3.3 - Transport in Plants Flashcards
Why do plants need transport systems?
Plants are multicellular and so have a small SA:Vol ratio and a relatively big metabolic rate.
Exchanging substances by direct diffusion (from the outer surface to the cells) would be too slow to meet their metabolic needs.
How is the xylem adapted to its function?
It contains lignified cell walls, which are waterproof, provides support and prevents collapse under tension.
Xylem vessels are empty and form a continuous column for ease of flow.
Continuous column also allows tension to pull water up.
Bordered pits in the cell wall allow movement of water sideways (lateral movement).
Compare and contrast vascular tissues.
Xylem: Dead; lignified walls; cells arranged end to end to form continuous vessel; no cross walls in vessels; bordered pits between vessels; no contents in vessels.
Phloem: Alive; walls not lignified; sieve tubes elements arranged end to end; sieve tube elements separated by sieve plates; plasmodesmata between companion cells and sieve tube elements; sieve tube elements contain cytoplasm only - kept alive by companion cell.
Outline the process by which water enters the cells of the root from the soil.
Root hair cells take up ions by active transport.
This lowers the water potential inside root hair cells.
Water moves down a water potential gradient from high ψ to low ψ by osmosis through channel proteins or aquaporins.
Once water has been taken up by the root hair cells how does it get across to the xylem vessels?
The apoplastic pathway goes via the cell walls but is blocked at the endodermis by the Casparian strip. Water is then forced into the endodermal cells.
The symplastic pathway goes through the cytoplasm via the plasmodesmata.
How does water move in the two different pathways?
Apoplast pathway – by mass flow.
Symplast pathway – by osmosis.
How does water move from the endodermal cells to the xylem?
Endodermal cells actively transport ions into the xylem.
This lowers the water potential of the xylem.
Water then moves from the endodermal cells into the xylem by osmosis.
What is transpiration?
The loss of water vapour from the leaves of a plant (via stomata) by diffusion down a water potential gradient.
Why is transpiration described as by diffusion and not osmosis?
As the movement is not across a partially permeable membrane.
Explain how water moves from the xylem and out of the leaves.
At the leaves, water leaves the xylem and moves into the cells, mainly by the apoplast (cell wall) pathway.
Water evaporates from the cell walls into the spaces in the leaf (spongy mesophyll cells).
When stomata are open this water diffuses out of the leaf, down a water potential gradient, into the surrounding air.
What is the transpiration stream?
The flow of water through a plant.
What are the mechanisms that move water?
Cohesion and tension, adhesion and root pressure.
What is the cohesion-tension theory?
As water molecules are cohesive (they stick together due to H bonding) when some are pulled into the leaf others follow.
Water evaporating from the leaves creates tension (suction) which pulls more water into the leaf.
Describe how adhesion moves water up the stem.
Water rises in the narrow vessels partly because water molecules are attracted to the walls of the vessels (capillary action).
Water also moves up the stem by mass flow. How is root pressure generated?
Active transport of ions into the xylem of root draws water into the stem by osmosis resulting in a high hydrostatic root pressure.
A pressure gradient is generated and will result in mass flow of water in the xylem.
What does a potometer measure?
Water uptake.
