Photosynthesis Photosynthesis ReactionsReactions

Electron carriersElectron carriers Electrons in chlorophyllElectrons in chlorophyll Sun excites themSun excites them Electrons gain EElectrons gain E These high E electrons require special These high E electrons require special

carriercarrier High E e- are similar to hot coals that need High E e- are similar to hot coals that need

to be transferredto be transferred Electron carriers needed to transport high-Electron carriers needed to transport high-

E e-E e- NADP+ NADP+

Nicotinamide adenine dinucleotide Nicotinamide adenine dinucleotide phosphate (phosphate (Just remember NADPJust remember NADP))

Accepts 2 high-E e- to become Accepts 2 high-E e- to become NADPHNADPH

This is how energy (e-) from sun can This is how energy (e-) from sun can be trapped in chemical formbe trapped in chemical form

NADPH carries high-E e- from NADPH carries high-E e- from chlorophyll to other parts of the chlorophyll to other parts of the chloroplastchloroplast

Help build molecules, such as glucoseHelp build molecules, such as glucose

PhotosystemsPhotosystems

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

Contain few hundred pigment moleculesContain few hundred pigment molecules Chlorophyll a (absorbs 680 nm wavelengths)Chlorophyll a (absorbs 680 nm wavelengths) Chlorophyll b (absorbs 700 nm wavelengths)Chlorophyll b (absorbs 700 nm wavelengths) Carotene/carotenoidsCarotene/carotenoids

Light-collecting unit of the cellLight-collecting unit of the cell Solar panelSolar panel Photosystem II (P680) and Photosystem II (P680) and Photosystem I (P700)Photosystem I (P700)

Light Dependent Light Dependent ReactionsReactions

Produce oxygen gas and convert ADP and NADP+ into the Produce 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 (this was discovered after photosystem I but actually occurs before it)it)

Photosystem II traps light E and transfers excited e- to an ETCPhotosystem II traps light E and transfers excited e- to an ETC ““water-splitting” photosystemwater-splitting” photosystem

Light absorbed by photosystem II is used to break-up water molecules Light absorbed by photosystem II is used to break-up water molecules into high-E electrons, oxygen, and H+ ionsinto high-E electrons, oxygen, and H+ ions

2 electrons2 electronsreplace lost e- in chlorophyllreplace lost e- in chlorophyll 2 H+ ions2 H+ ions released into the inside of the thylakoid membrane released into the inside of the thylakoid membrane 1 oxygen atom1 oxygen atom oxygen released into atmosphere oxygen released into atmosphere

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

electrons from “water splitting”electrons from “water splitting”

High-E electrons move through the ETC from High-E electrons move through the ETC from photosystem II to photosystem Iphotosystem II to photosystem I

Energy from electrons is used by molecules in Energy from electrons is used by molecules in the ETC to transport H+ ions from the stroma the ETC to transport H+ ions from the stroma into the inner thylakoid spaceinto the inner thylakoid space

Pigments in photosystem I use Energy from Pigments in photosystem I use Energy from light to re-energize the electronslight to re-energize the electrons

NADP+ then picks up these high-E electrons NADP+ then picks up these high-E electrons and H+ ions at the outer surface of the and H+ ions at the outer surface of the thylakoid membranethylakoid membrane

NADP+ becomes NADPHNADP+ becomes NADPH

As electrons are passed from chlorophyll to As electrons are passed from chlorophyll to NADP+, H+ ions are pumped across NADP+, H+ ions are pumped across membranemembrane

Inside of thylakoid membrane fills up with Inside of thylakoid membrane fills up with positive H+ ions, outside in negativepositive H+ ions, outside in negative

Chemosmosis occursChemosmosis occurs ATP synthase turns making ADPATP synthase turns making ADPATPATP

ETC proteinsETC proteins Photosystem II (recieves light & splits water)Photosystem II (recieves light & splits water)

Oxygen-evolving complexOxygen-evolving complex PlastoquinonePlastoquinone CytochromeCytochrome PlastocyaninPlastocyanin

Photosystem I (receives more light)Photosystem I (receives more light) FerredoxinFerredoxin Ferrdoxin-NADP reductaseFerrdoxin-NADP reductase

NADPNADPNADPHNADPH

ATP synthaseATP synthase ADPADPATPATP

Overview of Light Overview of Light Dependent RxnDependent Rxn

Use:Use: ADPADP NADP+NADP+ WaterWater

Produce:Produce: OxygenOxygen ATPATP NADPHNADPH

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

from low-energy compounds in Calvin cyclefrom low-energy compounds in Calvin cycle

