photosynthesis 2 revised!

8/9/2019 Photosynthesis 2 REVISED!

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PhotosystemsPhotosystems

Clusters of chlorophyll and other pigments inClusters of chlorophyll and other pigments inthethe thylakoidthylakoid membrane (organized by a set of membrane (organized by a set of proteins in the plant cell)proteins in the plant cell)

Contain few hundred pigment moleculesContain few hundred pigment molecules Chlorophyll aChlorophyll a

Chlorophyll bChlorophyll b

Carotene/Carotene/carotenoidscarotenoids

LightLight--collecting unit of the cellcollecting unit of the cell

Solar panelSolar panel

PhotosystemPhotosystem II andII and PhotosystemPhotosystem II



Light DependentLight Dependent

ReactionsReactions Produce oxygen gas and convert ADP and NADP+ into theProduce oxygen gas and convert ADP and NADP+ into the

energy carriers ATP and NADPHenergy carriers ATP and NADPH

Take place in the THYLAKOID membrane of chloroplastTake place in the THYLAKOID membrane of chloroplast

Begins with photosystem IIBegins with photosystem II (this was discovered after photosystem I but actually occurs before it)(this was discovered after photosystem I but actually occurs before it)

Photosystem II traps light E and transfers excited ePhotosystem II traps light E and transfers excited e-- to an ETCto an ETC

³water ³water--splitting´ photosystemsplitting´ photosystem Light absorbed by photosystem II is used to breakLight absorbed by photosystem II is used to break--up water moleculesup water molecules

into highinto high--E electrons, oxygen, and H+ ionsE electrons, oxygen, and H+ ions

2 electrons2 electronsreplace lost ereplace lost e-- in chlorophyllin chlorophyll

2 H+ ions2 H+ ions released into the inside of the thylakoid membranereleased into the inside of the thylakoid membrane 1 oxygen atom1 oxygen atom oxygen released into atmosphereoxygen released into atmosphere

Electrons in chlorophyll are excitedElectrons in chlorophyll are excitedpassed along ETCpassed along ETCdodoelectrons in chlorophyll run out?electrons in chlorophyll run out? No: the highNo: the high--E electrons lost by the chlorophyll are replaced by theE electrons lost by the chlorophyll are replaced by the

electrons from ³water splitting´electrons from ³water splitting´



HighHigh--E electrons move through the ETC fromE electrons move through the ETC from

photosystemphotosystem II toII to photosystemphotosystem II

Energy from electrons is used by molecules inEnergy from electrons is used by molecules inthe ETC to transport H+ ions from thethe ETC to transport H+ ions from the stromastroma

into the inner into the inner thylakoidthylakoid spacespace



Pigments inPigments in photosystemphotosystem I use Energy fromI use Energy from

light to relight to re--energize the electronsenergize the electrons

NADP+ then picks up these highNADP+ then picks up these high--E electronsE electronsand H+ ions at the outer surface of theand H+ ions at the outer surface of the

thylakoidthylakoid membranemembrane

NADP+ becomes NADPHNADP+ becomes NADPH



As electrons are passed from chlorophyll to As electrons are passed from chlorophyll to

NADP+, H+ ions are pumped acrossNADP+, H+ ions are pumped across

membranemembrane Inside of Inside of thylakoidthylakoid membrane fills up withmembrane fills up with

positive H+ ions, outside in negativepositive H+ ions, outside in negative

Chemosmosis

Chemosmosis occursoccurs

ATP ATP synthasesynthase turns making ADPturns making ADP ATP ATP



ETC proteinsETC proteins PhotosystemPhotosystem II (II (recievesrecieves light & splits water)light & splits water)

OxygenOxygen--evolving complexevolving complex

PlastoquinonePlastoquinone

CytochromeCytochrome

PlastocyaninPlastocyanin

PhotosystemPhotosystem I (receives more light)I (receives more light)

FerredoxinFerredoxin

FerrdoxinFerrdoxin--NADPNADP reductasereductase NADPNADPNADPHNADPH

ATP ATP synthasesynthase

ADP ADP ATP ATP



Over view of LightOver view of Light

Dependent RxnDependent Rxn Use:Use:

ADP ADP

NADP+NADP+

Water Water

Produce:Produce: OxygenOxygen

ATP ATP

NADPHNADPH

Why are these products important?Why are these products important? Provide energy to build energyProvide energy to build energy--containing sugarscontaining sugars

from lowfrom low--energy compounds in Calvin cycleenergy compounds in Calvin cycle



Calvin Cycle/ LightCalvin Cycle/ Light--

Independent ReactionsIndependent Reactions So what do we have from pour lightSo what do we have from pour light--dependentdependent

rxns?rxns? HighHigh--E electrons stored in ATP and NADPHE electrons stored in ATP and NADPH

³chemical energy´³chemical energy´ But plants cannot store this chemical energy for But plants cannot store this chemical energy for

more than a few minutes«must change thismore than a few minutes«must change thischemical energy into something that can be storedchemical energy into something that can be storedfor long periods of timefor long periods of time

Calvin cycleCalvin cycle Uses ATP and NADPH from lightUses ATP and NADPH from light--dependent rxn todependent rxn to

produce highproduce high--E sugarsE sugars



Step 1Step 1

Six carbon dioxide molecules enter cycleSix carbon dioxide molecules enter cycle

from atmospherefrom atmosphere

Enzyme adds each CO2 molecule to aEnzyme adds each CO2 molecule to aRibuloseRibulose biphosphatebiphosphate,, RuBPRuBP molecule (a 5molecule (a 5--

carbon molecule) making six unstable 6carbon molecule) making six unstable 6--

carbon moleculescarbon molecules

The six unstable 6The six unstable 6--carbon moleculescarbon molecules

immediately break off into 12 3immediately break off into 12 3--carboncarbon

molecules called 3molecules called 3--phosphoglycerate, 3phosphoglycerate, 3--

PGAPGA



Step 3Step 3

Two of the G3Ps (twelve 3Two of the G3Ps (twelve 3--carboncarbon

molecules) are removed from the cyclemolecules) are removed from the cycle

Plant uses these two G3Ps (3Plant uses these two G3Ps (3--carboncarbon

molecules) to make sugars, lipids, aminomolecules) to make sugars, lipids, amino

acids, and other compounds plant needsacids, and other compounds plant needs

for metabolism and growthfor metabolism and growth



Step 4Step 4

Remaining ten G3Ps (3Remaining ten G3Ps (3--carboncarbonmolecules) use ATP and rearrangemolecules) use ATP and rearrange

themselvesthemselves ADP and NADP+ go back to light rxns ADP and NADP+ go back to light rxns

Converted back into RuBP molecules (sixConverted back into RuBP molecules (six55--carbon molecules)carbon molecules)

Calvin cycle begins again with six newCalvin cycle begins again with six newCO2 moleculesCO2 molecules



Calvin cycle over viewCalvin cycle over view

Uses:Uses:

Six molecules of CO2Six molecules of CO2

NADPHNADPH

ATP ATP

Produces:Produces:

One 6One 6--carbon sugar ³glucose´carbon sugar ³glucose´



Photosynthesis over viewPhotosynthesis over view

Two sets of reactions work together Two sets of reactions work together

Light dependentLight dependent

Trap energy of sunlight into chemical formTrap energy of sunlight into chemical form

Calvin cycle/light independentCalvin cycle/light independent

Use chemical energy to produce stableUse chemical energy to produce stable--highhigh

energy sugars from carbon dioxide and water energy sugars from carbon dioxide and water



Factors that effect rate of Factors that effect rate of

photosyntehsisphotosyntehsis Water availabilityWater availability

Shortage of water can slow or stopShortage of water can slow or stop photosynphotosyn..

