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Chapter 10: Photosynthesis

Chapter Outline

Chapter 10: Photosynthesis


10.0 Introduction

  1. The Process of Photosynthesis
    1. Certain Organisms Capture Energy from Sunlight fig 10.1
    2. Use Energy to Build Energy-Rich Food Molecules


10.1 Photosynthesis takes place in chloroplasts

  1. The Chloroplast as a Photosynthetic Machine
    1. Life Is Powered by Sunshine
      1. Most living cells used energy captured from the sun by plants
      2. Radiant energy from sun equal to 1 million atomic bombs
      3. Photosynthesis captures only 1% of that energy to sustain all life
    2. The Photosynthetic Process
      1. Photosynthesis occurs in bacteria, algae, green plants fig 10.2
      2. In plant leaves, photosynthesis occurs in the chloroplasts
      3. Three stages of photosynthesis
        1. Capture energy from the sun
        2. Use energy to make ATP and NADPH
        3. Use ATP and NADPH to make carbon molecules from CO2 (carbon fixation)
      4. First two processes occur in presence of light: Light-dependent reactions
      5. Third process occurs with or without light: Light-independent reactions
      6. Carbon dioxide + water + light ® glucose + water + oxygen
    3. Inside the Chloroplast
      1. Internal membranes organized into flattened sacs called thylakoids
      2. Numerous thylakoids stacked in arrangements called grana
      3. Surrounding thalakoids is semiliquid stroma
      4. Photosystem: Network of photosynthetic pigments bound to membranes in thylakoids
        1. Each pigment molecule capable of capturing photons
        2. Protein lattice holds pigments in close contact
        3. Light strikes pigment molecule, energy passes from molecule to molecule
        4. Energy reaches key molecule in reaction center, touching membrane-bound protein
        5. Energy transferred to protein, passed to series of other proteins
        6. Makes ATP. NADPH and builds organic molecules
      5. Photosystem is antenna, amplifying power of single pigment molecules to gather light


10.2 Learning about photosynthesis is an experimental journey

  1. The Role of Soil and Water
    1. van Helmont's Plant Growth Experiments
      1. Weighed tree and soil in pot
      2. Plant grew five years, only water added
      3. Plant weight gain greater than weight loss of soil
      4. Thus determined that plant substance not derived from soil
      5. Incorrectly concluded weight gain due to water
    2. The Role of Water
      1. Experiments by Priestly to determine nature of air
        1. Sprig of mint restored air in jar that a burning candle had depleted
        2. Mouse could breathe in jar after plant but not before
        3. Living vegetation added something to the air
      2. Ingenhousz reproduced and expanded Priestley's experiments
        1. Air restored only in presence of sunlight
        2. Occurred only with green plant leaves, not roots
        3. Proposed that plants split CO2 into carbon and oxygen
        4. Oxygen released as gas into air
        5. Carbon and water combined to form carbohydrates
      3. van Niel examined photosynthesis in bacteria
        1. Purple sulfur bacteria convert H2S into sulfur, do not release oxygen
        2. Proposed H2A is an electron donor, product A comes from splitting H2A
        3. Thus O2 from photosynthesis comes from H2O not CO2
      4. Experiments reproduced using radioactive oxygen
      5. Carbohydrate typically produced by plants and algae is glucose
    3. The Role of Light
      1. Blackman's experiments determined that photosynthesis has two-stages
        1. Measured effects of changing light intensities and temperature
        2. In low light, higher temperature did not accelerate photosynthesis fig 10.3
        3. In strong light, higher temperature did accelerate it
        4. Postulated "light" reactions independent of temperature, "dark" reactions independent of light
        5. At temperatures above 35ø enzymes became denatured
      2. Present knowledge
        1. First stage requires light, reduces electron carriers, makes ATP from ADP
        2. In second stage carriers and ATP reduces C in CO2 and makes glucose
        3. Carbon fixation incorporates CO2 carbon into glucose in "dark" reaction
      3. Photosynthesis is a redox process
        1. Sun energy drives reduction of carrier molecules
        2. Reverse to the electron path in oxidative respiration
        3. Electrons in respiration loose energy going from sugar to oxygen
        4. Mitochondria use released energy to make ATP
        5. Electrons in photosynthesis must gain energy going from water to sugar
        6. Energy provided by the sun


