Figure 6.25 Relative Energy Gain of Different Types of Metabolism The left axis shows potential energy sources, ordered according to their relative tendency to give up electrons; those at the top lose electrons most easily.The right axis shows potential terminal electron acceptors, ordered according to their relative tendency to gain electrons; those at the bottom accept electrons most readily. Energy is released only when electrons are transferred from an energy source to a terminal electron acceptor that is lower on the chart; the greater the downward slope, the more energy that can be harvested to make ATP.

Chapter 6 Metabolism: Fueling Cell Growth

PERSPECTIVE 6.1 Mining with Microbes

Microorganisms have been used for thousands of years in the production of bread and wine. It is only in the past several decades, however, that they are being used with increasing frequency in another area of biotechnology, the mining industry. Although mining is a centuries-old industry, modern technologies have not yet been employed in mining as they have in medicine and agriculture.The mining process consists of digging crude ores from the earth, crushing them, and then extracting the desired minerals from the contaminants.The extraction process of such minerals as copper and gold frequently involves harsh conditions, such as smelting, and burning off the contaminants before extracting the metal, such as gold, with cyanide. Such activities are expensive and deleterious to the environment. With the development of biomining, some of these problems are being solved.

In the process of biomining copper, the low grade ore is dumped outside the mine and then treated with sulfuric acid. The acid conditions encourage the growth of the acidophilic bacterium Thiobacillus ferrooxidans, present naturally in the ore.This organism uses CO2 as a source of carbon and gains its energy by oxidizing sulfides of iron first to sulfur and then to sulfuric acid.The sulfuric acid dissolves the insoluble copper and gold from the ore. Currently, about 25% of all copper produced in the world comes from the process of biomining. Similar processes are being applied to gold mining.

The current process of biomining employs only the bacteria that are indigenous to the ore. Many improvements should be possible. For example, the oxidation of the minerals generates heat to the point that the bacteria may be killed.The use of thermophiles should overcome this problem. Further, many ores contain heavy metals, such as mercury, cadmium, and arsenic, which are toxic to the bacteria. It should be possible to isolate bacteria that are resistant to these metals. Biomining is still in its infancy.

6.8 Photosynthesis

Plants, algae, and several groups of bacteria are able to harvest the radiant energy of sunlight, and then use it to power the synthesis of organic carbon compounds from CO2. This capture and subsequent conversion of light energy into chemical energy is called photosynthesis. The general reaction of photosynthesis can be summarized as:

Light energy

Photosynthetic processes are generally considered in two distinct stages. The light-dependent reactions, often simply called the light reactions, are used to capture the energy from light and convert it to chemical energy in the form of ATP. The light-independent reactions, also termed the dark reactions, use that energy to synthesize organic carbon compounds. The processes used to convert carbon dioxide into organic compounds is called carbon fixation. We will describe the steps of carbon fixation in a separate section because a variety of prokaryotes other than photosynthetic bacteria use the process. Characteristics of various photosynthetic mechanisms of organisms we will describe are summarized in table 6.9.

Capturing Radiant Energy

Photosynthetic organisms are highly visible in their natural habitats because they possess pigments to capture light energy. These pigments vary in color because they absorb different wavelengths of light. The color we observe is due to the wavelengths that are reflected; for example, pigments that absorb only blue and red light will appear green (see figure 5.8). Multiple pigments are involved in photosynthesis, increasing the range of wavelengths of light that can be absorbed by a cell. The various pigments include:

■ Chlorophylls are found in plants, algae and a group of bacteria called cyanobacteria. The various types of chlorophylls are designated with a letter following the term, for example, chlorophyll a. The photosynthetic processes of organisms that employ chlorophylls are oxygenic, meaning they evolve oxygen.

■ Bacteriochlorophylls are found in two groups of bacteria, purple photosynthetic bacteria and green photosynthetic bacteria; bacteriophylls absorb wavelengths not absorbed by chlorophylls, enabling the purple and green bacteria to grow in habitats where other photosynthetic organisms cannot. The various types of bacteriochlorophylls are designated with a letter following the term, for example, bacteriochlorophyll a. The photosynthetic processes of organisms that use bacteriochlorophylls are anoxygenic, meaning that they do not evolve oxygen.

■ Accessory pigments include carotenoids and phycobilins. They increase the efficiency of light capture by absorbing wavelengths of light not absorbed by chlorophylls. Carotenoids are found in a wide variety of photosynthetic organisms, including both prokaryotes and eukaryotes. Mammals can use one of the carotenoids, ^-carotene as a source of vitamin A. Phycobilins are unique to cyanobacteria and red algae.

The photosynthetic pigments are located together in protein complexes called photosystems, which specialize in capturing and using light (figure 6.26). Within the photosystems, the various pigments play two very different roles:

■ Reaction-center pigments function as the electron donor in the photosynthetic process; in response to excitation by radiant energy, the molecule emits an electron, which is then passed to an electron transport chain similar to that used in respiration. The oxygenic photosynthetic organisms (plants, algae, and cyanobacteria) use chlorophyll a as the reaction-center pigment; the anoxygenic photosynthetic organisms (purple and green bacteria) use one of the bacteriochlorophylls.

■ Antennae pigments make up what is called the antenna complex, which acts as a funnel, capturing the energy of light and then transferring it to the reaction-center pigment.


Table 6.9 Comparison of the Photosynthetic Mechanisms Used by Different Organisms

Oxygenic Photosynthesis

Anoxygenic Photosynthesis

Table 6.9 Comparison of the Photosynthetic Mechanisms Used by Different Organisms

Oxygenic Photosynthesis

Anoxygenic Photosynthesis

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