Lecture Outlines
Energy-Acquiring Pathways

Sun, Rain, and Survival

    1. For life based on organic compounds, two questions can be raised:
      1. Where does the carbon come from?
      2. Where does the energy come from to link carbon and other atoms into organic compounds?
    2. Autotrophs are "self-nourishing."
      1. They obtain carbon from carbon dioxide.
      2. Photosynthetic autotrophs (plant, protistan, and bacterial members) harness light energy.
      3. Chemosynthetic autotrophs (a few bacteria) extract energy from chemical reactions involving inorganic substances (such as sulfur compounds).
    3. Heterotrophs feed on autotrophs, each other, and organic wastes.
      1. Heterotrophs acquire carbon and energy from autotrophs.
      2. Heterotrophs include animals, protistans, bacteria, and fungi.
    4. Carbon and energy enter the web of life by photosynthesis and in turn are released by glycolysis and aerobic respiration.
  1. Photosynthesis—An Overview
    1. Energy and Materials for the Reactions
      1. The light-dependent reactions convert light energy to chemical energy (which is then stored in ATP); the liberated electrons are picked up by NADPH.
      2. The light-independent reactions assemble sugars and other organic molecules using ATP, NADPH, and CO2.
      3. Overall, the equation for glucose formation is written:
            sunlight
          12H2O + 6CO2 –––––> 6O2 + C6H12O6 + 6H2O
  2. Where the Reactions Take Place
    1. The two stages of photosynthesis take place in the chloroplast.
    2. Light-dependent reactions occur in the thylakoid membrane system.
      1. The thylakoids are folded into grana (stacks of disks) and channels.
      2. The interior spaces of the thylakoid disks and channels are continuous and are filled with H+ needed during ATP synthesis.
    3. Carbohydrate formation occurs in the stroma (semifluid) area that surrounds the grana.
  • Sunlight as an Energy Source
    1. Properties of Light
      1. Energy from the sun radiates through space in wavelengths ranging from gamma rays to radio waves (the electromagnetic spectrum).
      2. Photoautotrophs use only a small range (400—750 nm) of wavelengths for photosynthesis; these wavelengths are the range of visible light.
      3. Light energy is packaged as photons, which vary in energy as a function of wavelength (most energetic in blue-violet; least energetic in red light).
    2. Pigments—The Molecular Bridge From Sunlight to Photosynthesis
      1. Pigments are the molecular bridge between sunlight and photosynthetic activity.
      2. Chlorophyll pigments absorb blue and red but reflect green (the color of leaves).
  • The Rainbow Catchers
    1. The Chemical Basis of Color
      1. Electrons in pigments absorb photons of specific energies, which correspond to specific colors of light.
      2. If the quantity of energy of an incoming photon matches the energy level required to boost an electron to a higher energy level, that wavelength will be absorbed; photons that are a mis-match will be transmitted (reflected) in the color visible to an observer.
    2. On the Variety of Photosynthetic Pigments
      1. Chlorophylls are the main pigments in all but one group of photoautotrophs.
        1. Chlorophyll a (green) is the main pigment inside chloroplasts.
        2. Chlorophyll b (bluish-green) occurs in plants, green algae, and photoautotrophic bacteria.
      2. Carotenoid pigments absorb blue-violet wavelengths but reflect yellow, orange, and red.
      3. Anthocyanins are pigments in many flowers.
      4. Phycobilins are the red and blue pigments of the red algae and cyanobacteria.
    3. What Happens to the Absorbed Energy?
      1. A photosystem is a cluster of 200 to 300 light-absorbing pigments located in the thylakoid membranes.
      2. The pigments "harvest" photon energy from sunlight.
        1. Absorbed photons of energy boost electrons to a higher level.
        2. The electrons quickly return to the lower level and release energy.
        3. Released energy is trapped by chlorophylls located in the photosystem’s reaction center.
        4. The trapped energy is then used to transfer a chlorophyll electron to an acceptor molecule.
      3. About Those Roving Pigments
        1. Carotenoids originate in photoautotrophs and move up the food chain as when algae are eaten by snails, which are in turn eaten by flamingos.
        2. Beta-carotene molecules are split to form vitamin A, the precursor of visual pigments used in the flamingo’s eyes.
  • The Light-Dependent Reactions
    1. Three events occur:
      1. Pigments absorb sunlight energy and give up excited electrons.
      2. Electron and hydrogen transfers lead to ATP and NADPH formation.
      3. The pigments that gave up the electrons in the first place get electron replacements.
    2. The ATP-Producing Machinery
      1. The chloroplast’s thylakoid membrane incorporates the light-harvesting photosystems, from which electrons are picked up and transferred to an adjacent electron transport system.
      2. Electron transport systems are organized sequences of enzymes and other proteins bound in a cell membrane
