Overview

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.

Plant 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.

Animal Metabolism

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.

Microbial Metabolism

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.

Procedure

Metabolism can be divided into catabolism and anabolism.

Catabolism is a process where complex food molecules like polysaccharides, lipids, and proteins are broken down into simple molecules, such as sugars, fatty acids, and amino acids.

These simpler molecules are then processed inside a cell via various biochemical pathways to produce ATP, the energy currency of the cell.

For example, starch, a complex carbohydrate, is broken down by digestive enzymes into  glucose, which is then metabolized to produce pyruvate and ATP via glycolysis.

Anabolism is the reverse of catabolism. Here, small and simple precursors use energy from ATP hydrolysis to synthesize complex polymers.

For example, in glycogenesis, glucose monomers are bonded together to form glycogen, an energy storage molecule in animals. 

Since cells need to generate energy to maintain cellular metabolism, it is important to keep an adequate pool of ATP in the cell by modifying rates of ATP producing and utilizing reactions.

Such balance in the cellular metabolism is achieved via various regulatory systems present in the body like the endocrine system, regulatory enzymes, feedback inhibition loops, and modulation of gene expression patterns.