1、,CHAPTER 10 PHOTOSYNTHESIS,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section A: Photosynthesis in Nature,1. Plants and other autotrophs are the producers of the biosphere 2. Chloroplasts are the site of photosynthesis in plants,Life on Earth is solar powered. The chloro
2、plasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules.,Introduction,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Photosynthesis nourishes almost all of the li
3、ving world directly or indirectly. All organisms require organic compounds for energy and for carbon skeletons. Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment. Autotrophs are the ultimate sources of organic compounds for all nonaut
4、otrophic organisms. Autotrophs are the producers of the biosphere.,1. Plants and other autotrophs are the producers of the biosphere,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Autotrophs can be separated by the source of energy that drives their metabolism. Photoautotrop
5、hs use light as the energy source. Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes. Chemoautotrophs harvest energy from oxidizing inorganic substances, including sulfur and ammonia. Chemoautotrophy is unique to bacteria.,Copyright 2002 Pearson Education, Inc., publi
6、shing as Benjamin Cummings,Fig. 9.1,Heterotrophs live on organic compounds produced by other organisms. These organisms are the consumers of the biosphere. The most obvious type of heterotrophs feed on plants and other animals. Other heterotrophs decompose and feed on dead organisms and on organic l
7、itter, like feces and fallen leaves. Almost all heterotrophs are completely dependent on photoautotrophs for food and for oxygen, a byproduct of photosynthesis.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Any green part of a plant has chloroplasts. However, the leaves are
8、 the major site of photosynthesis for most plants. There are about half a million chloroplasts per square millimeter of leaf surface. The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts. Chlorophyll plays an important role in the absorption of light energy during photos
9、ynthesis.,2. Chloroplasts are the sites of photosynthesis in plants,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf. O2 exits and CO2 enters the leaf through microscopic pores, st
10、omata, in the leaf. Veins deliver water from the roots and carry off sugar from mesophyll cells to other plant areas.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.2,A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long. Each ch
11、loroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. These have an internal aqueous space, the thylakoid lumen or thylakoid space. Thylakoids may be stacked into columns called grana.,Copyright 2002 Pearson Education, Inc., publis
12、hing as Benjamin Cummings,Fig. 10.2,CHAPTER 10 PHOTOSYNTHESIS,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Section A1: The Pathways of Photosynthesis,1. Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis 2. The light
13、reaction and the Calvin cycle cooperate in converting light energy to the chemical energy of food: an overview 3. The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look,Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and
14、H2O. Using glucose as our target product, the equation describing the net process of photosynthesis is: 6CO2 + 6H2O + light energy - C6H12O6 + 6O2 In reality, photosynthesis adds one CO2 at a time: CO2 + H2O + light energy - CH2O + O2 CH2O represents the general formula for a sugar.,1. Evidence that
15、 chloroplasts split water molecules enabled researchers to track atoms through photosynthesis,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not
16、CO2. Before the 1930s, the prevailing hypothesis was that photosynthesis occurred in two steps: Step 1: CO2 - C + O2 and Step 2: C + H2O - CH2O C.B. van Niel challenged this hypothesis. In the bacteria that he was studying, hydrogen sulfide (H2S), not water, is used in photosynthesis. They produce y
17、ellow globules of sulfur as a waste. Van Niel proposed this reaction: CO2 + 2H2S - CH2O + H2O + 2S,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis. CO2 + 2H2O - CH2O + H2O + O2
18、 Other scientists confirmed van Niels hypothesis. They used 18O, a heavy isotope, as a tracer. They could label either CO2 or H2O. They found that the 18O label only appeared if water was the source of the tracer. Essentially, hydrogen extracted from water is incorporated into sugar and the oxygen r
19、eleased to the atmosphere (where it will be used in respiration).,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Photosynthesis is a redox reaction. It reverses the direction of electron flow in respiration. Water is split and electrons transferred with H+ from water to CO2,
20、 reducing it to sugar. Polar covalent bonds (unequal sharing) are converted to nonpolar covalent bonds (equal sharing). Light boosts the potential energy of electrons as they move from water to sugar.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.3,Photosynthesis is
21、 two processes, each with multiple stages. The light reactions convert solar energy to chemical energy. The Calvin cycle incorporates CO2 from the atmosphere into an organic molecule and uses energy from the light reaction to reduce the new carbon piece to sugar.,2. The light reactions and the Calvi
22、n cycle cooperate in converting light energy to chemical energy of food: an overview,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,In the light reaction light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP+
23、(nicotinamide adenine dinucleotide phosphate), forming NADPH. NADPH, an electron acceptor, provides energized electrons, reducing power, to the Calvin cycle. The light reaction also generates ATP by photophosphorylation for the Calvin cycle.,Copyright 2002 Pearson Education, Inc., publishing as Benj
24、amin Cummings,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.4,The Calvin cycle is named for Melvin Calvin who, with his colleagues, worked out many of its steps in the 1940s. It begins with the incorporation of CO2 into an organic molecule via carbon fixation. This
25、new piece of carbon backbone is reduced with electrons provided by NADPH. ATP from the light reaction also powers parts of the Calvin cycle. While the light reactions occur at the thylakoids, the Calvin cycle occurs in the stroma.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummin
