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.
Types of bioenergetics reactions
There are two types of reactions-
1. Exergonic reaction-An exergonic reaction is a
spontaneous chemical reaction that releases energy. It is
thermodynamically favored, indexed by a negative value of ΔG (Gibbs free energy). Over the
course of a reaction, energy needs to be put in, and this activation energy
drives the reactants from a stable state to a highly energetically unstable
transition state to a more stable state that is lower in energy (see: reaction coordinate). The reactants
are usually complex molecules that are broken into simpler products. The entire
reaction is usually catabolic. The
release of energy (specifically of Gibbs free energy) is negative
(i.e. ΔG < 0) because
the energy of the reactants is higher than that of the products.
2. Endergonic reaction-An endergonic reaction is an
anabolic chemical reaction that consumes energy. It is the opposite of an
exergonic reaction. It has a positive ΔG,
for instance because ΔH >
0, which means that it takes more energy to break the bonds of the reactant
than the energy of the products offer, i.e. the products have weaker bonds than
the reactants. Thus, endergonic reactions are thermodynamically unfavorable and
will not occur on their own at constant temperature. Additionally, endergonic
reactions are usually anabolic.
Calorimetry
Calorimetry
is the method of measuring changes in state variables of
a body for the purpose of deriving the heat transfer associated
with changes of its state due, for example, to chemical reactions, physical changes, or phase transitions under
specified constraints. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the
Latin word calor meaning
heat and the Greek word metron meaning to measure. Scottish physician and
scientist Joseph Black,
who was the first to recognize the distinction between heat and temperature is said to be the
founder of the science of calorimetry.
Types of Calorimetry
It may
be Direct or Indirect calorimetry-
1.
Direct Calorimetry- This is done by putting the subject inside a specially prepared heat
proof chamber (Atwater-Benedict’s respiration calorimeter). Heat produced is
measured by changes in circulating water. This method gives accurate results
but can hardly be used in clinical setting due to elaborate apparatus and time
constraints.
2.
Indirect calorimetry-
Indirect calorimetry calculates heat that living organisms produce by measuring
either their production of carbon dioxide or from
their consumption of oxygen. Lavoisier noted in 1780
that heat production can be predicted from oxygen consumption this way,
using multiple regressions. The dynamic energy budget theory explains why this procedure is correct. There
are two methods for this a
a.
Closed circuit method-Different instruments are used for this purpose like
Benedict Roth apparatus to calculate heat production and O2 consumption.
b.
Open circuit method- Here different types of respirometers are used to
calculate O2 consumed and CO2 produced to calculate heat production.
Dynamic Energy Budget (DEB) theory
It is a formal metabolic theory which provides a single quantitative framework to
dynamically describe the aspects of metabolism (energy and mass budgets) of all living organisms at
the individual level, based on assumptions about energy uptake, storage, and
utilization of various substances. The theory specifies that an organism is
made up of two main compartments:
·
Structure
Thus
the DEB theory is as here under-
Assimilation of energy is proportional to surface area of the
structure and maintenance of energy reserve is proportional to its volume.
Reserve
does not require maintenance. Energy mobilization will depend on the relative
amount of the energy reserve, and on the interface between reserve and
structure of the body of an organism.
Gibbs
free energy
In thermodynamics, the Gibbs free
energy or Gibbs energy is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically
closed system at constant temperature and pressure. It also provides a necessary condition for processes such
as chemical reactions that may occur under these conditions.
The
concept of Gibbs free energy, originally called available energy, was developed in the 1870s by the American
scientist Josiah Willard Gibbs. In 1873, Gibbs described this available energy as
below–
The
greatest amount of mechanical work which can be obtained from a given quantity
of a certain substance in a given initial state, without increasing its
total volume or allowing heat to pass to or from external
bodies, except such as at the close of the processes are left in their initial
condition.
The Gibbs energy is thus the thermodynamic potential of a body in a closed system where exchange of heat takes
place without exchange of molecules. This is also applicable on all
thermodynamic reactions of body.
Gibbs–Helmholtz equation
The Gibbs–Helmholtz
equation is a thermodynamic equation used for calculating changes in
the Gibbs free energy of a system as a function
of temperature. It was originally presented in an
1882 paper entitled Die
Thermodynamik chemischer Vorgange by Hermann von
Helmholtz. It
describes how the Gibbs free energy, which was presented originally by Josiah Willard Gibbs, varies with temperature. It is
typically applicable to chemical reactions of the body.
The
equation is-
Where H
is the enthalpy, T the absolute temperature and G the Gibbs free energy of the
system, all at constant pressure p.
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