Sophisticated mass flow system in plants for water and dissolved solutes.
Movement of water through a plant.
Lost from the leaves through evaporation.
Water transported through xylem.
Composed of many dead cells joined together to form a long tube.
End cells walls are perforated or completely absent.
Has thick cell walls containing lignin, in rings or helices.
Lignin is very strong and doesnt collapse under presure.
Movement through Roots
Symplast Pathway - through living cytoplasms
Absorbed into root cells by osmosis. lower W.P. in soil than cells
Diffuses from epidermis through root to xylem down W.P. gradient
Cytoplasms connected by Plasmodesmata though holes in the cell walls. So no further osmosis til the xylem.
W.P. = Water potential
Apoplast Pathway - cell walls between cells
Cell walls are thick and open so water can diffuse easily down W.P. gradient.
No membranes to cross so is diffusion not osmosis.
Apoplast pathway stops at casparian strip (waterproof strip running through cell walls).
Water then has to cross cell membrane by osmosis into symplast.
Plant can control water uptake. 90% 0f water transport through apoplast
Root Pressure - Force created by uptake of water, by osmosis, that pushes water up xylem. (helps push water up young or short stems)
Casues Guttation (dew) when drops of water are forced out of leaves.
Movement through Stem
Xylem forms continuous pipe from roots to leaves.
No osmosis occurs in xylem, just mass flow.
Transpiration in leaves causes low presure in leaves and water is sucked up the stem to replace lost water.
Column of water in stem is under tension, water has a high tensile strength because water molecules stick together by H-bonds (Cohesion)
Called Cohesion-Tension Mechanism.
Very strong lignin walls stop xylem collapsing.
Movement through Leaves
Xylem vessles branch to form leaf veins.
Water diffuses from xylem to veins through adjacent cells down W.P. gradient (using symplast and apoplast pathway)
Water evaporates from spongy cells into sub-stomatal air space and diffuses out through stomata.
Evaporation is endothermic (requires energy) so it uses heat energy from the sun.
Therefore the sun is needed as a source of energy for all water movement in plants.
- Energy from sun
- Water evaporates from spongy cells and diffuses out of leaves
- Decresing W.P. in leaf cells
- Water diffuses out of xylem to leaves
- Decreasing pressure in xylem
- Water moves up by mass flow
- Decresing W.P. in roots xylem
- Water diffuses through root cells to xylem
- Decreasing W.P. in root epidemis cells
- Water diffuses into root hair cells from soil by osmosis