By the time you finish this unit, friend should have the ability to explain the core principles on this page and also in the corresponding sections of chapter 10 (Photosynthesis) in Campbell:

How the light reactions catch light energy and also convert that to chemistry energy.How the Calvin cycle renders use the the outputs that the irradiate reactions.Similarities and also differences between photosynthesis and also cellular respiration.

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For an ext detailed objectives, watch the testimonial section in ~ the bottom the this page.

The photosynthesis coverage in Campbell is excellent; i strongly recommend reading it. On this page, I"ll give a short account of several of the crucial photosynthesis ideas for Bio 6B, to plan to complement the chapter and also give you a clear idea the what to suppose for the quiz and midterm.

Before girlfriend start, ns recommend that you clock this video: Photosynthesis | HHMI BioInteractive Video. It"s terrific video that goes from simple overview to comprehensive mechanisms. This can be all you have to know, in 11 minutes!

Photosynthesis lecture video

I do a lecture video focusing ~ above the irradiate reactions, emphasizing the interaction in between light and matter.

Photosynthesis occurs in chloroplasts

Like to move respiration, eukaryotic photosynthesis wake up in specialized membrane-bound organelles. The chloroplast that perform photosynthesis are similar to mitochondria in numerous ways, especially because they room surrounded by two membranes and also contain a very huge surface area of folded inner membranes. In both cases, the interior membranes are connected in generating a proton gradient that drives ATP synthesis. In addition, both the mitochondrion and also the chloroplasts are obtained from bacterial cells v endosymbiosis and both save on computer their own bacteria-like DNA.


Image credit: Callahan/Wikimedia (micrograph); Molnar & Gair/Wikimedia (diagram).

In mitochondria, the proton gradient is generated throughout the deep folds (cristae) of the inside membrane. In chloroplasts, the proton gradient is generated throughout the membranes of the thylakoids, which room separate, however derived indigenous the within membrane. Thylakoids room flat and grouped into stacks (called grana). This structure provides a high proportion of surface room to volume: many of room for the photosynthesis complexes in the membrane, however a low interior volume (the lumen) come fill with protons.

The irradiate reactions power the Calvin cycle

Overall, photosynthesis catches light energy and uses it to produce organic molecules. The light reactions capture light energy and also use the to develop chemical energy, while the Calvin cycle offers this chemical energy to transform oxidized inorganic carbon (CO2) into reduced organic carbon (G3P, which can be convert to sugar).


The light reactions involve some mechanisms that are very similar to what you’ve seen in respiration. I’ll describe that in part detail. The Calvin cycle requires some interesting and facility biochemistry, yet I’ll only describe it in basic terms.

The irradiate reactions produce ATP and NADPH

There are two essential outputs that the light reactions that obtain used in the Calvin cycle: ATP and also NADPH, i beg your pardon carries reducing power in the form of high-energy electrons.

Photosystems absorb irradiate energy

Photosynthesis begins when photosynthetic pigment molecule in photosystems absorb photons of light energy. Photosynthetic pigments include chlorophyll and also a few accessory pigments, such as carotenes. As soon as these molecule absorb photons, electrons room excited, or pushed to higher-energy orbitals. Thanks to the complicated structure the photosystems, the high power of the electrons deserve to be shared across multiple pigment molecules by resonance transfer. The end result is that multiple photons can be absorbed, and their energy is passed come the reaction center chlorophylls at the heart of the photosystem. This increases those electrons to such a high power level that they are passed on to the electron transport chain, and also the reaction center chlorophylls room oxidized.

Photosystems space organized into photosynthetic supercomplexes. This enables many light-absorbing pigment molecules to cooperate in a large antenna complex, sharing energy throughout many electrons in one system. No individual chlorophyll molecule absorbs enough light power to strength photosynthesis, yet the supercomplex accomplishes something that can"t be excellent by its separation, personal, instance molecules. This is one of numerous examples in biology in i beg your pardon higher-level structures play critical role.

PS II splits water

A huge energy input native the pigments in the photosystem is compelled to oxidize the reaction facility chlorophylls of PS II. These oxidized chlorophylls consequently become an effective oxidizers, all set to change their shed electrons. The replacement electrons come from a surprising source: H2O.

In cellular respiration, H2O is the stable, low-energy final destination of the electrons from the electron transfer chain. That stable due to the fact that the electrons space primarily held by the oxygen atom, i beg your pardon is extremely electronegative. In various other words, it would certainly take a very powerful oxidizer come oxidize water. As it transforms out, that’s exactly what PS II provides, in the kind of those oxidized chlorophylls.

