Light-Dependent Reaction

12 Key excerpts on "Light-Dependent Reaction"

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  eBook - ePub

  Photosynthesis

  Solar Energy for Life

  • Dmitry Shevela, Lars Olof Björn, Govindjee(Authors)
  • 2018(Publication Date)
  • WSPC

    (Publisher)

  Chapter 3

  Basics of Photosynthesis: Light-Dependent Reactions

  3.1Overview: Harvesting Sunlight to Drive Redox Chemistry

  In all photosynthetic organisms (both oxygenic and anoxygenic) the photosynthetic light reactions begin with the absorption of light (photons) by pigments in light-harvesting complexes, embedded in the thylakoid membrane (or in case of cyanobacteria, also in phycobilisomes). The antenna (both outer and inner) systems deliver the energy of absorbed light (excitation energy; we shall discuss it in Section 3.2 ) to pigment–protein reaction center complexes, Photosystems II and I (PSII and PSI; see Section 3.3 ), both embedded in the thylakoid membrane (Fig. 3.1 ). As a consequence of primary photochemistry, which takes place after trapping of the excitation energy by special photoactive chlorophyll (Chl) molecules in the reaction centers of the two photosystems, light energy is converted into chemical energy. This energy drives the redox chemistry of the stepwise linear electron “transfer” from water to the oxidized form of nicotinamide adenine dinucleotide phosphate (NADP+ ), involving PSII and PSI as well as the Cyt

  b6 f

  complex (Fig. 3.1 ). In this chapter we shall also briefly describe photosynthetic ATP production by ATP synthase from ADP and inorganic phosphate (see Section 3.3.3 ).

  3.2Capturing the Energy of Light

  The first step in photosynthesis is the absorption of light by pigment molecules which include Chl

  a

  , other Chls, or phycobilins, or fucoxanthol, depending on the organism; this step occurs within femtoseconds (one femtosecond is 10−15 s; there are as many femtoseconds in a second as there are seconds in 31.54 million years!). This event means that a photon disappears, and the energy of the molecule increases, the pigment molecule is in an excited state. We need to consider only two kinds of energy of the molecule here: the electronic energy and the vibrational energy, and both are changed when the photon is absorbed, as a consequence of the so-called

  Franck-Condon principle

  : Upon absorption of a photon, the electronic transition from the ground state to the excited state occurs without a change in the position of the nuclei, because “the electrons are light and the nuclei are heavy,” and the molecule goes to a higher vibrational state (see Fig. 3.2 , and the discussion that follows; for Franck-Condon principle, see Rabinowitch and Govindjee [1969], and Atkins and Friedman [1999]). In addition, we have “vibronic” energy, which is a combination of “vibrational” and “electronic” energy: the “vibronic energy coupling” refers to the interaction between electronic and nuclear vibrational energy. This interaction, indeed, makes the light harvesting more efficient in several cases [Dean

  et al.

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  Concepts of Biology

  • Samantha Fowler, Rebecca Roush, James Wise(Authors)
  • 2016(Publication Date)
  • Openstax

    (Publisher)

  Each pigment can absorb different wavelengths of light, which allows the plant to absorb any light that passes through the taller trees. (credit: Jason Hollinger) How Light-Dependent Reactions Work The overall purpose of the Light-Dependent Reactions is to convert light energy into chemical energy. This chemical energy will be used by the Calvin cycle to fuel the assembly of sugar molecules. The Light-Dependent Reactions begin in a grouping of pigment molecules and proteins called a photosystem. Photosystems exist in the membranes of thylakoids. A pigment molecule in the photosystem absorbs one photon, a quantity or “packet” of light energy, at a time. A photon of light energy travels until it reaches a molecule of chlorophyll. The photon causes an electron in the chlorophyll to become “excited.” The energy given to the electron allows it to break free from an atom of the chlorophyll molecule. Chlorophyll is therefore said to “donate” an electron (Figure 5.12). To replace the electron in the chlorophyll, a molecule of water is split. This splitting releases an electron and results in the formation of oxygen (O 2 ) and hydrogen ions (H + ) in the thylakoid space. Technically, each breaking of a water molecule releases a pair of electrons, and therefore can replace two donated electrons. 124 Chapter 5 | Photosynthesis This OpenStax book is available for free at http://cnx.org/content/col11487/1.9 Figure 5.12 Light energy is absorbed by a chlorophyll molecule and is passed along a pathway to other chlorophyll molecules. The energy culminates in a molecule of chlorophyll found in the reaction center. The energy “excites” one of its electrons enough to leave the molecule and be transferred to a nearby primary electron acceptor. A molecule of water splits to release an electron, which is needed to replace the one donated. Oxygen and hydrogen ions are also formed from the splitting of water. The replacing of the electron enables chlorophyll to respond to another photon.

