Photosynthesis

6 Key excerpts on "Photosynthesis"

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  eBook - ePub

  Structure and Function of Plants

  • Jennifer W. MacAdam(Author)
  • 2011(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Chapter 10 Photosynthesis

  What is Photosynthesis? It is the metabolic process by which plants absorb solar radiation in the range of visible light and convert this light energy into chemical energy. There are steps called the photochemical (light) reactions of Photosynthesis (Figure 10.1 ) that produce NADPH (an electronacceptor molecule) and ATP (energy held in phosphate bonds), and further steps called the biochemical (dark) reactions (Figure 10.1 ) that utilize this NADPH and ATP to convert carbon dioxide (CO2 ) and water (H2 O) into sugars (C6 H12 O6 ); these sugars have the general formula CH2 O. These sugars are used for respiration or growth in the leaves where they are produced, they are transported for use in other parts of the plant, or they are stored for future use. These sugars form the basis of the food chain for all life.

  Light and Photosynthesis

  Within the range of wavelengths of all electromagnetic energy, the visible light used by Photosynthesis is bracketed by shorter wavelength ultraviolet (UV) and longer wavelength infrared (IR) light. UV radiation has such high energy that it damages biological molecules by knocking electrons off, thus UV radiation is called “ionizing radiation.” UV radiation is absorbed by glass, which is why you cannot get sunburn working in a greenhouse. IR radiation has too little energy to be useful in Photosynthesis. The energy of IR radiation is absorbed by cells and produces heat. IR radiation is not absorbed by glass but passes through, so a greenhouse or car will become heated by sunlight. Visible light has just the right amount of energy for Photosynthesis: enough to increase the energy level of electrons of the pigments that absorb light in the visible range but not enough to harm these molecules.

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  eBook - ePub

  Teaching Biology in Schools

  Global Research, Issues, and Trends

  • Kostas Kampourakis, Michael J. Reiss(Authors)
  • 2018(Publication Date)
  • Routledge

    (Publisher)

  The respective anabolic and catabolic biochemical processes merge changes of matter and energetic processes. In Photosynthesis, cyanobacteria, algae, and mainly green plants capture light energy and use it to synthesize energy-rich biomolecules. Organisms obtain energy-rich molecules such as glucose either through their own process of Photosynthesis, or by the intake of energy-rich molecules in food. The release of this energy in a controlled way inside cells is called respiration. Whilst the role of matter in both processes becomes concrete—as, for example, by an organism growing or losing weight—the shared energetic processes are abstract. In traditional biology education, the role of energy in these contexts is hardly made explicit and if so only marginally. But in conjunction with the introduction of new science standards and curricula in many countries, the overarching importance of energy in science has been strengthened (e.g. Germany: KMK, 2005; USA: NRC, 2012; see Table 12.1). As a disciplinary core idea energy is introduced as a basis for learning and revision of different content (e.g., Photosynthesis and respiration), aiming to stimulate the coherence and depth of understanding within each discipline, in this case the life sciences (NRC, 2012). Table 12.1 The concepts energy, Photosynthesis, and respiration (highlighted) as proposed by the Next Generation Science Standards for the life sciences through primary and secondary level (NGSS, 2013; Opitz et al., 2015) Content Area Grade Content Organization for matter and energy flow in organisms 3–5 “Food provides animals with the materials and energy they need for body repair, growth, warmth, and motion. Plants […] obtain energy from sunlight.” 6–8 “Plants use the energy from light to make sugars through Photosynthesis

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  eBook - ePub

  Plant Cells and their Organelles

  • William V. Dashek, Gurbachan S. Miglani(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  The energy used to drive this reaction is provided by the absorption of light. The molecular apparatus that performs this process—Photosynthesis—is located in thylakoid (from the Greek, “sac‐like”) membranes, which are confined within the chloroplast in plants. Photosynthesis is driven by absorption of light energy by chlorophyll (Chl). Wavelengths of light that contain levels of energy that can be accommodated by the electronic structure of Chl, primarily in the blue and red regions of the visible spectrum (photons with energy levels around 280 and 170 kJ/mol, respectively), are absorbed and generate “excited” states of Chl. The photosynthetic apparatus is designed to collect these packets of light energy and convert them to chemical compounds with sufficiently high levels of energy to drive synthesis of glucose from CO 2 and H 2 O. The major metabolically available end products that are stored in plants are starch, a polymeric form of glucose, or sucrose, a disaccharide. Another polymer of glucose is cellulose, which is metabolically available to several microorganisms but not to animals. Cellulose is used as a structural component by plants. Expanded descriptions of various aspects of Photosynthesis can be found in Raghavendra (1998), Blankenship (2002), Wise and Hoober (2006), and Rebeiz et al. (2010). Evolution of Photosynthesis Synthesis of tetrapyrroles Photosynthesis is nearly as old as life itself. Measurements of the carbon isotope content in early fossils suggest that a reaction similar to that catalyzed by the enzyme ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) fixed CO 2 into organic material. Because this reaction involves CO 2 as a reactant, CO 2 molecules containing the heavier isotopes of carbon, i.e., 13 C and 14 C, react slightly more slowly than 12 CO 2. Thus, direct incorporation of CO 2 into organic molecules, which depends upon diffusion, discriminates against the heavier isotopes