Calvin Cycle/ Light-Calvin Cycle/ Light-Independent ReactionsIndependent Reactions

So what do we have from pour light-dependent So what do we have from pour light-dependent rxns?rxns? High-E electrons stored in ATP and NADPHHigh-E electrons stored in ATP and NADPH ““chemical energy”chemical energy” But plants cannot store this chemical energy for But plants cannot store this chemical energy for

more than a few minutes…must change this more than a few minutes…must change this chemical energy into something that can be stored chemical energy into something that can be stored for long periods of timefor long periods of time

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

produce high-E sugarsproduce high-E sugars

Step 1Step 1

Six carbon dioxide molecules enter cycle Six carbon dioxide molecules enter cycle from atmospherefrom atmosphere Enzyme adds each CO2 molecule to a Ribulose Enzyme adds each CO2 molecule to a Ribulose

biphosphate, RuBP molecule (a 5-carbon biphosphate, RuBP molecule (a 5-carbon molecule) making six unstable 6-carbon molecule) making six unstable 6-carbon moleculesmolecules

The six unstable 6-carbon molecules The six unstable 6-carbon molecules immediately break off into 12 3-carbon immediately break off into 12 3-carbon molecules called 3-phosphoglycerate, 3-PGAmolecules called 3-phosphoglycerate, 3-PGA

Step 2Step 2

Twelve 3-carbon molecules are converted Twelve 3-carbon molecules are converted into higher energy forms using energy from into higher energy forms using energy from ATP and high-E electrons from NADPHATP and high-E electrons from NADPH Twelve 3-PGAs are converted into twelve Twelve 3-PGAs are converted into twelve

energized Glyceraldehyde 3-phosphates energized Glyceraldehyde 3-phosphates (G3P).(G3P). 3-PGAs + ATP3-PGAs + ATP1, 3-biphosphoglycerate1, 3-biphosphoglycerate 1, 3-biphosphoglycerate + NADPH1, 3-biphosphoglycerate + NADPH

Glyceraldehyde 3-phosphateGlyceraldehyde 3-phosphate

Step 3Step 3

Two of the G3Ps (twelve 3-carbon Two of the G3Ps (twelve 3-carbon molecules) are removed from the cyclemolecules) are removed from the cycle

Plant uses these two G3Ps (3-carbon Plant uses these two G3Ps (3-carbon molecules) to make sugars, lipids, amino molecules) to make sugars, lipids, amino acids, and other compounds plant needs acids, and other compounds plant needs for metabolism and growthfor metabolism and growth

Step 4Step 4

Remaining ten G3Ps (3-carbon Remaining ten G3Ps (3-carbon molecules) use ATP and rearrange molecules) use ATP and rearrange themselvesthemselves ADP and NADP+ go back to light rxnsADP and NADP+ go back to light rxns

Converted back into RuBP molecules (six Converted back into RuBP molecules (six 5-carbon molecules)5-carbon molecules)

Calvin cycle begins again with six new Calvin cycle begins again with six new CO2 moleculesCO2 molecules

Calvin cycle overviewCalvin cycle overview

Uses:Uses: Six molecules of CO2Six molecules of CO2 NADPHNADPH ATPATP

Produces:Produces: One 6-carbon sugar “glucose”One 6-carbon sugar “glucose”

Photosynthesis overviewPhotosynthesis overview

Two sets of reactions work togetherTwo 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 stable-high Use chemical energy to produce stable-high

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 stop photosyn.Shortage of water can slow or stop photosyn. Adaptations Adaptations

Desert plants and conifersDesert plants and conifers Waxy coatingWaxy coating

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

and 35*Cand 35*C Low temp. may cause photosyn. to stopLow temp. may cause photosyn. 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 Calvin cycle- RuBP binds with CO2 to make 6-C compound that Calvin cycle- RuBP binds with CO2 to make 6-C compound that

changes immediately into 3- Carbon compounds that eventually make changes immediately into 3- Carbon compounds that eventually make G3P then sugarG3P then sugar

Enzyme that catalyzes this reaction is called RubiscoEnzyme that catalyzes this reaction is called Rubisco Problem with Rubisco is that its not good at grabbing CO2…gets easily Problem with Rubisco is that its not good at grabbing CO2…gets easily

confused by oxygenconfused by oxygen When levels of CO2 inside cell are low, Rubisco starts grabbing oxygen When levels of CO2 inside cell are low, Rubisco starts grabbing oxygen This cause This cause photorespirationphotorespiration

