Z Scheme Photosynthesis Steps Essay

Free energy is released during the redox reactions of the electron transport chains. This energy is used in two ways.

In the electron transport chain that follows photosytem II, the movement of electrons through the chain releases energy that is used to move protons across the thylakoid membrane just as in the electron transport chain of cellular respiration in the mitochondria.  The protons are moved into the thylakoid space where they accumulate and produces a voltage across the membrane.  Also associated with this electron transport chain is the enzyme ATP synthase.  The ATP synthase uses the voltage formed by the build-up of protons in the thylakoid space to produce ATP.  Functionally, the electron transport chain that follows photosystem II is similar to the electron transport chain of respiration; the accumulation of protons on one side of a membrane provides the free energy that ATP synthase needs to drive the production of ATP.

In the electron transport chain that follows photosystem I, the electrons that move through the chain are used to reduce NADP+ *to *NADPH.

The two products of the light-dependent reactions of photosystem are ATP and NADPH.  The movement of high energy electrons releases the free energy that is needed to produce these molecules.  The ATP and NADPH are used in the light-independent reactions to make sugar.

Where do the electrons come from? Water molecules are split at the beginning of the first electron transport chain. The splitting of water provides electrons that become energized in the photosystems, and protons that accumulate across the membrane.

As water is split and electrons and protons are removed, oxygen is released. Thus, oxygen is a by-product of photosynthesis.  Recall from Tutorial 6 (Prokaryotes III - Evolution and Early Metabolism) the oxygen that is released as by-product of photosynthesis had a major impact on the early earth's atmosphere and is the source of the oxygen that we breathe.

Transcript for Photosynthesis I - Part II

Metabolism: Summary To Date

Basic concepts:

Oxidation/Reduction reactions

Enzymes, catalysts, and energy of activation

Phosphorylation: Substrate-level, photophosphorylation, respiration

Chemiosmotic hypothesis/proton pump

In microorganisms,  The source of energy may not always be the same as the source of carbon!

Cells need:


         Compounds for synthesis

Fermentationis the sum of anaerobic reactions that provide energy and carbon compounds for growth of microorganisms in the absence of O2.

In fermentation, one organic compound  donates  electrons and another compound is the electron acceptor.

ATP is formed viasubstrate-level phosphorylation

Fermentation reactions are balanced in that the oxidation levels of the substrates and products are equivalent. This limits the range of substances that can be fermentedó

Examples: glycolysis, and the fermentation of amino acids by Clostridia.

Product of glycolysis is pyruvate.


Anaerobic and aerobic respiration = the transfer of reducing equivalents from a donor, such as NADH or succinate from fermentations, to a terminal electron acceptor. This results in the translocation of protons to the exterior of the membrane.

There is great variation in electron transport chains among different bacterial groups, with the result that the amount of ATP generated varies. The final electron acceptor is oxygen in aerobes, and in anaerobes includes, for example, nitrate (NO3-), ferric iron (Fe3+), sulfate (SO42-), and even some organic compounds

The movement of protons back through the membrane drives the synthesis of ATP by the enzyme ATPase

This is electron transport, and has nothing to do with building or breaking down carbon compounds.

What's the difference between fermentation and respiration?

Strictly speaking, fermentation concerns the breakdown of carbon compounds, while respiration does not.

Tricarboxylic Acid Cycle


glucose -> pyruvic acid via glycolysis,

Now we can further break down the carbon compounds (aerobically), and generate:

         ï more A/GTP and NADH

         ï a number of smaller intermediate molecules used in biosynthesis

         ï final: CO2

The Krebs cycle = the TCA cycle (tricarboxylic acid cycle)

In fermentation, pyruvate was converted to various fermentation products. In the Krebs or TCA cycle, it's fully oxidized to CO2.

Interesting things about the Krebs cycle:

         ï It's really a cycle! Goes round and round...

         ï Where does the energy and CO2 come off?

ï Start with pyruvate from glycolysis (2 from each glucose)

ï Produce Acetyl Co-A (2-carbon compound) with loss of 1


ï this combined with 1 oxaloacetate- now have 6 carbons

ï in successive steps, lose two more carbons as CO2, end up

             with oxaloacetate which can now re-enter the cycle

What does the cell get out of this?

    1. The complete oxidation of pyruvate to CO2
    2. Many of the intermediates are necessary precursor molecules in cell synthesis
    3. Each turn of the cycle forms 1 GTP via substrate-level phosphorylation- this is turn forms 1 ATP- meaning for every glucose, 2 ATPs are formed via this cycle.
    4. The real yield of ATP comes when the NADH generated enters respiration!!!

A modified tricarboxylic acid cycle is employed by anaerobes, mostly to provide biosynthetic intermediates.


Additional reading: pp. 574-591

ï The process in which light energy is captured and converted to chemical energy

ï Usually also involves the reduction of CO2 and its incorporation into complex compounds

ï Photosynthetic organisms are the base of most terrestrial food chains (exceptions?)