Adaptations Adaptations Desert plants and conifersDesert plants and conifers

Waxy coatingWaxy coating

TemperatureTemperature PhotosynPhotosyn. Depends on enzymes that function between 0*C . Depends on enzymes that function between 0*C

and 35*Cand 35*C

Low temp. may causeLow temp. may cause photosynphotosyn. to stop. to stop

Intensity of lightIntensity of light Increase light intensity=increase rate of photosynthesisIncrease light intensity=increase rate of photosynthesis

After a certain level of intensity, plant reaches its max rate of After a certain level of intensity, plant reaches its max rate of photosynthesisphotosynthesis



ProblemsProblems C

alvin cycleC

alvin cycle-- RuBPRuBP binds withC

O2 to make 6binds withC

O2 to make 6--C

C

compound that changes immediately into 3compound that changes immediately into 3-- CarbonCarbon

compounds that eventually make G3P then sugar compounds that eventually make G3P then sugar

Enzyme that catalyzes this reaction is calledEnzyme that catalyzes this reaction is called RubiscoRubisco

Problem withProblem with RubiscoRubisco is that its not good at grabbingis that its not good at grabbing

CO2CO2

When levels of CO2 inside cell are low,When levels of CO2 inside cell are low, RubiscoRubisco startsstarts

grabbing oxygengrabbing oxygen

This causeThis cause photorespiration photorespiration

When plant uses light and consumes oxygen, producingWhen plant uses light and consumes oxygen, producingcarbon dioxidecarbon dioxide

This uses up energy«NOT goodThis uses up energy«NOT good

Big problem on hot, dry days when stomata close and can¶t getBig problem on hot, dry days when stomata close and can¶t get

CO2CO2

So how do plants deal?So how do plants deal?



Special PlantsSpecial Plants

C4 plantsC4 plants "four "four--carbon´carbon´

plants initially attachCOplants initially attachCO 22 to PEP (to PEP (phosphoenolpyruvatephosphoenolpyruvate) to form the) to form the

four four--carbon compoundcarbon compound oxaloacetateoxaloacetate using the enzyme PEPusing the enzyme PEP

carboxylasecarboxylase.. This takes place inThis takes place in mesophyllmesophyll cells.cells.

OxaloacetateOxaloacetate is then pumped to another set of cells, the bundleis then pumped to another set of cells, the bundle

sheath cells,sheath cells,

In the bundle sheath cells it releases theCOIn the bundle sheath cells it releases theCO 22 for use byfor use by RubiscoRubisco..

Now plant can useCalvin CycleNow plant can useCalvin Cycle

By concentratingCOBy concentratingCO 22 in the bundle sheath cells,C4 plants promotein the bundle sheath cells,C4 plants promote

the efficient operation of theCalvin cycle and minimizethe efficient operation of theCalvin cycle and minimize

photorespiration.photorespiration.

C4 plants include corn, sugar cane, and many other tropical grassesC4 plants include corn, sugar cane, and many other tropical grasses



Special PlantsSpecial Plants

C AM (C AM (crassulaceancrassulacean acid metabolism) plantsacid metabolism) plants plants initially attachCOplants initially attachCO 22 to PEP (to PEP (phosphoenolpyruvatephosphoenolpyruvate) to form the) to form the

four four--carbon compoundcarbon compound oxaloacetateoxaloacetate using the enzyme PEPusing the enzyme PEP

carboxylasecarboxylase.. They do not fix CO2 during the day and send it to other cells likeC4 They do not fix CO2 during the day and send it to other cells likeC4

plantsplants

C AM plants fix carbon at night and store theC AM plants fix carbon at night and store the oxaloaceteteoxaloacetete in largein large

vacuoles within the cell.vacuoles within the cell.

Then they can open their Then they can open their stomatasstomatas at night (cool weather) and letsat night (cool weather) and lets

CO2CO2 The avoid water loss and to use theCOThe avoid water loss and to use theCO 22 for the Calvin cycle duringfor the Calvin cycle during

the day when it can be driven by the sun's energy.the day when it can be driven by the sun's energy.

C AM plants are more common thanC4 plantsC AM plants are more common thanC4 plants

Ex. Are cacti, pineapples and other succulent plants.Ex. Are cacti, pineapples and other succulent plants.

photosynthesis 2 revised!

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