Photosynthesis

The most basic form of fermentation is called homolactic fermentation, in which the only end product is lactic acid. It starts with glycolysis and its end product, pyruvate, but goes to make lactic acid in a single redox reaction. This is one of the only respiratory pathways that doesn’t produce a gaseous byproduct. It happens in mammalian muscle cells and in lactobacilli and some fungi. The sour taste of yogurt comes from the lactic acid made by lactobacilli.

There is also heterolactic fermentation, in which lactic acid isn’t the only end product. Some goes on to make carbon dioxide and ethanol (using phosphoketolase). Some also goes on to make acetate or other waste products. The reasoning behind the need to do this at all is because lactic acid is too acidic to drive certain biological processes. This is why food is fermented in the first place—it drives out other bacteria and keeps them from taking hold, extending the shelf life of food.

Another reason to have heterolactic fermentation is because, if things like ethanol and CO2 are produced as end products, they are volatile, leaving the situation so that the rate of forward reactions is kept up. Too high a concentration of an end product like lactic acid will drive the concentration backward, slowing the growth of the organism itself. Things like propionic acid and butyric acid become better end products because they aren’t as acidic and won’t interfere with cellular growth. Several organisms will produce hydrogen gas as part of the fermentation process, including those that make butanol, butyric acid, caproate, and glyoxylate. The hydrogen gas will help to regenerate NAD from NADH. Remember that this is how NAD is recycled in many fermentation processes. The hydrogen gas is used by methanogens and organisms that are sulfate reducers as well and, in high concentrations, can be given off as a gaseous substance.

PHOTOSYNTHESIS

Photosynthesis is the process by which light energy becomes chemical energy— something common to plants, most algae, and cyanobacteria. The chemical energy is stored in carbohydrate molecules (sugars) that are essentially made by CO2 and water. In the majority of cases, the waste product of this reaction process is O2 gas. It is these

organisms that ultimately lead to the oxygen present in on earth as well as the organic molecules present on earth.

There is no single photosynthetic pathway for the different species that participate in it. It does, however, start out always with light being absorbed by proteins known as reaction centers. These reaction centers contain chlorophyll pigments located inside chloroplasts—organelles unique to higher order photosynthetic organisms. It can also be done in bacteria that have no chloroplasts but have pigmentation in their plasma membranes. Figure 25 shows the process of photosynthesis:

The light energy goes into the splitting of water, freeing hydrogen atoms and leaving oxygen behind as a waste product. These hydrogen atoms create two energy molecules: 1) ATP and 2) nicotinamide adenine dinucleotide phosphate (NADPH). These are shortterm energy storage molecules used to make long-term storage in the form of sugars. Some plants use the Calvin cycle, which takes the energy gotten from the initial light-

dependent reactions (NADPH and ATP) to make sugar molecules out of CO2. There are other organisms (especially bacteria) that use a type of “reverse Krebs cycle” to do the same thing.

In the Calvin cycle, the organic compound called ribulose bisphosphate is acted on along with CO2 and energy to make higher order organic molecules. Ultimately, the process goes on to make glucose and other six-carbon sugars. Photoautotrophs are organisms that can take CO2 alone to make organic molecules. Photoheterotrophs use other organic molecules besides CO2. Most organisms (algae, cyanobacteria, and plants) undergo oxygenic photosynthesis; that is, they make oxygen. Certain bacterial species undergo anoxygenic photosynthesis. They use light energy but do not give off oxygen.

In a vague sense, photosynthesis is the opposite of cellular respiration, although different enzymes are involved. This process, known as carbon fixation, is endothermic and involves the reduction of carbon dioxide. Reduction, as a rule, means to add hydrogen atoms to something. Rather than giving off energy, these endothermic reactions store energy in the form of carbohydrate molecules—an energy stored in the chemical bonds of the carbohydrate molecule. The process necessarily must involve the consumption of water in order to provide the hydrogen atoms necessary for reduction.

In oxygenic photosynthesis, the reaction proceeds as such:

CO2 plus 2 H20 molecules plus a photon of light energy goes to carbohydrate plus oxygen plus one water molecule. Note that, in actuality, water is both a reactant (starting molecule) and end-product of photosynthesis; it’s just that it takes more water to put in the reaction than is given out at the end of the reaction.

In anoxygenic photosynthesis, carbon dioxide mixes with arsenite instead of water. This leads to arsenate and carbon monoxide instead of oxygen.

There are two phases of photosynthesis. The first involves light-dependent reactions. Light is captured to make ATP and NADPH, which as mentioned are temporary storage molecules. The second in involves light-independent reactions, which uses the energy molecules made in the first phase to fix carbon.