ï Over half of photosynthesis on earth is carried out by microorganisms!


What did the first microbes do for a living?

The absence of oxidized compounds in the early earth would place limitations on the effectiveness of a proton gradient!

Maybe a simple photosystem evolved early? the ability to capture energy from sunlight.

 ferrous iron served as a trap for the O2 (banded iron formations)

Once reduced iron was used up, O2 began to accumulate in the atmosphere!

How photosynthesis works:

ï a quantum of energy from the sun drives an electron in a light-absorbing compound to a more excited state

ï an excitation-capturing molecule captures the energy from the light-absorbing compound as it falls back to its normal state

Three kinds of photosynthesis in microorganisms:

Uses chlorophyll:

1. oxygenic: splits water, produces oxygen

            Cyanobacteria, Prochlorophytes and chloroplasts.

2. anoxygenic: doesn't produce oxygen

Other photosynthetic bacteria such as purple bacteria and green sulfur bacteria

Doesn't use chlorophyll:

3. uses a carotenoid compound (bacteriorhodopsin)

Archaea, halobacteria, and unknown planktonic bacteria

Photosynthesis occurs in reaction centers:

Photosynthesis can be divided into light and dark reactions.

In the light reaction, light energy is used to generate ATP and NADPH.

In the dark reaction, ATP and NADPH are used to reduce CO2 and use it to synthesize carbohydrate. (Calvin-Benson cycle)


What do microbes need?

         ï C O H N S P trace elements... (MASS)

         ï ATP <--> proton gradient (ENERGY)

         ï (CARBON)

-heterotrophs can eat carbon compounds

- Autotrophs need reduced electrons for the following reaction:

CO2 -------> C6H12O6



the DARK REACTION of photosynthesis (Calvin or Calvin-Benson cycle) requires:

         ï ATP

         ï NADPH

         ï CO2 .

Most photosynthetic organisms have some type of chlorophyll.

Chlorophylls associated with photosynthetic membranes

(accessory) pigments : carotenoids, phycobiliproteins such as phycoerythrin and phycocyanin.

"reaction center". located in membranes:

ï eukaryotes: chloroplasts with sheetlike lamellae called thylakoids

ï prokaryotes: internal membrane systems of various kinds:

Light-mediated ATP synthesis in all phototrophs involves electron transport through a series of electron carriers, located in membranes, in a series, analogous to respiration (photophosphorylation)

The light reaction of oxygenic photosynthesis may occur in two stages: Photosystem Iand Photosystem II.

PSI: longer wavelength light (>680nm).

Cyclic phosphorylation: excited electron travels cyclically.

PSII: wavelength <680nm.

Noncyclic phosphorylation: excited electron travels from PSII to PSI (so-called Z-scheme)

Anoxygenic photosynthesis uses only PSI; therefore (mostly) cyclic phosphorylation (few weird other schemes).

What is Cyclic Phosphorylation?

Bacterial reaction center Light raises its reduction potential from = 0.5 to -1.0 V

The excited electron within the reaction center reduces a series of molecules, just as in the respiratory chain.

Synthesis of ATP during photosynthetic electron flow occurs as the result of the formation of a proton motive force-- which drives ATPase in the membrane.

Anoxygenic photosynthesis:

ï Photosynthetic bacteria other than cyanobacteria have bacteriochlorophylls: absorb at longer wavelengths than chlorophylls: infra-red light, which together with carotenoids give cells a red green or purple color.

            ï Anoxygenic photosynthesis is more primitive than the oxygenic photosynthesis of higher plants. No O2 is produced.

            ï One step photosynthesis (no photosystem II)

            ï Cyclic photophosphorylation only:

                        Electrons travel in a cyclic pathway. ATP is the product.

            ï Not in membranes, but closely associated with the cell membrane

Non-Cyclic Phosphorylation: The Z-scheme

A quantum of light hits the PSII reaction center. The first step is the splitting of water- an electron is donated to the PSII reaction center which is raised to an excited state.

The PSII reaction center donates an electron which travels through a series of donors and finally ends up on PSI. Its transfer is in a thermally favorable direction: releases energy, which is used to generate a proton motive force, from which ATP is produced.

Meanwhile, PSI has absorbed a quanta of light and donated an electron to a series of carriers. The final acceptor is NADP+ which is reduced to NADPH (reducing power)

This is NON-CYCLIC because the original electron doesn't cycle back to PSII, but instead ends up reducing NADP+.

Cyclic photophosphorylaton also occurs with PSI.

Oxygenic photosynthesis:

Two photosystems, both homologous (descended from) the single photosystem in anoxygenic photosynthesis. The familiar "Z-scheme" found in plants, cyanobacteria.

Cyanobacteria, Prochlorophytes and chloroplasts.


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