Chapter 9: Photosynthesis: Capturing Light Energy

Light and Photosynthesis

  • Because most life on this planet depends on light, either directly or indirectly, it is important to understand the nature of light and its essential role in photosynthesis.
  • Light consists of particles called photons that move as waves.
  • Photons with shorter wavelengths have more energy than those with longer wavelengths.
  • When a molecule absorbs a photon of light energy, one of its electrons becomes energized, which means that the electron shifts from a lower-energy atomic orbital to a high-energy orbital that is more distant from the atomic nucleus.
  • One of two things then happens to the energized electron, depending on the atom and its surroundings
    • The atom may return to its ground state, which is the condition in which all its electrons are in their normal, lowest-energy levels.
    • When an electron returns to its ground state, its energy dissipates as heat, and/or as an emission of light of a longer wavelength than the absorbed light; this emission of light is called fluorescence.
    • Alternatively, the energized electron may leave the atom and be accepted by an electron acceptor molecule, which becomes reduced in the process; this is what occurs in photosynthesis.

Chloroplasts

  • In plants photosynthesis occurs in chloroplasts, which are located mainly within mesophyll cells inside the leaf.
  • Chloroplasts are organelles enclosed by a double membrane; the inner membrane encloses the stroma in which membranous, saclike thylakoids are suspended.
    • Thylakoids enclose the thylakoid lumen.
    • Thylakoids arranged in stacks are called grana.
  • Chlorophyll a, chlorophyll b, carotenoids, and other photosynthetic pigments are components of the thylakoid membranes of chloroplasts.
  • Photons excite biological molecules such as chlorophyll and other photosynthetic pigments, causing one or more electrons to become energized.
    • These energized electrons may be accepted by electron acceptor compounds.
  • The combined absorption spectra of chlorophylls a and b are similar to the action spectrum for photosynthesis

Overview of Photosynthesis

  • During photosynthesis, light energy is captured and converted to the chemical energy of carbohydrates; hydrogens from water are used to reduce carbon, and oxygen derived from water becomes oxidized, forming molecular oxygen.
  • In the light-dependent reactions, electrons energized by light are used to generate ATP and NADPH; these compounds provide energy for the formation of carbohydrates during the carbon fixation reactions.

The Light-Dependent Reactions

  • Photosystems I and II are the two types of photosynthetic units involved in photosynthesis.
    • Each photosystem includes chlorophyll molecules and accessory pigments organized with pigment-binding proteins into antenna complexes.
  • Only a special pair of chlorophyll a molecules in the reaction center of an antenna complex give up energized electrons to a nearby electron acceptor.
    • P700 is in the reaction center for pho- tosystem I; P680 is in the reaction center for photosystem II.
  • During the noncyclic light-dependent reactions, known as noncyclic electron transport, ATP and NADPH are formed.
  • Electrons in photosystem I are energized by the absorption of light and passed through an electron transport chain to NADP+, forming NADPH.
    • Electrons given up by P700 in photo- system I are replaced by electrons from P680 in photosystem II.
  • A series of redox reactions takes place as energized electrons are passed along the electron transport chain from photosystem II to photosystem I.
    • Electrons given up by P680 in photosystem II are replaced by electrons made available by the photolysis of H2O; oxygen is released in the process.
  • During cyclic electron transport, electrons from photosystem I are eventually returned to photosystem I.
    • ATP is produced by chemiosmosis, but no NADPH or oxygen is generated.
  • Photophosphorylation is the synthesis of ATP coupled to the transport of electrons energized by photons of light.
    • Some of the energy of the electrons is used to pump protons across the thylakoid membrane, providing the energy to generate ATP by chemiosmosis.
  • As protons diffuse through ATP synthase, an enzyme complex in the thylakoid membrane, ADP is phosphorylated to form ATP.

The Carbon Fixation Reactions

  • The carbon fixation reactions proceed by way of the Calvin cycle, also known as the C3 pathway.
  • In the CO2 uptake phase of the Calvin cycle, CO2 is combined with ribulose bisphosphate (RuBP), a five-carbon sugar, by the enzyme ribulose bisphosphate carboxylase/oxygenase, commonly known as rubisco, forming the three-carbon molecule phosphoglycerate (PGA).
  • In the carbon reduction phase of the Calvin cycle, the energy and reducing power of ATP and NADPH are used to convert PGA molecules to glyceraldehyde-3-phosphate (G3P).
    • For every 6 CO2 molecules fixed, 12 molecules of G3P are produced, and 2 molecules of G3P leave the cycle to produce the equivalent of 1 molecule of glucose.
  • In the ruBP regeneration phase of the Calvin cycle, the remaining G3P molecules are modified to regenerate ruBP.
  • In photorespiration C3 plants consume oxygen and generate CO2 by degrading Calvin cycle intermediates but do not produce ATP.
    • Photorespiration is significant on bright, hot, dry days when plants close their stomata, conserving water but preventing the passage of CO2 into the leaf.
  • In the C4 pathway, the enzyme PEP carboxylase binds CO2 effectively, even when CO2 is at a low concentration.
    • C4 reactions take place within mesophyll cells.
    • The CO2 is fixed in oxaloacetate, which is then converted to malate.
    • The malate moves into a bundle sheath cell, and CO2 is removed from it.
    • The released CO2 then enters the Calvin cycle.
  • The crassulacean acid metabolism (CAM) pathway is similar to the C4 pathway.
    • PEP carboxylase fixes carbon at night in the mesophyll cells, and the Calvin cycle occurs during the day in the same cells.

Metabolic Diversity

  • Photoautotrophs use light as an energy source and are able to incorporate atmospheric CO2 into pre-existing carbon skeletons.
  • Chemoheterotrophs obtain energy by oxidizing chemicals and obtain carbon as organic molecules from other organisms.

Photosynthesis in Plants and in the Environment

  • Photosynthesis is the ultimate source of all chemical energy and organic molecules available to photoautotrophs, such as plants, and to virtually all other organisms as well.
    • It also constantly replenishes the supply of oxygen in the atmosphere, vital to all aerobic organisms.