Mass Transport in Plants

Question 1

Transpiration explained:

Answer 1

-the movement of water through a plants xylem occurs due to the evaporation of water
-the water potential gradient is the driving force behind the movement of water
-transpiration is important: it provides a cooling system, helpful in the uptake of mineral ions, the turgor pressure of the cells provides support to leaves and stems of non-woody plants

Question 2

movement of water through leaves:

Answer 2

-the water vapor lost by transpiration lowers the water potential in the air spaces surrounding the mesophyll cells
-the water with the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull
-this transpiration pull results in water moving through the mesophyll cell walls or out of the mesophyll cytoplasm
-the pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas) which cause water to move up the xylem vessels to replace this lost water - this movement is called the transpiration stream

Question 3

movement in the phloem:

Answer 3

-translocation that’s being transported (sieve tubes) is called phloem sap mainly made up of sucrose
-the loading and unloading of sucrose from the source to the phloem to the sink is an active process

Question 4

the sucrose loading mechanism:

Answer 4

-assimilates such as sucrose are transported from source to sink through the phloem sieve tubes
-carbs are generally transported in plants in the form of sucrose because it allows for efficient energy transfer and increased storage - disaccharide it’s less reactive than glucose

Question 5

unloading of assimilates e.g. sucrose:

Answer 5

-the unloading of assimilates occur at the sink
-simillar to the loading of sucrose with the sucrose being actively transported out of the companion cells and then moving out of the phloem tissue via apoplastic or symplastic pathways
-to maintain a concentration gradient in the sink tissue, sucrose is converted into other storage molecules like starch
-the intracellular space and walls of the sieve cells are adapted for the mass transport of sugars
-small volume of cytoplasm and vacuole present in the intramolecular space
-the phloem sap can flow easily by moving through the open corridor that has low resistance
-sieve cells have thick cell walls to help withstand pressure exerted by the mass flow of sugars

Question 6

mass flow in the phloem:

Answer 6

mass flow hypothesis: two partially permeable membranes containing solutions with different concentrations of ions, the two membranes were joined in a tubes, as the membranes were surrounded by water the water removed by osmosis across the membrane containing the more dilute solution where water was being forced out due to hydrostatic pressure
the pressure flow gradient

Question 7

hydrostatic flow gradiant:

Answer 7

-the advantages of mass flow is that it moves organic solutes faster than diffusion
-in the xylem tissue the pressure difference that causes mass flow occurs because of a water potential gradient between the soil and leaf however in the phloem tissue energy is required to create pressure differences for the mass flow of the organic solutes
the pressure difference is generated by actively loading sucrose in the sieve elements at the source which lowers the water potential in the sap
this results in water moving in the sieve elements as it travels down the water potential gradient by osmosis, the pressure within the sieve elements increases the hydrostatic pressure at the source and as the solutes are unloaded from the sieve elements causing water to follow by osmosis at the sink
pressure difference between the source and the sink results in the mass flow of water form high to low hydrostatic pressure
the mass flow of organic solutes within the phloem tissue occurs above and below the sources therefore sap flows upwards and downwards within a plant

Question 8

Explain how each of the following is related to the function of the Xylem tissue
1) xylem tissue contains hollow tubes
2) lignin is present in xylem cell walls

Answer 8

1) The hollow tubes enable efficient transportation of water and minerals
2) lignin provides strength and stability