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  Cambridge O Level Biology 5090

  • Azhar ul Haque Sario(Author)
  • 2023(Publication Date)
  • tredition

    (Publisher)

  Plant nutrition

  Photosynthesis

  Photosynthesis is a fascinating and intricate process that forms the foundation of life on Earth. It is the method by which plants, some bacteria, and algae transform light energy into chemical energy, creating carbohydrates from basic raw materials. This complex process is critical not only for the survival of plants but also for all animals, including humans, as it contributes to the oxygen we breathe and the food we consume.

  1. The Basics of Photosynthesis:

  The process of photosynthesis primarily occurs in the leaves of plants, within specialized cells called mesophyll cells. These cells contain chloroplasts, which house chlorophyll, a green pigment crucial for absorbing sunlight. The process begins when chlorophyll absorbs light, primarily from the blue and red spectrum, and uses this energy to drive the synthesis of carbohydrates.

  2. The Two Stages of Photosynthesis:

  Light-Dependent Reactions: These reactions require sunlight. When sunlight strikes the chlorophyll molecules, it excites the electrons, leading to a chain of reactions that ultimately produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), both energy-rich molecules. Water molecules are split during these reactions, releasing oxygen as a byproduct.

  Calvin Cycle (Light-Independent Reactions): This stage does not require sunlight directly but uses the ATP and NADPH generated in the Light-Dependent Reactions. The Calvin cycle occurs in the stroma, the fluid area surrounding the thylakoids in the chloroplasts. Here, carbon dioxide, which plants take in from the atmosphere, is fixed into glucose using the energy from ATP and NADPH. This cycle consists of a series of biochemical reactions and ultimately results in the production of glucose, a simple sugar.

  3. Factors Influencing Photosynthesis:

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  The Chemical Reactions of Life

  From Metabolism to Photosynthesis

  • Britannica Educational Publishing, Kara Rogers(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  CHAPTER 8The Process of Photosynthesis

  P hotosynthesis consists of a number of photochemical and enzymatic reactions and occurs in two stages. During the light-dependent stage, chlorophyll absorbs light energy, which excites some electrons in the pigment molecules to higher energy levels. These electrons leave the chlorophyll and pass along a series of molecules, generating formation of NADPH and high-energy ATP molecules. Oxygen, released as a by-product, passes into the atmosphere through pores in the leaves. NADPH and ATP drive the second stage, the dark reaction (or Calvin cycle, discovered by Melvin Calvin), which does not require light. During this stage glucose is generated using atmospheric carbon dioxide.

  THE LIGHT REACTIONS

  The process of photosynthesis begins with a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules. Following light absorption and the excitation of chlorophyll molecules, electrons are transferred along pigment molecules until they reach quinone. This process of electron transfer, similar to that occurring in mitochondria, ultimately results in the conversion of light energy to ATP.

  LIGHT ABSORPTION AND ENERGY TRANSFER

  The light energy absorbed by a chlorophyll molecule excites some electrons within the structure of the molecule to higher energy levels, or excited states. Light of shorter wavelength (such as blue) has more energy than light of longer wavelength (such as red), so that absorption of blue light creates an excited state of higher energy. A molecule raised to this higher energy state quickly gives up the “extra” energy as heat and falls to its lowest excited state. This lowest excited state is similar to that of a molecule that has just absorbed the longest wavelength light capable of exciting it. In the case of chlorophyll a

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  Biology for AP® Courses

  • Julianne Zedalis, John Eggebrecht(Authors)
  • 2018(Publication Date)
  • Openstax

    (Publisher)

  By harnessing energy from the sun, the evolution of photosynthesis allowed living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today. Only certain organisms, called photoautotrophs, can perform photosynthesis; they require the presence of chlorophyll, a specialized pigment that absorbs certain portions of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a waste product into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm. 8.2 The Light-Dependent Reaction of Photosynthesis The pigments of the first part of photosynthesis, the Light-Dependent Reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a and then to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of ions. The ions flow through ATP synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions. 8.3 Using Light to Make Organic Molecules Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO 2 from the environment.

