Water enters a plant through its root hair cells and then passes through the root cortex and the endodermis cell layer to reach the xylem.
Water cant travel to the xylem by two different paths:
The Symplast Pathway - goes through the cytoplasm and plasmodesmata into neighbouring cells. - living
The Apoplast Pathway - goes throught the cell walls and diffuses into neighbouring cells
-when it reaches the endodermis it's blocked by a casparian strip and take the Symplast Pathway
1) The apoplast pathway is used more regularly as there is less resistance. It's also used when water leaves the xylem and moves into cells.
2) Water evapourates from the leaves through the stomata when they are open.
- Water evaporates from the leaves - transpiration
- This creates tension and pulls more water into the leaf.
- Water molecules are cohesive so the column of water in the stem moves upwards.
- Water is also attracted to the walls of the xylem vessels so which helps it move up.
Stomata open to let CO2 in - and releases H2O at the same time - it moves down the concentration gradient.
Factors that Affect Transpiration Rate
- light - transpiration is faster as stomata open in light
- temperature - transpiration is faster as water evaporates out the cells faster so there is a higher concentration gradient for the water to diffuse faster
- Humidity - the lower, the faster the transpiration rate
-plants adapted to live in dry climates
- Stomata that are sunk in pits - sheltered from the wind
- hairs on the epidermis - traps air around stomata and reduces the water potential gradient
- curled leaves -traps moist air - lowers exposed surface area
- spines instead of leaves- reduce the surface area
- thin waxy layer - waterproof layer reduces evaporation
- reduced stomata - less places water can be lost
-the movement of dissolved substances from source to sink.
-substances are moved from source to sink
source - for sucrose is the leaves where it is made
sinks - where the sucrose is made up (roots, stems + leaves)
-enzymes maintain a concentration gradient between sources and sinks by dissolving the substances at the sink.
1) Sucrose is loaded into the seive tubes by active transport
2) this lowers the water potential - water enters the tubes by osmosis - the pressure increases
3) solutes are used up at the sinks - increasing the water potential inside - water leaves by osmosis - the pressure decreases
4) the result is a pressure gradient
Evidence for Mass Flow
- if a ring of bark is removed from a tree a buldge will form above it with a higher concentration of sugars than below the buldge - there is a concentration gradient
- if a metabolic inhinitor stops ATP prodcution, translocation stops - evidence of active transport
- ugar travels to many different sinks - not just the one with the highest water potential
- seive plates would create a barrier to mass flow