4.1: Overview of Metabolism
Living cells constantly carry out various chemical reactions which are necessary for their proper functioning. These reactions are interlinked to one another via multiple pathways. The collection of these chemical reactions is known as metabolism.
Sunlight, the primary source of energy in plants, is first absorbed by the chlorophyll pigments present in their leaves. Plants then use this energy to carry out photosynthesis, where water is oxidized into oxygen and carbon dioxide is reduced to glucose. Glucose acts as a precursor molecule to synthesize several other metabolites for the growth and survival of the plant. Primary metabolites are chemicals like amino acids and fatty acids, which are essential for the plant’s growth and development. In contrast, secondary metabolites offer survival advantages to the plant. For example, secondary metabolites like terpenes and phenolics are involved in plant defence against microbes and pests, whereas flavonoids are responsible for flower pigmentation.
All animals need food, water and oxygen to grow and reproduce. Here, oxygen acts as an oxidizing agent and the complex food molecules are broken down or catabolized to produce energy in the form of ATP, which is later used for the synthesis of necessary macromolecules or anabolism.
Enzymopathy or inborn errors of metabolism are rare genetic disorders in which the body lacks dedicated enzymes to break specific food molecules. Therefore, people with enzymopathy cannot efficiently utilize specific complex food molecules to produce energy. For example, patients with fructose intolerance disorder lack the enzyme aldolase-B required to metabolize fructose. Similarly, patients with galactosemia lack the enzyme galactose-1-phosphate uridyl transferase (GALT) required to metabolize galactose.
Microbes obtain energy and carbon from both organic as well as inorganic sources using different metabolic pathways. For example, Bacillus can metabolize organic molecules such as starch and cellulose, while Azotobacter and Rhizobium oxidize inorganic molecules such as nitrogen. Some other types of bacteria like Cyanobacteria contain chlorophyll and hence can produce glucose by photosynthesis.