10.3 Pigments capture energy from sunlight

  1. The Biophysics of Light
    1. The Photoelectric Effect
      1. Intensity of a generated spark was increased in the presence of light
      2. Photoelectric effect discovered by Heinrich Hertz
        1. Investigated spark generation and electromagnetic (radio) waves
        2. Strength intensified by the brightness and wavelength of light
      3. Phenomenon explained by Einstein
        1. Light consists of units of energy called photons
        2. Light blasted electrons from the wire hoop
        3. Create positive ions and facilitate passage of current across gap
    2. The Energy in Photons
      1. Photons possess different amounts of energy
      2. Energy content inversely proportional to the wavelength fig 10.4
        1. Highest energy wavelengths are short wavelength gamma rays
        2. Least energetic wavelengths are long wavelength radio waves
        3. Energy in visible light
        4. Violet has short wavelength and high energy photons
        5. Red has long wavelength and low energy photons
    3. Ultraviolet Light
      1. Sunlight contains short, energetic ultraviolet light
      2. Was a probable source of energy in the primitive earth
      3. Current earth shielded by the ozone layer
      4. Ultraviolet light causes sunburns
      5. Holes have appeared in ozone layer, threaten to increase incidence of skin cancers
    4. Absorption Spectra and Pigments
      1. Absorption of light energy dependent on its energy and kind of molecule it hits
      2. Electrons occupy discrete energy levels while orbiting in their atoms
      3. Specific atoms can absorb only certain photons of light
        1. Any given molecule has a characteristic absorption spectrum
        2. Can only absorb photons of certain energy level
      4. Pigments are molecules that absorb light, carotenoids and chlorophylls
      5. Chlorophylls absorb light within narrow ranges
        1. Chlorophyll a and b absorb violet-blue and red light fig 10.5
        2. Neither absorbs light between 500 and 600 nanometers, green light
        3. Wavelength not absorbed by chlorophylls reflected to eyes as green
        4. Chlorophyll a is primary photosynthetic pigment
          1. Directly converts light energy to chemical energy
          2. Cannot capture all wavelengths of light
        5. Chlorophyll b is an accessory pigment
          1. Has an absorption spectrum shifted toward green light
          2. Can absorb wavelengths that chlorophyll a cannot
        6. Carotenoids are also accessory pigments that expand energy capture
  2. Chlorophylls and Carotenoids
    1. Characteristics of Chlorophylls
      1. Absorb photons by excitation like the photoelectric effect
      2. Complex ring structure called a porphyrin ring
      3. Magnesium ion within a network of alternating single and double bonds
      4. Side groups of molecule alter absorption properties fig 10.6
    2. Englemann Experiments
      1. Attempted to characterize chlorophyll's absorption spectrum fig 10.7
        1. Arranged alga across a miniature spectrum on a microscope slide
        2. Used aerotactic bacteria to assess rate of oxygen production
        3. Most bacteria accumulated in red and violet-blue regions
      2. Chlorophyll a users include plants, algae and most photosynthetic bacteria
      3. Do not use retinal pigment because of its low photoefficiency
      4. Chlorophyll absorbs in two narrow bands, but with great efficiency
    3. Carotenoids fig 10.8
      1. Carbon ring linked to chains with alternating double, single bonds
      2. Absorb photons over a broad range, not highly efficient
      3. Include beta-carotene
        1. Two carbon rings connected by 18 carbon chain, alternating single and double bonds
        2. If split in half, two molecules of vitamin A produced
        3. Oxidation of vitamin A makes retinal, involved in vertebrate vision
        4. Connection between carotene (carrots) and enhanced vision
  3. Organizing Pigments into Photosystems
    1. Absorbing Light Energy
      1. Light reactions occur on photosynthetic membranes fig 10.9
        1. Photosynthesis occurs on cell membranes in bacteria
        2. In plants and algae, photosynthesis occurs in chloroplasts
          1. Evolutionary descendants of photosynthetic bacteria
          2. Photosynthetic membranes located within the chloroplasts
      2. Light reactions occur in three stages