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  eBook - ePub

  Physiology and Behaviour of Plants

  • Peter Scott(Author)
  • 2013(Publication Date)
  • Wiley-Interscience

    (Publisher)

  2

  Photosynthesis: the ultimate in autotrophy

  Plants make up more than 99% of the biomass of the earth and this dominance is based upon one feature of plants – the ability to photosynthesize. Without any doubt the most important ability plants possess is the capacity to photosynthesize, converting light energy into chemical energy in the form of carbohydrate molecules. Photosynthesis is crucial for supporting the life of the plant and all other living organisms.

  Photosynthesis is remarkable not only for the facility to convert light energy into chemical energy but also for the capacity to take CO2 from the air and convert it into a six-carbon ring carbohydrate (glucose or fructose, etc.). The CO2 in the atmosphere is present largely as a result of recycling from rotting organic material. However, billions of years ago CO2 appeared in our atmosphere as a result of volcanic activity. These levels have reduced over time through carbohydrate synthesis by plants. The six-carbon carbohydrate is crucial for the survival of most living organisms today, for by the action of enzymes on glucose, compounds such as the five-carbon molecule, ribose can be made (the basis for the sugar component of DNA), together with the three- and four-carbon compounds, which are the precursors of amino acids, and repeated two-carbon units, which make up the structure of fatty acids. The six-carbon ring is central to the production of all of these molecules and the pivotal reason why plants can make these is the presence of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO). At the same time as absorbing CO2 in the atmosphere and producing carbohydrates, plants also release oxygen as a byproduct of Photosynthesis. The reduction of atmospheric CO2

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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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  eBook - ePub

  Biochemistry

  • Raymond S. Ochs(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  12 Photosynthesis

  The carbon cycle is the global interchange of CO2 between animal production and plant utilization. It is a topical issue: currently, much more CO2 is being produced than is being utilized, so this cycle is out of balance. Global warming likely originates with the 19th-century Industrial Revolution and the increased burning of fossil fuels. At that time, it was postulated that an increase in atmospheric CO2 might cause increased global temperature. A steadily rising global CO2 concentration was experimentally confirmed in the middle of the 20th century.

  Organisms that consume organic compounds and produce CO2 are auxotrophic. CO2 is converted back to organic compounds by photosynthetic organisms. Our focus in this chapter will be on plants. A second cycle involves shuttling oxygen between its production by plants and its consumption by animals.

  Photosynthesis has two natural divisions: the light reactions, which include oxygen production and photochemical events, and the carbon reactions (formerly dark reactions; see the chapter addendum) by which CO2 is converted to organic compounds, such as sugars. While we discuss them separately, both proceed simultaneously in plants.

  12.1 Light and Carbon Reactions

  The overall reaction of Photosynthesis is:

  C O 2 + H 2 O + l i g h t → C H 2 O + O 2

  (12.1)

  The unique feature of Equation (12.1) is the incorporation of light energy to drive carbohydrate production (CH2 O) from CO2 . A slightly more detailed view of this process exposes the roles of the mobile cofactors ADP, Pi , ATP, NADP+ , and NADPH:

  ADP + P i + NADP + + H 2 O → O 2 + ATP + NADPH

  (12.2)

  CO 2 + ATP + NADPH → CH 2 O + ADP + P i + NADP +

  (12.3)

  Equation (12.2) summarizes the overall light reactions, whereas Equation (12.3) summarizes the overall carbon reactions. We have already encountered ATP, ADP, and Pi, mobile cofactors that represent the transfer of phosphate bond energy. The pair of mobile cofactors, NADP+ /NADPH, was introduced in Chapter 7 as virtually identical in structure and function to NAD+

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