{"id":414,"date":"2016-11-04T03:33:24","date_gmt":"2016-11-04T03:33:24","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/microbiology\/?post_type=chapter&#038;p=414"},"modified":"2018-07-11T18:58:15","modified_gmt":"2018-07-11T18:58:15","slug":"photosynthesis","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/chapter\/photosynthesis\/","title":{"raw":"Photosynthesis","rendered":"Photosynthesis"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>Describe the function and locations of photosynthetic pigments in eukaryotes and prokaryotes<\/li>\r\n \t<li>Describe the major products of the light-dependent and light-independent reactions<\/li>\r\n \t<li>Describe the reactions that produce glucose in a photosynthetic cell<\/li>\r\n \t<li>Compare and contrast cyclic and noncyclic photophosphorylation<\/li>\r\n<\/ul>\r\n<\/div>\r\nHeterotrophic organisms ranging from <em>E. coli<\/em> to humans rely on the chemical energy found mainly in carbohydrate molecules. Many of these carbohydrates are produced by <strong>photosynthesis<\/strong>, the biochemical process by which phototrophic organisms convert solar energy (sunlight) into chemical energy. Although photosynthesis is most commonly associated with plants, microbial photosynthesis is also a significant supplier of chemical energy, fueling many diverse ecosystems. In this section, we will focus on microbial photosynthesis.\r\n\r\nPhotosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions (Figure\u00a01). In the <strong>light-dependent reactions<\/strong>, energy from sunlight is absorbed by pigment molecules in photosynthetic membranes and converted into stored chemical energy. In the <strong>light-independent reactions<\/strong>, the chemical energy produced by the light-dependent reactions is used to drive the assembly of sugar molecules using CO<sub>2<\/sub>; however, these reactions are still light dependent because the products of the light-dependent reactions necessary for driving them are short-lived. The light-dependent reactions produce ATP and either NADPH or NADH to temporarily store energy. These energy carriers are used in the light-independent reactions to drive the energetically unfavorable process of \"fixing\" inorganic CO<sub>2<\/sub> in an organic form, sugar.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"650\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03164139\/OSC_Microbio_08_06_LightDepIn.jpg\" alt=\"Diagram of photosynthesis showing a chloroplast divided into the light-dependent reactions and CO2 fixation. There is an outer membrane, an inner membrane and a stack of membranes labeled granum (these are photosynthetic membranes). Light strikes the granum and H2A is converted to \u00bd A. This process produces ATP + NADPH\/NADH that is used in the CO2 fixation cycle. This cycle uses CO2 to produce organics. The CO2 cycle also produces ADP + Pi and NADP+ \/ NAD+ which are then used in the light-dependent reaction.\" width=\"650\" height=\"507\" \/> Figure\u00a01. The light-dependent reactions of photosynthesis (left) convert light energy into chemical energy, forming ATP and NADPH. These products are used by the light-independent reactions to fix CO<sub>2<\/sub>, producing organic carbon molecules.[\/caption]\r\n<h2>Photosynthetic Structures in Eukaryotes and Prokaryotes<\/h2>\r\nIn all <strong>phototrophic eukaryotes<\/strong>, photosynthesis takes place inside a <strong>chloroplast<\/strong>, an organelle that arose in eukaryotes by endosymbiosis of a photosynthetic bacterium (see <a href=\".\/chapter\/unique-characteristics-of-eukaryotic-cells\/\" target=\"_blank\" rel=\"noopener\">Unique Characteristics of Eukaryotic Cells<\/a>). These chloroplasts are enclosed by a double membrane with inner and outer layers. Within the chloroplast is a third membrane that forms stacked, disc-shaped photosynthetic structures called <strong>thylakoids<\/strong> (Figure\u00a02). A stack of thylakoids is called a <strong>granum<\/strong>, and the space surrounding the granum within the chloroplast is called <strong>stroma<\/strong>.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"750\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03164142\/OSC_Microbio_08_06_PhotoMemb.jpg\" alt=\"a) Drawing of a chloroplast, which is a bean shaped structure with an outer membrane and an inner membrane. Between these is the intermembrane space. Inside the inner membrane is an aqueous fluid called stroma and membranes (thylakoids) that form stacks called (grana). The thylakoids form disks with an inner thylakoid lumen. B) Micrograph and drawing of thyladoids which look like folded material. One of the thylakoid membranes is cleaved.\" width=\"750\" height=\"338\" \/> Figure\u00a02. (a) Photosynthesis in eukaryotes takes place in chloroplasts, which contain thylakoids stacked into grana. (b) A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. (credit: scale bar data from Matt Russell.)