon dioxide
- Over time this was absorbed and used by the plant during photosynthesis to create sugars that are all radioactively labelled
- Thin slices from these stems are then taken and placed on x ray film that turns black when exposed to radioactive material
- When this area of the stem is placed on it, the area carrying the sugars turned black correlating to where the phloem is
How is the xylem adapted to its function
They are hollow cells as they are dead with no organelles with no end walls
This means they can form a continuous column of water
What is the use of a potometer in investigating plants
A potometer measures transpiration and can therefore be used to measure the other enviromental variables on the rate of transpiration
Why must a sample of a plant be cut underwater in the potometer experiment
Prevent any air entering the xylem and breaking the water column
How do you set up the potometer experiment
Sample of a plant is cut underwater
Potometer is filled with water and all air bubbles are removed
Cut leafy plant is then attached to the potometer using seals and jelly
One air bubble is then introduced into the equipment
The distance the air bubble is moved towards the plant is recorded
Rate= The volume is divided by the time it took to lose that volume of water in order to get a rate
Apparatus can be then reset up and different conditions can be applied to the leaf to see if rate changes
Why are rubber seals used during the set up of the experiment
To make equipment airtight
Describe the structure of the phloem
Phloem is made up of sieve tube elements arranged end to end
End walls are perforated to form sieve plates
Sieve tube elements are associated with companion cells
How is sucrose transferred into sieve elements from photosynthesising tissue
- Sucrose is produced during photosynthesis in cells with chloroplasts
-The sucrose diffuses down a concentration gradient by facilitated diffusion from these cells into the companion cells using a carrier protein
-Hydrogen ions are then actively transported from companion cells to spaces in the cell wall in the phloem using ATP
-Hydrogen ions then diffuse down a conc gradient into the sieve tube elements through carrier proteins - Sucrose molecules are transported along with hydrogen ions in co transport using co transport proteins
How is mass flow mechanism used to transport sucrose through the sieve elements
- Once the sucrose is in the sieve tube elements, the water potential of the sieve tubes is lower
- The xylem however has a much higher water potential, meaning water moves down a conc gradient into the sieve tubes by osmosis creating a high hydrostatic pressure inside the sieve tubes
-At respiring cells, sucrose is used up duing respiration or used in storage meaning they have a low sucrose conc - This means sucrose is actively transported into them from sieve tubes lowering their water potential
- This means water will also move into the respiring cells by osmosis from sieve tubes
- This means hydrostatic pressure in this region of the sieve tubes is lowered
- This creates a pressure gradient down the sieve tubes, creating mass flow of sucrose solution down this gradient
Give evidence supporting the mass flow theory
- Pressure in sieve tubes shown by sap being released when they are cut
-Conc of sucrose is higher in source than sink
-Downward flow in the phloem occurs in daylight, ceases when leaves are shaded
-Inc in sucrose levels are followed by an inc in phloem time later
-Lack of oxygen and metabolic poisons inhibit translocation of sucrose in the phloem - Companion cells have many mitochondira and readily produce atp for active transport
Give evidence questioning the mass flow hypothesis
- Function of the sieve plates is unclear as they would seem to hinder mass flow
-Not all solutes move at the same speed, should do if movement is by mass flow
-Sucrose is delivered at same rate to all cells in the plant, however they should travel to where has the lowest sucrose conc according to mass flow
