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Photosynthesis - Other Approaches

Other Approaches to Energy Production


The Earth has a large collection of widely diverse environments in which living things manage to grow and survive. There are different approaches to photosynthesis that are more suitable than the standard approach in these different environments. Some organisms forgo the use of light for energy production entirely. However, all of these adaptations are variations on the same basic pathways of photosynthesis and respiration. In understanding these variations, you will better understand the pathways themselves.

C3 vs C4 plants

As mentioned previously, Rubisco is the most abundant enzyme on Earth. It is inarguable that this is a very important enzyme to all life, but it is believed that Rubisco is so abundant because of its inefficiencies.

Rubisco will sometimes recognize oxygen as a substrate instead of carbon dioxide. In that case, the following set of reactions occur:

As you can see, this result is very detrimental to photosynthesis. Instead of fixing carbon dioxide into a complex sugar, the plant has made extra work for itself in creating phosphoglycolate, a nearly useless compound. This reaction, using oxygen instead of carbon dioxide, directly competes with the regular reaction, at the same site on the enzyme.

At 25°C, the rate of the carboxylase reaction is four times that of the oxygenase reaction, so the plant is only about 20% inefficient. However, as temperature rises, the balance in the air between oxygen and carbon dioxide changes (due to changing solubility in the ocean), and the carboxylase reaction is less and less dominant. Plants living in warm climates have to overcome this handicap. In addition, plants living in arid climates have to close the pores in their leaves when it is particulalry dry, or they will wither. This also gives the effect of creating a closed environment in which, as carbon dioxide gets used up in photosynthesis, the relative concentration of oxygen increases and the oxygenase reaction begins to dominate.

A solution has evolved to combat this problem. Plants living in the abovementioned difficult conditions have discovered a way to make the carbon dioxide concentration very high in the immediate environment of Rubisco, so that the oxygenase reaction does not get a chance to happen.

This pathway is called the C4 pathway because it involves a 4 carbon intermediate in the outer cells. The 4-carbon intermediate brings a molecule of carbon dioxide right into the bundle sheath cells, where it is dropped right next to the location of the Calvin Cycle. In this way, the plant ensures that the concentration of carbon dioxide at the site of Rubisco is very high, so that only the carboxylase and not the oxygenase reaction can take place. The conventional pathway is called the C3 pathway because it involves only the 3-carbon sugars. Note that the C4 pathway still uses the conventional Calvin Cycle with its 3-carbon sugar intermediates; it makes use of 4-carbon sugars to bring the carbon dioxide closer to the site of fixation.

So why don't all plants adopt the C4 process? Or, more correctly, why don't the C4 plants out-compete the C3 plants, which are inefficient? Well, notice that it takes ATP to bring the carbon dioxide to the Rubisco. In moderate temperatures, the energy burden that this puts on the plant outweighs the advantage of eliminating the one in five times that Rubisco binds oxygen instead of carbon dioxide. In warmer climates, however, the C4 plants win with their novel strategy.

Lithotrophs

Some bacteria can derive reductive power by oxidizing compounds such as hydrogen gas, carbon monoxide, ammonia, nitrite, hydrosulphuric acid, sulphur, sulphate, or iron. These bacteria are called lithotrophs or rock-eaters, and their energy production system is called chemosynthesis.

Chemosynthesis works by oxidizing an inorganic substance (remember - oxidation is a loss of electrons, so this inorganic substance is the electron donor) and transporting these electrons through the membrane, like in oxidative phosphorylation and in photosynthesis. This electron transport pumps protons through the membrane, generating a proton gradient which can be used to form ATP.

These organisms make so much ATP through chemosynthesis that they can drive the electron transport chain backwards to generate NADH. This NADH provides the reducing power needed to synthesize carbon structures from carbon dioxide.

Bacterial Group Typical Species Electron Donor Carbon Source
Hydrogen-Oxidizing Alcaligenes eutrophus H2 CO2
CO-Oxidizing Pseudomonas carboxydovorans CO CO2
Ammonium-Oxidizing Nitrosomonas europaea NH4+ CO2
Nitrite-Oxidizing Nitrobacter winogradskyi NO2- CO2

Methanogens

The methanogens are a unique class of anaerobic bacteria. They derive energy by reducing carbon dioxide to methane, a quite different approach than other organisms. They use carbon dioxide as an energy source rather than treating it as an energy-depleted waste product. The methanogens are also special in that they can oxidize hydrogen gas to directly reduce NAD+ to NADH, rather than having to waste energy by making ATP through chemosynthesis and then driving it backwards through the electron transport chain. It is important to note that these organisms still incorporate their carbon into the Krebs Cycle for processing into amino acids, nucleic acids, and sugars. The respiratory pathway may have many variations, but they all build on the same backbone.

For more information about methanogenic metabolism, consult The Physiology and Biochemistry of Prokaryotes Chapter 12, David White, 1995 Oxford University Press.


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