Lecture 5

What is the light dependent reactions

Driven by light energy. e- move down a chain of proteins. Makes a proton gradient by releasing H+ into the thylakoid membrane. Gradient drives the synthesis of ATP. When e- reaches end of chain NADPH synthesized.

Step 1 at photosystem II

Light energy is absorbed by pigment in the antenna complex. Through resonance energy transfer, the energy is transferred to the P680 chlorophyll a in the reaction center. P680 chlorophyll a loses an e-. It is replaced through the oxidation of water which releases 2H+ in to the lumen of the thylakoid and O2 is produced.

Step 2 between photosystem II and cytochrome b6f

The e- freed from the P680 and travels down a chain of 3 proteins (pheophytin, plastoquinone(Qa), plastoquinone(Qb)through coupled oxidation/reduction reactions. 2H+ are brought into the thylakoid lumen.

Step 3 cytochrome b6f to plastocyanin

Plastoquinone donates electrons to the cytochrome b6f complex. Releases more H+ into the thylakoid lumen. Cytochrome f of the complex donates electrons to plastocyanin (a mol in the lumen). Plastocyanin carries electrons to the photosystem I complex.

Step 4 photosystem l

Plastocyanin transfers electrons to photosystem I.

Alternatively: photosystem l can act independently using a similar antenna complex with pigments and a reaction center. Uses light energy to generate e- in P700.

Step 5 photosystem l to ferredoxin

e- oxidized on photosystem l ultimately used to reduce NADP+ to NADPH. e- travel from P700 to ferredoxin. From here e- are transferred to NADP+ using the catalyst ferredoxin-NADP+-reductase. NADPH is produced. An H+ removed from the stroma. NADPH is an important molecule in carbon fixation.

Step 6 H+ gradient and ATP synthase

During e- transport chain accumulation of H+ in the thylakoid lumen which results in a proton gradient. Gradients are used to power protein mediated channels. H+ passes through ATP synthase and powers the phosphorylation of ADP to ATP(phosphate added).

What is the noncyclic electron flow

When e- pass down the e- transport chain from photosystem ll and produce NADPH. e- move unidirestionally from chlorophyll a to ferrodoxin

What happens with H+ during light dependent reactions

4H+ transferred into the thylakoid lumen from the stroma. 3H+ removed from the stroma (1 in the reduction of NADP and 2 during e- transfer on the plastoquinone to cytochrome.

Proton gradient powers what

Synthesis of ATP from ADP+phosphate group

2 methods for ATP synthesis

Proton gradient created by the electron transport chain (noncyclic phosphorlation).

Photosystem I functioning independently can generate a proton gradient with no need for the e- transport chain. Cyclic phosphorylation.

End product of noncyclic e- flow and non cyclic phosphorylation

6 NADPH and 6 ATP

Carbon fixation takes place where

The Calvin-Benson-bassham cycle aka calvin cycle

Compound that begins cycle and is regenerated at the end

Ribulose 1, 5-biphosphate (RuBP)

Where does the calvin cycle occur

Stroma

3 phases of calvin cycle

1. Fixation of CO2

2. Reduction

3. Regeneration of RuBP

Step 1 carbon fixation

3 CO2 bonded to 3 mols of RuBP. Fixation is catalized by enzyme Rubisco. Yields 6 mols of 3-phosphoglycerate(PGA). (no energy consumed yet). Rubisco: most abundant enzyme on earth(40% of soluble proteins on leaves)

Step 2 reduction 

6 mols of PGA converted to 6 mols of glyceraldehyde 3-phosphate(PGAL). 5PGAL continue in the cycle and one is converted to sugar. Consumes 6 ATP and 6 NADPH.

Step 3 regeneration of RuBP

5 mols of PGAL reduced to 3 mols of RuBP. Consumes 3 ATP.

Overall reaction of calvin cycle

3CO2+9ATP+6NADPH+6H⁺→PGAL+9ADP+8Pi+6NADP⁺+3H2O

What happens to the PGAL

Most are transported to the cytosol and converted into sucrose. Those that remain in the chloroplast is converted into starch and stored in the stroma. At night it is broken down into sucrose and transported to other parts of the plant.

Mistake in calvin cycle

Rubisco fixes O2 and photorespiration occurs

Why does rubisco make this mistake

In the past, much more CO2 than O2 in the atmosphere and rubisco did not evolve to distinguish them because there was just trace amount of O2 in the air.

What happens during photorespiration

Carbon which becomes phosphoglycolate does not directly produce sugars or contribute to the calvin cycle (lost energy). Carbon can be recovered but the plant expands energy to so. Photorespiration is very costly to the plant.

What planst evolved to minimize photorespiration

C4 photosynthesis (kranz anatomy and without it), C3-C4 intermediates and CAM photosynthesis

Extra step in C4 photosynthesis

To concentrate CO2 around rubisco enzyme to reduce chances that it will bind with O2. So it has a different cell organization.

C4 photosynthesis pathway

1. CO2 is fixed to PEP and forms oxaloacetate in cytosol of mesophyll cells.

2. Oxaloacetate is converted to malate.

3. Malate is moved from the mesophyll cell into a neighbouring bundle sheath cell.

4. Malate is converted to form CO2 and pyruvate.

5. CO2 enters the calvin cycle in the bundle sheath cell.

6. Pyruvate returns in the mesophyll sheath cell to react with ATP to regenerate PEP.

Kranz anatomy

Bundle sheath cells surround vascular bundle. Mesophyll cells which contain chloroplasts surround the bundle sheath.

C4 vs C3

C4 more efficient at high temperatures than at low ones.

Crassulacean acid metabolism (CAM)

Extreme adaptation to high temps and dry environment. During photosynthesis plants must transpire so stomatas stay open, plant loses water through stomata major disadvantage in dry environments, plants must choose between making sugar and drying out or dont photosynthesize and die. CAM can fix carbon in the absence of light at night.

CAM plants at night stomatas are open

1. CO2 is fixed to pep to form oxaloacetate.

2. Oxaloacetate is immediately converted to malate.

3. Malate is sorted in the vacuole as malic acid. Storing carbon until the morning when light is available to power photosystems

CAM during the day stomatas are closed

1. malic acid is transported out of the vacuole and into the cytosol as malate.

2. Malate is converted and the released CO2 enters the chloroplast.

3. CO2 is fixed to RuBP by rubisco calvin cycle begins and sugar and starch can be produced