Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 26

Organisms can also be classified according to their metabolic capabilities. While this method does not strictly follow evolutionary relationships, it is still very useful for understanding patterns of energy and metabolite flow, which is especially important in phototrophic organisms. A number of these patterns can be present simultaneously in a single organism, leading to names that are often intimidating. However, they are simply combinations of the individual metabolic patterns.

A fundamental metabolic distinction is between autotrophs and heterotrophs. The “troph” part is derived from a Greek word meaning “to feed” Autotrophs are “self‐feeding” organisms that derive all their cellular carbon from CO₂, whereas heterotrophs are organisms that derive cellular carbon from organic carbon compounds. A second pattern relates to the source of energy for cellular processes. Phototrophs derive their energy from sunlight, whereas chemotrophs derive energy from various types of chemical compounds. If these compounds are organic chemicals, the organisms are chemoorganotrophs. If they are inorganic chemicals, they are called chemolithotrophs.

An organism that derives its energy from light and all its cellular carbon from CO₂ is known as a photoautotroph. Most photosynthetic organisms can grow in this manner. If the organism grows by using light as an energy source, but assimilates organic carbon, it is known as a photoheterotroph. Many phototrophic organisms can also grow in this way, and in a large number of cases, a single organism can grow either photoautotrophically or photoheterotrophically, depending on the availability of organic matter. Table 2.1 summarizes some of the metabolic relationships among living organisms.

Table 2.1 Metabolic patterns in living organisms

Type of organism	Metabolic characteristics
Autotroph	Organism that is capable of living on CO2 as sole carbon source
Heterotroph	Organism that uses organic carbon as carbon source
Phototroph	Organism that uses light as source of energy
Chemotroph	Organism that uses chemicals as energy sources
Photoautotroph	Organism that uses light as source of energy and CO2 as sole carbon source
Photoheterotroph	Organism that uses light as source of energy and organic compounds as carbon source
Chemoorganotroph	Organism that obtains energetic needs from organic compounds
Chemolithotroph	Organism that obtains energetic needs from inorganic compounds
Chemolithoheterotroph	Organism that obtains energetic needs from inorganic compounds and uses organic compounds as carbon source
Chemolithoautotroph	Organism that obtains energetic needs from inorganic compounds and uses CO2 as sole carbon source

Oxygen is central to the metabolism of most cells. If an organism is capable of growing in the presence of oxygen, it is classed as an aerobe. If it cannot grow in the presence of oxygen, it is called an anaerobe. Some organisms can switch back and forth from aerobic and anaerobic metabolisms and are call facultative aerobes. In most cases, aerobes utilize organic molecules as electron donors and O₂ as an electron acceptor in a process called aerobic respiration. Other inorganic compounds can sometimes serve as electron acceptors, a process known as anaerobic respiration. Finally, organisms that use organic compounds as both electron donors and acceptors in the absence of oxygen live by carrying out fermentation.