[\/caption]\r\n\r\nPhotosynthetic membranes in prokaryotes, by contrast, are not organized into distinct membrane-enclosed organelles; rather, they are infolded regions of the plasma membrane. In cyanobacteria, for example, these infolded regions are also referred to as thylakoids. In either case, embedded within the thylakoid membranes or other photosynthetic bacterial membranes are <strong>photosynthetic pigment<\/strong> molecules organized into one or more photosystems, where light energy is actually converted into chemical energy.\r\n\r\nPhotosynthetic pigments within the photosynthetic membranes are organized into <strong>photosystems<\/strong>, each of which is composed of a light-harvesting (antennae) complex and a reaction center. The <strong>light-harvesting complex<\/strong> consists of multiple proteins and associated pigments that each may absorb light energy and, thus, become excited. This energy is transferred from one pigment molecule to another until eventually (after about a millionth of a second) it is delivered to the reaction center. Up to this point, only energy\u2014not electrons\u2014has been transferred between molecules. The <strong>reaction center<\/strong> contains a pigment molecule that can undergo oxidation upon excitation, actually giving up an electron. It is at this step in <strong>photosynthesis<\/strong> that light energy is converted into an excited electron.\r\n\r\nDifferent kinds of light-harvesting pigments absorb unique patterns of wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear the corresponding color. Examples of photosynthetic pigments (molecules used to absorb solar energy) are <strong>bacteriochlorophylls<\/strong> (green, purple, or red), <strong>carotenoids<\/strong> (orange, red, or yellow), <strong>chlorophylls<\/strong> (green), <strong>phycocyanins<\/strong> (blue), and <strong>phycoerythrins<\/strong> (red). By having mixtures of pigments, an organism can absorb energy from more wavelengths. Because photosynthetic bacteria commonly grow in competition for sunlight, each type of photosynthetic bacteria is optimized for harvesting the wavelengths of light to which it is commonly exposed, leading to stratification of microbial communities in aquatic and soil ecosystems by light quality and penetration.\r\n\r\nOnce the light harvesting complex transfers the energy to the reaction center, the reaction center delivers its high-energy electrons, one by one, to an electron carrier in an <strong>electron transport system<\/strong>, and electron transfer through the <strong>ETS<\/strong> is initiated. The ETS is similar to that used in <strong>cellular respiration<\/strong> and is embedded within the photosynthetic membrane. Ultimately, the electron is used to produce <strong>NADH<\/strong> or <strong>NADPH<\/strong>. The <strong>electrochemical gradient<\/strong> that forms across the photosynthetic membrane is used to generate <strong>ATP<\/strong> by chemiosmosis through the process of <strong>photophosphorylation<\/strong>, another example of <strong>oxidative phosphorylation<\/strong>.\r\n<h2>Oxygenic and Anoxygenic Photosynthesis<\/h2>\r\nFor photosynthesis to continue, the electron lost from the reaction center pigment must be replaced. The source of this electron (H<sub>2<\/sub>A) differentiates the <strong>oxygenic photosynthesis<\/strong> of plants and cyanobacteria from <strong>anoxygenic photosynthesis<\/strong> carried out by other types of bacterial phototrophs (Figure 3). In oxygenic photosynthesis, H<sub>2<\/sub>O is split and supplies the electron to the reaction center. Because oxygen is generated as a byproduct and is released, this type of photosynthesis is referred to as oxygenic photosynthesis. However, when other reduced compounds serve as the electron donor, oxygen is not generated; these types of photosynthesis are called anoxygenic photosynthesis. Hydrogen sulfide (H<sub>2<\/sub>S) or thiosulfate [latex]\\left({\\text{S}}_{2}\\text{O}_{3}^{2-}\\right)[\/latex] can serve as the electron donor, generating elemental sulfur and sulfate [latex]\\left({\\text{SO}}_{4}^{2-}\\right)[\/latex] ions, respectively, as a result.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"700\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03164147\/OSC_Microbio_08_06_PhotoEquat.jpg\" alt=\"In oxygenic photosynthesis 6 carbon dioxide 12 water and light energy is converted to glucose, 6 oxygen, and 6 water. In anoxygenic photosynthesis carbon dioxide, 2H2A and light energy is converted to a carbohydrate and water. H2A = water, H2S, H2, or other electron donor.\" width=\"700\" height=\"416\" \/> Figure 3. Eukaryotes and cyanobacteria carry out oxygenic photosynthesis, producing oxygen, whereas other bacteria carry out anoxygenic photosynthesis, which does not produce oxygen.