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Photosynthesis

Contents

• Light reactions of photosynthesis
• Dark reactions of photosynthesis
• Evolution of photosynthesis
• Diagrams

Light reaction of photosynthesis

Most energy entering the biosphere comes in via green-plant photosynthesis.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

In land plants and green algae (chlorophytes), photosynthesis has two distinct stages, the light reactions, which convert light energy to ATP and NADPH; and the dark reactions, which convert CO₂ to carbohydrate using ATP and NADPH. Both occur in the chloroplasts.

Light reactions:

12H₂O + 12NADP + 18ADP → 6O₂ + 12NADPH + 18ATP

Dark reactions

6CO₂ + 12NADPH + 18ATP → C₆H₁₂O₆ + 12NADP + 18ADP + 6H₂O

The light (dependent) reactions take place on chloroplast membranes and generate ATP and NADPH. These cofactors are mostly used for the reduction of CO₂ to carbohydrates, but some ATP and NADPH can be used for other metabolic processes. They only occur when light is present.

In green plants and other algae, photosynthesis goes on in the chloroplasts. These probably evolved by endosymbiosis. Some chloroplasts are probably secondary endosymbionts. As for mitochondria, the evidence is extensive.

• Chloroplasts have an outer (host) membrane and an inner (bacterial) membrane.
• The inner membrane is folded inwards to form sacs called lamellae. The membranes these lamellae are made of are called thylakoid membranes, which look just like those of cyanobacteria (blue-green algae).

The membranes of chloroplast are rather complex. The lamellae may occur singly, and these are called stromal or intergranal lamellae (intergrana); or they may be stacked like coins, when they are termed granal lamellae (grana), and the individual "coins" are called thylakoids. In cross section, the lamellae look like pairs of membranes separating a narrow internal space (the lumen) from the external stroma.

The membranes of the lamellae contain photosynthetic pigments and an electron transport system. The lumen of the lamellae contains the oxygen generating system. The region around the lamellae (stroma) contains the CO₂ fixing system (dark reactions).

To understand photosynthesis, we need to know a little about photochemistry and the pigments that are involved in converting light into chemical energy. Pigments look coloured because they absorb visible light. We see the colour which they do not absorb: chlorophyll absorbs red and blue light, but not green, so appears green. Light behaves like a stream of particles (quanta) called photons. Pigment molecules absorb light one photon at a time. When a pigment molecule absorbs a photon, it raises its electrons to higher energy levels. The pigment is excited and can perform photochemical reactions. This excitation energy is used in photochemical reactions. The energy (E) in a photon is determined by the wavelength (λ) of the light.

E = h c ⁄ λ

• h, Planck's constant, 6.6 × 10⁻³⁴ J s.
• c, speed of light, 3 × 10⁸ m s⁻¹.

Short wavelength photons (blue) have a higher energy than long wavelength (red) photons. Photons promote electrons to excited states: the difference in energy levels must be equal to the energy of the photon. More energetic photons (shorter wavelength) promote electrons to higher energy levels.

The main pigment in green plants is chlorophyll-a. It absorbs red and blue light to make it enter an excited state, and therefore appears green.

Action spectra can be used to show the wavelengths of light that make photosynthesis work. If we compare this to chlorophyll absorption spectrum, we see they are a good match. This implies chlorophyll is the main pigment in photosynthesis.

Chlorophyll molecules absorb light as individual photons. Each can cause a single photochemical reaction. If there is no direct photochemical reaction, chlorophyll may lose its excitation energy as heat and red-fluorescence, or by resonance transfer. In fluorescence a high energy (short wavelength) photon is absorbed, which promotes an electron. The electron then drops to the lowest vibrational sub-state of the excited state, releasing heat, before dropping to the ground state, emitting a photon of lower energy (longer wavelength) than that originally absorbed.