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  Botany For Dummies

  • Rene Fester Kratz(Author)
  • 2011(Publication Date)
  • For Dummies

    (Publisher)

  Photosynthesis gets more complicated when you tackle the process itself — in other words, the “how.” Photosynthesis doesn’t occur in one step as indicated by the summary reaction. Instead, it’s broken up into lots of little steps which are organized into two separate sets of reactions, called the

  light reactions and the light independent reactions, that occur in different parts of the chloroplast. Figure 7-4 presents an overview of the two parts of photosynthesis and shows how they’re connected.

  The light reactions are also called the light dependent reactions. The light independent reactions are also called the Calvin-Benson cycle, the Calvin cycle, and the Dark Reactions.

  Figure 7-4: The two halves of photosynthesis, the light reactions and the light independent reactions, are separate but linked.

  Each part of photosynthesis has a separate purpose, but they both contribute to the overall goal of storing matter and energy in sugar molecules:

  In the light reactions, plants capture light energy from the sun and transform it into chemical energy . The energy from light is transformed to chemical energy as it’s stored in the energy molecule ATP. During the light reactions, plants also take electrons from H 2 O and transfer them to the electron carrier NADPH.

  In the light independent reactions, plants capture carbon dioxide (CO 2 ) and convert it into carbohydrates. In this process, plants use electrons from NADPH and energy from ATP to reduce the CO 2 and convert it to a sugar.

  Plants make ATP and NADPH during the light reactions and then use these molecules during the light independent reactions to make carbohydrates from CO 2.

  Living things transfer and transform energy all the time. When organisms transfer energy, they move it from place to another. If you eat some noodles, you transfer chemical energy from the noodles to chemical energy in your own cells. When organisms transform

  energy, they change it from one type of energy to another. During photosynthesis, plants capture energy from the sun, which is light energy, and transform it into chemical energy in their cells. (Photosynthesis is actually a double whammy — plants transform light energy to chemical energy, and they also transfer the energy from the sun to their own cells!)

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  eBook - ePub

  Biochemistry

  • Donald Voet, Judith G. Voet(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  21 Photosynthesis: Bioinorganic Chemistry and Physiology

  1 Chloroplasts

  2 Light Reactions

  A. Absorption of Light

  B. Electron Transport in Purple Photosynthetic Bacteria

  C. Two-Center Electron Transport

  D. Photophosphorylation

  3 Dark Reactions

  A. The Calvin Cycle

  B. Control of the Calvin Cycle

  C. Photorespiration and the C4 Cycle

  Life on Earth depends on the sun. Plants and cyanobacteria chemically sequester light energy through photosynthesis, a light-driven process in which CO2 is “fixed” to yield carbohydrates (CH2 O).

  This process, in which CO2 is reduced and H2 O is oxidized to yield carbohydrates and O2 , is essentially the reverse of oxidative carbohydrate metabolism. ­Photosynthetically produced carbohydrates therefore serve as an energy source for the organism that produced them as well as for nonphotosynthetic organisms that directly or indirectly consume photosynthetic organisms. In fact, even modern industry is highly dependent on the products of photosynthesis because coal, oil, and gas (the so-called fossil fuels) are thought to be the remains of ancient organisms. It is estimated that photosynthesis annually fixes ∼1011 tons of carbon, which represents the storage of over 1018 kJ of energy. Moreover, photosynthesis, over the eons, has produced the O2 in Earth's atmosphere (Section 1-1C ).

  The notion that plants obtain nourishment from such insubstantial things as light and air took nearly two centuries to develop. In 1648, the Flemish physician Jean Baptiste van Helmont reported that growing a potted willow tree from a shoot caused an insignificant change in the weight of the soil in which the tree had been rooted. Although another century was to pass before the law of conservation of matter was formulated, van Helmont attributed the tree's weight gain to the water it had taken up. This idea was extended in 1727 by Stephen Hales, who proposed that plants extract some of their matter from the air.

