BIO 172 Lecture 16

What is the final electron acceptor in light reactions?

NADP+

What compounds are used for the reduction in Calvin Cycle?

3-phosphoglycerate, ATP, and NADPH.

What is RuBP?

5-carbon compound used in Calvin Cycle.

What is the overall summary of the photosynthetic reaction?

6CO2 + 6H2O + light = C6H12O6 + 6O2

NADP+ reductase Function

A enzyme that catalyzes the formation of NADPH by transferring two electrons from two molecules of reduced ferredoxin to NADP+ as well as a proton (H+) from the surrounding solution.

Cyclic electron transport. What is it? How does it work? Why do plants do it?

A light-driven flow of electrons through a photosynthetic reaction center with the electrons returning to the reaction center via an electron transport pathway. Cyclic electron transport will normally generate an electrochemical potential (typically pH ) gradient across a membrane but does not result in the net production of reductant. Pulls e- from H2O. Light excites e- to reduce NADP+ to NADPH. Linear electron transport (e- from H2O to NADP+ forming NADPH.

Cyto chrome b6f function

A protein in the electron transport chain through which electrons pass between photosystem II and photosystem I. It catalyzes the electron transfer between plastoquinone and plastocyanin, but also transports the two protons into the lumen of thylakoid discs. This proton transfer forms an electrochemical gradient, which is used by ATP synthase at the end of the light dependent reactions in order to form ATP from ADP and Pi.

What is the purpose of the regeneration?

Makes a high energy substrate through with high free energy to make CO2 addition exergonic.

Why is the waste product reduced?

So that NADH becomes NAD+ so that NAD+ can be recycled.

Why do some bacteria only have one photosystem?

Some bacteria only have one photsystem because they can't use water as a electron donor and the second photosystem was later evolved to allow bacteria to use water as a electron donor. They also only live in areas where more easily oxidized compounds are abundant.

PSII function

Supplies electrons to the beginning of the electron transport chain. Absorption of light energy allows it to pull an electron from water.

Chloroplasts AND mitochondria use electron transport and chemiosmosis to synthesize ATP

Both use energy from e- to pump H+ across the membrane •Diffusion of H+ down concentration gradient powers ATP synthase-chemiosomosis •The ATP synthase and some electron carriers are very similar •Intermembrane space ~ Thylakoid lumen and Matrix ~ Stroma •Oxidative phosphorylation ~ Photophosphorylation

Calvin Cycle Product

Carbohydrate like glucose or sucrose

Calvin Cycle

Carbon enters as CO2. In the first phase (Carboxylation/Carbon Fixation), the addition of CO2 to the 5-carbon compound, RuBP, is catalyzed by the enzyme rubisco. In the second phase (Reduction), NADPH transfers high energy electrons which generates 2 3-carbon compounds, G3P, per Carbon Dioxide. One G3P or triose phosphate exits and used to assemble glucose and sucrose in the cytoplasm. In the third and final phase (Regeneration of RUBP), a multistep process (12 reactions) which is used to regenerate RuBP from the remaining G3Ps or triose phosphates. Cost: 3CO2=G3P 9 ATP and 6 NADPH.

Where do the light reactions take place? Where do the dark reactions take place?

The light reactions take place on the Thylakoid Membranes. The dark reaction takes place in the stroma of the chloroplast.

How do we know that O2 comes from H2O and not from CO2?

There was an experiment in which the scientists used an isotope of oxygen(18O) for water in one flask and for carbon dioxide in the other flask and allowed the algae to photosynthesize. The ratio of 18O and 16O in the air above flask 1 was greater than the ratio in the air above flask 2 so therefore O2 comes from water not carbon dioxide.

Light Reactions Products

ATP, NADPH, O2

Describe the Z-scheme and how does each photosystem contribute to it?

Absorption of light energy by PS II energizes electrons pulled from water, allowing them to enter the photosynthetic electron transport chain. A second input of light energy by PS I raises the electrons to an even higher level so that they can be used to reduce NADP+.

How does fermentation generate energy?

Allows glucose to be continuously broken down to make ATP due to the recycling of NADH to NAD+.

What is rubisco?

An enzyme present in plant chloroplasts, involved in fixing atmospheric carbon dioxide during photosynthesis and in oxygenation of the resulting compound during photorespiration. -Source of vast majority of carbon fixation in all ecosystems • Most abundant enzyme in the world •Will also use O2 as substrate, but this consumes more of the energy that was generated in the light reactions - Rubisco catalyzes competing reactions with very different outcomes : RuBP + CO2 = 2 3-phosphoglycerate (used in Calvin cycle). RuBP + O2 = 1 3-phosphoglycerate (used in Calvin cycle) + 1 2-phosphoglycolate (when processed CO2 released, and ATP used).

What is fermentation?

Anaerobic reactions. Provide NAD+ to re-charge glycolysis. Lower energy yield - only 2 ATP per glucose. Produces Lactic Acid and ethanol in yeast.

Which of the following is TRUE regarding the Calvin cycle? A) CO2 is produced by Rubisco B) NADPH is consumed C) NADPH is produced D) Carbon fixation takes place in the cytosol E) ATP is producedLecture

B) NADPH is consumed

What is the product of the Calvin cycle?

Glucose and other organic compounds, NADP+, and ADP.

What is the electron donor in light reactions?

H2O

What is horizontal gene transfer and how did it contribute to the evolution of the PSII and PSI in plant cells?

Horizontal gene transfer is the transfer of genetic material between unicellular organisms. One hypothesis is that the gentic material associated with one photosystem was transferred to a bacterium that already had the other photosystem, resulting in a single bacterium with the genetic material to produce both types of photosystems.

Plastoquinone function

It accepts two electrons from Photosystem II and two hydrogen cations (H+) from the stroma of the chloroplast in the electron transport chain. After accepting the two electrons and two hydrogen cations, it is reduced to form plastoquinol. It transfers the electrons further down the electron transport chain to plastocyanin, a mobile, water-soluble electron carrier, through the cytochrome b6f protein complex.

Which reaction is wasteful?

Photorespiration: RuBP + O2 = 1 3-phosphoglycerate (used in Calvin cycle) + 1 2-phosphoglycolate (when processed CO2 released, and ATP used).

Which two reactions does rubisco catalyze?

Photosynthesis: Carbon fixation (1st step) of Calvin Cycle: RuBP + CO2 = 2 3-phosphoglycerate (used in Calvin cycle). Photorespiration: RuBP + O2 = 1 3-phosphoglycerate (used in Calvin cycle) + 1 2-phosphoglycolate (when processed CO2 released, and ATP used).

Plastocyanin Function

Protein located in Lumen of Thylakoid that transfers electrons from Cytochrome bf to PS1 in electron transport chain.

Photosystem I Function

Protein that energizes with a second input of light energy so they can be used to reduce NADP+.

What is carboxylation?

The addition of CO2 to 5-carbon compound, RuBP.

Light reactions

The first of two major stages in photosynthesis (preceding the Calvin cycle). These reactions, which occur on the thylakoid membranes of the chloroplast or on membranes of certain prokaryotes, convert solar energy to the chemical energy of ATP and NADPH, releasing oxygen in the process.

Why is 3-PGA reduced?

To make triose phosphate, G3P, so the cell can produce glucose and sucrose in the cytoplasm and so that RuBP can be regenerated to be used again in the Calvin Cycle.