Flashcards in Transport in Xylem and Phloem (Chapter 7) Deck (78)
What are the two possible pathways that water can move down the water potential gradient from cell to cell?
1) symplastic pathway
2) apoplastic pathway
Describe briefly the symplastic pathway
Water moves from cell to cell via the plasmodesmata
Describe briefly the apoplastic pathway
Water moves through the cell walls
What are 3 unusual characteristics of xylem tissue?
1) made from cells joined end to end to form tubes
2) the cells are dead
3) walls of the cells are thickened with lignin
What are the two functions of xylem?
Support and transport
What are the three types of cell in xylem tissue in flowering plants?
1) vessel elements and tracheids (cells involved with transport of water)
2) sclerenchyma fibres (dead, elongated cells with lignified walls that help support the plant)
3) parenchyma cells
What are xylem vessels made up of?
Many vessel elements (elongated cells) arranged end to end
How does a vessel element form?
1) each vessel element starts as a normal plant cell in whose wall lignin is laid down
2) as lignin builds up around the cell, the contents of the cell die, leaving a completely empty space (lumen) inside
What is lignin?
- A very hard, strong substance which is impermeable to water
What are pits, how are they formed, and describe their characteristics?
- Pits are non-lignified areas formed where no lignin is laid down in the original cell walls where groups of plasmodesmata are found
- They are seen as gaps in the thick walls of the xylem vessels
- They are not open pores bc they are crossed by permeable, unthickened cellulose cell wall
- The pits in one cell link with the those in the neighbouring cells, so water can pass freely from one cell to the next
How is a xylem vessel formed?
When the end walls of neighbouring vessel elements break down completely, to form a continuous tube running through the plant
What is hydrostatic pressure?
The pressure exerted by a liquid
How do changes in hydrostatic pressure cause water to move up the xylem vessels?
1) the removal of water from xylem vessels in the leaf reduces the hydrostatic pressure at the top of the xylem vessels
2) ∴ the hydrostatic pressure at the top of the xylem vessel becomes lower than the pressure at the bottom
3) this pressure difference causes water to move up the xylem vessels in continuous columns, as the higher pressure at the bottom pushes water up the vessel
4) the lower the hydrostatic pressure, the lower the water potential ∴ hydrostatic pressure gradient is also a water potential gradient
Why do xylem vessels have strong, lignified walls?
To stop them from collapsing as a result of pressure differences as the water in the xylem vessels is under tension
By what process does water move up xylem vessels?
By mass flow (i.e. all water molecules and any dissolved solutes move together as a body of fluid)
How do cohesion and adhesion allow mass flow of water?
1) Water molecules are attracted to each other (cohesion) and to the cellulose and lignin in the walls of the xylem vessels (adhesion)
2) Cohesion and adhesion ∴ help to keep the water in a xylem vessel moving as a continuous column
What is the benefit of xylem vessel elements being dead cells?
There is no protoplasm to get in the way of transport
Why do xylem vessels have a small diameter?
To help prevent breaks in the column of water (air locks) forming due to air bubbles
What is the benefit of having pits in xylem vessels?
- They allow water to move out into neighbouring vessels and ∴ bypass such an airlock
- Air bubbles cannot pass through pits
- Pits allow water to move out of xylem vessels to surrounding living cells
What is the effect of an airlock?
It prevents water from moving up the vessel as the column of water breaks and the difference in pressure between the water at the top and the water at the bottom cannot be transmitted through the vessel
How does the arrangement of xylem tissue in a series of roads around the centre of the stem help the plant?
It helps to support the stem
How else can plants increase the hydrostatic pressure difference between the top and the bottom of the xylem vessel?
By increasing the pressure at the base of the vessels (roots)
How is hydrostatic pressure raised at the bottom of the vessels?
1) the cells surrounding the vessels actively pump solutes across their membranes, into the water inside the xylem vessels in the root by active transport
2) the presence of the solutes lowers the water potential of the solution in the xylem ∴ drawing in water from the surrounding root cells
3) this influx of water increases the water pressure at the base of the xylem vessel
Why is water transport a largely passive process?
The water simply moves down a continuous water potential gradient from the soil to the air, driven by transpiration in the leaves
Where are xylem vessels in roots?
In the centre
How is water move across the root into the xylem?
1) water taken up by root hairs crosses the cortex of the root and enters the xylem in the centre of the root bc the water potential inside the xylem vessels is lower than the water potential in the root hairs
2) ∴ water moves down this water potential gradient across the root
Describe the apoplastic pathway in roots
Water soaks into the cell walls of the cells of the cortex and seeps across the root from cell wall to cell wall without ever entering the cytoplasm of the cortical cells
Outline the 3 steps of the apoplastic pathway?
1) water enters the cell in the cell wall
2) water moves through the cell wall
3) water moves from cell wall to cell wall through intracellular spaces directly
Describe the symplastic pathway in roots
Water moves into the cytoplasm/vacuole of a cortical cell by osmosis and then into adjacent cells through the interconnecting plasmodesmata, adjacent CSM or cells walls