Autotrophy and phototrophy in the microbial world.
organism derives all of its carbon needs from carbon dioxide.
CO2 usually “fixed” by a complex multistep process known as the Calvin cycle,
Most autotrophs (except for some bacteria) are also phototrophs, deriving energy from light
Essentials of phototrophy
1. Light energy is captured by special molecules called photosynthetic pigments.
2. Light energy thus captured is used to “excite” electrons
3.Electrons can now move through electron transport stimulating creation of proton gradient
4. Protons flowing “down” ATP synthase stimulate phosphorylation of ADP
Simplest (probably oldest) example of phototrophy:
in extreme halophilic Archaea
ex.: Halobacterium, Halococcus
Pigment is bacteriorhodopsin—similar to visual pigment in vertebrates
Light energy exites an electron in bacteriorhodopsin
Electron is transferred to ets where it gives up energy
Transfer of energy to ets stimulates proton gradient etc as above Electron then returns to the pigment molecule
THIS IS CALLED CYCLIC ELECTRON FLOW
Cyclic vs noncyclic
Note that what happens in Halobacterium etc is NOT autotrophy, hence NOT PHOTOSYNTHESIS. If anything, it’s photoheterotrophy
For photosynthesis, you must use light energy
not merely to make ATP but to
transfer electrons from some source, ultimately to carbon dioxide
AND HENCE MAKE CARBOHYDRATE
This is the “synthesis” in photosynthesis!!!!!!!!!
True photosynthesis requires some type of noncyclic electron flow
Cyclic flow will still probably be present, but electrons must move from point a to point b to make carbohydrate
Requirements for photosynthesis:
1. Electron donor: most common is water, other molecules also employed by some bacteria, especially H2S. General formula for electron donor is H2A.
2. Pigments: primary and accessory. Primary pigments absorb most of the energy, accessory pigments absorb at wavelengths where primary pigments can’t. Most important primary pigments are chlorophyll a and various bacteriochlorophylls. Accessory pigments include various carotenoids etc 3. Photosynthetic membrane. This contains pigments and ets components grouped into PHOTOSYSTEMS.
4. Ability to use ets to make ATP by chemiosmosis just like in respiration
5. Ability to use light energy to transfer electrons from donor to coenzyme. Coenzyme is either NAD or NADP.
6. A carbon dioxide fixation system: most common by far is Calvin cycle. Takes electrons on coenzyme and ATP energy and uses these to reduce carbon dioxide to carbohydrate
Oxygenic vs. anoxygenic
1. Occurs only in certain groups of photosynthetic bacteria (most groups customarily named after colors: green bacteria, purple bacteria etc)
2. Uses bacteriochlorophyll rather than chlorophyll (they differ in relatively minor chemical ways)
3. Uses only one photosystem
4. H2S is most common electron donor—generally anaerobic
5. Most (not all) use Calvin cycle for CO2 fixation, some use "reverse Krebs cycle"
6. Some (the "purple nonsulfur") live photoheterotrophically: can use light to make ATP but still need at least some preformed organic compounds. In absence of light some of these can just live as chemoheterotrophs.
Oxygenic vs. anoxygenic
1. Occurs in all photosynthetic eukaryotes plus the cyanobacteria
2. Uses water as electron source
3. Primary pigment is always chlorophyll a
4. Needs two photosystems to transfer electrons from water to coenzyme
5. Always employs calvin cycle to fix carbon dioxide
The Calvin Cycle
Occurs in chloroplast in eukaryotes, in cytoplasm in prokaryotes
Used by overwhelming majority of autotrophs to “fix” CO2
Complex process, can be thought of in three “phases”:
In carboxylation phase, carbon dioxide is actually fixed:
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