Lecture 13: Photosynthesis and Carbon Fixation

autotrophs

self feeders

heterotrophs

other-feeders

oxygenic photosynthesis generates

O2 in the atmosphere

photosynthesis converts what energy into what energy

light energy into chemical energy

What to the plants take in and what the they produce and release

Take in CO2 and water, produce carbohydrates, and release O2

photosynthesis equation

6CO2 + 6H2O -> C6H12O6 + 6O2

photosynthesis location

chloroplast

What happens if a molecule absorbs a photon?

thermal dissipation, fluorescence, energy transfer, electron transfer

Excited electrons properties

increased potential energy, transfer energy (excitons), easier to transfer (oxidation)

pigments

molecules that absorb specific wavelengths of light

Chlorophylls structure

porphyrin head (porphyrin ring with conjugated double bonds) and a phytol tail

chlorophyll function

absorb photons within the visible spectrum, minimize energy loss through thermal vibration (de-excitation)

accessory pigments

used to extend the range of light absorption

absorption spectra correlate with

photosynthetic activity (absorbed wavelengths power photosynthesis)

pigments are bound to the _____ ____ ____ within the ____ _____

light harvesting complexes; thylakoid membrane

reaction center

pigments where absorbed light energy drives electron transfer

antenna complex

provides a network of pigment molecules to capture and transfer energy

reaction center chlorophyll enables efficient

redox reaction

photosystems

energy hasvesting proyteins that faciliate transfer of excited electrons

energy is funneled to the _____ ____ ____ in the reaction center

chlorophyll special pair

excitation causes a _______ ____ ______ in the reaction center and initiates a ____ ___

separation of charges; redox chain

oxygen evolving complex

splits water to produce O2, electrons, and H+ in photosystem II

In the Oxygen Evolving Complex, water reduces what to produce O2 in a single step

Mn

Problem in cytochrom b6f complex

plastohydroquinone (PQH2) transports two e-, but plastocyanin (PC) can only transport one e- at a time

cytochrome b6f overall

remove 2H+ from the stroma, pump 4 total H+ into the thylakoid lumen, move both electrons from QH2

Ferradoxin (Fd)

shuttles e- NADP reductase to produce NADPH, shuttles e- to cyt b6 f complex to produce ATP

photosystem I

accepts e- from PC, excite e-, transfer e- to ferredoxin

track electrons through carriers in a

z scheme

photophosphorylation

light generates a proton gradient that drives ATP synthase

how many H drive a full rotation of CF0 subunit

12 H+

Two systems of electron transport

noncyclic and cyclic

noncyclic electron transport

ps I and psII required, produce NADPH and ATP

cyclic electron transport

PS I, produce ATP NOT NADPH

plants use cyclic and noncyclic to maintain what ra

3:2 ATP: NADPH for carbon fixation

cyclic electron transport recycles

electrons to generate additional ATP

Appressed thylakoids

stacked tight, packed with PSII and LHC to optimize light capture, LHC mediates connection between membranes

nonappressed thylakoids

unstacked regions with access to stroma (ferradoxin, NADP+, ADP), PSI and ATP synthase

how does thylakoid stacking work and change

dynamic in response to light

LHC mediates

connection between membranes

LHC (movement and job)

mobile, can help funnel energy to PSI when PSII is more active

CO2 assimilates into …

biomass in plants

carbon fixation aka the calvin cycle occurs where

stroma

calvin cycle: stage 1

CO2 fixation to 3-phosphoglycerate

Ribulose 1,5 bisphosphate carboxylase/oxygenase (RuBisCO)

very slow: large amounts are needed to achieve high carbon fixation rates

RuBisCO represents around what percent of soluble protein in chloroplast

50%

RuBisCO in the presence of oxygen forms

alternative products

Carboxylase activity of rubisco: problem

once it is primed for attack, cannot discriminate between CO2 and O2

Carboxylase activity of rubisco: solution

plants have evolved different mechanisms to increase [CO2] in leaf tissue

calvin cycle stage 2

reduce 3-PG to G-3-P

we need ___ G3P to regenerate RuBP

5

calvin cycle: stage 3

regeneration of ribulose 1,5 bisphosphate

Calvin cycle stage 3 problem

RuBP is pentose (5C), but G3P and DHAP are 3C

Triose phosphates interconvert to form

pentose phosphates

Calvin Cycle overall summary

for every 3 CO2 fixed, 9 ATP and 6 NADPH are consumed, 1 G3P is produced

enzymes in calvin cycle are ____ ___ by light

indirectly activated

when chloroplasts are illuminated

stromal [Mg2+] increases, stromal pH increases

fructose 1,6 bisphosphatase

stage 3, pH dependent, requires Mg2+, increase in light conditions