        1. Primary photoevent
          1. Photon of light captured by a pigment
          2. Electron within the pigment is excited
        2. Electron transport
          1. Excited electron shuttled along electron-carrier molecules
          2. Carrier molecules embedded within photosynthetic membrane
          3. Proton-pumping channel transports proton across membrane
          4. Electron induces event and is passed to an acceptor
        3. Chemiosmosis
          1. Passage of protons drives chemiosmotic synthesis of ATP
          2. Just like aerobic respiration
    2. Discovery of Photosystems
      1. Measure how much light produces how much photosynthesis
        1. Output increases linearly at low intensities
        2. Output lessens at high intensities
        3. Saturation occurs at high-intensity light
          1. At saturation all light-absorbing capacity is in use
          2. Adding more light does no good
      2. Emerson-Arnold experiments fig 10.10
        1. Test if at saturation all pigment molecules are in use
        2. Measure oxygen yield of Chlorella with microbursts of light
        3. If intensity of flashes increased, yield per flash increased to saturation
        4. Saturation achieved at one molecule of O2 per 2500 chlorophyll molecules
        5. Conclusion that photons absorbed by groups of molecules not individual molecules
        6. Clusters of chlorophyll and accessory pigments called photosystems
        7. Reaction center of photosystem acts as energy sink, traps excitation energy
        8. Emerson and Arnold observed individual reaction centers
    3. Architecture of a Photosystem
      1. Light captured by network of pigments called the photosystem fig 10.11
        1. Network of chlorophyll a and accessory pigments
        2. Held in protein matrix on surface of photosynthetic membrane
        3. Arrangement permits channeling of energy to a central point
        4. Molecule then passes energy out of photosystem to make ATP
      2. Consists of two closely linked components
        1. Antenna complex: Hundreds of pigment molecules
        2. Reaction center: One or more chlorophyll a molecules to pass energy out
      3. The antenna complex
        1. Captures photons from sunlight
        2. Web of chlorophyll molecules held to thalakoid membrane by protein matrix
        3. May contain a varying amount of carotenoids
      4. Photosystem protein matrix holds pigment in optimal orientation
        1. Excitation energy passes from one molecule to another
        2. After energy passes, excited electron returns to lower energy state
        3. Excited electrons do not physically pass from pigment to pigment, only energy
        4. Funnels energy from many electrons to reaction center
      5. The reaction center
        1. Transmembrane protein-pigment complex
        2. Model: Purple photosynthetic bacteria fig 10.12
          1. Two chlorophyll a molecules act as trap for photon energy
          2. Pass excited electron to a primary electron acceptor (quinone)
          3. Reduces quinone and makes it a strong electron donor
          4. Weak donor then donates a low-energy electron to chlorophyll
          5. Restores original condition of chlorophyll
        3. Weak donor is cytochrome in purple bacterium
        4. Weak donor is water in plant chloroplasts
  4. How Photosystems Convert Light to Chemical Energy
    1. Bacteria Use a Single Photosystem
      1. Sulfur bacteria fig 10.13
        1. Evolved photosynthetic units three billion years ago
        2. Absorption peak at 870 nanometers
        3. Sulfur bacteria extract proton from H2S, sulfur by-product
        4. Other organisms extract proton from H2O, oxygen by-product
      2. Ejection of an electron from bacterial reaction center (P870) leaves it one electron short
      3. Electron must be returned before it can function again
        1. Bacteria channel electron back via electron-transport system
        2. Passage drives proton pump, chemiosmotically generates an ATP per three electrons
      4. Overall process called cyclic photophosphorylation fig 10.14
        1. Process is not a true circle
        2. Returned electron is not same one that left, but has same energy
      5. Difference in energy is what drives proton pump
      6. Process is the fundamental component of photosynthesis
      7. Limitations of cyclic photophosphorylation
        1. Geared only towards energy production
