Phototrophy

The Purpose and Process of Photosynthesis

The process of photosynthesis converts light energy to chemical energy, which can be used by organisms for different metabolic processes.

Learning Objectives

Describe the process of photosynthesis

Key Takeaways

Key Points

  • Photosynthesis evolved as a way to store the energy in solar radiation as high-energy electrons in carbohydrate molecules.
  • Plants, algae, and cyanobacteria, known as photoautotrophs, are the only organisms capable of performing photosynthesis.
  • Heterotrophs, unable to produce their own food, rely on the carbohydrates produced by photosynthetic organisms for their energy needs.

Key Terms

  • photosynthesis: the process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts
  • photoautotroph: an organism that can synthesize its own food by using light as a source of energy
  • chemoautotroph: a simple organism, such as a protozoan, that derives its energy from chemical processes rather than photosynthesis

The Importance of Photosynthesis

The processes of all organisms—from bacteria to humans—require energy. To get this energy, many organisms access stored energy by eating food. Carnivores eat other animals and herbivores eat plants. But where does the stored energy in food originate? All of this energy can be traced back to the process of photosynthesis and light energy from the sun.

Photosynthesis is essential to all life on earth. It is the only biological process that captures energy from outer space (sunlight) and converts it into chemical energy in the form of G3P (
Glyceraldehyde 3-phosphate) which in turn can be made into sugars and other molecular compounds. Plants use these compounds in all of their metabolic processes; plants do not need to consume other organisms for food because they build all the molecules they need. Unlike plants, animals need to consume other organisms to consume the molecules they need for their metabolic processes.

The Process of Photosynthesis

During photosynthesis, molecules in leaves capture sunlight and energize electrons, which are then stored in the covalent bonds of carbohydrate molecules. That energy within those covalent bonds will be released when they are broken during cell respiration. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago.

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis. Because they use light to manufacture their own food, they are called photoautotrophs (“self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”) because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs.

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Photosynthetic and Chemosynthetic Organisms: Photoautotrophs, including (a) plants, (b) algae, and (c) cyanobacteria, synthesize their organic compounds via photosynthesis using sunlight as an energy source. Cyanobacteria and planktonic algae can grow over enormous areas in water, at times completely covering the surface. In a (d) deep sea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds. The ecosystem surrounding the vents has a diverse array of animals, such as tubeworms, crustaceans, and octopi that derive energy from the bacteria.

The importance of photosynthesis is not just that it can capture sunlight’s energy. A lizard sunning itself on a cold day can use the sun’s energy to warm up. Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo-” part) as high-energy electrons in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer, the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf.

Main Structures and Summary of Photosynthesis

In multicellular autotrophs, the main cellular structures that allow photosynthesis to take place include chloroplasts, thylakoids, and chlorophyll.

Learning Objectives

Describe the main structures involved in photosynthesis and recall the chemical equation that summarizes the process of photosynthesis

Key Takeaways

Key Points

  • The chemical equation for photosynthesis is [latex]6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2.[/latex]
  • In plants, the process of photosynthesis takes place in the mesophyll of the leaves, inside the chloroplasts.
  • Chloroplasts contain disc-shaped structures called thylakoids, which contain the pigment chlorophyll.
  • Chlorophyll absorbs certain portions of the visible spectrum and captures energy from sunlight.

Key Terms

  • chloroplast: An organelle found in the cells of green plants and photosynthetic algae where photosynthesis takes place.
  • mesophyll: A layer of cells that comprises most of the interior of the leaf between the upper and lower layers of epidermis.
  • stoma: A pore in the leaf and stem epidermis that is used for gaseous exchange.

Overview of Photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide, and water as substrates. It produces oxygen and glyceraldehyde-3-phosphate (G3P or GA3P), simple carbohydrate molecules that are high in energy and can subsequently be converted into glucose, sucrose, or other sugar molecules. These sugar molecules contain covalent bonds that store energy. Organisms break down these molecules to release energy for use in cellular work.

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Photosynthesis: Photosynthesis uses solar energy, carbon dioxide, and water to produce energy-storing carbohydrates. Oxygen is generated as a waste product of photosynthesis.

The energy from sunlight drives the reaction of carbon dioxide and water molecules to produce sugar and oxygen, as seen in the chemical equation for photosynthesis. Though the equation looks simple, it is carried out through many complex steps. Before learning the details of how photoautotrophs convert light energy into chemical energy, it is important to become familiar with the structures involved.

