• Exam I is tenatively scheduled for Wednesday, October 1st

• Covers materials in Lectures 1-11; Chapters 1, 2, 3 (3.1&3.2), 4, 13, 19 of Lehninger

• Comprised of:– Multiple choice (~10)– Biomolecule recognition (~4)– Short answer (~4-8)– Essay (1-2)

Photosynthesis/primary production

• CO2 + H20

O2 + (CH2O)

Oxygenic photosynthesis

6 CO2 + 12 H2S —>

+ 6 H2O + 12 S + (CH2O)

Anoxygenic photosynthesis

Distinguishing the reactions of photosynthesis

• Light reactions – chlorophyll

absorbs light and conserves the

energy in ATP and NADH

• Carbon fixation – Use ATP and

NADPH (NADH), to reduce

CO2 to form important bio-

molecules.

These reactions work in concert to cycle ATP and reducing power

For now, the light reactions• Occur in organelles of eukaryotes –

chloroplast of plants, algae; oxygenic photosynthetic bacteria have similar appearance

How is light converted to chemical energy?

• Absorption

• Electron Flow

• Establishing a proton-motive force

• ATP synthesis

Light energy

• The energy of a photon is greater at the violet end compared to the red end; shorter wavelength and high frequency = higher energy, the energy of visible light is on the order of 170-300 kJ

Distinct biomolecules absorb light at different wavelengths

Several biomolecules absorb light, those involved in photosynthesis include:

• Chlorophyll –porphyrin rings coordinating Mg, different types where modifications affect light absorbing properties

Chlorophylls are bound by proteins to form light-harvesting complexes

• Integral

membrane

protein containing

several pigments

or chromatophores

How it works in

a minute…

Some bacteria have alternative light harvesting molecules

• Phycobilins – an unwound porphyrin ring (no Mg, no circular structure)

Accessory pigments extend the range of light absorption

• Carotenoids and lutein – absorb light at wavelengths not absorbed by chlorophyll

These pigments are arranged in photosystems including antennas and a

reaction center

Light energy is transferred among antenna molecules by resonance

transfer

This energy ends up at the photochemical reaction center

• All the pigment models can absorb photons, but only a few chlorophyll molecules are involved in transducing light into chemical energy.

Light energy is captured by electron transfer at the reaction photochemical

center• Electrons of chlorophyll (and any molecule) have different

energy states depending on their distances from the + charged nuclei (the farther out, the higher the energy)

• The electrons in conjugated double bonds of chlorophyll are able to absorb the energy from photons. Conjugated double bonds are double bonds on alternating pairs of carbon molecules in a linear or cyclic organic molecule

• De-excitation has three paths: fluorescence, heat, and transfer

• Discrete jumps of electrons from one state to another occur with simultaneous Absorption (or release) of photons of appropriate energy (the energy representing the difference in energies of the electron states)

• Electrons in chlorophyll are high-energy, (note two excited states),unstable; can (a) fluoresce, (b) lose energy to solvent, releasing heat, (c) move to nearby, more stable electron acceptors (even if the electron acceptor is less stable than unexcited chlorophyll)

• If movement to acceptor occurs, light has "pumped" electron (uphill) from chlorophyll to acceptor.

Bottom line: Light kickstarts electron transfer

• CHL – chlorophyll molecules ; Z – electron acceptor

Two mechanisms for light energy transfer

• Resonance and electron

transfer

Steps from light absorption to electron transfer

• 1. Chlorophyll in the light harvesting complex is excited by light

• 2. Transfer of energy sequentially to neighbor pigments through resonance transfer (exciton transfer)

• 3. Reaching the photochemical reaction center, one of a special pair of chlorophyll a molecules is excited.

• 4. In this excited molecule, an electron is promoted to a higher energy orbital, and is passed to a nearby electron acceptor that is part of the electron transfer chain.

Continued…

• The electron leaves the reaction center chlorophyll with an empty orbital

• 5. The lost electron is replaced by a neighboring electron donor molecule

• Summary: Excitation by light causes electric charge separation and initiates an oxidation-reduction chain.

There are different types of photosystems

• Some bacteria have one of two types of photochemical reaction centers

• Some bacteria, and all plants have a two types of photochemical reaction centers that work in tandem

• A halophilic archaeal species has an all together different mechanism for light driven proton pumping

Type II reaction center

Purple bacteria photosynthetic machinery

• In addition to the reaction center and pheophytin, constituents of photosynthesis in this bacteria look like the oxidative phosphorylation pathway

• Pheophytin, a chlorophyll molecule lacking Mg, accepts an electron from the special pair of bacteriochlorophyll to initiate electron transfer

This photosynthesis mechanism has cyclic electron flow

Electron transfer is fast

• Light energy knocks an electron off the special pair bacteriochlorophyll producing two radicals, one positively charged (the special pair), the other negatively charged (the pheophytin)

• The pheophytin passes its electron to a tightly bound molecule of quinone, converting it to a semi-quinone

The result is reduced quinone along the cycle

• This quinone (QA), donates its electron to a second quinone (QB) via a non-heme Fe (Note this is the slowest step in the photochemical reaction center electron transfer)

• Two such transfers fully reduces QB, which diffuses away from the reaction center on it’s way to the bc1 oxidase complex

• The electron hole left on the special pair of bacteriochlorophylls is filled by an electron from a heme of cytochrome c (completing the cycle)

One proton pump in this cycle

• The fast kinetics and favorable thermodynamics make this process virutally irreversible and highly efficient.

• In this specific case, only cytochrome bc1 oxidase serves to pump protons, that ultimately drive ATP generation

Type I reaction center

• In this instance, cyclic electron flow is interrupted by transfer to a ferredoxin to make NADH

• These two “simple” bacterial photochemical reaction centers are evolutionarily related to the tandem reaction centers found in plants, algae, and cyanobacteria.

PSI and antenna