BIOL 600 Lecture Notes - Lecture 3: Metabolic Pathway, Kinetic Energy, Cellular Respiration
26 May 2018
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AP Bio Chapter 8 An Introduction to Metabolism
Lecture Outline
Overview: The Energy of Life
Concept 8.1 An organism’s metabolism transforms matter and energy, subject to the laws
of thermodynamics
The totality of an organism’s chemical reactions is called metabolism.
Metabolism is an emergent property of life that arises from interactions between
molecules within the orderly environment of the cell.
The chemistry of life is organized into metabolic pathways.
Metabolic pathways begin with a specific molecule, which is then altered in a series of
defined steps to form a specific product.
A specific enzyme catalyzes each step of the pathway.
Catabolic pathways release energy by breaking down complex molecules to simpler
compounds.
o A major pathway of catabolism is cellular respiration, in which the sugar glucose
is broken down in the presence of oxygen to carbon dioxide and water.
Anabolic pathways consume energy to build complicated molecules from simpler
compounds. They are also called biosynthetic pathways.
o The synthesis of protein from amino acids is an example of anabolism.
The energy released by catabolic pathways can be stored and then used to drive anabolic
pathways.
Energy is fundamental to all metabolic processes, and therefore an understanding of
energy is key to understanding how the living cell works.
o Bioenergetics is the study of how organisms manage their energy resources.
Organisms transform energy.
Energy is the capacity to do work.
o Energy exists in various forms, and cells transform energy from one type into
another.
Kinetic energy is the energy associated with the relative motion of objects.
o Objects in motion can perform work by imparting motion to other matter.
o Photons of light can be captured and their energy harnessed to power
photosynthesis in green plants.
o Heat or thermal energy is kinetic energy associated with the random movement of
atoms or molecules.
Potential energy is the energy that matter possesses because of its location or structure.
o Chemical energy is a form of potential energy stored in molecules because of the
arrangement of their atoms.
Energy can be converted from one form to another.
o For example, as a boy climbs stairs to a diving platform, he is releasing chemical
energy stored in his cells from the food he ate for lunch.
o The kinetic energy of his muscle movement is converted into potential energy as
he climbs higher.
o As he dives, the potential energy is converted back to kinetic energy.
o Kinetic energy is transferred to the water as he enters it.
o Some energy is converted to heat due to friction.
The energy transformations of life are subject to two laws of thermodynamics.
Thermodynamics is the study of energy transformations.
In this field, the term system refers to the matter under study and the surroundings
include everything outside the system.
A closed system, approximated by liquid in a thermos, is isolated from its surroundings.
In an open system, energy and matter can be transferred between the system and its
surroundings.
Organisms are open systems.
o They absorb energy—light or chemical energy in the form of organic
molecules—and release heat and metabolic waste products such as urea or CO2 to
their surroundings.
The first law of thermodynamics states that energy can be transferred and transformed,
but it cannot be created or destroyed.
o The first law is also known as the principle of conservation of energy.
o Plants do not produce energy; they transform light energy to chemical energy.
During every transfer or transformation of energy, some energy is converted to heat,
which is the energy associated with the random movement of atoms and molecules.
A system can use heat to do work only when there is a temperature difference that results
in heat flowing from a warmer location to a cooler one.
o If temperature is uniform, as in a living cell, heat can only be used to warm the
organism.
Energy transfers and transformations make the universe more disordered due to this loss
of usable energy.
Entropy is a quantity used as a measure of disorder or randomness.
o The more random a collection of matter, the greater its entropy.
The second law of thermodynamics states that every energy transfer or transformation
increases the entropy of the universe.
o While order can increase locally, there is an unstoppable trend toward
randomization of the universe.
o Much of the increased entropy of the universe takes the form of increasing heat,
which is the energy of random molecular motion.
In most energy transformations, ordered forms of energy are converted at least partly to
heat.
o Automobiles convert only 25% of the energy in gasoline into motion; the rest is
lost as heat.
o Living cells unavoidably convert organized forms of energy to heat.
For a process to occur on its own, without outside help in the form of energy input, it
must increase the entropy of the universe.
The word spontaneous describes a process that can occur without an input of energy.
o Spontaneous processes need not occur quickly.
o Some spontaneous processes are instantaneous, such as an explosion. Some are
very slow, such as the rusting of an old car.
Another way to state the second law of thermodynamics is for a process to occur
spontaneously, it must increase the entropy of the universe.
Living systems create ordered structures from less ordered starting materials.
o For example, amino acids are ordered into polypeptide chains.
o The structure of a multicellular body is organized and complex.
However, an organism also takes in organized forms of matter and energy from its
surroundings and replaces them with less ordered forms.
o For example, an animal consumes organic molecules as food and catabolizes them
to low-energy carbon dioxide and water.
Over evolutionary time, complex organisms have evolved from simpler ones.
o This increase in organization does not violate the second law of thermodynamics.
o The entropy of a particular system, such as an organism, may decrease as long as
the total entropy of the universe—the system plus its surroundings—increases.
o Organisms are islands of low entropy in an increasingly random universe.
o The evolution of biological order is perfectly consistent with the laws of
thermodynamics.
Concept 8.2 The free-energy change of a reaction tells us whether the reaction occurs
spontaneously
How can we determine which reactions occur spontaneously and which ones require an
input of energy?
The concept of free energy provides a useful function for measuring spontaneity of a
system.
Free energy is the portion of a system’s energy that is able to perform work when
temperature and pressure is uniform throughout the system, as in a living cell.
The free energy (G) in a system is related to the total enthalpy (in biological systems,
equivalent to energy) (H) and the entropy (S) by this relationship:
o G = H - TS, where T is temperature in Kelvin units.
o Increases in temperature amplify the entropy term.
o Not all the energy in a system is available for work because the entropy
component must be subtracted from the enthalpy component.
o What remains is the free energy that is available for work.
Free energy can be thought of as a measure of the stability of a system.
o Systems that are high in free energy—compressed springs, separated charges,
organic polymers—are unstable and tend to move toward a more stable state, one
with less free energy.
o Systems that tend to change spontaneously are those that have high enthalpy, low
entropy, or both.
In any spontaneous process, the free energy of a system decreases.