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Chemical equation for photosynthesis: The basic equation for photosynthesis is deceptively simple. In reality, the process includes many steps involving intermediate reactants and products. Glucose, the primary energy source in cells, is made from two three-carbon GA3P molecules.

Photosynthesis and the Leaf

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma ), which also play a role in the plant’s regulation of water balance. The stomata are typically located on the underside of the leaf, which minimizes water loss. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

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Structure of a leaf (cross-section): Photosynthesis takes place in the mesophyll. The palisade layer contains most of the chloroplast and principal region in which photosynthesis is carried out. The airy spongy layer is the region of storage and gas exchange. The stomata regulate carbon dioxide and water balance.

Photosynthesis within the Chloroplast

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. For plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double membrane envelope composed of an outer membrane and an inner membrane. Within the double membrane are stacked, disc-shaped structures called thylakoids.

Embedded in the thylakoid membrane is chlorophyll, a pigment that absorbs certain portions of the visible spectrum and captures energy from sunlight. Chlorophyll gives plants their green color and is responsible for the initial interaction between light and plant material, as well as numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen. A stack of thylakoids is called a granum, and the liquid-filled space surrounding the granum is the stroma or “bed.”

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Structure of the Chloroplast: Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer.

The Two Parts of Photosynthesis

Light-dependent and light-independent reactions are two successive reactions that occur during photosynthesis.

Learning Objectives

Distinguish between the two parts of photosynthesis

Key Takeaways

Key Points

  • In light-dependent reactions, the energy from sunlight is absorbed by chlorophyll and converted into chemical energy in the form of electron carrier molecules like ATP and NADPH.
  • Light energy is harnessed in Photosystems I and II, both of which are present in the thylakoid membranes of chloroplasts.
  • In light-independent reactions (the Calvin cycle), carbohydrate molecules are assembled from carbon dioxide using the chemical energy harvested during the light-dependent reactions.

Key Terms

  • photosystem: Either of two biochemical systems active in chloroplasts that are part of photosynthesis.

Photosynthesis takes place in two sequential stages:

  1. The light-dependent reactions;
  2. The light-independent reactions, or Calvin Cycle.

Light-Dependent Reactions

Just as the name implies, light-dependent reactions require sunlight. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy, in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy currency molecule ATP (adenosine triphosphate). The light-dependent reactions take place in the thylakoid membranes in the granum (stack of thylakoids), within the chloroplast.

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The two stages of photosynthesis: Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle (light-independent reactions). Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO2.

Photosystems

The process that converts light energy into chemical energy takes place in a multi-protein complex called a photosystem. Two types of photosystems are embedded in the thylakoid membrane: photosystem II ( PSII) and photosystem I (PSI). Each photosystem plays a key role in capturing the energy from sunlight by exciting electrons. These energized electrons are transported by “energy carrier” molecules, which power the light-independent reactions.

Photosystems consist of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In photosystem I, the electron comes from the chloroplast electron transport chain.

The two photosystems oxidize different sources of the low-energy electron supply,    deliver their energized electrons to different places, and respond to different wavelengths of light.

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Photosystems I & II: As explained above, the photosystems manipulate electrons with energy harvested from light.

Light-Independent Reactions

In the light-independent reactions or Calvin cycle, the energized electrons from the light-dependent reactions provide the energy to form carbohydrates from carbon dioxide molecules. The light-independent reactions are sometimes called the Calvin cycle because of the cyclical nature of the process.

Although the light-independent reactions do not use light as a reactant (and as a result can take place at day or night), they require the products of the light-dependent reactions to function. The light-independent molecules depend on the energy carrier molecules, ATP and NADPH, to drive the construction of new carbohydrate molecules. After the energy is transferred, the energy carrier molecules return to the light-dependent reactions to obtain more energized electrons. In addition, several enzymes of the light-independent reactions are activated by light.

Bacteriorhodopsin

Bacteriorhodopsin acts a proton pump, generating cellular energy in a manner independent of chlorophyll.

