Bioenergetics

Definition

The study of the various types of energy transformations that occur in living organisms

• Macromolecules of all types are assembled from raw materials, waste products are produced and excreted, genetic instructions flow from the nucleus to the cytoplasm, vesicles are moved along the secretory pathway, ions are pumped across cell membranes

• To maintain such a high level of activity, a cell must acquire and expend energy

• Energy may be defined as the ability to do work, that is, the capacity to change or move something

• The energy associated with chemical bonds can be harnessed to support chemical work and the physical movements of cells

• Thermodynamics is the study of the changes in energy that accompany events in the universe

The First Law of Thermodynamics

• The first law of thermodynamics is the law of conservation of energy

• It states that energy can neither be created nor destroyed

• Energy can, however, be converted (transduced ) from one form to another

• The transduction of electric energy to mechanical energy occurs when we plug in a clock (Figure 3.1a)

• the chemical energy stored in certain biological molecules, such as ATP, is converted to mechanical energy when organelles are moved from place to place in a cell, to electrical energy when ions flow across a membrane, or to thermal energy when heat is released during muscle contraction (Figure 3.1b).

• A number of animals, including fireflies and luminous fish, are able to convert chemical energy back into light (Figure 3.1c)

• The most important energy transduction in the biological world is the conversion of sunlight into chemical energy—the process of photosynthesis—which provides the fuel that directly or indirectly powers the activities of nearly all forms of life

• Regardless of the transduction process, however, the total amount of energy in the universe remains constant

• The energy of the system is termed the internal energy (E), and its change during a transformation is ΔE (delta E)

• One way to describe the first law of thermodynamics is that ΔE = Q - W, where Q is the heat energy and W is the work energy

• Depending on the process, the internal energy of the system at the end can be greater than, equal to, or less than its internal energy at the start, depending on its relationship to its surroundings (Figure 3.2)

• The internal energy at the end of the transformation will be greater than that at the beginning if heat is absorbed and less if heat is released

• Reactions that lose heat are termed exothermic, and ones that gain heat are endothermic, and there are many reactions of both types

The Second Law of Thermodynamics

• The second law of thermodynamics expresses the concept that events in the universe have direction; they tend to proceed “downhill” from a state of higher energy to a state of lower energy

• In any energy transformation, there is a decreasing availability of energy for doing additional work– Rocks fall off cliffs to the ground below, and once at the bottom,

their ability to do additional work is reduced; it is very unlikely that they will lift themselves back to the top of the cliff

• Such events are said to be spontaneous, a term that indicates they are thermodynamically favorable and can occur without the input of external energy

• The loss of available energy during a process is a result of a tendency for the randomness, or disorder, of the universe to increase every time there is a transfer of energy

• This gain in disorder is measured by the term entropy, and the loss of available energy is equal to TΔS, where Δ S is the change in entropy between the initial and final states

• According to the second law of thermodynamics, every event is accompanied by an increase in the entropy of the universe

• When a sugar cube is dropped into a cup of hot water, for example, there is a spontaneous shift of the molecules from an ordered state in the crystal to a much more disordered condition when the sugar molecules are spread throughout the solution (Figure 3.3a)

• The dissolved sugar in Figure 3.3a can decrease in entropy; it can be recrystallized by evaporating off the water (Figure 3.3b)

FIGURE 3.3 Events are accompanied by an increase in the entropy of the universe. (a) A sugar cube contains sucrose molecules in a highly ordered arrangement in which their freedom of movement is restricted. As the cube dissolves, the freedom of movement of the sucrose molecules is greatly increased, and their random movement causes them to become equally distributed throughout the available space. Once this occurs, there will be no further tendency for redistribution, and the entropy of the system is at a maximum. (b) Sugar molecules spread randomly through a solution can be returned to an ordered state, but only if theentropy of the surroundings is increased, as occurs when the more ordered water molecules of the liquid phase become disordered following evaporation

• Living organisms are able to decrease their own entropy by increasing the entropy of their environment

• Entropy is decreased in an organism when relatively simple molecules, such as amino acids, are ordered into more complex molecules, such as the protein myoglobin in a muscle cell

Free Energy (G)

• Together, the first and second laws of thermodynamics indicate that the energy of the universe is constant, but the entropy continues to increase toward a maximum

• the free energy G, named after J. W. Gibbs, who showed that “all systems change in such a way that free energy [G] is minimized

• In the case of a chemical reaction, reactants products, the change in free energy G is given by

• the free energy G, named after J. W. Gibbs, who showed that “all systems change in such a way that free energy [G] is minimized

