BIOL 205 :: Photosynthesis Lecture 1Introduction and the light reactions

What is photosynthesis?

• The process of converting solar energy into chemical energy.

• Can use only water and carbon dioxide to create sugars = chemical energy.

• Responsible for removal of ~ 200 billion tons of C from the atmosphere yearly.

What is photosynthesis?

6CO2 + 12 H2O + hv C6H12O6 +6O2 +6H2O

•hv designates light•you should memorize this equation!! you will see it again

O2 = oxygenic photosynthesis

What is photosynthesis?

• 2 stages of photosynthesis– Light reactions and Dark reactions

• Light reactions convert sunlight into chemical energy (ATP + NADPH)

• Dark reactions use those products to form sugars (stored chemical energy)

Overview of Photosynthesis

Light ReactionsH2O

+CO2

NADPH

ATP

Dark ReactionsCO2

Sugars

O2

Where does photosynthesis take place?

•Prokaryotes

•Prokaryotes have both anoxygenic and oxygenic•Cyanobacteria have oxygenic

-Photosynthesis on thylakoids (from plasma membrane)-Cyanobacteria are source of eukaryotic photosynthesis

•Eukaryotes•Oxygenic only•Takes place in chloroplasts

Chloroplast Morphology

Chloroplast Morphology-Terms

• Inner Membrane

• Outer Membrane

• Thylakoid

• Thylakoid lumen

• Stroma

• Granum

How can light provide energy for plants?

• Light is composed of particles: photons

• Light behaves like a wave– Can e described w/ wavelength &

frequency

• Only a small portion of the electromagnetic spectrum.

The electromagnetic spectrum

PAR = photosynthetically available radiation

Pigments

• Pigment = a light absorbing molecule• Associated with the thylakoid

membranes• Chlorophyll

– Chl a and Chl b (Chl c in some algae)

• Xanthophylls• Carotenoids

– ß-carotene

Chlorophyll

•Chl a has a methyl group •Chl b has a carbonyl group

Porphyrin ring delocalized e-

Phytol tail

Different pigments absorb light differently

Different pigments absorb light differently

2-minute quiz

1. What is the relationship between the structure of chlorophyll and its location in the chloroplast?

2. Why are plants green?

Light Reactions 1: Light capture and redox

• 2 spatially & functionally distinct units = Photosystems

• Photosystem II = 1st stage

• Photosystem I = 2nd stage

• Named after order of discovery

Light capture 1

LHCReaction Center

1. Most Chlorophyll is located in the Light Harvesting Complex

2. Sunlight is absorbed in the LHC and is passed from pigment to pigment

Remember the porphyrin ring?ß-carotene

Chl b

Chl a

Light capture - 2

LHCReaction Center

P680 chlorophylls*

3. Energy finally ends up in a pair of special chlorophyll a molecules: P680

4. e- in P680 Chl a goes to excited state and is shed = Charge separation

e-*

Optimal @ <680 nm

Light capture - 3

LHCReaction Center

+

e-*Q

5. High-energy e- accepted by quinone Q = primary e- acceptor

6. Q has been reduced; P680 Chl a has been oxidized

Oxidation by light = photo-oxidation

e- transport chain

7. The excited e- is shunted into the electron transport chain

Light capture - 4

H2O 1/2 O2

4H+

O2 evolving complex

8. The O2 evolving complex + Chl a+ strip e- from H2O and reduce Chl a+

Chl a+ = most powerful biological

oxidizing agent

e-

+

Light capture - 5

9. The reaction center is reset and ready to go again

Light Capture Movie

PSII electron transport chain

Qe- e-

H+ H+1. Q accepts 2 e- from P680 and removes 2 H+ from the stroma

2. Q passes the e- to cytochrome b/f complex & pumps the 2H+ into thylakoid lumen

b/f

cytochrome

complex

e- e-

3. As the e- moves through b/f more H+ are pumped into lumen

The PS II Electron transport chain

Qe- e-

H+ H+

2H+

b/f

cytochrome

H+

complex

plastocyanin

e-

PS II to PS I

plastocyanin

e- 4. e- end up on plastocyanin: a soluble electron carrier in the lumen

plastocyanin

e-

5. Plastocyanin serves as e- donor for PSI reaction center Chl a

Photosystem I

P700

e-*

+

plastocyanin

e-

1. Charge separation and photo-oxidation are similar to PSII

Optimal wavelength = 700nm

2. Plastocyanin acts as reducing agent on P700 Chl a

PSI electron transport chaine-Ferredoxin

Fe/S1. Ferredoxin recieves e- from P700*

FAD

NADP+

FAD-NAPD Reductase

H+

e-

2. e- moves throuh FAD (flavin adenine dinucleotide)

3. e- plus stromal H+ are used to reduce NADP+ to NADPH

NADPH = FINAL PRODUCT!

PSI electron transport chaine-

FerredoxinFe/S

FAD

NADP+

FAD-NAPD Reductase

H+

NADPH

e-e-

e-

ATP synthesis

stroma

lumen

H+

H+

H+

H+H+

H+ H+

H+H+

H+

H+

H+H+ 1. O2 evolving complex

liberates H+ into lumen from water

2. Q and Cyt b/f pump H+ from stroma into lumen.

3. NADP+ scavenges protons from the stroma

pH separation across membrane = Proton Motive Force!

ATP synthesis

stroma

lumen

H+

H+

H+

H+H+

H+ H+

H+H+

H+

H+

F0F1

complex

H+

ADP + Pi

ATP

The energy released as protons travel down their concentration gradient is used to fuel an ATP synthase

Light Reactions Products

In: CO2, H2O, sunlight

Out: O2, ATP, NADPH