Learning Objectives

Discuss the function of bacteriorhodopsin

Key Takeaways

Key Points

  • Bacteriorhodopsin is a proton pump found in Archaea, it takes light energy and coverts it into chemical energy, ATP, that can be used by the cell for cellular functions.
  • Bacteriorhodopsin forms chains, which contain retinal molecule within, it is the retinal molecule that absorbs a photon from light, it then changes the confirmation of the nearby Bacteriorhodopsin protein, allowing it to act as a proton pump.
  • While chlorophyll based ATP generation depends on a protein gradient, like bacteriorhodopsin, but with striking differences, suggesting that phototrophy evolved in bacteria and archaea independently of each other.

Key Terms

  • isomerized: converted from one isomer to another
  • retinal: One of several yellow or red carotenoid pigments formed from rhodopsin by the action of light; retinene
  • phototrophy: The synthesis of an organism’s food from inorganic material using light as a source of energy

Bacteriorhodopsin is a protein used by Archaea, the most notable one being Halobacteria. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy. The resulting proton gradient is subsequently converted into chemical energy.

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ATP generation via bacteriorhodopsin: Chemiosmotic coupling between sun energy, bacteriorhodopsin and phosphorylation by ATP synthase (chemical energy) during photosynthesis in Halobacterium salinarum (syn. H. halobium).

Bacteriorhodopsin is an integral membrane protein usually found in two-dimensional crystalline patches known as “purple membrane”, which can occupy up to nearly 50% of the surface area of the archaeal cell. The bacteriorhodopsin forms repeating elements that are arranged in chains. Each chain has seven transmembrane alpha helices and contains one molecule of retinal buried deep within, the typical structure for retinylidene proteins. It is the retinal molecule that changes its conformation when absorbing a photon, resulting in a conformational change of the surrounding protein and the proton pumping action. This releases a proton from a “holding site” into the extracellular side (EC) of the membrane. Reprotonation of the retinal molecule by restores its original isomerized form. This results in a second proton being released to the EC side. The releases the proton from its “holding site,” where a new cycle may begin.

The bacteriorhodopsin molecule is purple and is most efficient at absorbing green light (wavelength 500-650 nm, with the absorption maximum at 568 nm). Bacteriorhodopsin belongs to a family of bacterial proteins related to vertebrate rhodopsins, the pigments that sense light in the retina. Many molecules have homology to bacteriorhodopsin, including the light-driven chloride pump halorhodopsin, and some directly light-activated channels like channelrhodopsin. All other phototrophic systems in bacteria, algae, and plants use chlorophylls or bacteriochlorophylls rather than bacteriorhodopsin. These also produce a proton gradient, but in a quite different and more indirect way involving an electron transfer chain consisting of several other proteins. Furthermore, chlorophylls are aided in capturing light energy by other pigments known as “antennas”; these are not present in bacteriorhodopsin-based systems. Last, chlorophyll-based phototrophy is coupled to carbon fixation (the incorporation of carbon dioxide into larger organic molecules) and for that reason is photosynthesis, which is not true for bacteriorhodopsin-based system. Thus, it is likely that phototrophy independently evolved at least twice, once in bacteria and once in archaea.

Carotenoids and Phycobilins

To aid chlorophylls in the absorption of light not many photosynthetic organisms use carotenoids and phycobilins.

Learning Objectives

Distinguish between carotenoids and phycobilins

Key Takeaways

Key Points

  • Chlorophylls absorb light most efficiently at the ultraviolet end of the spectrum, however not all light that an organism gets is at that wavelength. Thus many photosynthetic organisms rely on accessory compounds to get light from different spectrums.
  • Caretenoids aid in the absorption of light in the blue-range spectrum, while at the same time help with the oxidative stress due to the photosynthetic process.
  • Phycobilins aid in the absorption of light in the red, orange, yellow, and green light, wavelengths.

Key Terms

  • isoprene: An unsaturated hydrocarbon, C5H8, that is readily polymerized; natural rubber (caoutchouc) is cis-1,4-polyisoprene, and trans-1,4-polyisoprene is present in gutta-percha and balata; it is the structural basis for the terpenes.
  • photosynthesis: The process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts.
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Microbial Mats Around the Grand Prismatic Spring: Thermophiles produce some of the bright colors of Grand Prismatic Spring, Yellowstone National Park. The color of the mats of algae and bacteria is due to the ratio of chlorophyll to carotenoid molecules produced by the organisms. During summertime the chlorophyll content of the organisms is low and thus the mats appear orange, red, or yellow. However during the winter, the mats are usually dark green, because sunlight is more scarce and the microbes produce more chlorophyll to compensate, thereby masking the carotenoid colors.