• The free energy of a chemical system can be defined as G = H - TS, where H is the bond energy, or enthalpy, of the system; T is its temperature in degrees Kelvin (K); and S is the entropy, a measure of its randomness or disorder

• If temperature remains constant, a reaction proceeds spontaneously only if the free-energy change G in the following equation is negative: ΔG = ΔH - TΔ S

• In an exothermic reaction, the products contain less bond energy than the reactants, the liberated energy is usually converted to heat (the energy of molecular motion), and Δ H is negative

• In an endothermic reaction, the products contain more bond energy than the reactants, heat is absorbed, and Δ H is positive

Hydrolysis of ATP Releases Substantial FreeEnergy and Drives Many Cellular Processes

In almost all organisms, adenosine triphosphate, or ATP, is the most important molecule for capturing, transiently storing, and subsequently transferring energy to perform work (e.g., biosynthesis, mechanical motion)

• The useful energy in an ATP molecule is contained in phosphoanhydride bonds, which are covalent bonds formed from the condensation of two molecules of phosphate by the loss of water

• An ATP molecule has two key phosphoanhydride bonds

Free-Energy Changes in Metabolic Reactions

• One of the most important chemical reactions in the cell is the hydrolysis of ATP (Figure 3.5). In the reaction

ATP + H2O → ADP + Pi

ATP Is Generated During Photosynthesisand Respiration

• To continue functioning cells must constantly replenish their ATP supply

• The initial energy source whose energy is ultimately transformed into the phosphoanhydride bonds of ATP and bonds in other compounds in nearly all cells is sunlight

• In photosynthesis, plants and certain microorganisms can trap the energy in light and use it to synthesize ATP from ADP and Pi

Much of the ATP produced in photosynthesis is hydrolyzed to provide energy for the conversion of carbon dioxide to six-carbon sugars, a process called carbon fixation:

• In animals, the free energy in sugars and other molecules derived from food is released in the process of respiration

• All synthesis of ATP in animal cells and in nonphotosynthetic microorganisms results from the chemical transformation of energy-rich compounds in the diet (e.g., glucose, starch)

• The complete oxidation of glucose to yield carbon dioxide

• Cells employ an elaborate set of enzyme-catalyzed reactions to couple the metabolism of 1 molecule of glucose to the synthesis of as many as 30 molecules of ATP from 30 molecules of ADP

• This oxygen-dependent (aerobic) degradation (catabolism) of glucose is the major pathway for generating ATP in all animal cells, nonphotosynthetic plant cells, and many bacterial cells

• Light energy captured in photosynthesis is not the only source of chemical energy for all cells

• Certain microorganisms that live in deep ocean vents, where sunlight is completely absent, derive the energy for converting ADP and Pi into ATP from the oxidation of reduced inorganic compounds

• These reduced compounds originate in the center of the earth and are released at the vents

NAD and FAD Couple Many BiologicalOxidation and Reduction Reactions

• In many chemical reactions, electrons are transferred from one atom or molecule to another; this transfer may or may not accompany the formation of new chemical bonds

• The loss of electrons from an atom or a molecule is called oxidation, and the gain of electrons by an atom or a molecule is called reduction

• Because electrons are neither created nor destroyed in a chemical reaction, if one atom or molecule is oxidized, another must be reduced

• For example, oxygen draws electrons from Fe2+ (ferrous) ions to form Fe3+ (ferric) ions, a reaction that occurs as part of the process by which carbohydrates are degraded in mitochondria. Each oxygen atom receives two electrons, one from each of two Fe2+ ions:

• Thus Fe2 is oxidized, and O2 is reduced

• Such reactions in which one molecule is reduced and another oxidized often are referred to as redox reactions

• Oxygen is an electron acceptor in many redox reactions in aerobic cells

• Many biologically important oxidation and reduction reactions involve the removal or the addition of hydrogen atoms (protons plus electrons) rather than the transfer of isolated electrons on their own

• Protons are soluble in aqueous solutions (as H3O+), but electrons are not and must be transferred directly from one atom or molecule to another without a water-dissolved intermediate

• In this type of oxidation reaction, electrons often are transferred to small electron-carrying molecules, sometimes referred to as coenzymes

• The most common of these electron carriers are NAD+ (nicotinamide adenine dinucleotide), which is reduced to NADH, and FAD (flavin adenine dinucleotide), which is reduced to FADH2

• The reduced forms of these coenzymes can transfer protons and electrons to other molecules, thereby reducing them