        2. Does not provide for biosynthesis
        3. Ultimate point of photosynthesis is to generate carbon compounds
          1. Sugars are more reduced than CO2, have more hydrogen atoms
          2. Bacteria inefficiently scavenge hydrogens from other sources
    2. Why Plants Use Two Photosystems
      1. Other bacteria evolved an improved version of the photosystem
      2. Solved the reducing power problem
        1. New process grafted on to original photosynthetic process
        2. New process used chlorophyll a
        3. Originated with the evolution of cyanobacteria
      3. Second system called photosystem II
        1. Molecules of chlorophyll a are arranged with a different geometry
        2. More of shorter wavelengths are absorbed than in earlier process
        3. In plants, the earlier process is called photosystem I
      4. Absorption peak of photosystem II pigment is 680 nanometers, called P680
      5. Absorption peak of photosystem I pigment is 700 nanometers, called P700
      6. Advantage of two photosystems
        1. Solves problem of obtaining reducing power
        2. Z diagram of photosystems I and II fig 10.15
  5. How the Two Photosystems Work Together
    1. The Two Stage System Is Called Noncyclic Photophosphorylation
      1. Path of electrons is not circular
        1. Electron does not return to origin, but goes to NADPH
        2. Electrons replenished by splitting water
      2. Photosystem II acts first
        1. Excited electron used to make ATP
        2. Passes electron on to photosystem I drives production of NADPH
        3. Two electrons from water makes one NADPH and slightly more than one ATP
    2. Photosystem II
      1. Reaction center resembles that of purple bacteria
        1. More than ten transmembrane protein subunits
        2. Antenna complex is more than 250 molecules of chlorophyll a and accessory pigments
      2. Oxygen atoms of two water molecules bind to manganese atoms in enzyme labeled Z
      3. Enzyme splits water, removes electrons one at a time
      4. Electrons fill void left in reaction center
      5. When four electrons removed, O2 is released
    3. The Path to Photosystem I
      1. Quinone is primary electron acceptor for electrons leaving photosystem II
      2. Reduced quinone, called plastoquinone (Q) is a strong electron donor
      3. Passes excited electron to proton pump called b6-f complex
      4. Complex located in thalakoid membrane fig 10.16
      5. Resembles bc1 complex in mitochondria respiratory electron transport chain
      6. With arrival of electron, complex pumps proton into thalakoid space
      7. Plastocyanin (PC) carries electron to photosystem I
    4. Making ATP: Chemiosmosis fig 10.17
      1. Thalakoid membrane is impermeable to most molecules and protons
      2. Proton transit occurs at ATP-synthetase proton channels
      3. Channels are knobs on external surface of thylakoid membrane
      4. ATP released into surrounding fluid within chloroplast, the stroma
      5. Stroma contains enzymes
        1. Catalyze reactions that catalyze light-independent reactions, fix carbon
    5. Photosystem I
      1. Reaction center called P700
        1. Transmembrane complex of at least 13 protein subunits
        2. Antenna complex of 130 chlorophyll a and accessory pigment molecules
      2. Accepts electron from plastocyanin to fill hole from exit of light-excited electron
      3. Boosts energy of exiting electron to very high level
      4. Passes electron to ferredoxin (Fd), an iron-sulfur protein
    6. Making NADPH
      1. Ferredoxin on outside (stromal side) of thalakoid membrane
      2. Reduced ferredoxin carries very high-potential electrons
      3. Two such electrons donated to NADP+ to make NADPH
      4. Reaction catalyzed by NADP reductase bound to the membrane
      5. Contributes further to proton gradient
    7. Making More ATP
      1. Above events make slightly more than 1 ATP
      2. One-and-one-half ATP per NADPH required to fix carbon
      3. Extra ATP made when plant switches to cyclic photophosphorylation mode
        1. Light-excited electron leaving photosystem I makes ATP instead of NADPH
        2. Energetic electron passed back to b6-f complex
        3. Complex pumps out proton, adding to proton gradient, driving chemiosmosis
      4. Proportions of cyclic and noncyclic photophosphorylation determine ATP and NADPH production