In resonance transfer, the energy of excitation is passed from one molecule to a neighbouring one if their spectra overlap. The whole of the energy of excitation (exciton) is transferred bodily to another ground-state molecule. This is how photosynthetic pigment molecules pass energy from one to another in the chloroplast. Resonance transfer can only occur when the two pigment molecules are close together and if the spectrum of the donor molecule overlaps with that of the receiver (they are 'tuned-in'). If the donor and receiver molecules are not identical, energy can only be transferred in one direction, producing a 'light pump', because a molecule that has absorbed a photon of short wavelength light has enough energy to excite one that absorbs long wavelength light, but not the other way around. Energy absorbed by accessory pigments of photosynthesis is channelled to chlorophyll-a because they all absorb at shorter wavelengths.

Accessory pigments are defined as any pigment, other than chlorophyll-a (or bacteriochlorophyll-a in cyanobacteria), which can gather light for photosynthesis.

They are protein-bound in particles and include:

Chlorophyll-b in green plants.

Phycobilins (below) in the red algae and cyanobacteria (blue-green algae).

Carotenoids. These are yellow/red pigments that absorb blue light. e.g. β-carotene.

The pigment molecules are arranged in blocks of about 50 called an antenna.

These channel energy to a central chlorophyll-a molecule, the reaction centre, where the photochemical reaction occurs. The excited chlorophyll-a ejects an electron, becoming an extremely strong oxidising agent, capable of pulling electrons out of water. The antenna plus the reaction centre taken together are termed a photosystem.

Plants also possess mobile light-harvesting complexes (LHCs) which serve to fine tune energy flux into the two photosystems. This is important because if photosystem I receives too little energy compared to PSII, electrons will 'back up' the transport chain, and prevent excited electrons from escaping photosystem II. Excited chlorophyll molecules in photosystem II will not be quenched by electrons from water, and will cause oxidative damage to the reaction centre. This causes the destruction of photosystem II, in a process known as photoinhibition. Photoinhibition is a particular problem when photon flux is very high. Fine tuning is achieved by the LHCs: LHC-II associates with PSII when phosphorylated, and dissociates when dephosphorylated. Phosphorylation is regulated by a kinase whose activity is high when light flux is low. This means that when the leaf is shade-adapted, the kinase is active, LHC-II is phosphorylated, and light is channeled into PSII. When light intensity increases, the kinase become inactive, and the LHC-II dissociates from PSII, preventing excessive energy input into PSII.

By theory, it appears to takes just four electrons (and four protons) to reduce CO₂ to carbohydrate. However, we find we need eight photons per CO₂, implying that two photochemical reactions are needed per electron, and that there are two kinds of photosystem operating in series, each physically separate in its own kind of particle. This is indeed the case:

• Photosystem I reduces NADP to NADPH. Its reaction centre absorbs maximally at 700 nm, and it is found mostly on intergrana.
• Photosystem II splits water (the Hill reaction). Its reaction centre absorbs maximally at 680 nm, and its is found mostly on grana, associated with light-harvesting complex II. It also contains more chlorophyll-b than PSI.

Unhelpfully enough, PSII comes 'earlier' in the reaction series than PSI.

Electrons excited out of the reaction centre in the photosystems are carried along a chain. The chain pumps protons, just like the respiratory complexes, and the electrons are eventually dumped onto NADP to form NADPH. Protons flow back through ATP synthase, generating ATP.

The electron transport chain of photophosphorylation has many similarities to that found in mitochondria: many of the chemical players are the same, and much of the gist will seem familiar but reversed. We will discuss the complexes found in the thylakoid membranes in turn.

Cyt-b₆/f complex

Plastocyanin reductase pumps 4 H⁺ per PQH₂ from the lumen to the stroma, and makes 2 PC_RED per PQH₂ oxidised. The copper in PC goes from Cu²⁺ to Cu⁺.

Like cytochrome reductase in mitochondria, this complex runs the Q-cycle, which delivers the two electrons from one PQH₂ to two plastocyanin molecules, which only carry one electron each. For each molecule of PQH₂ that arrives at the complex, one electron is sent back to another PQ, whilst the other is passed on down the chain, to a copper-containing protein called plastocyanin. Two rounds of this pump across 4 protons, and produce two reduced plastocyanin molecules.

Photosystem I

Rarely termed ferredoxin reductase, PSI is excited maximally by 700 nm light, and produces 2 Fd_RED per 2 PC_RED oxidised. The electrons are passed from P700 through a series of quinones and iron-sulfur centres to the iron-containing protein ferredoxin. Most of the ferredoxin is then immediately passed to a flavin (FMN) cofactor in NADP reductase, generating one of the raw materials for the dark reactions. The electron lost by P700 is replaced by one from plastocyanin. One NADPH is generated per 2 ferredoxin oxidised.

ATP synthase

Like mitochondria, chloroplasts have an F-type ATPase which generates ATP from a proton gradient. Thylakoids are permeable to Mg²⁺ and Cl⁻ so this is mostly a concentration effect (pH) rather than an electrical (charge separation) effect. In fact, the pH difference can be very marked: typically the lumen is at pH 4 and the stroma at pH 8.

Many herbicides act by damaging the photosynthetic chain. Atrazine damages the quinone binding area on PSII; paraquat takes electrons from ferredoxin and generates (lethal) hydrogen peroxide. Uncouplers like DNP (obviously) work in chloroplasts as well as mitochondria.

The whole process of the light reactions generates a proton gradient across the membrane. This is used for the chemiosmotic production of ATP. This is called Z-scheme (or straight chain) photo-phosphorylation.

Energy budget per water split:

• Water splitting adds 2H⁺ to the lumen.
• The Q-cycle pumps 4H⁺ into the lumen.
• (NADPH and dark reaction effects cancel).

We need to split two waters per CO₂ fixed, so we double this total to 12 protons, generating 3 or 4 ATP. Only 3 ATP are required per CO₂ in the dark reactions, so we probably make a slight profit of just under 1 ATP per CO₂ over that which is required. However, sometimes, some of the electrons from ferredoxin are fed back to cyt-b₆/f, short-circuiting the Z-scheme. This generates ATP via the Q-cycle but no NADPH. This is called cyclic phosphorylation and enables the cell to make even more ATP, even when NADPH is not required. Depending on the exact stoichiometry of the various complexes (still not nailed down completely), cyclic phosphorylation may be essential to satisfying the Calvin cycle's requirements.

Dark reaction of photosynthesis

There are certain synonyms and traps for the unwary here. The light reactions only occur in the light, and may also be termed light-dependent or thylakoid reactions. The dark reactions can occur in the dark (providing there is enough ATP and NADPH), but they usually occur in the light. They may also be termed light-independent reactions, or the stroma reactions, or the Calvin cycle, or the photosynthetic carbon assimilation (PCA) cycle, etc. Much like my preference for 'slaughterhouse' to 'livestock processing depot', you may see why I prefer 'dark reactions' despite the rather poor choice of name.

The dark reactions use the NADPH and ATP from the light reactions to fix CO₂. They were discovered by using radioactive CO₂ as a tracer. Experimenters saw what it was incorporated into when fed to photosynthesising plants. Ruben (1940) was the first to do this using ¹¹CO₂. This isotope has only a 20 minute half-life, which was only enough time to show that it went into the COOH group of an organic acid. Melvin Calvin (1945) used the longer-lived isotope ¹⁴C on microscopic algae. They were fed ¹⁴CO₂ in the light and killed in boiling ethanol after various times. The ethanol extracted the radioactive products, which could be concentrated by evaporation and separated by paper chromatography. The apparatus used for the algae was a disc-shaped vessel (lollipop), filled with algal suspension and fitted with a tap and an aeration system to bubble air through it. The algae were allowed to reach a steady state of photosynthesis in air. Na¹⁴CO₃ was then added, and the slightly acid buffer solution converted this to ¹⁴CO₂ which was photosynthesised. At intervals, the tap was opened, and samples of the algae run out into boiling ethanol. This stopped the reactions and extracted the soluble radioactive intermediates of photosynthesis.

Chromatography separates substances based on their solubility in two phases: One phase is stationary; the other phase is mobile, and moves, either by pumping or capillary action. A spot of mixture is put at the end of piece of filter paper, and the solvent is allowed to flow through the paper from that end by capillarity, carrying molecules from the spot. Less soluble molecules adsorb (stick) reversibly onto the paper, and move up the paper in a series of hops; more soluble materials get stuck less often and move faster. This effects separation of the mixture into its individual components along the plate. In simple paper chromatography, as used by Calvin, the stationary phase is hydrophilic paper; the mobile phase is a hydrophobic solvent (e.g. toluene). Hydrophobic compounds (X) move more quickly.

The R_f value is characteristic for particular compounds.

R_f = distance substance travelled ⁄ distance solvent travelled.

A better separation can be obtained by repeating the process using a different solvent and running it at right angles to the first. Spots can sometimes be identified by their own colour, or their R_f, or the paper sprayed with chemicals to make colourless compounds become coloured. Calvin identified his radioactive photosynthetic products by autoradiography. Dried chromatograms were held for against X-ray films (for weeks!). When developed, these showed the radioactive regions as black marks. The compounds from the radioactive areas were eluted from the paper and identified chemically, e.g. by mass spectroscopy and infra-red spectroscopy. Sometimes, they were also broken down to find which atoms were radioactive.

After short periods of photosynthesis, nearly all the radioactivity was found in the carboxyl group of phosphoglyceric acid (PGA). The other two atoms were non-radioactive, so the radioactive CO₂ had combined with a non-radioactive receptor molecule. In experiments on Scenedesmus, which was allowed to photosynthesise for only 5 seconds, 90% of the label was found in PGA. 95% percent of this radioactivity was found in carbon atom 1, but after 30 seconds, the other two carbons also became labelled. The labelling of the other carbons showed that the acceptor molecule was being regenerated from the radioactive products and this was taking about 30 seconds.

The nature of the CO₂ acceptor was not known for years, and a fruitless search was made for a C2 compound. Eventually, it was discovered that it was not a C2 compound, but a C5 compound. The receptor was eventually discovered to be ribulose 1,5-bis-phosphate (RuBP) [bis means two separate phosphates; di means two chained phosphates]. It combines with CO₂ to give 2 molecules of PGA (one of which is labelled). RuBP became labelled when ¹⁴CO₂ was being fixed at the right time to account for the label in carbon atoms 2 and 3 of PGA.

RuBP + CO₂ +H₂O → 2 PGA

An enzyme was later isolated from photosynthetic tissue which carboxylates RuBP. This enzyme was called ribulose 1,5-bis-phosphate carboxylase/oxygenase. (shortened to Rubisco).

The rest of the dark reactions have two purposes: firstly to regenerate the acceptor molecule RuBP from PGA; and secondly to generate some sugar profit in the form of glucose, sucrose or starch from PGA.

Two molecules of PGA are formed from the carboxylation of RuBP, one of which is labelled. After a while, other compounds became labelled. Many were known intermediates of glycolysis, and Calvin guessed that the energy from the light reactions of photosynthesis was driving glycolysis backwards to generate glucose from PGA. This is the part that generates our profit. PGA is phosphorylated to 1,3-bis-phosphoglycerate by ATP (from light reactions). This is then reduced to glyceraldehyde-3-phosphate (GAP), using NADPH. GAP dehydrogenase is confined to the chloroplast. Animals also use this pathway to make glucose from Krebs cycle acids by the process termed gluconeogenesis.

PGA + ATP + NADPH → GAP + ADP + P_i + NADP

Later, many sugar phosphates were found to be labelled. Everything from trioses to heptuloses: ribose, ribulose, fructose, etc. This appeared to be how the dark reactions regenerated RuBP. Sugar (phosphates) found to be labelled:

• C3 trioses.
  • Dihydroxyacetone-P.
• C4 tetroses
  • Erythrose-4-P.
• C5 pentoses
  • Ribose-5-P.
  • Ribulose-5-P.
  • Xylulose-5-P.
• C6 hexoses
  • Fructose-1,6-bP
  • Fructose-6-P.
• C7 heptuloses
  • Sedoheptulose-1,7-bP
  • Sedoheptulose-7-P

These sugars are all part of the pentose phosphate pathway, which is used to form ribose for nucleic acid synthesis in other organisms. The dark reactions have hijacked the pentose phosphate pathway to regenerate RuBP from GAP.

Adding these modified gluconeogenesis and pentose-phosphate pathways together, we get the Calvin cycle. This is an extremely complicated way of regenerating RuBP from GAP, which uses up 1 more ATP per RuBP. The upshot is:

5 trioses → 3 pentoses.

5 GAP + 3ATP → 3 RuBP + 3ADP + 2P_i

For every 3 RuBP we carboxylate, we get 6 PGA. 5 of these are used to regenerate RuBP via the Calvin cycle (pentose phosphate pathway). 1 of these is used to generate sugar profit (gluconeogenesis pathway). Hence each turn of the Calvin cycle fixes 6 molecules of CO₂, uses and regenerates 6 RuBP, and generates one hexose profit, using 12 NADPH and F18 ATP from the light reactions.

Evolution of photosynthesis

Evolution builds on what already exists, in small steps, each one having a selective advantage over preexisting adaptations. Using these principles and our knowledge of modern organisms we will try to explain why trees are green; how the Calvin cycle evolved; and how photosynthetic oxygen-production evolved.

Theories of the origin of life are continuously changing. We join the story half-way through: living things have evolved protein-based metabolisms and DNA-based replication. We can state several characteristics of our Ur-Life. They have ATPase pumps that can interconvert proton gradients with ATP. They rely on external sources of reduced carbon for all their energy, electron and carbon needs.

Archaeal photosynthesis

The earliest photosynthetic organisms lived in an anaerobic atmosphere. They did not fix CO₂ or generate O₂ or even use chlorophyll. They used light to make ATP and absorb nutrients. We have seen this already. Archaeal photosynthesis in Halobacterium halobium uses the purple pigment bacteriorhodopsin in the external membrane and light energy to expel protons from the cell. The proton gradient used for nutrient absorption (by co-transport with protons) and for ATP production. This has a selective advantage.

Why was a purple pigment selectively advantageous to the Archaea? A purple pigment absorbs in the middle of the visible spectrum, where the sun emits light most copiously. It absorbs green light, but reflects red and blue so it looks purple.

Without the ability to fix CO₂ Archaea cannot photosynthesise without external reducing agents and organic carbon. So, they uses light to make energy, but need reduced carbon (food) on the menu.

Some handy terminology for nutritional modes will help us here:

Carbon to make biomass.

• Carbon dioxide: autotroph.
• Organic molecules: heterotroph.

Energy to make ATP.

• (Sun)light: phototroph.
• Oxidation of chemicals: chemotroph.

Electrons to make NADH (usually considered as subdivisions of chemotrophy).

• Water/hydrogen/etc.: lithotroph.
• Organic molecules: organotroph.

The evolution of photosynthesis is a story of self-reliance: from needing food to using light for energy; from needing food to using carbon dioxide for biomass; and from needing food to using water for reducing power (electrons). Ur-Life (and humans) are hetero-chemo-organotrophs and get all their carbon, energy and reducing power from food. Photosynthetic archaea like Halobium are hetero-photo-organotrophs, getting ATP from light, but all carbon and electrons still come from organic sources. Green plants are auto-photo-lithotrophs, getting everything from light, water and CO₂.

The first green photosynthetic organisms may have lived in sediments, where they could gather the organic matter from falling dead organisms. Because they competed for light with Archaea, they specialised in absorbing blue and red light and hence needed a green pigment.

Chlorophylls specifically absorb blue and red light, so appear green. They are porphyrins, like haem in cytochromes, and indeed, chlorophyll based photosystems probably evolved from cytochromes. These organisms were probably also the first organisms to use resonance transfer. This let them make the best use of the very dim light in the sediments. It was from these organisms that virtually all modern photosynthesising plants evolved, which is why trees are green.

Evolution of the Calvin cycle

Sulfur bacteria are two phylogenetically distinct groups of photosynthetic bacteria that get their reducing power from hydrogen sulfide or hydrogen. There are two sorts: purple sulfur bacteria (Chromatium, which is in the γ-Proteobacteria) and green sulfur bacteria (Chlorobium, which is in the Chlorobi).

Chlorophyll probably evolved in the ancestors of the green sulfur bacteria (and perhaps long before that). It occurs in the bacterial cell membrane and transfers electrons photochemically onto NAD, and the NADH so produced is used to run glycolysis backwards. How did this evolve? The respiration in early sulfur bacteria generated NADH. This must be converted back to NAD, but could not be oxidised by oxygen in an anaerobic atmosphere. Instead the electrons were passed via ferredoxin to hydrogenase. Hydrogenase combines the electrons from ferredoxin and with protons inside the cell to make hydrogen gas. This removes protons from within the cell and generates a gradient that can be used to make ATP. If this system could be run backwards, the bacteria could generate sugars by gluconeogenesis, using hydrogen as an electron donor.

All the bacteria needed was a way of generating PGA, and they could run glycolysis backwards, turning a waste-route into a way of making sugar for 'free'. The green sulfur bacteria evolved an enzyme called ATP citrate lyase, which cleaves citrate into acetyl-CoA and oxaloacetate (this is called the reductive Krebs cycle). This allowed these bacteria to run the Krebs cycle backwards and generate PGA. The enzyme was required because the Krebs cycle is essentially irreversible, so some of the 'backwards' steps needed new enzymes, in much the same way that fructose bisphosphatase acts as a work-around in gluconeogenesis for the irreversibility of the phosphofructokinase reaction in glycolysis. [Note that a number of other enzymes were required to bypass these irreversible steps, not just the lyase].

The purple sulfur bacteria chanced upon another way. All cells already contained enzymes to convert GAP to pentose phosphates (the pentose phosphate pathway) because this is needed to make nucleic acid. RuBP became the substrate for a new enzyme (Rubisco) which made it combine with CO₂ to generate PGA. Running glycolysis backwards uses hydrogen gas as a reducing agent, and this destroys the proton gradient. Modern sulfur bacteria solve this by dehydrogenating H₂S using sulfide dehydrogenase, to produce solid sulfur, protons and electrons outside the cell. This generates the proton gradient for ATP synthesis.

The two groups of sulfur bacteria can therefore fix CO₂ (in two distinct ways), and/or make ATP photosynthetically. However, they depend on a supply of electrons from H₂S in the external medium for both. They could not use electrons from water, because there is not enough energy available from just one photosystem. They are therefore auto-photo-chemotrophs, but tied to sources of (scarce) sulfides.

Evolution of oxygenic photosynthesis

Purple non-sulfur bacteria such as Rhodospirillum are closely related to dull things like Escherichia coli 9and also to the purple sulfur bacteria). They run the Q-cycle and generate ATP production from it. Purple non-sulfur bacteria have done away with the need for external H₂S for ATP synthesis by recycling their electrons.

Quinone accepts electrons from chlorophyll and delivers them to a cytochrome-b/c running the Q-cycle. This pumps protons across the membrane for ATP generation, and the electrons are carried back to the chlorophyll by a soluble cytochrome. The cytochrome isn't lost to the outside because in these bacteria photosynthesis occurs in infoldings of the membrane called chromatophores (somewhat akin to thylakoids). The purple non-sulfur bacteria have a cyclical pathway that takes electrons ejected from chlorophyll via quinone, FeS and cytochrome-c centres back to chlorophyll. Because this transport is coupled to a Q-cycle, this allows for proton pumping and therefore ATP generation. This is advantageous because it allows chemiosmotic ATP production without using up precious sulphur compounds; however, without ferredoxin, they can't fix CO₂. Some modern non-sulphur bacteria can fix CO₂, but this is by a more recently evolved mechanism.

Cyanobacteria

The green sulphur bacteria could fix CO₂ but only make small amounts of ATP (with electrons from H₂S).

The non-sulphur bacteria could make large amounts of ATP with electrons from water, but they couldn't fix CO₂.

Genetic recombination would give an organism with both photosystems that could both fix CO₂ and make large amounts of ATP. Photosystem I is just like the single photosystem in the green sulphur bacteria, with iron-sulfur centres passing electrons to ferredoxin; whilst photosystem II is like the purple non-sulphur bacteria, with a phaeophytin-quinone system pulling electrons out of water (rather than sulfides).

Organisms with coupled PSI and PSII would have a selective advantage over both parents, as they could generate lots of ATP and lots of sugar, with little expenditure of sulfides. With the two photosystems operating independently, it would still not have been able to generate oxygen. However, mutations involving the electron carriers would eventually put the two photosystems in series, generating enough energy to split water.

Exactly what this means is still a subject of debate. It is probable that both photosystems evolved from a common ancestor (most likely a modified cytochrome). The 'nice' story would be that cyanobacteria are the result of some horizontal gene transfer of PSI from GSBs to purple bacteria, which, by this time had also developed the Calvin cycle, and an ability to use sulfides/hydrogen as an electron donor. However, it now seems just as likely that both photosystems were present in the common ancestor of the GSBs, PNSBs, and cyanobacteria, and that photosystems were lost in the branches leading to the GSBs and PNSBs. The evolutionary tree below shows many of the inconvenient details: there are actually six groups of photosynthetic eubacteria, with a plethora of different ways of making a living, all embedded in groups that contain both phototrophic and chemotrophic bacteria, and we're still not even sure if the branching is correct!

These water-splitting auto-photo-lithotrophs became ubiquitous and gave rise to the cyanobacteria and chloroplasts. They filled the atmosphere with oxygen, and allowed the evolution of efficient, aerobic life, as we know it. On the other hand, this was a colossal act of environmental vandalism as far as the poor anaerobes were concerned…

That concludes this little foray into the joys of photosynthesis (and respiration). Although these processes may seem very complex at first glance, I hope this essay has helped to show their underlying similarities, and the fantastically ad hoc nature of evolved systems.

Diagrams

You may find these diagrams useful:

• Pentose phosphate pathway.
• Calvin cycle.

Test yourself

1. Compare and contrast the electron transport chains in oxidative and photo-phosphorylation.
2. Draw a diagram to summarise the dark reactions. Highlight the carboxylation, gluconeogenesis and pentose-phosphate parts. Ensure you show where ATP and NADPH are consumed.
3. Where and why did the following features of photosynthesis evolve?
   • Light-driven ATPases.
   • The action spectrum of photosynthesis.
   • Photosystem I.
   • Calvin cycle.
   • Photosystem II.
   • Water splitting manganoprotein.

Answers

Bibliography

• Berg, J. M., Tymoczko, J. L. and Stryer, L. (2006). Biochemistry. 6th edition. W. H. Freeman and Company, New York. 458-464. "Glucose can be synthesised from noncarbohydrate precursors"
• Berg, J. M., Tymoczko, J. L. and Stryer, L. (2006). Biochemistry. 6th edition. W. H. Freeman and Company, New York. 541-564. "The light reactions of photosynthesis"
• Berg, J. M., Tymoczko, J. L. and Stryer, L. (2006). Biochemistry. 6th edition. W. H. Freeman and Company, New York. 566-573. "The Calvin cycle synthesizes hexoses from carbon dioxide and water"
• Berg, J. M., Tymoczko, J. L. and Stryer, L. (2006). Biochemistry. 6th edition. W. H. Freeman and Company, New York. 577-585. "The pentose phosphate pathway generates NADPH and synthesizes five-carbon sugars"
• Ferreira, K. N., et al. (2004). Architecture of the photosynthetic oxygen-evolving center. Science 303:1831-1838. http://dx.doi.org/10.1126/science.1093087
• Taiz, L. and Zeiger, E. (2002). Plant Physiology. 3rd edition. Sinauer Associates Incorporated, Sunderland, Massachusetts. 111-143. "Photosynthesis: the light reactions"
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• Wilson, A. T. and Calvin, M. (1955). The photosynthetic cycle: CO₂ dependent transients. Journal of the American Chemical Society 77:5948-5957. http://pubs.acs.org/cgi-bin/sample.cgi/jacsat/1955/77/i22/pdf/ja01627a050.pdf