Introduction to Energy Metabolism

Probably the most important concept concerning cells and all of life itself is that living organisms are highly ordered, highly structured systems. The universe as a whole is constantly becoming less and less orderly; its disorder (entropy) is increasing. Bacteria, protists, fungi, plants, and animals, however, represent phenomena in which particles become more orderly. A plant absorbs diffusely scattered molecules of carbon dioxide, water, and minerals and organizes them into organic molecules, cells, tissues, and organs. Each plant carries this out with such precision that each species of plant is easily distinguishable from others. After death, decay is the process by which an organism's molecules become more disordered and scattered—their entropy increases.

Because living organisms are part of the natural world that is described by the laws of physics and chemistry, the decrease in the entropy of living organisms must obey physical laws. This is accomplished by putting energy into the living system, the source of energy being sunlight. To be accurate, we must consider the sun and life together: The atomic reactions that generate sunlight cause greater disorder in the sun than sunlight causes order in living organisms. The whole system (sun + life) becomes more disordered. Because there is no means of putting energy into an organism's body after death, an increase in entropy cannot be prevented.

Sunlight maintains and increases the orderliness of life by two methods: (1) directly, in the process of photosynthesis, which produces complex organic compounds, and (2) indirectly, in the respiration of those organic compounds, either by the organism itself or by another organism that eats it. These two methods of supplying energy and maintaining orderliness—photosynthesis and respiration—are the basis for a major, fundamental dis- faction in the types of organisms. Photoautotrophs are organisms that gather energy directly from light and use it to assimilate small inorganic molecules into their own tissues. Photoautotrophs include all green plants, all cyanobacteria, and the few bacteria capable of photosynthesis. Heterotrophs are organisms that cannot do this but instead take in organic molecules and respire them, obtaining the energy available in them. Heterotrophs include all animals, all completely parasitic plants (Fig.), all fungi, and the nonphotosynthetic bacteria. Gathering energy by taking in organic material has the advantage that part of the material can be used as construction material instead of fuel. At least some of the amino acids, fatty acids, and sugars in food can be built into the organism's own polymers and the rest respired for energy. Photoautotrophs must build all their own molecules using just carbon dioxide, water, and various nitrates, sulfates, and other minerals.

FIGURE :Total parasites such as this broomrape (Orobanche) are heterotrophs like animals. Like parasitic tapeworms or blood flukes, they absorb monomers such as monosaccharides and amino acids.

Tremendously important consequences follow from the fact that photoautotrophs and heterotrophs differ in their sources of energy and building material (Table 10.1). Sunlight and carbon dioxide do not need to be stalked, hunted, and captured, so sensory organs, muscles, and central nervous systems like those of animals are unnecessary. Conversely, the ocean is full of microscopic bits of food, and animals such as sponges and corals can gather it the way plants gather carbon dioxide. The mode of nutrition has had overriding influence on the bodies and metabolisms of plants and animals.

Tissues and organs are also either photoautotrophic or heterotrophic. Chlorophyllous leaves and stems are photoautotrophic, whereas roots, wood, and flowers are heterotrophic and survive on carbohydrates imported through phloem. During winter, if all leaves have abscised, the entire plant may be composed of heterotrophic tissues, and it maintains its metabolism by respiring stored starch.

Tissues often change their type of metabolism; young seedlings are white and heterotrophic while germinating underground; they survive on nutrients stored in cotyledons or endosperm. Seedlings become photoautotrophic only after they emerge into sunlight. Immature fruits may be green and photosynthetic, but in the last stages of maturation, chloroplasts are converted to chromoplasts and metabolism then depends on imported or stored nutrients (chromoplasts are plastids that contain large amounts of pigments other than chlorophyll). Young leaf primordia are green, but they grow more rapidly than their own photosynthesis would permit; they have a mixed metabolism of photosynthesis and carbohydrate import.

Photosynthesis is a complex process by which carbon dioxide is converted to carbohydrate This involves endergonic reactions driven by ATP and requiring new bonding orbitals filled by electrons carried to the reaction by NADPH. Before this can happen, ATP and NADPH themselves must be formed in highly endergonic reactions driven by light energy. In order to understand this, you must first understand the nature of light and pigments along with the concept of reducing power.