save my exams mass trasnport in plants Flashcards
whats transported in the xylem
mineral ions and organic compounds
are transported while dissolved in water. The dissolved mineral ions are transported
plant roots
uptake of water and mineral ions and can have root hairs to increase the surface area for absorption of the substance
two pathways that water can take to move across the cortex
Apoplast (also known as apoplastic)
Symplast (also known as symplastic)
symplast
A smaller volume of water travels via the symplast pathway, which involves the cytoplasm and plasmodesmata, and vacuole of the cells
osmosis into the cell and between cells through the plasmodesmata
movement of water in the symplast pathway is slower than the apoplast pathway
apoplast
series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
diffusion
from cell wall to cell wall directly or through the spaces between cells
Casparian strip and forms an impassable barrier for the water
When the water and dissolved minerals reach the Casparian strip they must take the symplast pathway
trasnpiration
1)water vapour diffuses from air spaces through a stoma through trasnpiration to lower water p
2)water evapourates from a mesophyll cell wall into the air spaces creating a tranpiration pull
3)water moves through the apoplastic pathway or our of cytoplasm through symplastic way
4)water leaves xylem through pit and travel by symplasic/apoplastic pathway
5)water moves up the xylem vessels to replace water lost from leaf
role of stomata
When the stomata are open there is a greater rate of transpiration and of gaseous exchange
When the stomata close, transpiration and gaseous exchange decrease
As stomata allow gaseous exchange (CO2 in and O2 out) they are generally open during the day
movement in the phloem
translocation
requires the input of metabolic energy (ATP)
liquid that is being transported-phloem sap
source of the assimilates could be
Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose, as sucrose has less of an osmotic effect than glucose)
Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)
Food stores in seeds (which are germinating)
what a sink could be
Meristems (apical or lateral) that are actively dividing
Roots that are growing and / or actively absorbing mineral ions
Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)
carbohydrates
generally transported in plants in the form of sucrose because:
It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)
It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported
loading of assimilates
may move by the symplastic pathway and apoplastic pathway
unloading of assmilates
occurs at the sinks
sucrose being actively transported out of the companion cells and then moving out of the phloem tissue via apoplastic or symplastic pathways
maintain a concentration gradient in the sink tissue, sucrose is converted into other storage molecules such as starch. This is a metabolic reaction so requires enzymes
sieve cells
dapted for the mass transport of sugars
thick cell walls
These walls help them to withstand the pressure exerted by the mass flow of sugars
what happens when sucrose takes apoplastic pathway
modified companion cells-
pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. This is an active process and therefore requires ATP as an energy source
large concentration of hydrogen ions in the cell wall of the companion cell results in the hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell
The hydrogen ions move through a cotransporter protein. While transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose
The sucrose molecules then move into the sieve tubes via the plasmodesmata from the companion cells
A high concentration of sucrose decreases the water potential in the phloem and water enters by osmosis
The entry of water results in a high pressure (pressure gradient) which enables the mass flow of sugars towards sink tissues
At the sink tissues, sugars are unloaded
pressure
In xylem tissue the pressure difference that causes mass flow occurs because of a water potential gradient between the soil and leaf (this requires no energy input by the plant)
However in 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 into the sieve elements at the source (usually a photosynthesising leaf or storage organ) which lowers the water potential in the sap
This results in water moving into the sieve elements as it travels down the water potential gradient by osmosis
The pressure difference between the source and the sink results in the mass flow of water (containing the dissolved organic solutes) from the high hydrostatic pressure area to the low hydrostatic pressure area
mass flow hypothesis
used to explain the movement of assimilates in the phloem tissue
Two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated)
These two membranes were placed into two chambers containing water and were connected via a passageway
The two membranes were joined via a tube
As the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution which forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)
evidence supporting mass flow
When the phloem sieve tube is punctured phloem sap oozes out
This suggests that it is under pressure
When the virus is applied in the dark it is not transported
This suggests that for translocation to occur photosynthesis and the production of sucrose is required in the source tissue
evidence against mass flow
The rate of translocation of different organic substances was measured and the results showed that amino acids appeared to travel more slowly than sucrose
The mass flow hypothesis states should be flowing at the same rate
t has been suggested that some sieve tubes translocate at different times
The mass flow hypothesis states that nearly all sieve tubes should be involved in translocation at the same time as they are all connected to the same leaves
Transpiration definition
the loss of water from the leaves of the plant
most of this occurs from the underside of a leaf where there are many sto
Transpiration stream
water evapourated from the leaves htia decreases the water potential of the air space insdie the mesophyll
water moved into the air spaces adjacent cells
watwr moves out of the xylem into the cells in the leaves
water hydorgen bonds toitself by cohesion and bond to the walls of the xylem vessel capillary action
cohesion
when water to sticks to other water molecules
cohesion - tension theory
water is a polar molecule meaning thnat its positivr snd negative charged are not evenly dirtsibuted
the oxygen atom has a slight neh=gative charge while the two hydorgn atoms are slightly positivr
this means tht in the xylem water molevules spolnateilsy arrange so that neg and pos poles lie next to eachother f
