Concept Of Exergonic And Endergonic Reactions And Modes Of Metabolisma

Introduction

Metabolism is a complex enzymatic process. Either there is formation of complex molecule from simpler molecules (anabolism) or there is the breakdown of a complex molecules to simpler form (catabolism). Most of the microbial metabolisms we are going to talk come under catabolism. Catabolic reactions thus either require or release energy mainly in the form of heat.

Endergonic reactions:

Endergonic reactions are those reactions in which reactions do not have enough potential energy to convert into products. In other words, those reactions that require certain amount of energy from external source are called endergonic reactions. In terms of heat endergonic reactions are called endothermic reactions. For example: Conversion of conversion of glucose to glucose-6 phosphate during glycolysis. Endergonic reactions are non-spontaneous in nature as reactants have less free energy than that of product. So value of change in free energy is positive.

General endergonic reaction can be represented as:

A + B + energy → C + D

Example: Glucose + ATP → Glucose-6 phosphate + ADP

In above reactions one mole ATP is consumed as source energy by the hydrolysis of one phosphodiester bond of ATP (i.e. 7.3 kcal per molecule) and ATP is reduced to ADP. Hence above reaction is a typical example of endergonic reaction.

Exergonic reactions:

Exergonic reactions on other hand are those types of reactions in which the involved reactions have enough potential energy to convert into products. These types of reactions do not require external energy source and hence are spontaneous in nature. For example: conversion of fructose-1,6 biphosphate into fructose-6 phosphate .As reactants have more energy than that of product, the value of free energy change becomes negative.

General exergonic reaction can be represented as:

A + B → C + D + energy

Examples:

Fructose-1, 6 bi phosphate + ADP/phosphofructokisase → Fructose-6 phosphate + ATP

Phosphoenol pyruvate + ADP/ pyruvate kinase → Pyruvate

Here 1 mole of ATP is generated by substrate level phosphorylation of ADP in above reactions. Hence both reactions are typical exergonic reactions. Energy in ATP is stored in the form of phosphodiester bond which when hydrolyzed yields 7.2 KCal energy that is utilized by organisms for the regular life process.

Adenosine Triphosphate (ATP):

In case of living cells they acquire energy by the breakdown of complex molecules such as polysaccharides, fats, proteins etc. Prototrophic organism utilize sunlight as the source of energy, whereas chemotrophic organisms utilize chemical compounds for the same purpose. In case of heterotrophic organisms, energy is obtained by the degradation of complex molecules from which alot of energy can be captured in the form of chemical bonds that can be utilized in need. The rest of energy which could not be captured is lost via different means (heat in most cases). Living cells capture energy in the form of Adenosine Triphosphate (ATP).

Adenosine triphosphate (ATP) is a nucleotide and it consists of adenine, a triphosphate unit and ribose pentose sugar unit. Here energy is stored in the form of phosphodiester bond that links one phosphate group with other. ATP is the outcome of the exergonic reactions during microbial metabolism and is utilized in endergonic reactions as power generator or energy source.

Autotrophic and Heterotrophic Metabolisms

Biologically overall mass of organisms are divided into two major classes on the basis of nutritional mode. They are:

• Autotrophic organisms: - Those organisms that are capable to prepare their food by themselves utilizing free available energy (e.g. sunlight, chemical energy etc.) are called autotrophic organisms. All green plants (photoautotrophs) and cyanobacteria (chemoautotrophs) are the examples of autotrophic organisms. Energy rich glucose molecule is obtained as the major product which can be utilized forth ward for energy generation.
• Heterotrophic organisms: - Those organisms that depend upon other organisms for energy(food) are called heterotrophic organisms. Most of all animals belong to heterotrophic organisms. They utilize the complex molecules (glucose) obtained from other organisms, break them down and ultimately obtain energy in the form of ATP.

Autotrophs and heterotrophs have somehow different nature of metabolism. Autotrophic metabolism is anabolic as well as catabolic in nature whereas heterotrophic metabolism is only catabolic in nature. In case of autotrophs, metabolism occur in a pathway called photosynthesis (in green plants). But in heterotrophs, metabolic pathway is generally termed as respiration.

Photosynthesis is the complex mechanism of utilizing carbon dioxide (CO₂) and water (H₂o) in presence of sunlight to prepare food.

CO₂ + H₂O + sunlight/chlorophyll → C₆H₁₂O₆ +H₂O+ O₂

Autotrophic metabolism is much more advanced than heterotrophic metabolism. After the photosynthesis, also called light reaction is completed, consumption of glucose starts, which is called dark reaction, which follows the catabolic pathway which is called Calvin cycle or C₃ cycle. Here we do not talk much about autotrophic metabolism and deal more with heterotrophic metabolisms. We just build a concept in our mind that every metabolism ultimately goals for the production of ATP.

Every heterotrophic metabolism initiates with the energy rich macro molecules such as carbohydrates, lipids, proteins etc. These all energy rich molecules are the ultimate products of autotrophic metabolism. The most common overall metabolic flow chart is given below:

Autotrophs

↓

Photosynthesis

↓

Glucose

↓ Glycolysis→ Calvin cycle/Electron Transport chain → ATP ↓

Heterotrophs

↓

Glycolysis ↓

ATP ← Fermentation← Pyruvate→ Krebs cycle → ATP

This how autotrophic and heterotrophic metabolism interconnected. As said many times the ultimate goal of overall metabolism is to generate ATP. Above given flow chart is just a skeletal diagram of inter-connection between autotrophic and heterotrophic metabolism. Many of us may be bearing confusion about how these two modes of metabolisms are interconnected. So now this was a little effort to erase that confusion.

Absorbing the above diagram one can ask "can heterotrophs get glucose molecule directly from autotrophs?", and answer is "may or may not be". In fact heterotrophs get food in the form of polymers such as starch. Starch is hydrolyzed and glucose is obtained as monomer which undergoes series of splitting and conversion reactions ultimately forming three carbon molecule pyruvate. Pyruvate thus formed undergoes different pathways for ATP synthesis. In above diagram we can see two pathways followed by pyruvate molecule: Krebs cycle and Fermentation.

Krebs cycle: It is an aerobic pathway of metabolism and is the major metabolic pathway as large numbers of ATP can be yielded.

Fermentation: It is an anaerobic pathway of metabolism and considered as minor metabolic pathway as low numbers of ATP is produced.

As per our course of study we will study the heterotrophic metabolism in much detail. There we will study about the modes of metabolism, different pathways of metabolism and the likes so detail and conceptually that we will not get in confusion in this topic any more. Starting from the food we eat to yield of energy is not that easy step but much more interesting topic to study.