absorption of water and minerals water and minerals enter the plant through the epidermis of the...
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Absorption of water and mineralsWater and minerals enter the plant through the epidermis of the
root, through the cortex, and into the stele (vascular xylem)Diagram on pg. 755Endodermis is very selective in what gets to the xylem.The minerals are “screened” by the symplast when they enter
at the epidermisThose that enter the endodermis through the apoplast, are
stopped by the Casparian strip – a waxy barrierHowever, the water can cross the membrane and enter with
symplast water.Then the water enters the tracheid and vessels elements of the
xylem – these are apoplastic – solutes are transferred from symplasts to apoplast.
Now, the water and minerals can be transported throughout the plant.
Transport of xylem sapWater is being “pulled” by transpiration, so
must be replenished by soil water.At night, root cells push mineral ions into
the xylemThis lowers the water potential causing
water to flow in and force water up the xylem – root pressure
This cause guttation fluid in the morningRoot pressure cannot compete with
transpiration at sunrise.
Guttation
Cohesion and AdhesionThe cohesive and adhesive nature of water
contribute to transpiration
Guard cellsIn dicots, guard cells take on water, become turgid, and
expand due to the position of the cellulose microfibrils.This opens the stomataInflux of K ions also causes water to enter the guard
cells and the stomata to open. Light triggers this.Stomatal opening also correlates with H ion being
transported out of the cellBlue light receptors in the guard cells are triggered at
dawn to power proton pumps and promote K ion uptakeAlso, guard cells begin to photosynthesize, making ATP
for proton pumps.Guard cells also contain an internal clock that open and
close stomata
C4 Plants C4 plants minimize the cost
of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
This step requires the enzyme PEP carboxylase
PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle
© 2011 Pearson Education, Inc.
Figure 10.20
C4 leaf anatomy The C4 pathway
Photosyntheticcells of C4 plant leaf
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Stoma
Mesophyll cell PEP carboxylase
CO2
Oxaloacetate (4C)PEP (3C)
Malate (4C)
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
ADP
ATP
CAM Plants
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night, incorporating CO2 into organic acids
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
© 2011 Pearson Education, Inc.
Sugarcane
Mesophyllcell
Bundle-sheathcell
C4 CO2
Organic acid
CO2
CalvinCycle
Sugar
(a) Spatial separation of steps (b) Temporal separation of steps
CO2
Organic acid
CO2
CalvinCycle
Sugar
Day
Night
CAM
Pineapple
CO2 incorporated(carbon fixation)
CO2 releasedto the Calvincycle
2
1
Figure 10.21
Transport of Sugars Translocation is the movement of
carbohydrates through the phloem from a source to a sink. The source is leaves, the sink is where the carbohydrate will be used.
Pressure-flow hypothesis1. Soluble sugars like
fructose and sucrose move from palisade mesophyll to sieve tube members by active transport
2. Water then diffuses into the cells
3. Pressure in sieve tube causes water and sugar to flow toward sink
4. Sugars are moved by active transport into neighboring cells
5. Water diffuses back to xylem
StarchAny cell can act as a sink if they convert
soluble sugar into starch.Any cell can act as a source if they break
down starch into glucose.