[\/caption]\r\n\r\nPhotosystems have been classified into two types: <strong>photosystem I (PSI)<\/strong> and <strong>photosystem II (PSII)<\/strong>\u00a0. Cyanobacteria and plant chloroplasts have both photosystems, whereas anoxygenic photosynthetic bacteria use only one of the photosystems. Both photosystems are excited by light energy simultaneously. If the cell requires both ATP and NADPH for biosynthesis, then it will carry out <strong>noncyclic photophosphorylation<\/strong>. Upon passing of the PSII reaction center electron to the ETS that connects PSII and PSI, the lost electron from the PSII reaction center is replaced by the splitting of water. The excited PSI reaction center electron is used to reduce NADP<sup>+<\/sup> to NADPH and is replaced by the electron exiting the ETS. The flow of electrons in this way is called the <strong>Z-scheme<\/strong>.\r\n\r\nIf a cell\u2019s need for ATP is significantly greater than its need for NADPH, it may bypass the production of reducing power through <strong>cyclic photophosphorylation<\/strong>. Only PSI is used during cyclic photophosphorylation; the high-energy electron of the PSI reaction center is passed to an ETS carrier and then ultimately returns to the oxidized PSI reaction center pigment, thereby reducing it.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>Why would a photosynthetic bacterium have different pigments?<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Light-Independent Reactions<\/h2>\r\nAfter the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules (having lifespans of millionths of a second), photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term energy storage. The carbon comes from CO<sub>2<\/sub>, the gas that is a waste product of cellular respiration.\r\n\r\nThe <strong>Calvin-Benson cycle<\/strong> (named for Melvin Calvin [1911\u20131997] and Andrew Benson [1917\u20132015]), the biochemical pathway used for fixation of CO<sub>2<\/sub>, is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts. The <strong>light-independent reactions<\/strong> of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration (see <a href=\".\/chapter\/metabolic-pathways\/\" target=\"_blank\" rel=\"noopener\">Metabolic Pathways<\/a>\u00a0for a detailed illustration of the Calvin cycle).\r\n<ul>\r\n \t<li><strong>Fixation<\/strong>: The enzyme <strong>ribulose bisphosphate carboxylase (RuBisCO)<\/strong> catalyzes the addition of a CO<sub>2<\/sub> to <strong>ribulose bisphosphate (RuBP)<\/strong>. This results in the production of <strong>3-phosphoglycerate (3-PGA)<\/strong>.<\/li>\r\n \t<li><strong>Reduction<\/strong>: Six molecules of both ATP and NADPH (from the light-dependent reactions) are used to convert 3-PGA into glyceraldehyde 3-phosphate (G3P). Some G3P is then used to build glucose.<\/li>\r\n \t<li><strong>Regeneration<\/strong>: The remaining G3P not used to synthesize glucose is used to regenerate RuBP, enabling the system to continue CO<sub>2<\/sub> fixation. Three more molecules of ATP are used in these regeneration reactions.<\/li>\r\n<\/ul>\r\nThe Calvin cycle is used extensively by plants and photoautotrophic bacteria, and the enzyme RuBisCO is said to be the most plentiful enzyme on earth, composing 30%\u201350% of the total soluble protein in plant chloroplasts.[footnote]A. Dhingra et al. \"Enhanced Translation of a Chloroplast-Expressed <em>Rbc<\/em>S Gene Restores Small Subunit Levels and Photosynthesis in Nuclear <em>Rbc<\/em>S Antisense Plants.\" <em>Proceedings of the National Academy of Sciences of the United States of America<\/em> 101 no. 16 (2004):6315\u20136320.[\/footnote] However, besides its prevalent use in photoautotrophs, the Calvin cycle is also used by many nonphotosynthetic chemoautotrophs to fix CO<sub>2<\/sub>. Additionally, other bacteria and archaea use alternative systems for CO<sub>2<\/sub> fixation. Although most bacteria using Calvin cycle alternatives are chemoautotrophic, certain green sulfur photoautotrophic bacteria have been also shown to use an alternative CO<sub>2<\/sub> fixation pathway.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Concepts and Summary<\/h3>\r\n<ul>\r\n \t<li>Heterotrophs depend on the carbohydrates produced by autotrophs, many of which are photosynthetic, converting solar energy into chemical energy.<\/li>\r\n \t<li>Different photosynthetic organisms use different mixtures of <strong>photosynthetic pigments<\/strong>, which increase the range of the wavelengths of light an organism can absorb.<\/li>\r\n \t<li><strong>Photosystems<\/strong> (PSI and PSII) each contain a <strong>light-harvesting complex<\/strong>, composed of multiple proteins and associated pigments that absorb light energy. The <strong>light-dependent reactions<\/strong> of photosynthesis convert solar energy into chemical energy, producing ATP and NADPH or NADH to temporarily store this energy.<\/li>\r\n \t<li>In <strong>oxygenic photosynthesis<\/strong>, H<sub>2<\/sub>O serves as the electron donor to replace the reaction center electron, and oxygen is formed as a byproduct. In <strong>anoxygenic photosynthesis<\/strong>, other reduced molecules like H<sub>2<\/sub>S or thiosulfate may be used as the electron donor; as such, oxygen is not formed as a byproduct.