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October 07, 2013 Introduction to Photosynthesis
For this discussion, we will focus on Photosynthesis in eukaryotes, specifically in the chloroplast.
Chloroplast
Like the mitochondria, the chloroplast is an endosymbiont. The cyanobacteria (photosynthetic bacteria) is the prokaryotic relative of the modern chloroplast. The cyanobacteria became an endosymbiont after the formation of the mitochondria (remember, all eukaryotes carry mitochondria (or at least mitochondrial DNA), but only photosynthetic eukaryotes have chloroplasts). There are different lineages of chloroplasts, just as there are different lineages of mitochondria. NOTE: Cyanobacteria is a phyllum of bacteria. The chloroplast is considered a part of that phyllum. |
| Figure 1: The Chloroplast - http://en.wikipedia.org/wiki/File:Chloroplast_II.svg |
The outer chloroplast membrane is eukaryotic in nature, while the inner membrane is bacterial in origin and composition. In some plants, there is even a peptidoglycan cell wall (bacterial cell wall) between the outer and inner membrane.
Inside of the inner membrane, the chloroplast can be divided into the Stroma and the Thylakoid Membranes.
The stroma is the fluid filled inner compartment of the chloroplast, while the thylakoid membrane is an internal membrane that specializes in the harvesting light (photons) and converting the energy into reducing power.The thylakoid membrane is divided into two main types: Stromal Thylakoids (aka Lamellae or Frets) and Granal Thylakoids. The granal yhylakoids make up the stacks of thylakoids known as Granum, while the stromal thylakoids create supporting structure. The entire system of thylakoid membranes is suspended inside of the stroma.
The thylakoid membrane system has its origin in the Cyanobacteria (see figure 2). The cyanobacteria are one of the bacterial groups able to create internal membranes. They form Photosynthetic Lamellae, which are the precursors to the thylakoid membrane. This lamellae provides an increased surface area for pigments (phycobilisomes). This increases the chance that a photon will strike a pigment in the correct orientation. If it were not for the increased surface area, the bacterium would have very little energy to carry out carbon fixation. (Remember: To get enough energy, a phototroph needs an increased surface area).
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| Figure 2: Cross-Section Structure of Cyanobacteria-http://images.tutorvista.com/content/kingdoms-living-world/cyanobacteria-cell-structure.jpeg |
Photosynthesis
Photosynthesis is divided into two stages: the Light Dependent Reactions and the Calvin Cycle (aka Light Independent or Dark reactions).
The light dependent reactions involve the pigments (photosystems) and proteins of the thylakoid membranes. Within the system of thylakoid membranes, NADPH + H+ and ATP will be produced. You will see the formation of a proton motor force, which will be used to make ATP. Note that we are not using NAD+, but instead NADP+. As a generality, NAD+is the electron carrier used in catabolic reactions, while NADP+ is the electron carrier used in anabolic reactions (why? does it have to do with the enzymes being used?).
The energy (reducing power and ATP) created during the Light Dependent Reactions will be used in the Calvin Cycle to reduce carbon compounds, with the end result being the production of glucose. In your studies of biology, you may have seen the following reactions:
Cellular Respiration
C6H12O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + heat
Photosynthesis
light + 6CO2 + 12H20 --> C6H12O6 + 6O2 + 6H20
They look simple, but as we have discussed with cellular respiration, it takes many steps to get to the final product. Likewise, with photosynthesis, we are not going to complete this general reaction in one step. One common misconception is that Cellular Respiration is the reverse of Photosynthesis. On the surface they may appear to be reversals, but you're not going to see cellular respiration solely in reverse (there are times that you will see some reactions from glycolysis).
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