10.4 Cells use the energy and reducing power captured by the light reactions to make organic molecules

  1. The Light-Independent Reactions
    1. Products of the Light-Dependent Reactions Used to Build Organic Molecules
      1. Energy: Photosystem II ATP drives endergonic reactions
      2. Reducing power: Photosystem I NADPH provides hydrogens and energetic electrons
    2. Carbon Fixation
      1. CO2 must be attached to an organic molecule
      2. Atmospheric CO2 is reduced during carbon fixation
      3. Two intermediates of glycolysis reassembled
        1. Fructose 6-phosphate (F6P) + glyceraldehyde 3-phosphate (G3P)
        2. Reassembled to form five-carbon molecule ribulose 1,5 bisphosphate (RuBP)
      4. CO2 binds to RuBP during carbon fixation
      5. Forms two molecules of 3-C phosphoglycerate (PGA) fig 10.18
      6. Catalyzed by ribulose bisphosphate carboxylase (rubisco)
        1. Enzyme is comparatively slow
        2. Many copies are needed
        3. May be most abundant protein on earth
    3. The Calvin Cycle
      1. Light-independent reactions can occur readily in the dark
      2. Carbon-fixing reaction proceeds because RuBP is energy-rich
      3. Reactions consume ATP and NADPH
      4. Enzyme catalyzed steps similar to Krebs cycle
      5. Process called C3 photosynthesis (PGA contains 3 carbon atoms)
      6. Steps of reaction fig 10.19
        1. Three CO2 (3 Cs) fixed to RuBP (15 Cs)by rubisco to form 6 PGA (18 Cs)
        2. Complex cycle of rearrangements result in
          1. Reforming RuBP
          2. Producing glyceraldehyde 3-phosphate (G3P)
        3. Called the Calvin cycle
        4. Three turns of cycle use three CO2, make 3 G3P, reform 3 RuBP
    4. Output of the Calvin Cycle
      1. G3P is intermediate in glycolysis
      2. If exported from cell, converted to fructose 6-phosphate, glucose 1-phosphate
      3. F6P and G1P further converted to sucrose
      4. If G3P levels very high
        1. Some G3P converted into G1P by reversing glycolysis reactions
        2. G1p combined into insoluble polymer of long chains of starch
        3. Stored as starch grains in chloroplast
  2. Photorespiration
    1. Evolution Favors Workable, Not Always Optimal Solutions
      1. RuBP carboxylase (rubisco) secondarily interferes with Calvin cycle fig 10.20
        1. Initiates oxidation of RuBP
        2. CO2 is released without the production of ATP or NADPH
        3. Process called photorespiration, acts to undo photosynthesis
      2. Both reactions occur at the same active site
        1. Decarboxylation reaction of photorespiration requires oxygen
        2. Little photorespiration occurred prior to the O2 atmosphere
      3. C3 plants lose one fourth to one half of their fixed carbon in this way
        1. Loss is related to increased temperature
        2. Oxidation of RuBP increases more than its photosynthesis
      4. Tropical plants adapted to counteract this problem
    2. The Crassulacean Acid Pathway
      1. Crassulacean acid metabolism (CAM) also used by plants in hot climates fig 10.21
      2. Succulents open their stomata at night and close them during the day
      3. Reduces photorespiration by reducing CO2 available
      4. CO2 needed to produce sugar provided by organic molecules from Calvin cycle
    3. The C4 Pathway
      1. Include grasses and other plants
      2. Called C4 pathway since first product is a four-carbon molecule
      3. Concentrate CO2 by carboxylating phosphoenolpyruvate (PEP) fig 10.22
        1. Resulting four-carbon oxaloacetate converted to malate
        2. Malate conveyed to bundle-sheath cells, impermeable to CO2 fig 10.23
        3. Malate decarboxylated to pyruvate, releasing CO2 in the cell
        4. Pyruvate returns to leaf cell, changed back to phosphoenolpyruvate
        5. Requires two high energy bonds, ATP becomes AMP
      4. C4 plants are found in hot climates
        1. Process uses 30 ATP, normal photosynthesis uses 18 ATP
        2. Saves the loss of fixed carbon via photorespiration
  3. The Energy Cycle
    1. Photosynthesis and Energy-Capturing Metabolisms Are Related fig 10.24
      1. Photosynthesis uses respiration products as starting substrates
      2. Respiration uses photosynthesis products as starting substrates
      3. Calvin cycle uses part of the glycolytic process in reverse
      4. Electron transport proteins in plants and mitochondria are related fig 10.25
    2. Photosynthesis Is One Important Aspect of Plant Biology

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