<\/li>\r\n \t<li><strong>Noncyclic photophosphorylation<\/strong> is used in oxygenic photosynthesis when there is a need for both ATP and NADPH production. If a cell\u2019s needs for ATP outweigh its needs for NADPH, then it may carry out <strong>cyclic photophosphorylation<\/strong> instead, producing only ATP.<\/li>\r\n \t<li>The <strong>light-independent reactions<\/strong> of photosynthesis use the ATP and NADPH from the light-dependent reactions to fix CO<sub>2<\/sub> into organic sugar molecules.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Multiple Choice<\/h3>\r\nDuring the light-dependent reactions, which molecule loses an electron?\r\n<ol style=\"list-style-type: lower-alpha\">\r\n \t<li>a light-harvesting pigment molecule<\/li>\r\n \t<li>a reaction center pigment molecule<\/li>\r\n \t<li>NADPH<\/li>\r\n \t<li>3-phosphoglycerate<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"661577\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"661577\"]Answer b. During the light-dependent reactions,\u00a0a reaction center pigment molecule loses an electron.[\/hidden-answer]\r\n\r\nIn prokaryotes, in which direction are hydrogen ions pumped by the electron transport system of photosynthetic membranes?\r\n<ol style=\"list-style-type: lower-alpha\">\r\n \t<li>to the outside of the plasma membrane<\/li>\r\n \t<li>to the inside (cytoplasm) of the cell<\/li>\r\n \t<li>to the stroma<\/li>\r\n \t<li>to the intermembrane space of the chloroplast<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"786038\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"786038\"]Answer b. In prokaryotes, hydrogen ions are pumped by the electron transport system of photosynthetic membranes toward\u00a0the inside (cytoplasm) of the cell.[\/hidden-answer]\r\n\r\nWhich of the following does not occur during cyclic photophosphorylation in cyanobacteria?\r\n<ol style=\"list-style-type: lower-alpha\">\r\n \t<li>electron transport through an ETS<\/li>\r\n \t<li>photosystem I use<\/li>\r\n \t<li>ATP synthesis<\/li>\r\n \t<li>NADPH formation<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"198741\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"198741\"]Answer d. NADPH formation\u00a0does not occur during cyclic photophosphorylation in cyanobacteria.[\/hidden-answer]\r\n\r\nWhich are two products of the light-dependent reactions?\r\n<ol style=\"list-style-type: lower-alpha\">\r\n \t<li>glucose and NADPH<\/li>\r\n \t<li>NADPH and ATP<\/li>\r\n \t<li>glyceraldehyde 3-phosphate and CO<sub>2<\/sub><\/li>\r\n \t<li>glucose and oxygen<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"464234\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"464234\"]Answer b. The two products of the light-dependent reactions are NADPH and ATP.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>True\/False<\/h3>\r\nPhotosynthesis always results in the formation of oxygen.\r\n\r\n[reveal-answer q=\"253450\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"253450\"]False[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Fill in the Blank<\/h3>\r\nThe enzyme responsible for CO<sub>2<\/sub> fixation during the Calvin cycle is called ________.\r\n\r\n[reveal-answer q=\"634759\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"634759\"]The enzyme responsible for CO<sub>2<\/sub> fixation during the Calvin cycle is called <strong>ribulose bisphosphate carboxylase (RuBisCO)<\/strong>.[\/hidden-answer]\r\n\r\nThe types of pigment molecules found in plants, algae, and cyanobacteria are ________ and ________.\r\n\r\n[reveal-answer q=\"618857\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"618857\"]The types of pigment molecules found in plants, algae, and cyanobacteria are <strong>chlorophylls<\/strong> and <strong>carotenoids<\/strong>.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Short Answer<\/h3>\r\n<ol>\r\n \t<li>Why would an organism perform cyclic phosphorylation instead of noncyclic phosphorylation?<\/li>\r\n \t<li>What is the function of photosynthetic pigments in the light-harvesting complex?<\/li>\r\n \t<li>Is life dependent on the carbon fixation that occurs during the light-independent reactions of photosynthesis? Explain.<\/li>\r\n<\/ol>\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>Describe the function and locations of photosynthetic pigments in eukaryotes and prokaryotes<\/li>\n<li>Describe the major products of the light-dependent and light-independent reactions<\/li>\n<li>Describe the reactions that produce glucose in a photosynthetic cell<\/li>\n<li>Compare and contrast cyclic and noncyclic photophosphorylation<\/li>\n<\/ul>\n<\/div>\n<p>Heterotrophic organisms ranging from <em>E. coli<\/em> to humans rely on the chemical energy found mainly in carbohydrate molecules. Many of these carbohydrates are produced by <strong>photosynthesis<\/strong>, the biochemical process by which phototrophic organisms convert solar energy (sunlight) into chemical energy. Although photosynthesis is most commonly associated with plants, microbial photosynthesis is also a significant supplier of chemical energy, fueling many diverse ecosystems. In this section, we will focus on microbial photosynthesis.