26、gs,The thylakoids convert light energy into the chemical energy of ATP and NADPH. Light, like other form of electromagnetic energy, travels in rhythmic waves. The distance between crests of electromagnetic waves is called the wavelength. Wavelengths of electromagnetic radiation range from less than
27、a nanometer (gamma rays) to over a kilometer (radio waves).,3. The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,The entire range of electromagnetic radiation is the electromagnetic
28、spectrum. The most important segment for life is a narrow band between 380 to 750 nm, visible light.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.5,While light travels as a wave, many of its properties are those of a discrete particle, the photon. Photons are not t
29、angible objects, but they do have fixed quantities of energy. The amount of energy packaged in a photon is inversely related to its wavelength. Photons with shorter wavelengths pack more energy. While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavel
30、engths, permitting only visible light to pass in significant quantities.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,When light meets matter, it may be reflected, transmitted, or absorbed. Different pigments absorb photons of different wavelengths. A leaf looks green beca
31、use chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.6,A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light. It beams n
32、arrow wavelengths of light through a solution containing a pigment and measures the fraction of light transmitted at each wavelength. An absorption spectrum plots a pigments light absorption versus wavelength.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.7,The ligh
33、t reaction can perform work with those wavelengths of light that are absorbed. In the thylakoid are several pigments that differ in their absorption spectrum. Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green. Other pigments with different stru
34、ctures have different absorption spectra.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.8a,Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis. An action spectrum measures changes in some measure of photosynthetic acti
35、vity (for example, O2 release) as the wavelength is varied.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.8b,The action spectrum of photosynthesis was first demonstrated in 1883 by an elegant experiment by Thomas Engelmann. In this experiment, different segments of
36、a filamentous alga were exposed to different wavelengths of light. Areas receiving wavelengths favorable to photosynthesis should produce excess O2. Engelmann used the abundance of aerobic bacteria clustered along the alga as a measure of O2 production.,Copyright 2002 Pearson Education, Inc., publis
37、hing as Benjamin Cummings,Fig. 10.8c,The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one photosynthetic pigment, including chlorophyll a. Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and
38、 transfer energy to chlorophyll a. Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a. Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also pa
39、rticipate in photoprotection against excessive light.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,When a molecule absorbs a photon, one of that molecules electrons is elevated to an orbital with more potential energy. The electron moves from its ground state to an excited
40、 state. The only photons that a molecule can absorb are those whose energy matches exactly the energy difference between the ground state and excited state of this electron. Because this energy difference varies among atoms and molecules, a particular compound absorbs only photons corresponding to s
41、pecific wavelengths. Thus, each pigment has a unique absorption spectrum.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Photons are absorbed by clusters of pigment molecules in the thylakoid membranes. The energy of the photon is converted to the potential energy of an elec
42、tron raised from its ground state to an excited state. In chlorophyll a and b, it is an electron from magnesium in the porphyrin ring that is excited.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig.
43、10.9,Excited electrons are unstable. Generally, they drop to their ground state in a billionth of a second, releasing heat energy. Some pigments, including chlorophyll, release a photon of light, in a process called fluorescence, as well as heat.,Copyright 2002 Pearson Education, Inc., publishing as
44、 Benjamin Cummings,Fig. 10.10,In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems. A photosystem acts like a light-gathering “antenna complex” consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules.,Copyr
45、ight 2002 Pearson Education, Inc., publishing as Benjamin Cummings,Fig. 10.11,When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center. At the reaction center is a primary electron acceptor which
46、 removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Each photosystem - reaction-center chlorophyll and primary electron acceptor surrounded by an antenna complex - functions in the chloroplast as a light-harvesting unit.,Copyright 2002 Pearson Educat
47、ion, Inc., publishing as Benjamin Cummings,There are two types of photosystems. Photosystem I has a reaction center chlorophyll, the P700 center, that has an absorption peak at 700nm. Photosystem II has a reaction center with a peak at 680nm. The differences between these reaction centers (and their
48、 absorption spectra) lie not in the chlorophyll molecules, but in the proteins associated with each reaction center. These two photosystems work together to use light energy to generate ATP and NADPH.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,During the light reactions,
49、 there are two possible routes for electron flow: cyclic and noncyclic. Noncyclic electron flow, the predominant route, produces both ATP and NADPH. 1. When photosystem II absorbs light, an excited electron is captured by the primary electron acceptor, leaving the reaction center oxidized. 2. An enzyme extracts electrons from water and supplies them to the oxidized reaction center. This reaction splits water into two hydrogen ions and an oxygen atom which combines with another to form O2.,Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings,
