Photosynthesis 06112012.pdf

7/23/2019 Photosynthesis 06112012.pdf

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PHOTOSYNTHESIS :

Regulation of Calvin Cycle

PhotorespirationHatch and Slack Pathway

Crassulacean Acid Metabolism



Regulation of Calvin Cycle



• Regulation prevents the Calvin Cycle from beingactive in the dark, when it might function in a futile

cycle with Glycolysis & Pentose Phosphate Pathway,wasting ATP & NADPH.

• Light activates, or dark inhibits, the Calvin Cycle(previously called the “dark reaction”) in several

ways.

• Light-activated e- transfer is linked to pumping of H+ into thylakoid disks. pH in the stroma increases toabout 8.

• Alkaline pH activates stromal Calvin Cycle enzymesRuBP Carboxylase, Fructose-1,6-Bisphosphatase &Sedoheptulose Bisphosphatase.



• The light-activated H+ shift is countered by Mg++ releasefrom thylakoids to stroma. RuBP Carboxylase (in stroma)requires Mg++ binding to carbamate at the active site.

• Some plants synthesize a transition-state inhibitor,

carboxyarabinitol-1-phosphate (CA1P), in the dark.

• RuBP Carboxylase Activase facilitates release of CA1Pfrom RuBP Carboxylase, when it is activated underconditions of light by thioredoxin.

stroma(alkaline)

Chloroplast

H2O OH-

+ H+

h

(acid inside

thylakoid disks)



• Thioredoxin is a small protein with a disulfide that is

reduced in chloroplasts via light-activated electron transfer.

• During illumination, the thioredoxin disulfide is reduced

to a dithiol by ferredoxin, a constituent of thephotosynthetic light reaction pathway, via an enzyme

Ferredoxin-Thioredoxin Reductase.

• Reduced thioredoxin activates several Calvin Cycle

enzymes, including Fructose-1,6-bisphosphatase,Sedoheptulose-1,7-bisphosphatase, and RuBP Carboxylase

Activase, by reducing disulfides in those enzymes to thiols.

disulfide

Thioredoxin f PDB 1FAA

t h i o r e d o x i n

-S

-S t h i o r e d o x i n

-SH

-SH|

ferredoxinRed ferredoxinOx

Ferredoxin-

Thioredoxin

Reductase



PHOTORESPIRATION



• Photorespiration occurs when the CO2 levels inside a leaf

become low. This happens on hot dry days. On hot dry days the

plant is forced to close its stomata to prevent excess water loss.

• The plant continues fix CO2 when its stomata are closed, the

CO2 will get used up and the O2 ratio in the leaf will increaserelative to CO2 concentrations. When the CO2 levels inside the

leaf drop to around 50 ppm, RuBisCO starts to combine O2 with

RuBP instead of CO2.



• The net result of this is that instead of producing 2 x 3C PGA

molecules, only one molecule of PGA is produced and a toxic 2C

molecule called phosphoglycolate is produced





• The plant must get rid of the phosphoglycolate since it

is highly toxic. It converts the molecule to glycolic acid.

• The glycolic acid is then transported to the peroxisomeand there converted to glycine.

• Glycine is transported to mitochondria and converted

to serine

• The serine is then used to make other organicmolecules.

• All these conversions cost the plant energy and results

in the net loss of CO2 from the plant

• To prevent this process, two specialized biochemical

additions have been evolved in the plant world: C4 and

CAM metabolism.



Hatch and Slack (C4)Pathway



C4 Leaf Anatomy

• The C4 plants vascular bundlesare surrounded by two rings ofcells- the inner ring, called bundlesheath cells, contain starch-

rich chloroplasts lacking grana- mesophyll cells present asthe outer ring.This peculiar anatomy is calledkranz anatomy .

• kranz anatomy is to provide a

site in which CO2 can beconcentrated around RuBisCO,thereby avoidingphotorespiration



• The C4 pathway is

designed to efficiently fix

CO2 at low concentrationsand plants that use this

pathway are known as C4

plants.

• These plants fix CO2

into a

four carbon compound

(C4) called oxaloacetate.

This occurs in cells called

mesophyll cells.



1. CO2 is fixed to a three-carbon

compound calledphosphoenolpyruvate to produce

the four-carbon compound

oxaloacetate. The enzyme

catalyzing this reaction, PEP

carboxylase, fixes CO2 very

efficiently so the C4 plants don't

need to have their stomata open as

much. The oxaloacetate is then

converted to another four-carboncompound called malate in a step

requiring the reducing power of

NADPH

STEPS IN C4 CYCLE



2. The malate then exits the mesophyll

cells and enters the chloroplasts of

specialized cells called bundle sheath

cells. Here the four-carbon malate is

decarboxylated to produce CO2, a

three-carbon compound called pyruvate, and NADPH . The CO2

combines with ribulose bisphosphate

and goes through the Calvin cycle.

3. The pyruvate re-enters the mesophyllcells, reacts with ATP, and is

converted back to

phosphoenolpyruvate, the starting

compound of the C4

cycle.

STEPS IN C4 CYCLE



Differences between Calvin (C3) and C4 cycles

C3 C4

• Temp 15-250 C

•

Absence of malate• One carboxylation reaction

• CO2 is the substrate

• Usual leaf structures

• Temp 30-350 C

•

Presence of malate• 2 carboxylation reactions

• HCO3 is the substrate

• Closed stomata to reduce

water loss andconcentrating CO2 in the

bundle sheet cells

• Additional ATP is required



Crassulacean Acid Metabolism



• CAM plants live in very dry condition and,unlike other plants, open their stomata to fixCO2 only at night.

•Like C4 plants, the use PEP carboxylase to fixCO2, forming oxaloacetate.

• The oxaloacetate is converted to malate whichis stored in cell vacuoles. During the day whenthe stomata are closed, CO2 is removed fromthe stored malate and enters the Calvin cycle







C3 Plants C4 Plants CAM Plants

Stomata Open at Night

Closed during Day

Partially closed

during day and

partially closed at

night

Closed during day

and Open at night

Carbon fixation Carbon dioxide is“fixed” into a three

carbon compound

that is stable

Carbon dioxide istemporarily stored

as a 4 carbon stable

compound

Carbon dioxide istemporarily stored

as an organic acid.

Water Loss Has trouble withwater loss due to

transpiration.

Trouble with

photorespiration

Less water loss thanC3 plants.

Photorespiration is

not a problem

Grow very slowlyand no problems

with

photorespiration

Comparison

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Photosynthesis 06112012.pdf