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  Postharvest Physiology and Biochemistry of Fruits and Vegetables

  • Elhadi M. Yahia, Armando Carrillo-Lopez(Authors)
  • 2018(Publication Date)
  • Woodhead Publishing

    (Publisher)

  As a result, phosphorylation reactions are activated, producing photosynthetic ATP and reduced coenzyme NADPH as well. This photochemical process is named the Hill reaction (Fig. 3.4), and its final products (ATP and NADPH +) are the metabolic energy necessary to incorporate atmospheric carbon (CO 2) into organic compounds derived from the Calvin cycle. Figure 3.4 Light-Dependent Reaction in photosynthesis (Hill reaction). At the first event in the Hill reaction, when chlorophyll excitation occurs, two water molecules are oxidized by four successive charge separation reactions by PSII to produce one molecule of O 2 (diatomic oxygen) and four hydrogen ions. The produced electrons are transferred to a redox active tyrosine residue which then reduces the oxidized chlorophyll a (called P680) which serves as the electron donor driven by the light in the reaction center of PSII. That photosensitive receiver is in effect restored and is then capable of repeating the absorption of another photon and the release of another photo-dissociated electron. The oxidation of water is catalyzed in PSII by an active redox structure containing four manganese ions and one calcium ion. This oxygen-avoiding complex binds to two molecules of water and contains the four oxidizing equivalents that are used to drive the oxidation reaction of water. PSII is the only biological multisubunit enzyme known to carry out this oxidation of water. The released hydrogen ions contribute to the transmembrane chemiosmotic potential leading to ATP synthesis in a process called photophosphorylation. 3.4.1 Cyclic and Noncyclic Photophosphorylation Photophosphorylation is the conversion of ADP to ATP using the energy of sunlight by activation of PSII. This involves the splitting of the water molecule in oxygen and hydrogen protons (H +), a process known as photolysis. Subsequently, a continuous unidirectional flow of electron from water to PSI is performed (Fig. 3.5)

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  eBook - ePub

  Plant Biochemistry

  • P. M. Dey, J. B. Harborne(Authors)
  • 1997(Publication Date)
  • Academic Press

    (Publisher)

  In plants, photosynthesis occurs primarily in leaf cells in organelles called chloroplasts, which are about 5 μm long and bound by two membrane envelopes. The number of chloroplasts per leaf cell varies from 1 to over 100, depending on cell type, species and growth conditions.

  The utilization of light energy to drive the synthesis of NADPH and ATP takes place in a complex system of membrane-enclosed sacs within the chloroplast referred to as thylakoid membranes, and the reactions involved in CO2 fixation and reduction to sugar are catalyzed by soluble enzymes in the chloroplast matrix referred to as the stroma.

  The first part of this chapter will address the current state of knowledge on the structure, function and regulation of the thylakoidal light energy-conversion apparatus and the second part will deal with the stromal reactions involved in CO2 fixation and reduction. For excellent introductions to these topics, the reader is referred to a number of standard undergraduate biochemistry teaching texts (Stryer, 1995 ; Zubay, 1993 ), and to Nicholls & Ferguson (1992) and Ho (1995) for more advanced reading.

  2.2 LIGHT ENERGY UTILIZATION TO PRODUCE ATP AND NADPH

  2.2.1 Introduction and background information

  The principal light-absorbing pigments in the thylakoid membranes are the cyclic tetrapyrrole derivatives chlorophylls (Chl) a and b. Their structures are shown in Fig. 2.1 and absorption spectra in Fig. 2.2 . Carotenoids (linear polyenes) are also important light-absorbing pigments found in the thylakoid membranes. Structures of the major carotenoids found in plants are shown in Chapter 11 . Chla performs two functions, in light harvesting and in photochemistry. These processes take place in integral membrane protein complexes referred to as light-harvesting (or antenna) complexes and reaction centers respectively.

  Figure 2.1 Structure of Chla . The dotted lines indicate the extent of the delocalized π orbital system which is responsible for the absorption of visible light. Chlb differs from Chla in having a formyl (-CHO) group instead of methyl (-CH3 ) on ring II. Modified from Lawlor (1993).

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  Photophysiology

  General Principles; Action of Light on Plants

  • Arthur C. Giese(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 6 PHYSICAL ASPECTS OF THE LIGHT REACTION IN PHOTOSYNTHESIS Roderick K. Clayton Biology Division, Oak Ridge National Laboratory, 1 Oak Ridge, Tennessee 1. Introduction The central physical problem of photosynthesis is concerned with the manner in which light energy is absorbed by chlorophyll, transmitted to a photochemical site, and converted to chemical energy. Recent years have seen a great proliferation of mechanisms, some conceptual and some demonstrated, which could be important for these primary events. The problem is to learn what does take place. It is therefore essential to consider what restrictions are imposed by existing knowledge, much of which is biochemical. To this end we shall first examine some biochemical aspects of photosynthesis, as related to photochemical systems and reaction centers. It will then be possible to formulate the biophysical problems with some clarity. The next step will be to survey the physical and chemical properties of molecules and molecular aggregates of chlorophyll and to see which of these properties are exhibited in vivo. The hypotheses that can be entertained will then be self-evident; a brief evaluation of these will bring this chapter to a close. 2. The Biophysical Problem Delineated 2.1 Biochemical Outlines Photosynthesis in green plants and algae can be defined in terms of three consecutive processes: 1. The energy of light quanta affords a separation of oxidizing and reducing entities; chlorophyll mediates this primary photochemical process. 1 Operated by Union Carbide Corporation for the United States Atomic Energy Commission. 155 156 RODERICK K. CLAYTON 2. The primary oxidizing and reducing entities provide starting points for a variety of electron-transfer reactions. As a result, chemical bond energy is stored, high-potential reducing substances are generated, and oxygen is released from water.