When plant uses light and consumes oxygen, producing carbon dioxideWhen plant uses light and consumes oxygen, producing carbon dioxide This uses up energy…NOT goodThis uses up energy…NOT good Big problem on hot, dry days when stomata close and can’t get CO2Big problem on hot, dry days when stomata close and can’t get CO2

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

Reducing photorespiration Separate carbon fixation from Calvin cycle

C4 plants PHYSICALLY separate carbon fixation from Calvin cycle

different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds

different enzyme to capture CO2 (fix carbon) PEP carboxylase

different leaf structure

CAM plants separate carbon fixation from Calvin cycle by TIME OF DAY fix carbon during night

store carbon in 4C compounds

perform Calvin cycle during day

Special PlantsSpecial Plants C4 plantsC4 plants

"four-carbon”"four-carbon” plants initially attach CO plants initially attach CO 2 2 to PEP (phosphoenolpyruvate) to form the to PEP (phosphoenolpyruvate) to form the

four-carbon compound oxaloacetate using the enzyme PEP four-carbon compound oxaloacetate using the enzyme PEP carboxylase. carboxylase.

This takes place in mesophyll cells. This takes place in mesophyll cells. Oxaloacetate is then pumped to another set of cells, the bundle sheath Oxaloacetate is then pumped to another set of cells, the bundle sheath

cells,cells, In the bundle sheath cells it releases the CO In the bundle sheath cells it releases the CO 2 2 for use by Rubisco.for use by Rubisco.

Now plant can use Calvin CycleNow plant can use Calvin Cycle By concentrating CO By concentrating CO 2 2 in the bundle sheath cells, C4 plants promote in the bundle sheath cells, C4 plants promote

the efficient operation of the Calvin cycle and minimize photorespiration. the efficient operation of the Calvin cycle and minimize photorespiration. C4 plants include corn, sugar cane, and many other tropical grasses C4 plants include corn, sugar cane, and many other tropical grasses

C4 plants A better way to capture CO2

1st step before Calvin cycle, fix carbon with enzymePEP carboxylase store as 4C compound

adaptation to hot, dry climates have to close stomates a lot different leaf anatomy

sugar cane, corn, other grasses…

sugar cane

corn

Comparative anatomy

C3 C4

Location,location,location!

PHYSICALLY separate C fixation from Calvin cycle

Special PlantsSpecial Plants

CAM (crassulacean acid metabolism) plantsCAM (crassulacean acid metabolism) plants plants initially attach CO plants initially attach CO 2 2 to PEP (phosphoenolpyruvate) to form the four-to PEP (phosphoenolpyruvate) to form the four-

carbon compound oxaloacetate using the enzyme PEP carboxylase.carbon compound oxaloacetate using the enzyme PEP carboxylase. CAM plants fix carbon at night and store the oxaloacetete (organic acid) in CAM plants fix carbon at night and store the oxaloacetete (organic acid) in

large vacuoles within the cell. large vacuoles within the cell. During the day, CO2 is released from these organic acids and used in the During the day, CO2 is released from these organic acids and used in the

Calvin cycleCalvin cycle

Then they can open their stomatas at night (cool weather) and lets CO2 Then they can open their stomatas at night (cool weather) and lets CO2 The avoid water loss and to use the CO The avoid water loss and to use the CO 2 2 for the Calvin cycle during the for the Calvin cycle during the

day when it can be driven by the sun's energy. day when it can be driven by the sun's energy. CAM plants are more common than C4 plants CAM plants are more common than C4 plants

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

CAM (Crassulacean Acid Metabolism) plants Adaptation to hot, dry climates

separate carbon fixation from Calvin cycle by TIME close stomates during day

open stomates during night

at night: open stomates & fix carbonin 4C “storage” compounds

in day: release CO2 from 4C acids to Calvin cycle increases concentration of CO2 in cells

succulents, some cacti, pineapple

It’s all inthe timing!

CAM plants

succulents

cacti

pineapple

C4 vs CAM Summary

C4 plants separate 2 steps of C fixation anatomically in 2 different cells

CAM plants separate 2 steps of C fixation temporally =2 different times

night vs. day

solves CO2 / O2 gas exchange vs. H2O loss challenge