Organismal Physiology Lecture No. 4: Energy & Metabolism
Thursday September 20 , 2012
-Energy is defined in mechanics as the capacity to do mechanical work, measured as the product of force
and distance. E.g. A small wave hitting against the shore is energy moving, not an actual object.
-Metabolism is defined as the set of processes by which cells and organisms acquire, rearrange and void
commodities in ways that sustain life.
-Energy metabolism is defined as the set of processes by which organisms acquire energy (or the
capacity to do work), channel energy into useful functions, and dissipate energy from their bodies.
-There are two categories of energy metabolism: catabolic processes (which break down organic
molecules and release energy) and anabolic processes (which build organic molecules and use energy).
Laws Of Thermodynamics:
-The 1 Law of Thermodynamics states that energy cannot be created or destroyed it can only change
forms. Energy is always conserved and escapes (flows) as waves in the form of heat and sound.
-The 2 Law of Thermodynamics states that entropy (disorder or randomness) always increases. Even in
closed systems, unless there is an input of energy maintaining order, randomness will always increase.
This is the very reason that perpetual motion machines do not live up to their title as friction will bring
the system to a halt (provided there is no energy input).
Importance Of Energy In Biology:
-A broader definition that is often more useful for biologists is that energy is that capacity to increase
order. This is because organisms are organized and need to maintain order if they are to survive. In this
context, energy carries the capacity to increase order. According to the 2 Law of Thermodynamics,
organisms can only maintain their organization as open systems provided an energy input from the
-Organisms are dynamic objects that are constantly exchanging their bodily atoms with those of the
environment or are replacing and recycling them.
Different Forms Of Energy In Existence:
-Chemical energy: - the energy stored in chemical bonds. When the atoms within molecules are
rearranged, energy may be required, or it may be released. E.g. ATP, glucose, etc. -Electrical energy: - the energy of an electrical gradient. E.g. Separation of positive and negative charges,
set-up of currents across membranes, etc.
-Mechanical energy: - the energy of organized motion. E.g. Blood flow from heart, limb movement, etc.
-Molecular kinetic energy (heat): - the energy of the random motion of atoms and molecules.
The Differences Of High-Grade Low-Grade Energy:
-*All forms of energy are not equally capable of doing physiological work in living organisms. That is why
energy forms are further subdivided into high-grade and low-grade. High-grade energy forms include
chemical energy (which can be used to do all forms of physiological work) and electrical and mechanical
energy (which can be used to do some forms of physiological work). Low grade energy forms consist
exclusively of heat or molecular kinetic energy, which cannot be used to do any physiological work.
-This is not to say that heat is not important to organisms (it is in fact, very important) only that heat
cannot be used to do work in organisms because heat dissipates before a great differential median can
be established. Heat cannot fuel the firing of nerve cells, movement of muscles, transport of fluids, etc.
The Transformation Of Energy & Its Relative Efficiency:
-Within organisms, energy can be transformed from one form into another. For instance, the chemical
bonds of the food an animal eats can eventually be transformed into the energy of the animal’s motion.
-In living organisms, energy transformation is not 100% efficient. This does not mean that energy is lost
(according to the 1 Law of Thermodynamics) only that useful (high-grade) energy is converted to heat
(low-grade) energy. Every time one form of high-grade energy is converted to another within an
organism, some of that energy is lost as heat. Low-grade heat energy can never be converted back to
any form of high-grade energy.
The Possible Functional Consequences Of Acquired Energy:
-Biosynthesis: - making and growing the parts of the body, as well as any products that the organism
might secrete. E.g. breastfeeding and reproduction require an enormous amount of energy.
-Maintenance: - maintaining the integrity of the organism’s body, through processes such as circulation,
respiration, tissue repair and includes “internal work” or any work done inside the body. E.g. The
movement of fluids through circulation.
-Generation of “External Work”: - applying mechanical forces to objects outside the body, including
movement (running, jumping and swimming), biting, and lifting objects.