<\/p>\n<p>Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions (Figure\u00a01). In the <strong>light-dependent reactions<\/strong>, energy from sunlight is absorbed by pigment molecules in photosynthetic membranes and converted into stored chemical energy. In the <strong>light-independent reactions<\/strong>, the chemical energy produced by the light-dependent reactions is used to drive the assembly of sugar molecules using CO<sub>2<\/sub>; however, these reactions are still light dependent because the products of the light-dependent reactions necessary for driving them are short-lived. The light-dependent reactions produce ATP and either NADPH or NADH to temporarily store energy. These energy carriers are used in the light-independent reactions to drive the energetically unfavorable process of &#8220;fixing&#8221; inorganic CO<sub>2<\/sub> in an organic form, sugar.<\/p>\n<div style=\"width: 660px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03164139\/OSC_Microbio_08_06_LightDepIn.jpg\" alt=\"Diagram of photosynthesis showing a chloroplast divided into the light-dependent reactions and CO2 fixation. There is an outer membrane, an inner membrane and a stack of membranes labeled granum (these are photosynthetic membranes). Light strikes the granum and H2A is converted to \u00bd A. This process produces ATP + NADPH\/NADH that is used in the CO2 fixation cycle. This cycle uses CO2 to produce organics. The CO2 cycle also produces ADP + Pi and NADP+ \/ NAD+ which are then used in the light-dependent reaction.\" width=\"650\" height=\"507\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a01. The light-dependent reactions of photosynthesis (left) convert light energy into chemical energy, forming ATP and NADPH. These products are used by the light-independent reactions to fix CO<sub>2<\/sub>, producing organic carbon molecules.<\/p>\n<\/div>\n<h2>Photosynthetic Structures in Eukaryotes and Prokaryotes<\/h2>\n<p>In all <strong>phototrophic eukaryotes<\/strong>, photosynthesis takes place inside a <strong>chloroplast<\/strong>, an organelle that arose in eukaryotes by endosymbiosis of a photosynthetic bacterium (see <a href=\".\/chapter\/unique-characteristics-of-eukaryotic-cells\/\" target=\"_blank\" rel=\"noopener\">Unique Characteristics of Eukaryotic Cells<\/a>). These chloroplasts are enclosed by a double membrane with inner and outer layers. Within the chloroplast is a third membrane that forms stacked, disc-shaped photosynthetic structures called <strong>thylakoids<\/strong> (Figure\u00a02). A stack of thylakoids is called a <strong>granum<\/strong>, and the space surrounding the granum within the chloroplast is called <strong>stroma<\/strong>.<\/p>\n<div style=\"width: 760px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03164142\/OSC_Microbio_08_06_PhotoMemb.jpg\" alt=\"a) Drawing of a chloroplast, which is a bean shaped structure with an outer membrane and an inner membrane. Between these is the intermembrane space. Inside the inner membrane is an aqueous fluid called stroma and membranes (thylakoids) that form stacks called (grana). The thylakoids form disks with an inner thylakoid lumen. B) Micrograph and drawing of thyladoids which look like folded material. One of the thylakoid membranes is cleaved.\" width=\"750\" height=\"338\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a02. (a) Photosynthesis in eukaryotes takes place in chloroplasts, which contain thylakoids stacked into grana. (b) A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. (credit: scale bar data from Matt Russell.)<\/p>\n<\/div>\n<p>Photosynthetic membranes in prokaryotes, by contrast, are not organized into distinct membrane-enclosed organelles; rather, they are infolded regions of the plasma membrane. In cyanobacteria, for example, these infolded regions are also referred to as thylakoids. In either case, embedded within the thylakoid membranes or other photosynthetic bacterial membranes are <strong>photosynthetic pigment<\/strong> molecules organized into one or more photosystems, where light energy is actually converted into chemical energy.<\/p>\n<p>Photosynthetic pigments within the photosynthetic membranes are organized into <strong>photosystems<\/strong>, each of which is composed of a light-harvesting (antennae) complex and a reaction center. The <strong>light-harvesting complex<\/strong> consists of multiple proteins and associated pigments that each may absorb light energy and, thus, become excited. This energy is transferred from one pigment molecule to another until eventually (after about a millionth of a second) it is delivered to the reaction center. Up to this point, only energy\u2014not electrons\u2014has been transferred between molecules. The <strong>reaction center<\/strong> contains a pigment molecule that can undergo oxidation upon excitation, actually giving up an electron. It is at this step in <strong>photosynthesis<\/strong> that light energy is converted into an excited electron.