Introduction to metabolism

Introduction 

We know that life of any organism is driven by energy to perform its activities primary source of energy is food that contain chemical energy of biomolecules such as carbohydrates, lipids and proteins .major biochemical task of living cells is to convert this chemical energy to a form that can be used by the cells is to perform their life supporting activities. this conversion is accomplished by series of chemical reactions which constitute metabolism.

In this unit, you will study about metabolism and its function .

You would also learn that in spite of the apparent complexity ,there are common reaction and themes followed in almost all living organism which from the basic design of metabolism . a brief outline of the organization  of metabolism pathway ,the role of ATP and redox carries will also be discussed in the next unit, you will study about glycolysis , the central pathway of glucose oxidation.

Autotrophs and Hetrotrophs

The biomolecules found in living organisms are essentially carbon based. the other elements  incorporated in the carbon backbone create structural and functional diversity essential for sustaining life. Living organismare divided into large group based on the chemical form in which they get carbon from the environment :autotrophs and heterotrophs.

Autotrophs

Autotrophs use carbon dioxide (the most oxidized from of carbon )from the atmosphere as their sole source of carbon and reduce it to glucose .they are capable of synthesizing all the require carbon and reduce it to glucose. The autotrophs are also known as producers as they bring in fixed usable carbon into the biosphere for themselves and directly or indirectly support life in this planet.

 this group includes photosynthesis bacteria and plant .

heterotrophs

heterotrophs cannot fix atmosphere carbon dioxide. Instead they use organic molecules such as glucose and other complex carbohydrates as source of carbon. hetrotrophs are also known as consumer as these feed on plants and other animals and synthesize the required biomolecules by transforming the food they consume.

Both autotrophs and heterotrophs are further classified into two group based on source of energy :

Phototrophs capture radient  energy from the sun. they are known as photosynthetic  organisms.

Chemotrophs use oxidation –reduction reactions to extract energy from organic molecules like glucose or oxidization inorganic substances like Fe²⁺NO₂ ^-,NH₄⁺element sulphur .summarises the four major groups of organism based on the source of carbon and energy.

The obligate aerobes have an absolute dependence on oxygen .bacteria like Azotobacter  vinelandii and most eukaryotes couple generation of energy in the form of reducing equivalents NADH andFADH₂the re-oxidation of NADH and FADH₂ Via the electron transport chain (ETC) with oxygen as the terminal receptor is highly exergonic . there is another class of microbes that also have an ETC but the terminal electron accepter is other than oxygen (may be nitrate sulphate ,etc ).this is called anaerobic respiration. It is less efficient than aerobic respiration.

The extreme group is of obligate anaerobes that represent by many prokaryotes belonging to archaea and eubacteria. They cannot tolerate oxygen and survive in specialized niches. In fact oxygen is a poison for them. Clostridium botulinum is an example of an obligate anaerobe between these two extremes are some aerobic organism that can adapt to anaerobic condition by shifting from respiration to fermentation. The alternative mode is no doubt inefficient but the organism survives. These organism are called facultative anaerobes ,for example Escherichia coli yeast and human skeletal system.

Metabolism

Metabolism is the sum of all enzyme catalyzed chemical reaction taking place in an organism. It is also called intermediately metabolism as a substrate goes through multiple intermediary step to from an end product. The outcome of  these interconnected pathway is to support the followings:

The breakdown of complex biomolecules obtained from environment into simpler usable compounds.

The efficient extraction of chemical energy of biomolecules  into ATP and reducing equivalents for biosynthesis  and other cellular activities.

Synthesis of complex boimolecules from simple precursors in accordance with the changing needs of the organism.

Synthesis and storage of long and short term energy reserves in condition of excess.

Catabolism and anabolism

All metabolism reaction participate either in catabolism ,anabolism or both. The overall free energy change of a metabolic pathway is negative.

Catabolism (Greek Cata ,down and ballein to throw )refers to reaction involved in the breakdown of complex biomolecules such as co₂ NH₃(ammonia)and H₂O  these reaction are energy generating and oxidative in nature some part of the energy released is utilized for the synthesis of ATP or reduced energy carriers (NADH NADPH AND FADH₂ )and the rest is lost as heat. A less apparent role of catabolism is to provide a variety of anabolic precursors. Finally ,catabolism sequences are convergent in nature. The TCA cycle is a convergent cycle for the complete oxidation of carbon.

Anabolism (Greek Cata ,up and ballein to throw) on the other hand, is involved in step biosynthesis of simple and complex biomolecules ,starting from step by step biosynthesis of simple and complex biomolecules starting from simple precursors. anabolic pathways are generally divergent and depand on a source of energy that is made available in the form of activated precursors or ATP.