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Radboud universityFaculty of ScienceBiologyHOMEWeb modulesThe  leaf >     Photosynthesis

    Photosynthesis

Plants occupy a fundamental position in the food web, since they serve as the primary source (producers) of organic compounds for animals and humans. Plants utilize their leaves to produce sugars in a process that involves light and which is called photosynthesis. Simplified (Fig. 1), water (H2O, the source of H and O) and carbon dioxide (CO2, the source of C) are transformed into sucrose (a sugar = C6H12O6) and oxygen (= O2 to the atmosphere) in a process in which light energy is consumed:
 
Fig. 1; photosynthesis reaction
A. Flow of photosynthetic components in the leaf, B. Chlorofyll molecuule (thanks to Dennis Vriezema), C. Absorption curves of chlorofyll a, b and caronetoiuml;ds, D. Chloroplast and details of thylakoids (Transmission electron microscopy), E. Very simplified representation of the photosynthesis reactions in a mesophyll cell
1 Cell wall, 2 Cytoplasm, 3 Vacuole, 4 Chloroplast envelope (2 membranes), 5 Tonoplast, 6 Plasma membrane, 7 Grana, 8 Stroma thylakoids, 9 Starch grains, 10 Stroma

 
Movie 'Plants: photosynthesis' of Britannica.com


  • Photosynthesis starts with a so-called light reaction. Light energy from the sun is captured by pigment molecules (chlorophyll a, chlorophyll b en carotenes; Fig B and C) that are located in the internal membranes (thylakoids) of chloroplasts (Fig. D and E). This energy is consumed to produce reducing power (H+ ions = protons) and molecular oxygen (O2) from water molecules (H2O). The reducing power (the protons) is used to generate ATP (adenosine triphosphate) and NADPH (reduced nicotine amide adenine dinucleotide phosphate; fig F).
  • Then, in the so-called dark reaction (a process that does not require light,but does not require darkness neither) ATP and NADPH are invested to include CO2 into organic molecules.
  • The CO2 fixation and net production of carbohydrates (sugar-like) molecules occurs in the stroma (the fluid part of the chloroplasts) in a complex cascade of reactions named the Calvin cycle (Fig. F). Most angiosperms and gymnosperms are so-called C3 plants, because the first product of CO2 incorporation is a three-carbon-compound: 3-phosphoglycerate.
  • In C4 plants, like corn and sugarcane, the Calvin cycle is preceded by fixation of CO2 into a four-carbon-molecule (oxaloacetate) under control of an enzyme (phosphoenolpyruvate [=PEP] carboxylase) with a high affinity for CO2. The efficient C4 mechanism is correlated to a unique leaf anatomy: Kranz anatomy (Kranz = wreath: see micrograph), in which veins are surrounded by a CO2 fixing bundle sheath, which in turn stays in direct contact with the mesophyll cells where the Calvin cycle occurs.
     

  • Go to the Bioplek for beautiful animations and explanations on photosynthesis (secondary school level, in Dutch but very visual)
  • In autumn most leaves loose their green color and become yellowish or reddish. How does this come? (see answer)
    last modified: 1 Oct 2013