        1. Electrons expelled from a chlorophyll molecule go through one or two electron transport systems in the thylakoid membranes.
        2. As the electron passes from one molecule to another in each system, phosphate is added to ADP to form ATP.
    3. Cyclic Pathway of ATP Formation
      1. In the cyclic pathway of ATP formation, excited electrons leave the P700 reaction center, pass through an electron transport system, and then return to the original photosystem I.
      2. Energy associated with the electron flow drives the formation of ATP from ADP.
      3. The cyclic pathway is probably the oldest means of ATP production, being used by early bacteria.
    4. Noncyclic Pathway of ATP Formation
      1. The noncyclic pathway of ATP formation transfers electrons through two photosystems and two electron transport systems (ETS) in the thylakoid membranes.
      2. The pathway begins when chlorophyll P680 in photosystem II absorbs energy.
        1. Boosted electron moves through a transport system that releases energy for ADP + Pi ––> ATP.
        2. Electron fills "hole" left by electron boost in P700 of photosystem I.
        3. Electron from photolysis of water fills "electron hole" left in P680 and produces oxygen byproduct.
      3. Pathway continues when chlorophyll P700 in photosystem I absorbs energy.
        1. Energy hole is filled by electron from P680.
        2. Boosted electron from P700 passes to acceptor, then ETS; it finally joins NADP to form NADPH (which along with ATP can be used in synthesis of organic compounds).
      4. The Legacy–A New Atmosphere
        1. Oxygen is a by-product of the noncyclic pathway.
        2. Beginning about 1.5 billion years ago, large amounts of oxygen began accumulating in the atmosphere, which up to that time had been oxygen-free.
  • A Closer Look at ATP Formation in Chloroplasts
    1. Hydrogen ions from photolysis of water accumulate inside the thylakoid compartment of chloroplasts to set up concentration and electric gradients.
      1. Oxygen atoms from photolysis combine to form O2 which is released into the atmosphere.
      2. As the hydrogen ions flow out through channels into the stroma, enzyme action links Pi to ADP to form ATP.
    2. This mechanism is called the chemiosmotic theory of ATP formation.
  • Light-Independent Reactions
    1. These reactions are the "synthesis" of photosynthesis.
      1. The participants and their roles in the synthesis of carbohydrate are:
        1. ATP, which provides energy;
        2. NADPH, which provides hydrogen atoms and electrons;
        3. Atmospheric air, which provides carbon dioxide (the source of carbon and oxygen).
      2. The reactions are not dependent on sunlight directly.
    2. Capturing Carbon
      1. Carbon dioxide diffuses from the air, across the plasma membrane of the plant cell and into the stroma.
      2. Carbon fixation occurs when the carbon atom of CO2 becomes attached to ribulose bisphosphate (RuBP) to form a six-carbon intermediate; this is the first step in the Calvin-Benson cycle that will ultimately lead to sugar phosphate formation.
      3. The six-carbon intermediate splits at once to form two PGA (phosphoglycerate) molecules.
    3. Building the Glucose Subunits
      1. Each PGA then receives a Pi from ATP plus H+ and electrons from NADPH to form PGAL (phosphoglyceraldehyde).
        1. Most of the PGAL molecules continue in the cycle to fix more carbon dioxide, but two PGAL join to form a sugar phosphate, which will be modified to sucrose, starch, and cellulose.
        2. Final tally: 12H2O + 6CO2 + 18ATP + 12NADPH –––> C6H12O6 + 18ADP + 18Pi + 12NADP+ + 6H2O + 12H+
      2. Sugar phosphates are used as cellular fuel and as building blocks in synthesis of sucrose or starch.
        1. Sucrose is the most easily transportable.
        2. Starch is the main storage form, but it can be converted back to sucrose for distribution to leaves, stems, and roots.
      3. Photosynthesis also yields intermediates and products that can be used in lipid and amino acid synthesis.
  • Fixing Carbon–So Near, Yet So Far
    1. C4 Plants
      1. Plants in hot, dry environments close their stomata to conserve water but in so doing retard carbon dioxide entry and permit oxygen buildup inside the leaves.
      2. Thus, oxygen–not carbon dioxide–becomes attached to RuBP to yield one PGA (instead of two) and one phosphoglycolate (not useful); this nonproductive process is called photorespiration.
      3. To overcome this fate, crabgrass, sugarcane, corn, and other plants fix carbon twice (in mesophyll cells then in bundle-sheath cells) to produce oxaloacetate (a four-carbon, hence C4) compound, which can then donate the carbon dioxide to the Calvin-Benson cycle.
    2. CAM Plants
      1. Succulents, such as cacti, open their stomata and fix CO2 only at night, storing the intermediate product for use in photosynthesis the next day.
      2. These plants are known as CAM plants because, unlike C4 species, they do not fix carbon in separate cells but at different times in the same cell.

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    (c)1998 Brooks/Cole Publishing Company/ITP 1-800-590-9951
    Last Updated August 1998

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