1. Light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts.

• The plant chloroplast contains grana, which are stacks of hollow disks. The membrane surrounding the grana is the thylakoid membrane. (fig. 12-29, Lodish)
• Like in the mitochondria, electron transfer reactions occur in internal membranes. Cyclic carbon fixation reactions, like the breakdown reactions in the citric acid cycle, occur in the stroma, the analog of the matrix in mitochondria. (fig. 12-30, Lodish)Notice that functionally, this entire process looks a lot like the citric acid cycle run backwards, though the individual components can be quite different.
• Notice that photosynthesis uses NADPH and NADP⁺ instead of NADH and NAD⁺.
• Chlorophyll contains a hydrophilic head group and a hydrophobic tail region. (fig. 12-31, Lodish)A magnesium atom is held in the center of a cyclic, conjugated double bond porphyrin ring which is responsible for absorbing red light. (There also is an absorption band in the blue. Thus red and blue are absorbed and green passes through, giving plants a characteristic green color. (fig. 12-32, Lodish)
• Light is absorbed by antenna chlorophyll molecules, then transferred to the reaction center chlorophylls. Some 100's of antenna chlorophyll molecules transfer energy to a reaction center, with transfer times of about 10^-10 sec from the edge of the unit to the center. (fig. 12-34a, Lodish - cyanobacteria), (fig. 12-34bc, Lodish - plants)
• The energy from light is used to pump H⁺ ions from the stroma into the thylakoid space and to reduce NADP⁺ to NADPH. (fig. 12-37, Lodish) Flow of H⁺ from the intermembrane space back into the stroma releases energy which is used to phosphorylate ADP to ATP. The chemiosmotic coupling here is similar way to the mechanism of ATP generation used in mitochondria.
• The PSII reaction center, where oxygen is evolved, is complex, containing two molecules of chlorophyll a, phenophytins, quinones, and bound ions of calcium, chloride, and Manganese. (fig. 12-38, Lodish)
• The electron transport chain can be shown on an energy diagram. Notice how two photons of light are used to generate NADPH and only the first system, which is called photosystem II, participates in the breakdown of water to release electrons.The process shown in this figure is known as noncyclic photophosphorylation. Another process, known as cyclic photophosphorylation, uses only photosystem I. (fig. 12-41, Lodish)

2. Carbon fixation is catalyzed by ribulose bisphosphate carboxylase (RuBP carboxylase), the world's most abundant enzyme. (fig. 12-43, Lodish)

• The Calvin cycle combines three carbon dioxide molecules into one molecule of three carbon glyceraldehyde 3-phosphate. (fig. 12-44, Lodish)

3. Some plants, particularly many which live in hot, dry climates, have a mechanism for storing carbon dioxide by combining it with a three carbon molecule to form a four carbon molecule. This pathway is known as the C4 or Hatch-Slack pathway.

• Leaves which do C4 photosynthesis have Kranz anatomy. (fig. 12-46a, Lodish)(This figure is courtesy of Gary Anderson, Department of Biological Sciences, University of Southern Mississippi.)

• In the mesophyll cells, the carbon dioxide combines with phosphoenolpyruvate (PEP) to form oxaloacetate. The oxaloacetate forms malic acid which moves into the photosynthetic bundle sheath cells where carbon dioxide is released and enters the Calvin cycle. (fig. 12-46b, Lodish)
• C4 biochemistry requires ATP and NADPH for each turn of the oxaloacetate-malate-pyruvate cycle. Why then is it used, since it seems to be less efficient than standard C3 photosynthesis? Well, RuBP carboxylase will bind oxygen instead of carbon dioxide, in a process known as photorespiration. This is a dead-end process, tying up RuBP carboxylase without doing anything useful. (RuBP carboxylase is quite slow, so lots of it is present. So, if we tie a bunch up doing something useless, it could become quite a problem.)

  By shielding the bundle sheath cells from the outside gaseous atmosphere, oxygen concentrations are lowered in the stroma of the chloroplasts where RuBP carboxylase is found. It turns out that the C4 process generally is more efficient than the plain C3 process at 30 degrees C. or more. So, lots of plants which are found in the tropics and those exposed to hot summer temperatures as in Kansas, have C4 mechanisms. These plants include corn, sugar cane, and some weeds, like bermuda and crab grass. Some plants use C4 processes on their top leaves which are exposed to the sun but lower leaves use C3.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2007, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.