Chapter 19: The Light Reactions of Photosynthesis

Chapter 19: The Light Reactions of Photosynthesis

  • The mechanisms by which organisms obtain energy from their environment are dealt with by the authors.
    • Light energy is transduced into the same forms of chemical energy, which in turn leads to the conversion of CO2 into carbohydrate by organisms.
    • Chapter 20 of the text will cover carbon fixation and sugar synthesis.
  • An overview of the process and the basic equation of photosynthesis are what the authors begin with.
    • Next comes a description of the bacterium and its reaction center.
    • They explain how the light reactions lead to the creation of protons.
  • A review of the basic concepts of metabolism in Chapter 14 will help you understand this chapter.
  • You should be able to complete the objectives when you have mastered this chapter.
  • The structures are associated with the functions they perform.
  • Compare theories of the origin of mitochondria and the probable origin of the chloroplast.
  • Discuss the common features of diverse reaction centers.
  • List electron donors used bybacteria The electron donor is H2S.
  • The similarities between the two are obvious.
  • The photosystem II contains a description of the L, M, and H subunits.
    • D1 contains the "loose" plastoquinone.
  • Use the diagram of photosystem II to identify the appropriate compo nents, sites, and functions.
  • D1 contains the "loose" plastoquinone.
  • Match the photosystems of the purple sulfur bacterium or of green plants with the ap propriate properties listed in the right column.
  • The net equation of the reaction catalyzed by photosystem I should be written.
  • The descriptions should be matched with the A.
  • Light absorption, light collection in the antennae, and reaction center chemistry are all done by chlorophylls.
  • Photosystems I and II work together to create a transmembrane force.
  • The direction of the two genes are reversed.
  • The electron can travel from higher to lower in the mitochondria, but only with the help of a photon.
  • The visible region of the spectrum has absorption bands caused by the polyene structure.
    • The peak absorption coefficients are higher than 105 cm-1 M-1.
    • The iron porphyrins return to the ground state much more quickly than excited magnesium tetrapyrroles.
    • The high-energy electron can be transferred before it is dissipated as heat.
  • Draw a vertical line dividing the box in half.
    • The half on the left is the half that shows electron transfers in the L subunit.
    • On the right side of the M is the QB.
    • The special pair is shown at the bottom of Figure 19.10 and it is upside-down.
    • The text doesn't mention the fact that D1 contains the exchangeable plastoquinone QB.
  • The answer is incorrect because a cluster of four manganese ion serves as a charge accumulator by interacting with the strong oxidant P680+ and H2O to form O2.
  • The answer is incorrect.
    • Five oxidation states can be adopted by the center of four Mn atoms.
    • There are four oxidation states in which the manganese ion can exist.
  • The transmembrane protons are used to synthesise.
  • PC is plastocyanin and the net reaction is PC(Cu+) + ferredoxinoxidized PC(Cu2+) +ferredoxinreduced.
    • A powerful reductant is reduced ferredoxin.
    • Two reduced ferredoxins oxidize two ferredoxins in a reaction.
    • FAD is a group on theidase that is used to collect two electrons from two reduced ferredoxin molecules and transfer them to a single NADP+ molecule.
  • In Jagendorf's experiment, chloroplasts were equilibrated with a buffer at pH 4.
    • The suspension was quickly brought to pH 8.
    • The stroma's pH went from 8 to 8 in a matter of seconds, while the thylakoid space's went from 4 to 4.
  • The synthesis occurred in the dark as the pH gradient dissipated.
  • The answer is incorrect because photo system I provides electrons for pho tophosphorylation.
  • One O2, two NADPH, and three ATP would be produced by eight photons.
  • The yellow color of jaundiced skin is caused by the yellow color of the "bilin" pigments.
    • To compare these structures, look at pages 688 and 699 in the text.
  • The answer is incorrect because the differentiation into stacked and unstacked regions probably prevents direct interaction between the excited reaction center chlorophylls.
  • It is easier to promote hydrogen sul fide's electrons to the level of NADPH than it is to promote water's electrons.
    • The free energy change to move two electrons from oxygen to NADH would cost more if we reversed the calculation.
    • This value is cut in half by hydrogen sulfide, so a single photo system suffices.
  • There are two components in the electron-transport chain associated with pho to system I.
    • A - 0 is a chlorophyll that carries a single elec tron.
  • There are other useful data in the text.
  • The free-energy change that occurs in mitochondria is caused by a pair of electrons being transferred from H+ to NADH.

