See also metabolism

The process of bacterial metabolism begins with hydrolysis of large macromolecules in the external cellular environment by specific enzymes. Small molecules produced by this hydrolysis (i.e, fatty acids from lipids, monosaccharides from polysaccharides, short peptides from proteins) are transported into bacteria using a variety of transport mechanisms which all require permeases (binding proteins). These transport mechanisms include (1) facilitated diffusion (passive transport) down a concentration gradient (e.g., glycerol), (2) phosphorylation-linked transport where molecules are chemically altered during uptake (e.g., glucose) by phosphorylation, (3) active transport which requires a proton motive force (e.g, lactose) and (4) siderophores which traps Fe+3 from transferrin and transport it into the cell.

The process of substrate breakdown and conversion into usable energy is known as catabolism. The energy produced may then be used in the synthesis of cellular constituents (cell walls, proteins, nucleic acids, etc.) in a process called anabolism.  The specific  used by bacteria for catabolism and anabolism are, for the most part, shared by both prokaryotic and eukaryotic cells.

The primary pathway used by both bacteria and eukaryotic cells for the conversion of glucose to pyruvate (a universal intermediate) is the glycolytic or Embden-Myerhof-Parnas (EMP) pathway. The reactions which make up this pathway occur under both anaerobic and aerobic conditions. There is a net production of 2 molecules of ATP.

In the absence of oxygen, pyruvate undergoes fermentation wherein organic molecules are e acceptors and NAD is regenerated. Synthesis of key intermediates (anabolism) include malate, succinate, oxaloacetate, etc.

Under aerobic conditions, in the TCA cycle, pyruvate is oxidized to water and CO2 along with the generation of GTP and substrates such as alpha ketoglutarate, citrate, etc.. Additional ATP is also generated  in the electron transport chain from the oxidation of NADH which is provided by the TCA cycle.

Nitrogen Fixation of Some Bacteria:

Nitrogen fixation or the reduction of oxidized form of nitrogen into usable forms is the most energetically epxensive reaction to occur in any cell, requiring 16 ATP to make two molecules of NH3. This is due to the triple bond in N2.

Plants need ammonia (NH3) or nitrate (NO3-) to build amino acids, but most of the nitrogen in the atomosphere is in the form of gaseous nitrogen (N2). Plants lack the biochemical pathways (including the enzyme nitrogenase) necessary to cnvert N2 to NH3. Some bacteria have this capacity. Some nitrogen fixing bacteria such as Rhizbium fix nitrogen in exchange for carbohydrates from plants. Symboiotic relationships ahve evolved between some plant groups and bacteria that can fix atomospheric nitrogen into usable forms. Legumes for example can form root nodules which host hese bacteria in exchange for carbohydrates. This is important where the soil lacks nitrogen compounds.