Photosynthesis in many plants and algae depend on chlorophylls which absorb light closer to the ultraviolet side of the spectrum, and emit light in the green end of the spectrum. However during certain times of the year or in various location most of the light may be shifted to other wavelengths away from the ultraviolet spectrum. To deal with these problems, organisms dependent on photosynthesis express various compounds that allow them to absorb different spectrum of light. Notably are carotenoids and phycobilins.

Chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some fungi. Carotenoids can be produced from fats and other basic organic metabolic building blocks by all these organisms. Carotenoids generally cannot be manufactured by species in the animal kingdom so animals obtain carotenoids in their diets, and may employ them in various ways in metabolism.There are over 600 known carotenoids; they are split into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons, and contain no oxygen). All carotenoids are tetraterpenoids, meaning that they are produced from 8 isoprene molecules and contain 40 carbon atoms. Carotenoids in general absorb blue light. They serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect chlorophyll from photodamage.

Phycobilins (from Greek: φ (phykos) meaning “alga”, and from Latin: bilis meaning “bile”) are chromophores (light-capturing molecules) found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae and higher plants). They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble proteins, known as phycobiliproteins. Phycobiliproteins then pass the light energy to chlorophylls for photosynthesis.The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a. Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light, while those at greater depth often contain more of the phycobilins that can capture green light, which is relatively more abundant there.

Facultative Phototrophy

A facultative phototroph can rely on photosynthesis and alternative energy sources to survive and grow.

Learning Objectives

Recognize the traits associated with the classification of facultative phototrophy

Key Takeaways

Key Points

  • Phototrophs can obtain cellular energy from light as well as using light to fix carbon to make complex macromolecules on which to survive.
  • Chlamydomonas reinhardtii is an organism that can rely on photosynthetic and chemical energy sources, depending on conditions.
  • Facultative means optional, in terms of biology it refers to an organism that can switch energy sources for survival.

Key Terms

  • autotroph: Any organism that can synthesize its food from inorganic substances, using heat or light as a source of energy.
  • pyrenoid: any of several transparent structures found in the chloroplast of certain algae etc.; they are responsible for the fixation of carbon dioxide and the formation of starch

An autotroph or “producer”, is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in its surroundings, generally using energy from light ( photosynthesis ) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water. They are able to make their own food, and do not need a living energy or carbon source. Autotrophs can reduce carbon dioxide to make organic compounds, creating a store of chemical energy. Phototrophs, a type of autotroph, convert physical energy from sunlight (in case of green plants) into chemical energy in the form of reduced carbon.

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Chlamydomanas reinhardtii: Scanning electron microscope image, showing an example of green algae (Chlorophyta).

In terms of biology facultative means “optional” or “discretionary” the antonym of which is obligate meaning “by necessity”. Thus facultative phototrophy means an organism that can switch between phototrophy to make organix compounds and other means of getting cellular energy. Probably the best studied example of a facultative phototrophy is Chlamydomonas reinhardtii.

Chlamydomonas reinhardtii is a single celled green alga about 10 micrometres in diameter that swims with two flagella. It has a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an “eyespot” that senses light. Although widely distributed worldwide in soil and fresh water, C. reinhardtii is primarily used as a model organism in biology in a wide range of subfields. When illuminated, C. reinhardtii can grow in media lacking organic carbon and chemical energy sources, and can also grow in the dark when supplied with these. C. reinhardtii is also of interest in the biofuel field, as a source of hydrogen. As one can imagine switching energy sources under varying conditions allows facultative microbes to live in different conditions, in the case of a facultative phototroph it can rely of light other energy sources.

Oxygenic Photosynthesis

Oxygenic photosynthesis, provides energy to organism and allows for carbon fixation, all the while producing oxygen as a byproduct.

Learning Objectives

Describe oxygenic photosynthesis

Key Takeaways

Key Points

  • Plants, algae and cyanobacteria release oxygen during photosynthesis.
  • Photosynthesis is also needed for carbon fixation.
  • While different organisms may have differences during oxygenic photosynthesis, they all follow the general equation of, carbon dioxide + water + light energy → carbohydrate + oxygen.

