Autotrophy and Phototrophy

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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

Anoxygenic photosynthesis

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

Oxygenic photosynthesis:

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”:

1. Carboxylation

2. Reduction

3. Regeneration

In carboxylation phase, carbon dioxide is actually fixed:

Ribulose...
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