<\/p>\n<p>Different kinds of light-harvesting pigments absorb unique patterns of wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear the corresponding color. Examples of photosynthetic pigments (molecules used to absorb solar energy) are <strong>bacteriochlorophylls<\/strong> (green, purple, or red), <strong>carotenoids<\/strong> (orange, red, or yellow), <strong>chlorophylls<\/strong> (green), <strong>phycocyanins<\/strong> (blue), and <strong>phycoerythrins<\/strong> (red). By having mixtures of pigments, an organism can absorb energy from more wavelengths. Because photosynthetic bacteria commonly grow in competition for sunlight, each type of photosynthetic bacteria is optimized for harvesting the wavelengths of light to which it is commonly exposed, leading to stratification of microbial communities in aquatic and soil ecosystems by light quality and penetration.<\/p>\n<p>Once the light harvesting complex transfers the energy to the reaction center, the reaction center delivers its high-energy electrons, one by one, to an electron carrier in an <strong>electron transport system<\/strong>, and electron transfer through the <strong>ETS<\/strong> is initiated. The ETS is similar to that used in <strong>cellular respiration<\/strong> and is embedded within the photosynthetic membrane. Ultimately, the electron is used to produce <strong>NADH<\/strong> or <strong>NADPH<\/strong>. The <strong>electrochemical gradient<\/strong> that forms across the photosynthetic membrane is used to generate <strong>ATP<\/strong> by chemiosmosis through the process of <strong>photophosphorylation<\/strong>, another example of <strong>oxidative phosphorylation<\/strong>.<\/p>\n<h2>Oxygenic and Anoxygenic Photosynthesis<\/h2>\n<p>For photosynthesis to continue, the electron lost from the reaction center pigment must be replaced. The source of this electron (H<sub>2<\/sub>A) differentiates the <strong>oxygenic photosynthesis<\/strong> of plants and cyanobacteria from <strong>anoxygenic photosynthesis<\/strong> carried out by other types of bacterial phototrophs (Figure 3). In oxygenic photosynthesis, H<sub>2<\/sub>O is split and supplies the electron to the reaction center. Because oxygen is generated as a byproduct and is released, this type of photosynthesis is referred to as oxygenic photosynthesis. However, when other reduced compounds serve as the electron donor, oxygen is not generated; these types of photosynthesis are called anoxygenic photosynthesis. Hydrogen sulfide (H<sub>2<\/sub>S) or thiosulfate [latex]\\left({\\text{S}}_{2}\\text{O}_{3}^{2-}\\right)[\/latex] can serve as the electron donor, generating elemental sulfur and sulfate [latex]\\left({\\text{SO}}_{4}^{2-}\\right)[\/latex] ions, respectively, as a result.<\/p>\n<div style=\"width: 710px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03164147\/OSC_Microbio_08_06_PhotoEquat.jpg\" alt=\"In oxygenic photosynthesis 6 carbon dioxide 12 water and light energy is converted to glucose, 6 oxygen, and 6 water. In anoxygenic photosynthesis carbon dioxide, 2H2A and light energy is converted to a carbohydrate and water. H2A = water, H2S, H2, or other electron donor.\" width=\"700\" height=\"416\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 3. Eukaryotes and cyanobacteria carry out oxygenic photosynthesis, producing oxygen, whereas other bacteria carry out anoxygenic photosynthesis, which does not produce oxygen.<\/p>\n<\/div>\n<p>Photosystems have been classified into two types: <strong>photosystem I (PSI)<\/strong> and <strong>photosystem II (PSII)<\/strong>\u00a0. Cyanobacteria and plant chloroplasts have both photosystems, whereas anoxygenic photosynthetic bacteria use only one of the photosystems. Both photosystems are excited by light energy simultaneously. If the cell requires both ATP and NADPH for biosynthesis, then it will carry out <strong>noncyclic photophosphorylation<\/strong>. Upon passing of the PSII reaction center electron to the ETS that connects PSII and PSI, the lost electron from the PSII reaction center is replaced by the splitting of water. The excited PSI reaction center electron is used to reduce NADP<sup>+<\/sup> to NADPH and is replaced by the electron exiting the ETS. The flow of electrons in this way is called the <strong>Z-scheme<\/strong>.<\/p>\n<p>If a cell\u2019s need for ATP is significantly greater than its need for NADPH, it may bypass the production of reducing power through <strong>cyclic photophosphorylation<\/strong>. Only PSI is used during cyclic photophosphorylation; the high-energy electron of the PSI reaction center is passed to an ETS carrier and then ultimately returns to the oxidized PSI reaction center pigment, thereby reducing it.