  • It is possible to produce compounds that can kill plants while keeping toxicity to animals to a minimum.
  • The other structures in the thylakoid Membrane are also "upside down" compared to the other systems.
  • A - 0 is the stronger reductant because it has a more negative standard re ducing potential.
    • Under standard conditions, a - 0 will reduce NADP+ to NADPH.
  • The number of electrons must be equalized by the equation 2 by 2 to get the reaction.
  • 0 values do not make the mistake of using factors to equalize half-cell reduction potentials.
    • In a house with an adequate electrical power supply, the potential difference at the fuse box is approximately 117 V regardless of whether the house is in total darkness or not.
  • The electrons are moved down an electron-transport chain.
    • The free-energy change as a pair of electrons is transferred from NADH + H+ to oxygen is -52.6 kcal/mol.
    • In both cases the large "span" of free energy is used.
  • This is not possible.
    • The redox potential of X* must be more negative than that of Y for electrons to flow spontaneously.
    • The free-energy change must be large enough to allow for the formation of ATP.
    • In order to make electrons flow from X* to Y as depicted in the scheme, ATP would have to be consumed.
  • The synthesis of ATP is driven by the force across the membranes.
    • During electron transport, a membrane potential of 0.14 V is established.
    • Light-generated potential is close to 0.
    • The same free-energy yield must be given by a greater pH gradient in the chloroplast.
  • Oxygen would change.
    • The process will be driven to the right by adding NADP+.
  • When 700-nm light is used, little oxygen would be created.
    • Oxygen is not maximally excited by 700-nm light because it is evolved by photosystem II.
  • The most sensitive spectrophotometric assays would be provided by these wavelengths of maximum absorbance.
  • Green light is transmitted by chlorophyll because it has no significant absorption in the green re gion of the spectrum.
  • The preferred solvent is acetone.
    • They are insoluble in water because of the porphyrin ring and the very hydrophobic phytol tail of the chlorophylls.

  • Higher animals have limited biosynthetic abilities because their diet contains plants and sometimes other animals.
    • Animals can't synthesise many of the amino acids.
    • Glyphosate is one of the most popular herbicides in use today.
    • For more discussion, see page 679 in the text.
    • There are a number of herbicides that block the synthesis of amino acid.
    • They are not very harmful to animals.
  • The 4-manganese center which interacts with water is complex, and might be related to a 2 manganese center found in catalase.
    • There is evidence that some O2 appeared in the atmosphere more than two billion years ago.
    • Water-based photosynthesis can't be younger than this, and could be older if the oxygen evolved locally.
    • Stromatolites similar to those found in Shark Bay, Australia, can be found in layers that are more than three billion years old.
  • Because radiation is an efficient way to transfer energy, it is likely that photon would be involved in bringing the energy to the places where life exists.
  • For example, H2, H2S, or other small organic molelcules can be used as electron donors.
  • O2 evolution can occur if an artificial electron acceptor can accept electrons from Q.
  • Cyclic photophosphorylation could happen.
    • Outside this cycle is the site of DCMU inhibition.
  • 1000-nm light has an energy content of 600/1000 x 47.6 kcal/einstein.
  • A minimum of 0.42 photon is needed to drive the synthesis.
  • The electron transfer rate is 1013 s-1 for groups in contact and falls by a factor of 10 for every 1.7 A through a protein environment.
    • An estimated decrease of about 12.94 orders of magnitude is given byDividing 22 A by 1.7 A.
    • The estimated rate is 100.06 events per second.
  • For every 1.7 A in crease in the separation distance, the rate will decrease by 10.
    • The time for electron transfer will increase to 10 ps.
  • The Hill reaction uses a photo system.
    • The evolution of O2 is due to the electrons from P680 being excited.
    • The excited electrons pass to pheophytin, then to Q, and finally to the artificial acceptor such as ferricyanide.
  • Since the DTT alone should provide enough reducing power, the enhancement with thioredoxin could be due to another effect.
    • It is possible that thioredoxin could bind to the enzyme and cause a conformational change to a more active state.
  • The segment that was removed and replaced is responsible for the redox reg ulation that is not found in mitochondria.
  • The levels of reducing agents and the amount of ongoing photosynthesis are what the Enzymes respond to when they don't respond to light directly.
  • The Cys side chains can be found in reduced and disulfide forms.