Photosynthesis

Life from Light and Air

Phot

osyn

thes

is w

ith

Ha

nk!

Obtaining the materials– sunlight• leaves = solar collectors

– CO2• stomates = gas

exchange– H2O• uptake from roots

– nutrients• N, P, K, S, Mg, Fe… • uptake from roots

Concurrent Reactions

Light-Dependent Reactions*Key to these rxns is light absorption

Success lies in photosystems – 2 part complex

1. Antenna complex – site of absorption; excites electrons & passes them on

2. Reaction center – redox-rxns occur here; excited ele- transferred to electron acceptor

reactioncenter

antennapigments

PS

680

H+ + ½ O2 +

PS

700

NADP + + 2H +

NADPH

ADP + P

ATP!

- pheophytin- plastoquinone

- Cytochro

mecomplex

- H + ion

- ferredoxin- ele -

- NADP+ Reductase

- ATP Syntha

se

To Calvin Cycle!!!

Stroma

Lumen

Step 1: Light absorbed in PS II excites ele- (from H2O splitting) → passed to electron acceptor (pheophytin)Step 2: Plastoquinone(PQ) shuttles high energy ele- through series of redox rxns (ETC 1); passes through cytochrome complex, driving in H+, ↑ [H+] = ↓ pHStep 3: De-energized ele- are passed off to plastocyanin (PC) → PS I re-excites ele- and ferredoxin shuttles through ETC 2

Step 4: a) Ele- reduce NADP+ to NADPH at NADP+ reductase

b) H+ pumped out via ATP Synthase – activateschemiosmosis → production of ATP from ADP

Regulation of the Light Reaction

– Calvin cycle uses more ATP than NADPH

– If there is enough, NADPH production is turned off

18 ATP +12 NADPH

1 C6H12O6

ATP

Review Animations

• Light dependent reactions

Light-INdependent Reactions

•Want to make C6H12O6 → synthesis**How? From what? What raw materials are available?

CO2

C6H12O6

NADPH

NADPreduces CO2

carbon fixation

NADP

From CO2 C6H12O6

• CO2 has very little chemical energy– fully oxidized

• C6H12O6 has a LOT of chemical energy– highly reduced

• Synthesis = endergonic process– put in a lot of energy

• Reduction of CO2 C6H12O6 proceeds in many small uphill steps– each catalyzed by specific enzyme– uses energy stored in ATP & NADPH

←from light rxns!

starch,sucrose,cellulose& more

1C CO2

Calvin cycle

5CRuBP

3C

RuBisCo

1. Carbon fixation

2. Reduction

3. Regenerationof RuBP

ribulose bisphosphateribulose

bisphosphate

carboxylase

6 NADP6 NADPH 6 ADP

6 ATP

3 ADP3 ATP

usedto makeglucose

3C

3CG3P aka PGAL

glyceraldehyde-3-P

C C C C C

C C C C C

C C C C C

6CC C C C C C

C C C C C CC C C C C C

C C C

C C CC C CC C CC C CC C C

PGAphosphoglycerate

C C C

C C CC C CC C CC C CC C CC C C

CC

C

C C C= =

|H

|H

|H

|H

|H

|HC C C– –

5C

To G3P and Beyond!• Resulting 3-C Sugar

- important intermediate• G3P glucose carbohydrates

lipids phospholipids, fats, waxes amino acids proteins nucleic acids DNA, RNA

To G3Pand beyond!

A little more about RuBisCo…

•Enzyme which fixes carbon from air•ribulose bisphosphate carboxylase

• the most important enzyme in the world!

**It makes life out of air!**

• most abundant enzyme

“Banking”• Keeping the books straight…

– 6 turns of Calvin cycle = 1 C6H12O6 (6C)– 6 CO2 1 C6H12O6 (6C)– 18 ATP + 12 NADPH 1 C6H12O6

– Left over ATP from light reactions used elsewhere by cell

Sum it Up: Light Reactions

O2

H2O

Energy BuildingReactions

ATP

produces ATP & NADPH

releases O2 as waste

sunlight

H2O ATP O2lightenergy ++ + NADPH

NADPH

Calvin Cycle

sugars

CO2

SugarBuildingReactions

ADP

builds sugars

uses ATP & NADPH

recycles ADP & NADP+ ATP

NADPH

NADP

CO2 C6H12O6 ++ + NADPATP + NADPH ADP

Supporting a biosphere

• Big Picture: photosynthesis is THE most important process for continuation of life on Earth• Each year photosynthesis…• captures 121 billion tons of CO2

• synthesizes 160 billion tons of carbohydrates

• heterotrophs depend on plants for food, fuel, & raw materials

Light Depedent Review:

• Where did the energy come from?• Where did the electrons come from?

• Where did the H2O come from?• Where did the O2 come from?

• Where did the O2 go? • Where did the H+ come from?• Where did the ATP come from?• What will the ATP be used for?

• Where did the NADPH come from?• What will the NADPH be used for?

Light Independent Review II:

• Where did the CO2 come from?• Where did the CO2 go?

• Where did the H2O come from?• Where did the H2O go?

• Where did the energy come from?• What’s the energy used for?

• What will the C6H12O6 be used for?• Where did the O2 come from?

• Where will the O2 go?• What else is involved…not listed in this

equation?

6CO2 6H2O C6H12O6lightenergy + ++ 6O2

Adaptations in Alternative Environments C4 Path• CO2 stored is mesophyll & released in vascular tissue

Adaptations in Alternative EnvironmentsCAM Path• Stomata open at night – CO2 stored in vacuoles for later use

Combating Photorespiration

Photorespiration – rxn between RuBP & O2 when [O2] is high → uses ATP & produces CO2

Favored ONLY when [O2]

exceeds [CO2]

C4 Path CAM Plants

Store CO2