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  Energetics of Photosynthesis

  • Govindjee, Unknown Govindjee(Authors)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  4. S U M M A R Y Photosynthesis is an oxidation-reduction process in which light energy is converted into chemical energy. In green plants, this amounts to the reduction of C 0 2 into ( C H 2 0 ) and the oxidation of H 2 0 to molecular 0 2 . In photo-synthetic bacteria various H donors replace H 2 0 , and no 0 2 is evolved. Several hundred pigment molecules (e.g., Chi b and Chi a in a green plants, or BChl in photosynthetic bacteria) somehow cooperate to perform photo-synthesis. This collection of pigment molecules, along with other necessary 40 Govindjee and Rajni Govindjee components, is called a photosynthetic unit (PSU). In green plants, there are two pigment systems, and thus two types of PSU's. Each PSU has its own energy trap or reaction center (P700 in PS I and P680 in PS II). Certain photo-synthetic bacteria possess at least one type of reaction center, labeled P870. Light energy absorbed by any of the pigment molecules in a PSU is probably transferred, as an exciton to the reaction centers where the primary light reaction occurs. The primary light reaction is essentially the oxidation of the reaction center and the reduction of a primary electron acceptor. In green plants, one light reaction (I) leads to the oxidation of a cytochrome (f) and reduction of pyridine nucleotide. The other light reaction leads to the oxidation of H 2 0 to molecular 0 2 and reduction of certain intermediate(s). The transfer of electrons from the latter to Cyt f completes the chain ; this step is exergonic and is coupled to ATP production (noncyclic phosphorylation). A cyclic flow of electrons around reaction I can also lead to ATP production (cyclic photophosphorylation). ATP and reduced pyridine nucleotide pro-duced by the two light reactions are sufficient to run the carbon fixation cycle that leads to the production of organic matter ( C H 2 0 ) from C 0 2 .

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  Light

  Physical and Biological Action

  • Howard H. Seliger, William D. McElroy(Authors)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  In making use of the free energy of sunlight, the green plant is able to fix carbon dioxide and split water to evolve oxygen. Through a series of reactions which are reasonably well known, carbohydrates and other molecules are synthesized. Thus light energy serves indirectly as a source of free energy for all living things. The immediate end products of metabolism are carbon dioxide and water. Consequently the combination of photosynthetic organisms and heterotrophic organisms gives us a biological machine which continually turns over C0 2 and H 2 0 on the surface of the earth. It has been calcu-lated that each carbon dioxide molecule in the atmosphere is fixed and incorporated into some plant structure once in every 200 years and that all the oxygen is renewed by plants every 2000 years. During the past 15 years considerable progress has been made in understanding the proc-esses involved in carbon dioxide reduction and fixation in green plants as well as the primary photochemical events. It is not possible for us to review all of the pertinent data concerning the physiological, biophysical, and biochemical investigations on photosynthesis, and consequently the reader is referred to the extensive reviews and monographs: Rabinowitch (1945, 1951, 1956), Bassham (1963), Gaffron (1960), Jagendorf (1962), Whatley and Losada (1964), Clayton (1964) and Blinks (1964). Photosynthesis in green plants occurs in the chloroplast which consists of a lamellar phase imbedded in a matrix (see Fig. 5.9), surrounded by a membrane. The lamellar structures can be separated from the matrix and recent Studies by Park and Pon (1961, 1962), Lichtenthaler and Park (1963) and Park and Biggins (1964) indicate that it is made up of subunits which Calvin (1962) has called quantasomes. Aggregates of 6 to 7 quantasomes are capable of performing the light reactions and the asso-ciated electron transport processes of photosynthesis. Recently, Lieh-

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