When the reaction center of PS II oxidizes H2O, two electrons are passed come the reaction center chlorophylls. This will eventually be excited and also passed to the ETC. In addition, two protons (H+) are released right into the thylakoid space, contributing to the proton gradient. Finally, each oxidized H2O release one oxygen, contributing come the formation of the O2 us breathe. Without photosynthesis, there would be no eukaryotes. (If friend think this through, it additionally tells you that oxygen-producing photosynthesis must an initial have been performed by bacteria, prior to the evolution of prokaryotes. In fact, few of those oxygen-producing photosynthesis bacteria later took increase residence inside eukaryotic bio cells and also became chloroplasts.)

ATP synthase is it is provided by a proton gradient

The light reactions produce ATP, i m sorry is supplied as an energy source in the Calvin cycle. The mechanism for producing this ATP is the same as in moving respiration: a proton gradient cd driver the rotor ~ above ATP synthase, leading to the enzyme come phosphorylate ADP. As with in respiration, the proton gradient is generated throughout an interior membrane of the organelle. In photosynthesis, there are two sources of protons: some space transported into the thylakoid room by the electron transfer chain, and also some are developed in the thylakoid once PS II splits water.

The light reactions strength the Calvin cycle: details

We can put the irradiate reactions and the Calvin cycle together in one diagram, like this:


Detailed steps:

Photon energy is caught by Photosystem II (PS II). The soaked up energy excites electron from chlorophylls within the photosystem. When the electrons get sufficiently excited, the reaction center chlorophylls within PS II acquire oxidized.When PS II loser electrons, it it s okay replacement electron by oxidizing H2O. This supplies more electrons for the electron deliver chain, and also releasing protons right into the thylakoid space, including to the proton gradient.Excited electrons from PS II space passed come the cytochrome complex through Pq. The cytochrome complicated passes the electrons with a collection of exergonic redox actions while catching energy to pump protons into the optical membrane space. (You might recall that cytochromes also play crucial roles in cellular respiration. In enhancement to the mobile electron transport cytochrome C, there are cytochrome proteins in the proton-pumping complexes.)The protons released through PS II and also those pumped through the cytochrome complicated build up a strong electrochemical proton gradient. The proton gradient powers ATP synthase, i beg your pardon produces ATP. (The ATP created in this action will be supplied in the Calvin cycle; photosynthetic cells still must perform to move respiration to fulfill their other ATP needs.)Electrons native the cytochrome complicated are passed to PS i at low energy. PS ns captures much more photon energy and also excites these electrons to a high power state.High-energy electron from PS i are used to do NADPH by the enzyme NADP+ reductase. The overall process of electron being recorded from H2O, passed with the electron carry chain, and also ultimately passed to NADPH, is referred to as linear (or noncyclic) electron flow.Alternatively, the excited electron from PS I deserve to be passed ago to the cytochrome complex, giving redox energy to power much more proton pumping. This process, dubbed cyclic electron flow, adds to the proton gradient and powers an ext ATP production. Cyclic electron flow occurs when the Calvin bicycle slows because of a lack of ATP, since linear electron circulation alone doesn"t produce enough ATP relative to the quantity of NADPH. Slowly the Calvin cycle outcomes in a lessened NADP+ concentration, for this reason electrons space passed come the cytochrome complicated instead.Carbon fixation, in which a CO2 molecule is included to an existing essential molecule, is one exergonic process. In an earlier step, CO2 agree molecules (RuBP) with incredibly high totally free energy were generated. In a reaction including multiple molecules at once, high-energy bonds native RuBP room broken, while lower-energy binding are developed when the CO2 is added to come to be a carboxyl team (COO-). This reaction is catalytic analysis by the enzyme Rubisco (Ribulose Bisphosphate Carboxylase). Not natural carbon (CO2) i do not care organic carbon in this step.In the reduction phase the the Calvin cycle, NADPH reduces the carboxyl groups and also ATP energy drives the development of new, higher-energy bonds, ultimately creating G3P. This product deserve to be supplied to produce sugars or offered in the following phase the the Calvin cycle.In the regeneration phase of the Calvin cycle, part G3P molecules space reorganized to come to be high-energy RuBP molecule that deserve to act together CO2 acceptors. ATP is forced for this process.


The irradiate reactions create ATP and also excited electrons, brought by NADPH. The Calvin cycle uses these commodities to turn CO2 into lessened organic molecules.

Photosynthesis contrasted to respiration

It"s tempting to see photosynthesis and cellular respiration together opposite processes, due to the fact that they have actually opposite inputs and outputs. However, the mechanisms space much much more similar than they room different. Features common to both procedures include:

Double membrane-bound oribel that create an electrochemical proton gradient.The proton gradient is created by an electron transfer chain the couples redox reactions to proton transport.The electron move chain uses cytochrome proteins.ATP synthase produces ATP, powered by proton gradient.Electrons are carried by dinucleotides (NADPH or NADH).There is a oxidization cycle for organic molecule (oxidizing in the citric acid cycle, reduce in the Calvin cycle).

