- rate of diffusion and evaporation increases
- as temp increases, rate of transpiration increases
Light intensity
- stomata open at daylight to absorb CO2
- more H2O evaporates
- increases in light intensity increases rate of transpiration
Wind speed
- still air saturated with water vapour
- wind 'sweeps' water vapour away
- this increases water potential gradient between leaf and air
- increase in wind speed increases rate of transpiration
Humidity
- humid air saturated with water vapour
- shallow gradient between air and leaf
- increase in humidity decreases rate of transpiration
Xylem Vessel - structure and function
Hollow with no cell content
lessressistance to water
Large Lumen
lessressistance to water
Lack of end walls
- creates continous column of water
Pits-
- lateral movement to plant cells
Thick walls of lignin-
- prevents inward collapse
- waterproof
- adhesion
Water Transport
From soil to root hair
- soil has few solutes therefore higher water potential
- cytoplasm and cell sap have lots of solutes therefore lower water potantial
- water therefore moves in by osmosis from a region of high water potential to a region of low water potential, down conc. gradient.
Across Cortex
- APOPLAST pathway: along cellulose cel walls
- SYMPLAST pathway: through living contents i.e vacuole, cytoplasm, plasmadesmata
- At the stele, apoplast pathway is blocked
- Because of casparian strip of the endodermis prevents water therefore forced to go by symplast pathway
- casparian strip is made of suberin=> waxy and waterproof
Up the Xylem
- water is drawn out the top of the xylem to replace those lost in mesophyll cells
- this reduces hydrostatic pressure at the top of xylem
- pressure at bottom>pressure at top
- water therefore moves up xylem by transpiration pull
water kept in a continouscoumn by:
1) cohesion - water molecules stick to eaach other
2) adhesion - water molecules stick to lignin
From leaf to atmosphere
- Mesophyll tissue has air spaces
- walls of mesophyll cells are wet > this water evaporate into air spaces
- creating water potential between outside and inside
- water diffuses out through stomata
Translocation - transport of soluble organic substances from source to sink
- source - leaf
- sink - roots
- sucrose loaded at source by companion cells
- H+ is pumped out of the cell by carrier proteins
- H+ rapidly diffuses back establishing H+ gradient across membrane
- sucrose moves in through by co-transport
- sucroseduffuses down concentration gradient through plasmodesmata
- water follows in by osmosis => raises hydrostatic pressure
- high pressure and mass flow generate => sucrose unloaded at sink
