7.7 Transport in Plants

Water lost from mesophyll cells to spaces in leaf through the cell walls
Water molecules %%form hydrogen bonds%%, stick together, %%cohesion%%
Water forms %%continuous column%% up stem
As water evaporates from mesophyll cells, more molecules are %%drawn up due to cohesion%%
%%Transpiration pull%%
Transpiration pull %%puts xylem under tension,%% so there is %%negative pressure%% within xylem
%%Cohesion tension theory%%
There will be a %%change in diameter of tree trunk%% throughout day due to transpiration
During day, %%when transpiration is high%%, there is %%more negative pressure%% inside cells so %%walls of xylem pulled inwards so diameter decreases%%
If xylem broken and air enters it, water can no longer move up tree as column of water is broken and water cannot be pulled up
If xylem broken, %%water does not leak out as it is not under pressure, instead air is drawn in which is consistent with it being under pressure%%
Transpiration pull is a passive process
%%Xylem cells dead, have no ends to produce a continuous column%%
Energy via heat energy from sun required for transpiration

%%Translocation%%
https://www.youtube.com/watch?v=5jAxyj1ZDhA
%%Sucrose moves from companion cells into sieve tube elements%% by %%active transport%%.
This %%reduces the water potential%% of the sieve tube element.
%%Water moves into the phloem by osmosis%%, which %%increases the hydrostatic pressure%%.
There is a %%pressure gradient with high hydrostatic pressure near the source cell and lower hydrostatic pressure near the sink cell%%s.
Solutes move down the pressure gradient %%towards the sink end of the phloem%%.
%%Solutes move into sink cells%% and are converted into other molecules (e.g. starch).
The removal of solutes %%increases the water potential at the sink end,%% causing %%water to move out of the phloem by osmosis.%%
This %%maintains the hydrostatic pressure gradient%% between the source and the sink
%%Mass flow hypothesis%%
%%Active loading%%
The %%companion cell actively transports hydrogen ions into the surrounding cells.%%
This creates a %%hydrogen ion gradient%% between the surrounding cells and the companion cell.
%%Hydrogen ions move back into the companion cell%% down their concentration gradient through a %%co-transporter protein%%.
Whenever a %%hydrogen ion moves through the co-transporter, a sucrose molecule is also transported into the companion cel%%l, against its concentration gradient.
The %%same process occurs to transport sucrose from the companion cell into the sieve tube element.%%

There is %%pressure in the sieve tube elements, as shown by sap being released when the stem of a plant is cut%%.
The %%concentration of sucrose is higher in the leaves (source) of plants than in roots(sink).%%
%%Increases in sucrose levels in the leaves%% are followed by a %%similar increase in sucrose concentration in the phloem%%.
%%Metabolic poisons/a lack of oxygen inhibit translocation of sucrose in the phloem%%

%%The function of the sieve plates is unclear%%
  as they would %%appear to hinder mass flow%%
  (some suggest though they have a structural
  function to help prevent bursting under
  pressure).
%%Not all solutes move at the same speed%%,
they should do if it is 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.

In order to investigate if the phloem is responsible for mass flow a ringing experiment can be used.
In this the %%bark and phloem of a tree are removed leaving just the xylem%% in the centre.
Overtime the %%tissues above the missing ring swell with sucrose solution and the tissue below dies.%%
This shows that sucrose is transported in the phloem

Tracer experiments can also be used to investigate the transport of sucrose in plants. Plants are grown in a environment that contains %%radioactivity labelled carbon dioxide%% (14CO2).
The presence of this means that they are %%incorporated into the sugar produced in photosynthesis.%%
The %%movement of these sugars can now be traced%% through the plant using autoradiography.
Those %%areas that have been exposed to the radiation produced by the 14C in the sugars will appear black%%.
It follows that these regions correspond to the area where the phloem is and therefore suggest that this is where the sugars are transported.