All this similarities don"t exist by chance. Both the mitochondrion and also the chloroplastic are acquired from bacterial cells, and those cells mutual many materials by common ancestry. This is a common theme in biology: new pathways can evolve by reconfiguring pre-existing genes and also proteins to create different results.


The structures of NADPH and also NADH are really similar; both consist of two adenine nucleotides joined together. They do comparable jobs: receiving electrons from one molecule and passing lock on to another. Why is it that NADPH is provided in photosynthesis, if NADH is used in to move respiration? Why couldn"t among these electron carrier be provided for both processes? The most important difference in between NADPH and NADH isn"t in the molecules themselves, however in the method they are regulated within the cell:

NAD+ greatly exists in the oxidized form. to move respiration depends on quick oxidation of food molecule, so the / ratio is kept very high. There is normally a high concentration of NAD+, all set to act together an oxidant in glycolysis, pyruvate oxidation, or the citric mountain cycle. This increases respiration. As quickly asNAD+ gets lessened to NADH, it gets oxidized again by the high electron need of the electron carry chain.NADPH mainly exists in its reduced form. on the various other hand, a high concentration that NADPH is necessary to journey the Calvin bike forward. The light reactions continually reduce NADP+, so it is always ready to act as a to reduce agent.

These two dinucleotide electron carriers role in countless other processes in cells, with NAD+ usually prepared to act as an oxidizer and NADPH ready to act as a reducing agent.

Plants have mitochondria, too

The light reactions create ATP, so numerous students finish up thinking that photosynthetic cell don"t should do moving respiration to generate ATP. This is mistaken, due to the fact that the ATP produced in the light reactions gets offered in the Calvin cycle. Cell still require ATP for various other processes, so they need to perform respiration. Mesophyll cell in the leaves of plants have plenty that chloroplasts, however they also have mitochondria.


Terms & concepts

Carbon fixationChloroplastCytochrome complexElectron move chain in photosynthesisExcited electronsNADP+/NADPHPhotosynthetic pigmentsPhotosystems I and also II (Note that, for historical reasons, the one that comes first is referred to as PS II.)Proton gradient in photosynthesisReaction center chlorophyllsRedox processes in photosynthesisResonance transferStromaThylakoid

Review questions

In the overall procedure of photosynthesis, CO2 (fully oxidized carbon) becomes decreased to type sugar molecules. Acquiring reduced means that electrons room added. Where perform the electron come from?What commodities of the light reactions gain used in the Calvin cycle?Where go PS II acquire the electrons that will get excited by light energy? whereby does it happen these electrons after they"re excited?Where go PS I obtain the electrons the will obtain excited by irradiate energy? wherein does it happen these electrons after they"re excited?In one step of photosynthesis, water gets split. Is this process endergonic or exergonic? What provides it happen?Is the whole procedure of the Calvin cycle, including all inputs and outputs, endergonic or exergonic? Explain.In the Calvin cycle, what it s okay oxidized and what it s okay reduced? (You don"t have to memorize the surname of the assorted intermediates; think in regards to inputs and outputs.)Describe the pathway that electrons take with the electron carry chain and the remainder of photosynthesis, from beginning to end."Rubisco" is brief for ribulose bisphosphate carboxylase. What does carboxylase mean?Is the carbon fixation action of the Calvin bicycle endergonic or exergonic? Explain.Why is ATP required in the rejuvenation of RuBP, the CO2 acceptor, in the Calvn cycle?Compare and also contrast the attributes of the electron move chains in photosynthesis and respiration.Does photosynthesis in its entirety produce a net gain of ATP?Where walk the O2 come native in photosynthesis?What is cyclic vs. Straight (or noncyclic) electron flow in photosynthesis? Why space both pathways necessary?For the proton gradient in photosynthesis, where carry out the protons come from?Why perform the photosynthetic cell of plants need mitochondria?There"s a photograph in Campbell reflecting a flask the dissoled chlorophyll create bright red fluorescence ~ being engendered by short-wavelength light. Why doesn"t this happen throughout photosynthesis?What colors of irradiate work finest for providing power for photosynthesis? Why?

References & additional reading

Photosynthesis in general

Photosynthesis | HHMI BioInteractive Video. An excellent video the goes from simple overview to in-depth mechanisms. This can be every you have to know, in 11 minutes!

Photosynthesis native OpenStax. A thing from a totally free online textbook; this publication is design to it is in the equivalent of Campbell Biology.

See more: What Does The Phrase The Rose That Grew From Concrete Mean Figuratively As Used In This Poem

Photosynthesis from khan Academy. A complete unit, covering more or less the same set of ideas as Campbell.

Photosynthesis on PDB-101

The an excellent protein website PDB-101 has actually some exciting resources regarded the proteins connected in photosynthesis: