Photosynthesis Overview

Overview of Photosynthesis:

Photosynthesis involves the conversion, by various phototrophic organisms, of light energy into organic molecules, with or without the production of oxygen.

Nonoxygenic, anaerobic photosynthesis evolved^ in prokaryotes before oxygenic photosynthesis, and is found in green filamentous, green sulfur, purple sulfur, and purple nonsulfur bacteria. Photosynthesis in green sulfur bacteria such as Chlorobium tepidum takes place in the chlorosome.

Oxygenic photosynthesis apparently developed several billion years ago in an ancestor of present day Cyanobacteria. Oxygenic photosynthesis occurs in plants, which possess plastids derived from Cyanobacteria, and in photosynthetic oxygenic prokaryotes, which utilise H2O as electron donor (Cyanobacteria, prochlorophytes).

The photosystem machinery of oxygenic photosynthesis is located within the specialized internal thylakoid membrane system. The chloroplast is the site of oxygenic photosynthesis in eukaryotic cells.

The current consensus is that chloroplasts originated from Cyanobacteria that have become endosymbionts. This is an origin analogous to the endosymbiotic (im) origin of mitochondria, which are believed derived from the "purple bacteria" (alpha-proteobacteria).

The thylakoid membrane, with its embedded membrane-bound pigments, is the structural unit of photosynthesis. Both photosynthetic prokaryotes and photosynthetic eukaryotes possess membranes with embedded photosynthetic pigments. Only eukaryotes, which have a nuclear membrane and membrane-bound organelles, have chloroplasts with an encapsulating membrane.

Light-dependent photosynthetic reactions employ the thylakoid membrane-embedded antenna system to harness energy delivered by a photon. The light-dependent, photophosphorylation reactions of photosystems ultimately transduce the energy of light to generate molecules of ATP and NADPH, which act as energy-transfer molecules in the light independent (“dark") reactions of the Calvin cycle. Table ~ comparison photosynthesis & respiration : Table ~ comparison plant bacterial photosynthesis : : Table ~ comparison of C-3, C-4, CAM plants :

The overall reaction of oxygenic photosynthesis is:

6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O
or,
CO2 + 2 H2O = CH2O + H2O + O2

Microbial Photosynthesis
Cyanobacteria and prochlorophytes conduct oxygenic photosynthesis. Several photosynthetic bacteria, such as the sulfur bacteria, do not generate oxygen. Nonoxygenic photosynthesis, or cyclic photophosphorylation differs from oxygenic photosynthesis in several ways other than lack of oxygen production.

Hydrogen sulfide (H2S) is utilized by the purple sulfur and green sulfur bacteria:

CO2 + 2H2S = CH2O + H2O + 2So

Nonoxygenic photosynthesis in prokaryotes, utilizing S– or So or H2 as electron donor include green filamentous, green sulfur, purple sulfur, and purple nonsulfur.

Tables  Overview of Photosynthesis  Comparison of C-3, C-4, CAM plants  Comparison of plant and bacterial photosynthesis  Structure of bacteriochlorophylls  Comparison of Photosynthesis and Respiration

Diagram  Z-scheme of noncyclic photophosphorylation :


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Photophosphorylation

Photophosphorylation is the light dependent process by which a proton gradient generates ATP from ADP and Pi.

ADP + Pi →ATP synthase→ ATP

The process of generating a proton gradient resembles that of the electron transport chain of respiration.

Photophosphorylation may be cyclic or noncyclic:

In cyclic photophosphorylation, excited electrons resulting from the absorption of light in Photosystem I are received by the primary electron acceptor and then transferred to the cytb6-f complex, which acts as an electron transport chain. The electrons are returned back to the reaction center of Photosystem I. The excited electrons of cyclic photophosphorylation generate the proton gradient that the ATPase employs to synthesize ATP. No reduction of NADP+ occurs in cyclic photophosphorylation.

In noncyclic photophosphorylation, ATP is generated by the protons gradient created across the thylakoid membranes during the Z-scheme (diagram). The Cytochrome b6-f complex acts as an electron transfer chain. As the electrons release energy during a series of redox reactions, protons are pumped into the thylakoid space. This proton gradient is used for chemiosmotic generation of ATP. The excited electrons are passed on to Photosystem II, where an extra photon of light is harnessed for the reduction of NADP+.

Image :

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Light-reactions

The photosynthetic reactions that require light occur within the thylakoid membrane. These light-dependent photosynthetic reactions employ the thylakoid membrane-embedded antenna system to harness energy delivered by a photon. The light-dependent, photosystem reactions ultimately transduce the energy of light to generate molecules of ATP and NADPH, which act as energy-transfer molecules in the light-independent, “dark” reactions of the Calvin cycle.

The thylakoid-embedded antenna complex comprises closely packed pigment molecules and the reaction center. Any of the pigment molecules can be excited by a photon of light, and pass their energy along to nearby pigment molecules until the excitation eventually reaches a specialized molecule of chlorophyll called the reaction center, which delivers an excited electron to the electron acceptor molecule in an electron transfer chain. The antenna, including the reaction center, and the electron transport molecules together make up a photosystem.

There are two kinds of photosystems in eukaryotes – PSI and PSII. The reaction center chlorophyll (im) molecule within the antenna of photosystem I responds most strongly to 700 nm light, and is therefore termed P700. The reaction center within the antenna of photosystem II responds most to 680 nm light, and is accordingly called P680. Photosystem I evolved very early, and it is found in nonoxygenic phototrophs; photosystem II evolved later. Because the PSII photosystem is most sensitive to shorter wavelength 680 nm light, it absorbs slightly more energy than the P700-PSI system.

The electron transport system of each photosystem is embedded within the thylakoid membrane and functions in the production of ATP. The system comprises membrane-bound electron carriers that pass electrons from one molecule to the next.

Water is split to generate an electron, hydrogen ions, and oxygen:
2H2O → 4e^- + 4H⁺ + O2

Mechanism of Photophosphorylation
By receiving the energized electron (reduction), the first carrier of the P680 (PSII) electron transport system gains energy. It utilizes some of the energy to pump H⁺ into the thylakoid lumen, then passes the less energetic electron to the second of four carrier molecules. Each successive carrier in the electron transport chain utilizes some of the energy of the received electron to pump H⁺ from the stroma into the thylakoid lumen, and then passes the further depleted electron along to the next carrier. Thus, the electron transport system functions to generate a concentration gradient of H⁺ inside the thylakoid. The chemical potential energy of the H⁺ concentration gradient is employed to synthesize ATP.

In the process called photophosphorylation, ATP synthase produces ATP from ADP and Pi when hydrogen ions pass out of the thylakoid. The electron, with its energy almost spent, is passed to the P700 antenna of the PSI photosystem and a second electron transport chain. The P680-PSII system has thus generated ATP, a H⁺ concentration gradient, and energy (Z-scheme (image) of noncyclic photophosphorylation).

The antenna of the evolutionarily older PSI system absorbs a photon of light, and, like the PSII antenna, passes the energy along to the PSI reaction center. Like the PSII system, the P700 reaction center passes the energized electron to an electron acceptor. However, unlike noncyclic photophosphorylation, the electron is retained in the PSI system (cyclic photophosphorylation). So, unlike the PSII system, the electron acceptor of the PSI system utilizes the delivered energy to reduce only molecules of NADP⁺ to NADPH.

The “light” reactions of noncyclic photophosphorylation produce both ATP and NADPH, which act as energy-transfer molecules in the light-independent (“dark”) reactions of the Calvin cycle. The “dark”, or light-independent Calvin anabolic reactions occur in the stroma of the chloroplast in either light or dark conditions. The light-independent reactions function to reduce CO2 to glucose:

6CO2 + 6H2O → Energy + C6H12O6 + 6O2.

The manganese-calcium oxide cluster, also referred to as the "Oxygen Evolving Complex", OEC, or photosynthetic water oxidase. The OEC is located on the oxidizing side of Photosystem II (PSII).

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External link Light-reactions :

Nonoxygenic photosynthesis

Several photosynthetic bacteria, such as the sulfur bacteria, do not generate oxygen. Nonoxygenic, or aerobic photosynthesis differs from oxygenic photosynthesis in other ways than lack of oxygen production:

1) Pigments called bacteriochlorophylls (bacteriochlorophyll-a and carotenoids in purple bacteria) absorb light of longer wavelengths than the chlorophylls and accessory pigments of oxygenic Cyanobacteria or photosynthetic eukaryotes.
2) Reduced compounds such as hydrogen sulfide or organic molecules provide the necessary electrons for the reduction of carbon dioxide. (The oxygenic phototrophs employ reduced oxygen – water – as electron donor for reduction of CO2.)
3) The purple bacteria utilize only one photosystem, PSI, while oxygenic phototrophs utilize two photosystems (PSI and PSII).
4) Photophosphorylation is cyclic in nonoxygenic photosynthesis.

The photosynthetic machinery of nonoxygenic photosynthetic purple bacteria is often located in intracytoplasmic membranes. It is not yet known whether or not these membranes are similar to the thylakoid membrane of oxygenic phototrophs, or whether these intracytoplasmic membranes of nonoxygenic phototrophs are merely an extension of the plasma membrane.

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External link Nonoxygenic photosynthesis

Oxygenic photosynthesis

Oxygenic photosynthesis utilizes light energy to generate ‘excited’ electrons from water:
2H2O → 4e^- + 4H⁺ + O2

A photon is absorbed by Photosystem II and the two resultant excited electrons are passed to Photosystem I, which employs a second photon to further boost their energy for the overall reaction: NADP⁺ + H⁺ + 2e^- → NADPH.

Thus, the excited electron is subsequently passed to an electron acceptor rather than cycling as in nonoxygenic photophosphorylation. Photophosphorylation is the process of creating ATP using a proton gradient created by the energy gathered from sunlight

Oxygenic photosynthesis by photosystem II (P680-PSII) incorporates photobiochemical capacity specific to cyanobacteria and chloroplasts. Protons (H+) are pumped into the thylakoid lumen, generating a concentration gradient that induces thylakoid ATP synthase to phosphorylate ADP to the energy-storage moiety ATP.

ADP + Pi →ATP synthase→ ATP

Utilization of proton movement to join ADP and Pi is termed chemiosmosis. This is accomplished by enzymes called ATP synthases or ATPases.

Diagram • Z-scheme of noncyclic photophosphorylation :

Oxygenic photosynthesis apparently developed several billion years ago in an ancestor of present cyanobacteria. The oxygenic photosynthetic machinery is located within the specialized internal thylakoid membrane system. The ability to construct and modify this thylakoid membrane system appears to be an important feature of oxygenic photosynthesis.

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External link Oxygenic photosynthesis

C-3

Left (click to enlarge):
The C-3, Calvin, photosynthetic carbon reduction (PCR), reductive pentose phosphate cycle (RPP), carbon fixation.

This reaction takes place in the leaves of most plants. During C-3, carbon is fixed, reduced, and utilized via the formation of intermediate 3-carbon sugar phosphates in a cyclic sequence. One complete RPP cycle incorporates three molecules of carbon dioxide and produces one molecule of the three-carbon compound glyceraldehyde 3-phosphate.

The C-3 pathway proceeds in three stages:
1. CO2 fixation (carboxylation) by Rubisco,
2. Carbon reduction to (CH2O)
12 PGA + 12 ATP -> 12 bisPGA + 12 NADPH + 12 H+ -> 12 GAP + 12 NADP+ + 12 Pi, and
3. Regeneration of the CO2 acceptor moleucle (ribulose 1,5-bisphosphate).

Some of the glyceraldehyde 3-phosphate generated in the reductive stage undergoes gluconeogenesis to form glucose. In plants, glucose is converted to sucrose or starch for later use.

The C-3 reactions are sometimes called the "dark reactions" of photosynthesis because photon energy is not used directly – the reactions are light-independent. However, ATP and NADPH generated by light-dependent photophosphorylation reactions are required.

Table ~ comparison of C-3, C-4, CAM plants :

The reaction cycle employs combinations of different length sugar-phosphates and eventually regenerates RuBP in addition to sugar-phosphate for sucrose/starch synthesis.
3-GAP (3 C) → DHAP (3 C)
DHAP (3C) + GAP (3 C) → fructose-1,6-bisphosphate (6C)
fructose-1,6-bisP (6C) + H2O → fructose-6-P (6C) + Pi
fructose-6-P (6 C) + GAP (3 C) → Xylulose-5-phosphate (5 C) + erythrose-4-phosphate (4 C)
erythrose 4-P (4 C) + DHAP (3 C) → sedoheptulose 1,7-bisphosphate (7 C)
sedoheptulose 1,7-bisP + H2O → sedoheptulose 7-P + Pi
sedoheptulose 7-P (7 C) + GAP (3 C) → xylulose 5-phosphate (5 C) + ribose 5-phosphate (5 C)
xylulose 5-P or ribose 5-P → ribulose 5-P
ribulose 5-P + ATP → RuBP (more)

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Calvin cycle

The Calvin cycle is a light independent ("dark-reaction") process of carbon fixation that takes place in the chloroplasts of all plants, whether C3, C4, or CAM plants. The Calvin cycle utilizes light energy stored as ATP and NADPH to convert CO₂ and H₂O to organic compounds.

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

Enzymes of the Calvin cycle are functionally equivalent to many enzymes involved in other metabolic pathways such as glycolysis and gluconeogenesis, but they are located in the chloroplast stroma rather than in the cytoplasm, thus functionally separating the reactions.

The enzymes are activated by light (hence light independent rather than "dark reaction") and by products of the light-dependent photosynthetic reactions. These regulatory mechanisms prevent the Calvin cycle from operating in reverse to respiration, thus preventing a continuous cycle of CO₂ reduction to carbohydrates from occurring simultaneously with carbohydrate oxidation to CO₂ (respiration). This regulation prevents the waste of energy (as ATP) in simultaneous reverse reactions that would have no net productivity.

Table ~ comparison photosynthesis & respiration : Table ~ comparison plant & bacterial photosynthesis : Table ~ comparison of C-3, C-4, CAM plants :

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C-4

In warm, dry conditions the C-4 pathway permits fixation of CO2 with
reduced losses due to photorespiration. The initial CO2 fixing enzyme in the C-4 pathway is phosphoenolpyruvate carboxylase, or PEPcase, which has a higher affinity for CO2 than Rubisco. Fixation of carbon in the mesophyll of C-4 plants prevents wasteful photorespiration by Rubisco.

In the mesophyll, PEPcase fixes CO2 as 4-carbon compounds:

PEP carboxylase + PEP + CO2 → oxaloacetate (C4)
oxaloacetate → malate (C4)

CO2 is taken up by cells of the mesophyll, where PEPcase fixes CO2 as 4-carbon oxaloacetate and malate before transport to the bundle sheath, a specialized tissue in which photosynthetic carbon reduction (C-3) takes place in bundle sheath cell chloroplasts.

Conversion of 4-carbon malate to 3-carbon pyruvate releases CO2 to the bundle cell's Calvin cycle, where 3-phosphoglycerate is furmed under the action of Rubisco. Pyruvate is phosphorylated in the mesophyll into PEP by the phosphorus group donated by a single molecule of ATP. PEP is again utilized to fix CO2 and form PEP carboxylase under the enzymatic action of PEPcase (phosphoenolpyruvate carboxylase)

The C-4 pathway consumes 30 ATP for the synthesis of one molecule of glucose, while the C-3 pathway consumes 18 ATP for the synthesis of one molecule of glucose. However, the reduction of wasteful photorespiration by Rubisco, in which tropical plants lose more than half photosynthecized carbon, more than compensates for the extra cost of ATP.

Table ~ comparison of C-3, C-4, CAM plants :

The C-4 pathway is also called the Hatch-Slack pathway for its Australian co-discoverers. The pathway is a more recent evolutionary development than the C-3 cycle, having arisen during the Cenozoic. Because C-4 carbon fixation has evolved on several occasions in different groups of plants, it is an example of convergent evolution. C4 plants, with their characteristic dimorphic chloroplasts, became more common during the Mesozoic, and today represent about 5% of plant biomass. Plants that employ C-4 metabolism include maize, sorghum, sugarcane, Eleusine, Amaranthus, and switchgrass (Panicum virgatum).

C4 photosynthesis: principles of CO2 concentration and prospects for its introduction into C3 plants.
C4 photosynthesis has a number of distinct properties that enable the capture of CO2 and its concentration in the vicinity of Rubisco, so as to reduce the oxygenase activity of Rubisco, and hence the rate of photorespiration.
Richard C. Leegood, C4 photosynthesis: principles of CO2 concentration and prospects for its introduction into C3 plants (Free Full Text Review Article), Journal of Experimental Botany, Vol. 53, No. 369, pp. 581-590, April 1, 2002.

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CAM

CAM is the acronym for Crassulacean Acid Metabolism, named for the Crassulaceae plant family in which it was discovered.

The chemical reaction of CO2 accumulation is similar to that of C4 plants, but in CAM plants CO2 fixation and its assimilation are separated temporally rather than spatially. CAM plants occur mainly in arid regions, where the opening of stomata to take up CO2 would be connected with large losses of water. To reduce this trans-stomatal loss during intense sun (transpiration via the cuticle continues), CAM plants utilize a mechanism that permits nocturnal uptake of CO2. Prefixed CO2 is stored in the vacuoles as malate (and isocitrate) and is subsequently utilized during the daytime in the C-3, Calvin cycle.

Table ~ comparison of C-3, C-4, CAM plants :

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