Bioenergetics: Energy and Metabolism (With Diagram)

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., Fe^{2 +}), 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.