Kohlenstoff-Aufnahme: Photosynthese

by louisa L.

Biochemistry of Photosynthesis

  • Photosynthesis involves two major groups of reactions: The light-harvesting reactions (Lichtreaktion) transform light energy into temporary forms of chemical energy (ATP and NADPH), The carbon-fixation reactions (Dunkelreaktion) use the products of the light harvesting reactions to convert CO2 into sugars, a more permanent form of chemical energy that can be stored, transported or metabolized.

  • Both groups of reactions occur simultaneously in the light in chloroplasts, which are organells inside phyotosynthetic cells

  • In the light-harvesting reactions, chlorophyll (a light absorbing pigment) captures energy from visible light. Absorbed radiation is converted to chemical energy (NADPH and ATP), and oxygen is produced as a waste product. Visible radiation accouts for 40% of incoming solar radiation, which places an upper limit on the potential efficiency of photosynthesis in converting solar radiation into chemical energy.

  • The carbon-fixation reactions of photosynthesis use the chemical energy (ATP and NADPH) from the light-harvesting reactions to reduce CO2 to sugars. The rate-limiting step in the carbon fixation reactions is the reaction of a five-carbon-sugar (Ribulose-biphosphate (RuBP)) with CO2 to form two three-carbon organic acids (phosphoglycerate), which are then reduced using ATP and NADPH from the light reactions to form three carbon sugars (glyceraldehyde 3-phosphate). The initial attachment of CO2 to a carbon skeleton is catalyzed by the enzyme ribulose-biphosphate carboxylase-oxygenase (Rubisco).

  • In terrestrial plants CO2 used in photosynthesis diffuses along a concentration gradient from the atmosphere outside the leaf to the chloroplast. The boundary layer, stromata and cellular water all influence the overall diffusion of CO2 from the free air to Rubisco, but Stromata are the largest (and most variable) component of this resistance. Water readily evaporates, and water vapour diffuses out through the stromata to the atmosphere. The open stromata that are necessary for plants to gain carbon are therefore an avenue for water.

  • Zusammenfassung:

    • 1. CO2 Diffusion

    • 2. Lichtreaktion

    • 3. Kohlenstofffixierung

    • 4. Photorespiration


louisa L.


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