Explain the general reaction for photosynthesis in terms of water, light, oxygen and carbon dioxide and carbohydrate.
Describe the balance between respiration and photosynthesis that sustains life in the biosphere.
Describe the structure of the chloroplast.
Explain the difference between the light reactions and the Calvin cycle.
Explain how pigments are important to photosynthesis.
Relate the absorption spectrum of a pigment to its color.
Compare the function of the two photosystems in green plants.
Explain how the light reactions generate ATP and NADPH.
Explain the molecular basis of energy generated as a result of electron transport.
Describe what is meant by the term ‘carbon fixation.’
Explain how the Calvin cycle produces glucose.
Explain the action of rubisco in oxidizing RuBP.
Compare and contrast major variations in photosynthesis, including the conditions under which these variations are most likely encountered.
Describe the path of energy from sunlight to covalent bonds formed during the light reactions of photosynthesis.
Describe the path of energy from the light reactions through the Calvin Cycle.
Chapter Summary
Eukaryotic chloroplasts are composed of stacks of thylakoid disks called grana located within the stroma, a fluid matrix. The photosynthetic pigments are bound to the thylakoid membrane which pumps protons from the stroma to the interior. ATP molecules are generated as the protons diffuse back out to the stroma. The enzymes of the Calvin cycle are in the stroma.
Photosynthesis is composed of two very different processes: the light reactions and the Calvin cycle. The light reactions occur in eukaryotic chloroplasts on specific photosynthetic membranes of bacteria. The pigment captures a photon of light and excites one of its electrons. The excited electron shuttles through various carrier molecules to a final acceptor and chemiosmotically generates ATP and NADPH. The Calvin cycle fixes carbon by using the products of the light reactions to chemically reduce carbon dioxide into organic molecules.
Early research in plant physiology showed that plants did not derive major nutrients from the soil to support their growth, but that the sun’s energy and carbon dioxide were required. Light energy exists in the form of packets called photons. Photons of short wavelength light are more energetic than photons of long wavelength light. The energy in these photons is captured by carotenoid or chlorophyll pigments. The former absorb photons with a broad range of energy and are not highly efficient, while the latter absorb a narrow range of photons very efficiently. Most photosynthetic organisms use chlorophylls as their light gathering pigment.
Early photosynthetic bacteria exhibited cyclic photophosphorylation, a process that only produces ATP and does not provide for biosynthesis. The bacterial reaction center channels its light energy to P870, which then passes to a primary electron acceptor. The electron returns to the pigment through an electron transport chain. This drives a proton pump and produces ATP through chemiosmosis. Other bacteria improved on this photosystem, utilizing chlorophyll a to absorb the more energetic photons associated with shorter wavelengths of light. The P680 pigment of photosystem II became the first stage of a two stage photosystem while P700 remained as the pigment of photosystem I. The excited electron of photosystem II drives a proton pump and chemiosmotically generates ATP. The electron then passes on to photosystem I where it absorbs another photon of energy. This electron is channeled to the primary electron acceptor where it generates reducing power by reducing NADP+ to NADPH. The electron removed from photosystem II is replaced by an electron obtained from the splitting of a molecule of water. Oxygen is a byproduct of this reaction, called noncyclic photophosphorylation.
Carbon fixation is similar to glycolysis, but runs in reverse. The Calvin cycle uses ATP energy and NADPH reducing power to make organic molecules from carbon dioxide. Carbon dioxide attaches to the five-carbon molecule ribulose bisphosphate (RuBP) and is then split into two molecules of three-carbon phosphoglycerate (PGA) by an enzyme called rubisco. Some of these molecules are used to reconstitute RuBP; others are assembled into sugars via glyceraldehyde-3phosphate (G3P). Six turns of the cycle are needed to form glucose.
Plants that exhibit C3 photosynthesis lose much of their fixed carbon when RuBP carboxylase interferes with the Calvin cycle, a process called photorespiration. C4 plants expend ATP to concentrate carbon dioxide in the cells that carry out the Calvin cycle. This high concentration of carbon dioxide prevents RuBP carboxylase from binding oxygen and thus reduces photorespiration. The loss of ATP greatly outweighs the potential loss of fixed carbon. Many succulent plants reduce photorespiration by closing their stomata and thus decrease the amount of carbon dioxide present during the day. These plants are called CAM plants; they use both C3 and C4 pathways within the same cells. C4 plants use both pathways, but do each in a different cell.
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