Metabolism and metabolic pathways


Metabolism is a biological process that helps in creating a win-win situation by means of providing the energy necessary for the body as well as excreting the wastes. It is a broad concept constituting the series of chemical reactions . A metabolic pathway is a series of chemical reactions in a cell that build and breakdown molecules for cellular processes which ultimately release energy and carbon dioxide. The energy is synthesized by chemical oxidation of nutrients. This article is intended to deal with the basic concepts of metabolism, its course, types and the major pathways through which metabolism takes place. Furthermore, a brief explanation for concepts of energy and rates of metabolism is also included.



Components of metabolism

Metabolism takes a cyclic course involving both metabolism and catabolism. Catabolism is a destructive process whereas anabolism, a constructive process.  Catabolism helps in breaking down the complex nutritive molecules into simpler ones, consequently, the chemical energy is released in the form of adenosine triphosphate (ATP), which is the unit of bio-energy. Additionally, heat is released to maintain core body temperature at approximately 37°C.  On the other hand, anabolism is a  constructive process aids in building up, or synthesis of more complex molecules with the help of smaller ones. However, in the anabolic process,  instead of releasing the energy, there is a consumption of ATP.  The cycle of anabolism and catabolism continues to keep the body in balance. The balance is achieved through a series of chemical reactions called metabolic pathways. They provide a simplified path through which energy is formed in an efficient and gradual manner at cell level rather than large intracellular ‘explosions’. Metabolic pathways are predominantly led by a number of enzymes, hormones, and factors providing control over the process. Invariably, the active components involved in both the processes are our cells,  with the help of bodily processes like respiration and cardiac activity.



The energy in biological systems

The energy synthesized in the body is measured and expressed through the units joules or as units of heat energy (kilocalories). One kilocalorie is the total amount of heat required to raise the temperature of one litre of water by 1 degree Celsius. About 3 million kilocalories of energy is generated by our body cells in total. Mathematically, l kilocalorie is  = 4184 joules (J) = 4.184 kilojoules (kJ). Different foods release the varied amount of energy; one gram of carbohydrate provides 4 kilocalories or  17 kilojoules, whereas 1 gram of protein yield  17 kilojoules (4 kcal) but 1 gram fat provides as high as 38 kilojoules or 9 kilocalories. On the calories viewpoint, every carbohydrate molecule is converted into simple glucose before it is being utilized by our body cells. Similarly, fats get lysed to its simpler units of triglycerides while the excessive triglycerides are converted into glucose. The process in which glucose is formed by other nutrients such as proteins and fats is called glucogenesis. The stored glycogen is broken down into glucose called Glycogenolysis. All in all, energy is the main intention behind various chemical processes falls under metabolism.



Calorie balance and body weight

Anything excess can seriously hamper the health and well-being of our body hence, the energy stored must be in balance with various other factors such as our weight, activity levels, and age. Our body weight remains constant when energy intake and utilization are almost equal. On the contrary, if the intake exceeds our requirement, the ultimate result is obesity or overweight. Similarly, our body weight declines if the intake is too less than what our body demands. Furthermore, age, gender, hormones, body`s special physiological status such as pregnancy etc. can also influence on the body weight and calorie balance.



Rate of Metabolism

The metabolic rate is the rate at which our body cells release energy with the help of metabolic pathways. It takes into account of all cells in our body.  Most of the animals including human beings generate wastes and carbon dioxide by using oxygen whereas plants produce oxygen as their by-product after utilizing the carbon dioxide. The basal metabolic rate (BMR) is the metabolism of an individual at rest. It is the total energy being utilized for bodily processes such as respiration, neurological activities, cardiac activities when we are at rest. Therefore, irrespective of whether we are involved in active exercises or not, our body consumes some proportion of energy.  The following paragraph briefs various factors that influence the metabolic rate of human beings.



Factor affecting metabolic rate

Metabolism is variable rather than the static. This is because a variety of factors can either increase or decrease the rate of metabolism. Some of the factors that can demand more energy are age, gender, height and weight, special physiological changes such as pregnancy, menstruation and lactation. This can, in turnincrease the rate of BMR as the factors move in ascending fashion; for example, more of the age, height and weight higher are the BMR. In addition, type of work, degree of muscular activity, physical exertion Increased can change the BMR requirements considerably. Increase in the thyroid level, body temperatures hormones can also increase the energy demands.



Metabolic pathways

The fundamental purpose of the metabolic effort of cells is to manage the energy demands of cellular activities as well as the energy needs for working.  A few common pathways are central to this function. The nutrients enter these central energy-producing pathways in a phased manner, during which a series of intermediary molecules are formed which ultimately release the energy. A substantial proportion of our energy is stored as ATP. The process of converting energy into glycogen (stored energy) is assisted by the liver and skeletal muscles as they provide space for the storage. The most primary and preferred fuel of energy is the glucose, however other ways from which our body can deal with the energy needs are amino acids, fatty acids, glycerol and occasionally nucleic acids. There are three central metabolic pathways; namely, Glycolysis, Citric acid (Krebs) cycle and Oxidative phosphorylation. These topics will be discussed in detail in different articles. But to brief in short, the products from glycolysis enter the citric acid cycle, and products from the citric acid cycle move into the oxidative phosphorylation.



Further reading


Carbohydrate metabolism      


Vitamins and their deficiency disorders




Proteins in our body




  1. What is the major purpose of metabolism?

  2. Differentiate between anabolism and catabolism.

  3. What are the end -products of metabolism?

  4. List the factors affecting metabolism.

  5. What are the common metabolic pathways in human beings?





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