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>Why would a photosynthetic bacterium have different pigments?<\/li>\n<\/ul>\n<\/div>\n<h2>Light-Independent Reactions<\/h2>\n<p>After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules (having lifespans of millionths of a second), photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term energy storage. The carbon comes from CO<sub>2<\/sub>, the gas that is a waste product of cellular respiration.<\/p>\n<p>The <strong>Calvin-Benson cycle<\/strong> (named for Melvin Calvin [1911\u20131997] and Andrew Benson [1917\u20132015]), the biochemical pathway used for fixation of CO<sub>2<\/sub>, is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts. The <strong>light-independent reactions<\/strong> of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration (see <a href=\".\/chapter\/metabolic-pathways\/\" target=\"_blank\" rel=\"noopener\">Metabolic Pathways<\/a>\u00a0for a detailed illustration of the Calvin cycle).<\/p>\n<ul>\n<li><strong>Fixation<\/strong>: The enzyme <strong>ribulose bisphosphate carboxylase (RuBisCO)<\/strong> catalyzes the addition of a CO<sub>2<\/sub> to <strong>ribulose bisphosphate (RuBP)<\/strong>. This results in the production of <strong>3-phosphoglycerate (3-PGA)<\/strong>.<\/li>\n<li><strong>Reduction<\/strong>: Six molecules of both ATP and NADPH (from the light-dependent reactions) are used to convert 3-PGA into glyceraldehyde 3-phosphate (G3P). Some G3P is then used to build glucose.<\/li>\n<li><strong>Regeneration<\/strong>: The remaining G3P not used to synthesize glucose is used to regenerate RuBP, enabling the system to continue CO<sub>2<\/sub> fixation. Three more molecules of ATP are used in these regeneration reactions.<\/li>\n<\/ul>\n<p>The Calvin cycle is used extensively by plants and photoautotrophic bacteria, and the enzyme RuBisCO is said to be the most plentiful enzyme on earth, composing 30%\u201350% of the total soluble protein in plant chloroplasts.<a class=\"footnote\" title=\"A. Dhingra et al. &quot;Enhanced Translation of a Chloroplast-Expressed RbcS Gene Restores Small Subunit Levels and Photosynthesis in Nuclear RbcS Antisense Plants.&quot; Proceedings of the National Academy of Sciences of the United States of America 101 no. 16 (2004):6315\u20136320.\" id=\"return-footnote-414-1\" href=\"#footnote-414-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a> However, besides its prevalent use in photoautotrophs, the Calvin cycle is also used by many nonphotosynthetic chemoautotrophs to fix CO<sub>2<\/sub>. Additionally, other bacteria and archaea use alternative systems for CO<sub>2<\/sub> fixation. Although most bacteria using Calvin cycle alternatives are chemoautotrophic, certain green sulfur photoautotrophic bacteria have been also shown to use an alternative CO<sub>2<\/sub> fixation pathway.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Key Concepts and Summary<\/h3>\n<ul>\n<li>Heterotrophs depend on the carbohydrates produced by autotrophs, many of which are photosynthetic, converting solar energy into chemical energy.<\/li>\n<li>Different photosynthetic organisms use different mixtures of <strong>photosynthetic pigments<\/strong>, which increase the range of the wavelengths of light an organism can absorb.<\/li>\n<li><strong>Photosystems<\/strong> (PSI and PSII) each contain a <strong>light-harvesting complex<\/strong>, composed of multiple proteins and associated pigments that absorb light energy. The <strong>light-dependent reactions<\/strong> of photosynthesis convert solar energy into chemical energy, producing ATP and NADPH or NADH to temporarily store this energy.<\/li>\n<li>In <strong>oxygenic photosynthesis<\/strong>, H<sub>2<\/sub>O serves as the electron donor to replace the reaction center electron, and oxygen is formed as a byproduct. In <strong>anoxygenic photosynthesis<\/strong>, other reduced molecules like H<sub>2<\/sub>S or thiosulfate may be used as the electron donor; as such, oxygen is not formed as a byproduct.<\/li>\n<li><strong>Noncyclic photophosphorylation<\/strong> is used in oxygenic photosynthesis when there is a need for both ATP and NADPH production. If a cell\u2019s needs for ATP outweigh its needs for NADPH, then it may carry out <strong>cyclic photophosphorylation<\/strong> instead, producing only ATP.<\/li>\n<li>The <strong>light-independent reactions<\/strong> of photosynthesis use the ATP and NADPH from the light-dependent reactions to fix CO<sub>2<\/sub> into organic sugar molecules.<\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Multiple Choice<\/h3>\n<p>During the light-dependent reactions, which molecule loses an electron?<\/p>\n<ol style=\"list-style-type: lower-alpha\">\n<li>a light-harvesting pigment molecule<\/li>\n<li>a reaction center pigment molecule<\/li>\n<li>NADPH<\/li>\n<li>3-phosphoglycerate<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q661577\">Show Answer<\/span><\/p>\n<div id=\"q661577\" class=\"hidden-answer\" style=\"display: none\">Answer b. During the light-dependent reactions,\u00a0a reaction center pigment molecule loses an electron.