Absorption of water and minerals Water and minerals enter the plant through the epidermis of the root, through the cortex, and into the stele (vascular.

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

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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.