Lecture 1 - April 1, 2004

WHERE DO YOU GET YOUR ENERGY?
WHAT IS ENERGY?
THE DIFFERENT FORMS OF ENERGY
PROPERTIES OF ENERGY
THE FIRST AND SECOND LAWS OF THERMODYNAMICS
1. FIRST LAW - CONSERVATION OF ENERGY
2. ENTROPY AND THE SECOND LAW OF THERMODYNAMICS
GIBBS FREE ENERGY, ΔG
BIOLOGICAL ENERGY - OVERVIEW
THE STRUCTURE OF MITOCHONDRIA

1. WHERE DO YOU GET YOUR ENERGY?

   1) All of our energy comes from the sun, via plants, which capture electromagnetic radiation and synthesize macromolecules, which we ingest as food.

   2) The macromolecules are digested by an overall process called catabolism.

   3) Energy conversion: The first step in releasing energy from macromolecules is hydrolysis into simple molecules - amino acids, fatty acids, sugars. then transformed via glycolysis and other cytosolic processes to Acetyl CoA, which delivers “energy” to the Krebs citric acid cycle in the mitochondrial matrix. This is followed by oxidative phosphorylation (also mitochondrial) and the main topic of this course.

   4) Organization of metabolism in the cell.

2. WHAT IS ENERGY?

With perhaps the exception of light, which is pure energy, energy is not a thing. It refers to a condition or state of a thing.

The energy of an object, or of a system, is how much work the object or system can do on some other object or system. In other words, energy measures the capability of an object or system to do work on another system or object.

3. THE DIFFERENT FORMS OF ENERGY

Kinetic Energy: An object in motion can do work by virtue of its motion. (I gave the example of my former automobile).

Potential Energy: A book sitting on a table is said to have "potential energy", because if it is nudged off, gravity will accelerate the book, giving the book kinetic energy. Potential energy is converted into kinetic energy as the book falls.

Thermal, or heat energy: Consider a hot cup of coffee. The coffee is said to possess "thermal energy", or "heat energy" which is really the collective, microscopic, kinetic and potential energy of the molecules in the coffee (the molecules have kinetic energy because they are moving and vibrating, and they have potential energy due their mutual attraction for one another - much the same way that the book and the Earth have potential energy because they attract each other). Temperature is a measure of how much thermal energy something has. The higher the temperature, the faster the molecules are moving around and/or vibrating, i.e. the more kinetic and potential energy the molecules have.

Chemical Energy: Consider the ability of your body to do work. The glucose (blood sugar) in your body is said to have "chemical energy" because the glucose releases energy when chemically reacted (combusted) with oxygen. Your muscles use this energy to generate mechanical force and also heat. Chemical energy is really a form of microscopic potential energy, which exists because of the electric and magnetic forces of attraction exerted between the different parts of each molecule - the same attractive forces involved in thermal vibrations. These parts get rearranged in chemical reactions, releasing or adding to this potential energy.

Electrical Energy: All matter is made up of atoms, and atoms are made up of smaller particles, called protons (which have positive charge), neutrons (which have neutral charge), and electrons (which are negatively charged). Electrons orbit around the nucleus, which is made up of neutrons and protons. Metals have certain electrons that are only loosely attached to their atoms. They can easily be made to move from one atom to another if an electric field is applied to them. When those electrons move among the atoms of matter, a current of electricity is created (which can do work).

Electrochemical Energy: A battery also stores energy in a chemical way. But electricity is also involved, so we say that the battery stores energy "electro-chemically". Mitochondria are also electron chemical devices.

Sound Energy:   Sound waves are compression waves associated with the potential and kinetic energy of air molecules.

Nuclear Energy: The Sun, nuclear reactors, and the interior of the Earth, all have "nuclear reactions" as the source of their energy, that is, reactions that involve changes in the structure of the nuclei of atoms. In the Sun, hydrogen nuclei undergo fusion to make helium nuclei, which releases energy. In a nuclear reactor, or in the interior of the Earth, Uranium nuclei (and certain other heavy elements in the Earth's interior) split apart, in a process called fission. If this didn't happen, the Earth's interior would have long gone cold! The energy released by fission and fusion is not just a product of the potential energy released by rearranging the nuclei. In fusion or fission, some of the matter making up the nuclei is actually converted into energy. How can this be? The answer is that matter itself is a form of energy!

      E = mc2.

This formula was discovered by Einstein as part of his "Theory of Special Relativity". The equation means that the energy intrinsically stored in a piece of matter at rest equals its mass times the speed of light squared.

Electromagnetic Energy (light): This is the form in which energy is transmitted to the Earth from the Sun. Light can be thought of as oscillating, coupled electric and magnetic fields that travel freely through space (without there having to be charged particles of some kind around). Light may also be thought of as little packets of energy called photons (that is, as particles, instead of waves).

The Einstein-Planck Equation (1900-1905)

                   E   = hν

where E is the energy contained in a quantum of light with frequency, ν, and h is the Planck constant. Planck’s constant, h, was the last fundamental constant of physics to be discovered. This was the first experimental proof of quantum theory, but it was not understood by Planck.

In 1905, Einstein wrote, “The energy of a light ray emitted from a point source is not distributed continuously over ever-increasing volumes of space but consists of a finite number of energy quanta localized at points of space that move without dividing, and can be absorbed or generated only as complete units.”

In 1923, de Broglie proposed that Einstein’s quantization of light applies to all matter, including electrons. This was proved to be correct in 1926 - the birth of quantum mechanics.

4. PROPERTIES OF ENERGY

   1. The total energy of a system and its surroundings is a constant - energy is conserved. The First Law of Thermodynamics.