<\/div>\n<\/div>\n<p>In prokaryotes, in which direction are hydrogen ions pumped by the electron transport system of photosynthetic membranes?<\/p>\n<ol style=\"list-style-type: lower-alpha\">\n<li>to the outside of the plasma membrane<\/li>\n<li>to the inside (cytoplasm) of the cell<\/li>\n<li>to the stroma<\/li>\n<li>to the intermembrane space of the chloroplast<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q786038\">Show Answer<\/span><\/p>\n<div id=\"q786038\" class=\"hidden-answer\" style=\"display: none\">Answer b. In prokaryotes, hydrogen ions are pumped by the electron transport system of photosynthetic membranes toward\u00a0the inside (cytoplasm) of the cell.<\/div>\n<\/div>\n<p>Which of the following does not occur during cyclic photophosphorylation in cyanobacteria?<\/p>\n<ol style=\"list-style-type: lower-alpha\">\n<li>electron transport through an ETS<\/li>\n<li>photosystem I use<\/li>\n<li>ATP synthesis<\/li>\n<li>NADPH formation<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q198741\">Show Answer<\/span><\/p>\n<div id=\"q198741\" class=\"hidden-answer\" style=\"display: none\">Answer d. NADPH formation\u00a0does not occur during cyclic photophosphorylation in cyanobacteria.<\/div>\n<\/div>\n<p>Which are two products of the light-dependent reactions?<\/p>\n<ol style=\"list-style-type: lower-alpha\">\n<li>glucose and NADPH<\/li>\n<li>NADPH and ATP<\/li>\n<li>glyceraldehyde 3-phosphate and CO<sub>2<\/sub><\/li>\n<li>glucose and oxygen<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q464234\">Show Answer<\/span><\/p>\n<div id=\"q464234\" class=\"hidden-answer\" style=\"display: none\">Answer b. The two products of the light-dependent reactions are NADPH and ATP.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>True\/False<\/h3>\n<p>Photosynthesis always results in the formation of oxygen.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q253450\">Show Answer<\/span><\/p>\n<div id=\"q253450\" class=\"hidden-answer\" style=\"display: none\">False<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Fill in the Blank<\/h3>\n<p>The enzyme responsible for CO<sub>2<\/sub> fixation during the Calvin cycle is called ________.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q634759\">Show Answer<\/span><\/p>\n<div id=\"q634759\" class=\"hidden-answer\" style=\"display: none\">The enzyme responsible for CO<sub>2<\/sub> fixation during the Calvin cycle is called <strong>ribulose bisphosphate carboxylase (RuBisCO)<\/strong>.<\/div>\n<\/div>\n<p>The types of pigment molecules found in plants, algae, and cyanobacteria are ________ and ________.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q618857\">Show Answer<\/span><\/p>\n<div id=\"q618857\" class=\"hidden-answer\" style=\"display: none\">The types of pigment molecules found in plants, algae, and cyanobacteria are <strong>chlorophylls<\/strong> and <strong>carotenoids<\/strong>.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Short Answer<\/h3>\n<ol>\n<li>Why would an organism perform cyclic phosphorylation instead of noncyclic phosphorylation?<\/li>\n<li>What is the function of photosynthetic pigments in the light-harvesting complex?<\/li>\n<li>Is life dependent on the carbon fixation that occurs during the light-independent reactions of photosynthesis? Explain.<\/li>\n<\/ol>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-414\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>OpenStax Microbiology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\">http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-414-1\">A. Dhingra et al. \"Enhanced Translation of a Chloroplast-Expressed <em>Rbc<\/em>S Gene Restores Small Subunit Levels and Photosynthesis in Nuclear <em>Rbc<\/em>S Antisense Plants.\" <em>Proceedings of the National Academy of Sciences of the United States of America<\/em> 101 no. 16 (2004):6315\u20136320. <a href=\"#return-footnote-414-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":7,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"OpenStax Microbiology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-414","chapter","type-chapter","status-publish","hentry"],"part":384,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/414","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":7,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/414\/revisions"}],"predecessor-version":[{"id":2166,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/414\/revisions\/2166"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/parts\/384"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/414\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/media?parent=414"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapter-type?post=414"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/contributor?post=414"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/license?post=414"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}