Energy Metabolism

 Energy Metabolism

There is a continuous exchange of energy between a living organism and its environment as laws of thermodynamics are applicable to both. Unlike plants humans get their energy from food and may store it in their bodies. The energy exchange of body is based on input and output of energy which is based on first law of thermodynamics by Mayer, Joule and Helmholtz, made applicable on living body by Voit, Pattenkofer and Rubner which states that energy is neither gained nor lost when converted from one form to another i.e. thermal, chemical, mechanical or electrical.

Calorie- The unit of energy is expressed as Calorie which is the amount of heat required to raise the temperature of one gram of water from 15 to 16℃. However in Physiology and medicine the unit used is kilo calorie which is equal to 1000 calories. The measurement of heat is known as calorimetry.

Bioenergetics 

Bioenergetics is a field in biochemistry and cell biology that concerns with the energy flow through living systems. This is an active area of biological research that includes the study of the transformation of energy in living organisms and the study of thousands of different cellular processes such as cellular respiration and the many other metabolic and enzymatic processes that lead to production and utilization of energy in forms such as adenosine triphosphate (ATP) molecules.  Bioenergetics describes how living organisms acquire and transform energy in order to perform biological work. The study of metabolic pathways is thus essential to bioenergetics.

Bioenergetics is the part of biochemistry concerned with the energy involved in making and breaking of chemical bonds in the molecules found in biological organisms. It can be defined as follows-

Study of energy relationships and energy transformations and transductions in living organisms is called Bioenergetics.

The ability to harness energy from a variety of metabolic pathways is a property of all living organisms. Growth, development, anabolism and catabolism are some of the central processes in the study of biological organisms, because the role of energy is fundamental to such biological processes. Life is dependent on energy transformations; living organisms survive because of exchange of energy between living tissues/ cells and the environment outside the cells.

In a living organism, chemical bonds are broken and made as part of the exchange and transformation of energy. Energy is available for work (such as mechanical work) or for other processes (such as chemical synthesis and anabolic processes in growth), when weak bonds are broken and stronger bonds are made. The production of stronger bonds allows release of usable energy.

Adenosine triphosphate (ATP) is the main energy molecule for organisms; the goal of metabolic and catabolic processes are to synthesize ATP from available starting materials from the environment, and to break- down ATP into adenosine diphosphate (ADP) and inorganic phosphate by utilizing it in biological processes. In a cell, the ratio of ATP to ADP concentrations is known as the energy charge of the cell. A cell can use this energy charge to relay information about cellular needs; if there is more ATP than ADP available, the cell can use ATP to do work, but if there is more ADP than ATP available, the cell must synthesize ATP via oxidative Phosphorylation.

Living organisms produce ATP from energy sources, mostly sunlight or O₂, mainly via oxidative Phosphorylation. The terminal phosphate bonds of ATP are relatively weak compared with the stronger bonds formed when ATP is hydrolyzed (broken down by water) to adenosine diphosphate and inorganic phosphate.

Here it is the thermodynamically favorable free energy of hydrolysis that results in energy release; the phosphor anhydride bond between the terminal phosphate group and the rest of the ATP molecule does not itself contain this energy. An organism's stockpile of ATP is used as a battery to store energy in cells. Utilization of chemical energy from such molecular bond rearrangement powers biological processes in every biological organism.

Living organisms obtain energy from organic and inorganic materials; i.e. ATP can be synthesized from a variety of biochemical precursors. For example, lithotrophs can oxidize minerals such as nitrites or forms of sulfur, such as elemental sulfur, sulfites, and hydrogen sulfide to produce ATP.

During  photosynthesis, autotrophs produce ATP using light energy, where as  heterotrophs must consume mostly organic compounds including carbohydrates, fats, and proteins. The amount of energy actually obtained by the organism is lower than the amount released in combustion of the food; there are losses in digestion, metabolism, and thermogenesis.

Environmental materials that an organism takes in are generally combined with oxygen to release energy, although some can also be oxidized an aerobically. The bonds holding the molecules of nutrients together and in particular the bonds holding molecules of free oxygen together are relatively weak compared with the chemical bonds holding carbon dioxide and water together. 

The utilization of these materials is a form of slow combustion because the nutrients are reacted with oxygen (the materials are oxidized slowly enough that the organisms do not actually produce fire). The oxidation releases energy because stronger bonds (bonds within water and carbon dioxide) have been formed. This net energy may evolve as heat, which may be used by the organism for other purposes, such as breaking other bonds.