   2. Energy can be transferred from one system to another through the interaction of forces between the systems. (Energy transduction, such as converting food to ATP).

   3. Energy comes in multiple forms that are interchangeable within the bounds of the Second Law of Thermodaynamics.

5. THE FIRST AND SECOND LAWS OF THERMODYNAMICS

Clausius: “The Energy of the universe is constant (First law); the Entropy of the universe is constantly increasing (second law).

FIRST LAW - CONSERVATION OF ENERGY

Consider two interacting systems that exchange energy as work or heat. The first law states that the loss of energy by one system equals the gain in the other. (An important development in the 1700s was an observation on the boring of cannons in Munich - it was found that the mechanical energy expended was always proportional to the heat produced, demonstrating the equivalence of work and heat).   Mathematically:

ΔE   =   Q   -   W

where Q is the heat absorbed by the system and W is the work done by the system. This is another statement of the First Law.

Note that we can only look at the CHANGE in energy.

In thermodynamics, it is common to focus on pressure-volume work, i.e.,

W = Δ(PV)

Other forms of work, such as electrical work can also be added as needed.

It is also common to work at constant pressure:

W = PΔV

Calorimetric experiments to measure heat are conveniently performed at constant pressure, leading to the definition of ENTHALPY:

ΔH = QP

Therefore, at constant pressure, the energy change is

ΔE   =   ΔH   -   PΔV

ENTROPY AND THE SECOND LAW OF THERMODYNAMICS

Entropy (S) is another state variable of the system, like energy.

The Entropy of the Universe is increasing S >= 0.

ΔS (system) + ΔS (surroundings) >= 0

> for any spontaneous process; = for ideal reversible process.

Consider diffusion of salt through a permeable membrane containing two solutions. The salt is initially on one side only. We know that the salt will diffuse SPONTANEOUSLY AND IRREVERSIBLY until equilibrium is reached. Moreover, the system CANNOT RESTORE ITSELF TO THE INITIAL STATE. In other words, the system as it proceeds to equilibrium LOSES ITS ABILITY to do work on the environment. We have not lost energy, which remains constant. Entropy was introduced to account for this progression toward equilibrium

Entropy is a measure of the disorder of the system. In statistical mechanics, it reflects the number of possible microscopic states that the system can be in.

   S = k log (N),

where k = Boltzmann’s constant and N is the number of possible states. Thus, a highly ordered system (few possible states) has low entropy, while a highly disordered system (many possible states) has high entropy.

It can be shown that for a REVERSIBLE process, the increase in entropy of a CLOSED system is given by

ΔS   =   Qrev/T

In general, ΔS = Qrev/T   + ΔS (irreversible)

Therefore, for a reversible system,

ΔE   <= TΔS   +   PΔV

The equal sign means REVERSIBLE REACTION

NOTE: The energy for life comes from the sun as light (E   =   hν). Plants use this energy to split H2O into O and 2H, which reduce CO2 to form glucose. Glucose is oxidized to H2O and CO2 to provide the source of life energy in animals. In this overall process of energy conservation, the entropy (disorder) of the sun increases, while the entropy (order) on the earth decreases.

6. GIBBS FREE ENERGY, ΔG

Dealing with total energy, E, is not very practical, because E = E (S,V), and S is not always easy to measure and because E does not give information about the irreversible progress of the reaction.

Josiah Willard Gibbs introduced the State Function ΔG, which is DEFINED as follows:

ΔG = ΔH - TΔS

Notice that ΔG = ΔE - TΔS + PΔV (algebra)

For chemical reactions, ΔV is small, so

ΔG ~ ΔE - TΔS

NOW, we have an energy function that depends both on the change in internal energy AND on the change in entropy of the system. Because of this the value of ΔG can tell us whether or not a reaction can change SPONTANEOUSLY.

General properties of ΔG:

1. A system is at equilibrium if ΔG = 0 (very important)

2. A reaction can occur spontaneously ONLY if ΔG is negative.

3. A reaction CANNOT occur spontaneously if ΔG is positive.

4. ΔG is a STATE function. Therefore it depends only on the free energy of the products minus the free energy of the reactants.

5. ΔG is INDEPENDENT OF THE PATH of the transformation. (The same whether or not catalyzed by an enzyme)

6. ΔG provides no information about the RATE of the reaction

This is a brief review of physics and biochemistry to bring us to the point of discussing bioenergetics.

7. BIOLOGICAL ENERGY - OVERVIEW

Consider the chemical reaction

A + B   – C + D

ΔG = ΔG o   + RT log    [C][D]
                  [A][B]

– is the standard free energy change
R is the gas constant,
T is absolute temperature,
[A], [B], [C], [D ] are molar concentrations of reactants and products.

The chemical potentials - μ

G = G(T, P, N1,....Nn)

We will want to know the free energy associated with a given component of the system. This is called the chemical potential or partial molal free energy and is defined thus:

μj   ≡ (∂G/∂Nj )T,P,Ni

(This partial derivative means: the derivative of G with respect to quantity, N, of a particular component, holding all other properties constant).

The chemical potential is particularly importand and will be used throughout the course.

8. THE STRUCTURE OF MITOCHONDRIA

Mitochondria are small, vesicular organelles. The internal aqueous compartment is called the matrix, which contains the enzymes of the Krebs tricarboxylic acid cycle. The matrix is enclosed by a highly folded, insulating membrane called the inner membrane, which contains the enzymic machinery of oxidative phosphorylation. The inner membrane is separated from the cytosol by a more permeable outer membrane, and the aqueous compartment between the inner and outer membranes is called the intermembrane space.

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