Key Terms

  • cyanobacteria: Cyanobacteria, also known as blue-green bacteria, blue-green algae, and Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis.
  • oxygenic: of, relating to, containing or producing oxygen

In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. Photosynthesis is not only needed by photosynthetic organism for energy but also for carbon fixation.

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Photosynthesis overview: Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is a redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert carbon dioxide into a carbohydrate, which is a reduction reaction. In general outline, photosynthesis is the opposite of cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, and release chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments. The general equation for photosynthesis is therefore:

2n CO2 + 2n DH2 + photons → 2(CH2O)n + 2n DO

Carbon dioxide + electron donor + light energy → carbohydrate + oxidized electron donor.

In oxygenic photosynthesis water is the electron donor and, since its hydrolysis releases oxygen, the equation for this process is:

2n CO2 + 4n H2O + photons → 2(CH2O)n + 2n O2 + 2n H2O

carbon dioxide + water + light energy → carbohydrate + oxygen + water

Often 2n water molecules are cancelled on both sides, yielding:

2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2

carbon dioxide + water + light energy → carbohydrate + oxygen

In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms.

Anoxygenic Photosynthesis

Photosynthetic reactions can be anoxygenic, thus they do not produce oxygen.

Learning Objectives

Discuss the characteristics that classify a specific type of chlorophototrophy as anoxygenic photosynthesis

Key Takeaways

Key Points

  • Anoxygenic photosynthesis produces cellular energy ( ATP ), without oxygen as a by-product.
  • As opposed to eukaryotic organisms, which rely on chlorophylls for photosynthesis, anoxygenic organisms rely on bacteriochlorophylls.
  • The electron transport chain of anoxygenic phototrophs is cyclic, meaning the electrons used during photosynthesis are fed back into the system, therefore no electrons are left over to oxidize water into oxygen.

Key Terms

  • electron transport chain: An electron transport chain (ETC) couples electron transfer between an electron donor (such as NADH) and an electron acceptor (such as O2) with the transfer of H+ ions (protons) across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate (ATP). Electron transport chains are the cellular mechanisms used for extracting energy from sunlight in photosynthesis and also from redox reactions, such as the oxidation of sugars (respiration).
  • electron donor: An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process.
  • anoxygenic: That does not involve the production of oxygen

Phototrophy is the process by which organisms trap light energy (photons) and store it as chemical energy in the form of ATP and/or reducing power in NADPH. There are two major types of phototrophy: chlorophyll-based chlorophototrophy and rhodopsin-based retinalophototrophy. Chlorophototrophy can further be divided into oxygenic photosynthesis and anoxygenic phototrophy. Oxygenic and anoxygenic photosynthesizing organisms undergo different reactions either in the presence of light or with no direct contribution of light to the chemical reaction (colloquially called “light reactions” and “dark reactions”, respectively).

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Green d winogradsky: A column containing green sulfur bacteria which uses anoxygenic photosynthesis.

Anoxygenic photosynthesis is the phototrophic process where light energy is captured and converted to ATP, without the production of oxygen. Water is therefore not used as an electron donor. There are several groups of bacteria that undergo anoxygenic photosynthesis: Green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic Acidobacteria, and phototrophic heliobacteria. Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls (similar to chlorophyll found in eukaryotes). Bacteriochlorophyll a and b have wavelengths of maximum absorption at 775 nm and 790 nm, respectively in ether. In vivo however, due to shared extended resonance structures, these pigments were found to maximally absorb wavelengths out further into the near-infrared. Bacteriochlorophylls c-g have the corresponding “peak” absorbance at more blue wavelengths when dissolved in an organic solvent, but are similarly red-shifted within their natural environment (with the exception of bacteriochlorophyll f, which has not been naturally observed).Unlike oxygenic phototrophs, anoxygenic photosynthesis only functions using (by phylum) either one of two possible types of photosystem. This restricts them to cyclic electron flow and are therefore unable to produce O2 from the oxidization of H2O.

The cyclic nature of the electron flow is typified in purple non-sulfur bacteria. The electron transport chain of purple non-sulfur bacteria begins when the reaction centre bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to Bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate a proton motor force (PMF) which can then be used to synthesize ATP by oxidative phosphorylation. The electron returns to P870 at the end of the chain so it can be used again once light excites the reaction-center. Therefore electrons are not left over to oxidize H2O into O2.