In the study of physiology and biochemistry, cells are often thought of as tiny machines in which all events may be explained in terms of either chemical reactions, fluid dynamics, electrical fluxes across partitions, or the absorption or emission of light.
In other words, cellular activities, regardless of their level of complexity, are ultimately founded on the known laws of physics and chemistry.
Those laws of physics and chemistry those are fundamental to an understanding of cellular activities, especially the production and consumption of energy during cell metabolism.
Energy and Metabolism:
The metabolism of a cell is characterized by a myriad of chemical reactions in which energy is either consumed produced, or transduced (i.e., converted) from one form into another. Metabolism can be subdivided into two broad categories: catabolism and anabolism.
During catabolic reactions (or reaction sequences), molecules are broken down by the cell into simpler forms; whereas during anabolism, complex molecules are formed from simpler ones. The catabolic and anabolic reactions that proceed in cells are accompanied by energy changes and it is the study of these changes that constitutes the field of bioenergetics.
Consider, for example, an anabolic process such as the synthesis of new membranes within the cell. Such biosynthesis requires (consumes) energy, and this energy must ultimately be obtained from the cell’s environment in some form. Within the cell, the energy (perhaps in a new form) is consumed to “drive” the cell’s membrane-synthesizing processes.
Whole organisms and individual cells may be assigned to different groups according to the nature of the materials that they must acquire from their surroundings in order to support their metabolic needs. Most plant cells (i.e., those that contain chlorophyll) and many different kinds of bacteria require only CO2, H2O (or H2S), simple nitrogen-containing compounds like NH3, and trace mineral elements from their environment in order to fulfill their minimum metabolic needs.
These cells or organisms are called autotrophs. With the exception of trace amounts of certain vitamins, they can live and grow in the complete absence of an exogenous supply of organic materials. (Indeed, most autotrophs do not even need an external source of vitamins.) When an autotroph can utilize light as a source of energy, it is called a photoautotroph. Other autotrophs can obtain their energy from the oxidation of inorganic substances such as ammonium ions (i.e., NH4+), ferrous iron (i.e., Fe2 +), or elemental sulfur (S). This kind of autotroph is called a chemoautotroph (see Table 9-1 for examples).
All animal cells (and certain plant cells and most bacteria) depend on an external source of organic compounds and specific vitamins for their metabolism and are therefore called heterotrophs. Some heterotrophs (e.g., a few algae and bacteria) can also use light as an energy source and are called photo heterotrophs. However, most heterotrophs require organic compounds both as a source of energy and as raw materials for the synthesis of intracellular components; such heterotrophs are called chemo heterotrophs (Table 9-1). Energy that is derived by the catabolism of organic materials is used to meet anabolic needs.
The primary sources of energy and raw materials for heterotrophic metabolism are polysaccharides, lipids, and proteins. Organisms that remove these macromolecules from their environment break them down in the successive catabolic stages of metabolism.
As these compounds are chemically degraded, the chemical energy that is inherent in their molecular structure is both released in the form of heat and used to create the bonds that form new molecules, as in the attachment of free (inorganic) phosphate to ADP to form ATP (Fig. 9-1). The ultimate primary products of catabolism are NH3, CO2, and H2O.
Although autotrophic organisms can use CO2, H2O and small nitrogenous compounds from their environment, these small compounds do not by themselves contain enough extractable chemical energy to sustain the organisms. Consequently, autotrophs also absorb energy in the form of light and, using the light energy, they synthesize simple organic acids from CO2 and water (i.e., photosynthesis), phosphorylate ADP to form ATP, and synthesize amino acids from the organic acids using incorporated NH3 (a process called animation).
Drawing from the pool of ATP as a source of chemical energy, and using these simpler molecules, cells then synthesize complex molecules such as proteins, polysaccharides, nucleic acids, and lipids (Fig. 9-2). All these metabolic activities are accompanied by the loss of some unusable chemical energy as heat.
Autotrophic organisms not only possess the enzymes for the anabolic processes just described but, like heterotrophs, they also have catabolic enzyme systems. Consequently, autotrophs can produce ATP by breaking down polysaccharides, lipids, and proteins.
Like autotrophs, heterotrophs have ATP-dependent anabolic enzyme systems that synthesize macromolecules, but most heterotrophs are unable to carry out photosynthesis. Compounds are cycled within cells and also between cells and between whole organisms as depicted in Figure 9-3. Each transition is accompanied by a specific energy change.