Plant Transport Flashcards
How is water taken up the plant?
It travels from the soil through the roots and is transported to the leaves where it maintains turgidity and is a reactant in photosynthesis.
What process is a lot of water lost by?
Transpiration via the stomata. The water loss must be offset by a constant replacement from the soil.
Which region has the greatest uptake?
The root hair cell
What features does the root hair cell have?
Large surface area for water to enter via osmosis. Cellulose cell wall which is freely permeable to water.
A large number of mitochondria to provide ATP for active transport of mineral ions.
A large number of protein carriers in bedded in the membrane for active transport of mineral ions.
Does soil water have a dilute or concentrated solution of mineral ions?
It contains a very dilutes solution so has a high water potential.
Does the vacuole and cytoplasm of the root hair cell have a dilute or concentrated solution of mineral ions?
It contains a concentrated solution of solutes therefore has a low water potential. Water moves into the root hair cell down water potential gradient by osmosis.
What is the apoplast pathway?
It’s when the source solution soaked into the walls of the epidermal cells and travels across the cortex to the cell walls or through spaces between cells, drawn by the transpiration stream.
What is the symplast pathway?
It’s when water moves to the cytoplasm of cells via the plasmodesmata. The plasmodesmata are strands of cytoplasm through pits in the cell wall joining adjacent cells so that the simplest route is continuous across the root cortex.
What is the vacuolar pathway?
It’s when water travels through the cell vacuoles.
What are the two main pathways?
The apoplast and symplast pathway is with the apoplast past rate being faster and more significant.
Why can’t water into the xylem from the apoplast route?
The ligin makes the xylem waterproof. Water can only pass from the symplast or vacuolar pathways into the xylem so must leave the apoplast pathway.
Describe the structure of the epidermis?
The vascular tissue in the centre of the root is surrounded by the pericycle where the pericycle is surrounded by a single layer of the cells called endodermis.
What are the endodermis cell walls impregnated by?
A waxy cuticle called the Suberin.
What does the Suberin form?
A distinctive band on the radical and tangential walls called the casparian band.
What does the casparian band do?
It blocks the apoplast route so drives the water into the cytoplasm. At the band water passes across the plasma membrane and continues along the symplast route. As the xylem lacks cell contents the water is transferred to the apoplast in the pericycle.
Why does most water enter the cytoplasm of the root hair cell?
By osmosis because the active uptake of mineral ions lowers the solute potential.
How does water cross the cortex of the plant?
Via the cell walls. Water molecules are attracted to cellulose (adhesion) and to each other (cohesion)
How far can water and ions travel along the apoplast route?
It can travel along that route until it reaches the casparian band which prevents it moving over, so must cross into the symplast route.
How are ions transported across the membrane?
By active transport, with water following into the symplast route down a WP gradient.
What happens when water and ions enter the pericycle?
They enter from the epidermis where the ions are actively pumped into the xylem and water follows by osmosis.
Describe the movement of water from the root endodermis to the xylem?
It moves via osmosis across the endodermal cell membrane. Water potential in the xylem need to be more negative than the water potential in the endodermal cells.
How is a lower water potential in the xylem than the endodermal cells activated?
The water potential in the endodermis cells is raised by water driven in by the casparian strip.
The water potential in the xylem is lowered by active transport of mineral salts mainly sodium ions from the endodermis and the pericycle into the xylem.
How does water move into the xylem?
Down the water pressure gradient via osmosis. Water coming into the xylem generates an upward push of root pressure and water already in the xylem.
How are minerals absorbed into the cytoplasm of root hair cells?
By active transport against a concentration gradient.
Describe the uptake of nitrogen?
Nitrogen usually enters the plan is nitrate/ammonium ions which diffuse along a concentration gradient into the apoplast stream but enters the symplast by active transport against a concentration gradient and then flows in the cytoplasm through the plasmodesmata. Active transport allows the plan to absorb the iron selectively at the endodermis.
Describe the movement of water from the roots to the leaves?
Water always moves down the water tension gradient. Air has a very low water potential and the soil water has a very high water potential due to the dilute solution of solutes. Therefore water moves up the soil through the plants into the air it passes from the route to the xylem up to the leaves when most is transpired
What are the three main mechanisms for the movement of water?
Cohesion tension, capillarity, root pressure.
Where does water evaporate from and what does this cause?
Evaporates from the leaves cells into the air spaces and defuses out through the stomata. They stored water across the cells of the leaf in the apoplast, symplast and vacuolar pathways from the xylem.
What is cohesion?
It’s when the water molecules are attracted to each other due to the polar nature of the hydrogen bonds between the water molecules, which then ‘cling’ to each other as they move up the xylem.
What does cohesion cause?
It causes a continuous pole which produces tension in the water column. The columns of water in the xylem and held open by the cohesive forces between water molecules and the adhesive forces between the water molecules and the hydrophilic lining of the xylem vessels.
What is adhesion?
The charges of the molecules with water being polar it adheres to the ligin which lines the inner surface of the xylem vessels which contributes to the movement up the xylem.
How does transpiration cause tension?
As transpiration is the loss of water from the leaves this creates a negative pressure which gives rise to the transpiration stream. They continued removal of water molecules from the top of the xylem vessels results in tension causing a pull in the xylem column.
What does the cohesion tension theory describe?
The movement of water up the xylem by a combination of adhesion of water molecules and tension in the column resulting in their cohesion.
What is capillarity?
It’s the movement of water up narrow tubes (xylem) by capillary action. It only operates at short distances i.e. up to 1 m.
What is root pressure?
ROOT pressure is when mineral ions are actively transported into the xylem, decreasing the water potential within the xylem. Water moves into the xylem by osmosis increasing the hydrostatic pressure.
In the transpiration stream what is water drawn up by?
Cohesive forces between the water molecules. Adhesive forces between the water molecules and the hydrophilic lining in the xylem vessels.
What are the two main types of water conducting tissues in the xylem?
Xylem vessels and tracheids.
What do you xylem vessels and tracheids form?
Continuous tubes (no end walls where cells join) a column of water can only travel up one direction.
What is the role of dead cells within the xylem?
These dead cells have no cell contents, so passive with no impediment to fly so it’s easier for water to flow up. They are dead due to the decomposition of ligin on the cellulose cell walls which makes them impermeable.
How is the ligin deposited?
As spring/spirals which provide mechanical strength as it prevents the xylem from collapsing due to tension (negative pressure) this supports the plant and allows adhesion. Pit where there are no ligin is deposited = plasmodesmata (allows sideways movement between vessels)
What are xylem vessels?
They form main conduct in tubes with wide cells that have reduced or absent end walls so form a continuous tube. The ligin builds up their cell wall as the contents die which leaves an empty space. As a tissue develops the end walls break down leaving a hollow tube which water can travel up (cellulose stains red in the xylem so its easily identifiable)
What are xylem tracheids?
They are slightly narrower with perforated end walls so water flow is more obstructed than vessels but provides more support. They are less adapted than vessels.
