Overview: The Energy of Life

       The living cell is a miniature chemical factory where thousands of reactions occur

       The cell extracts energy and applies energy to perform work

       Some organisms even convert energy to light, as in bioluminescence

Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics

       Metabolism is the totality of an organism’s chemical reactions

       Metabolism is an emergent property of life that arises from interactions between molecules within the cell

Organization of the Chemistry of Life into Metabolic Pathways

       A metabolic pathway begins with a specific molecule and ends with a product

       Each step is catalyzed by a specific enzyme

 

       Catabolic pathways release energy by breaking down complex molecules into simpler compounds

       Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism

       Anabolic pathways consume energy to build complex molecules from simpler ones

       The synthesis of protein from amino acids is an example of anabolism

       Bioenergetics is the study of how organisms manage their energy resources

Forms of Energy

       Energy is the capacity to cause change

       Energy exists in various forms, some of which can perform work

       Kinetic energy is energy associated with motion

       Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules

       Potential energy is energy that matter possesses because of its location or structure

       Chemical energy is potential energy available    for release in a chemical reaction             

       Energy can be converted from one form to another

The Laws of Energy Transformation

       Thermodynamics is the study of energy transformations

       A isolated system, such as that 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

The First Law of Thermodynamics

       According to the first law of thermodynamics, the energy of the universe is constant

    Energy can be transferred and transformed, but it cannot be created or destroyed

       The first law is also called the principle of conservation of energy

The Second Law of Thermodynamics

       During every energy transfer or transformation, some energy is unusable, and is often lost as heat

       According to the second law of thermodynamics

    Every energy transfer or transformation increases the entropy (disorder) of the universe

       Living cells unavoidably convert organized forms of energy to heat

       Spontaneous processes occur without energy input; they can happen quickly or slowly

       For a process to occur without energy input, it must increase the entropy of the universe

Biological Order and Disorder

       Cells create ordered structures from less ordered materials

       Organisms also replace ordered forms of matter and energy with less ordered forms

       Energy flows into an ecosystem in the form of light and exits in the form of heat

       The evolution of more complex organisms does not violate the second law of thermodynamics

       Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases

Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously

       Biologists want to know which reactions occur spontaneously and which require input of energy

       To do so, they need to determine energy changes that occur in chemical reactions

Free-Energy Change, DG

       A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell

       The change in free energy (G) during a process is related to the change in enthalpy, or change in total energy (H), change in entropy (S), and temperature in Kelvin (T)

                                                       G = H – TS

       Only processes with a negative G are spontaneous

       Spontaneous processes can be harnessed to perform work

Free Energy, Stability, and Equilibrium

       Free energy is a measure of a system’s instability, its tendency to change to a more stable state

       During a spontaneous change, free energy decreases and the stability of a system increases

       Equilibrium is a state of maximum stability

       A process is spontaneous and can perform work only when it is moving toward equilibrium

Free Energy and Metabolism

       The concept of free energy can be applied to the chemistry of life’s processes

Exergonic and Endergonic Reactions in Metabolism

       An exergonic reaction proceeds with a net release of free energy and is spontaneous

       An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous

Equilibrium and Metabolism

       Reactions in a closed system eventually reach equilibrium and then do no work

       Cells are not in equilibrium; they are open systems experiencing a constant flow of materials

       A defining feature of life is that metabolism is never at equilibrium

       A catabolic pathway in a cell releases free energy in a series of reactions

       Closed and open hydroelectric systems can serve as analogies

Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions

       A cell does three main kinds of work

    Chemical

    Transport

    Mechanical

       To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one

       Most energy coupling in cells is mediated by ATP

The Structure and Hydrolysis of ATP

       ATP (adenosine triphosphate) is the cell’s energy shuttle

       ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups

       The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis

       Energy is released from ATP when the terminal phosphate bond is broken

       This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves

How the Hydrolysis of ATP Performs Work

       The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP

       In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction

       Overall, the coupled reactions are exergonic

       ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant

       The recipient molecule is now called a phosphorylated intermediate

The Regeneration of ATP

       ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)

       The energy to phosphorylate ADP comes from catabolic reactions in the cell

       The ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways

Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers

       A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction

       An enzyme is a catalytic protein

       Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

The Activation Energy Barrier

       Every chemical reaction between molecules involves bond breaking and bond forming

       The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)

       Activation energy is often supplied in the form of thermal energy that the reactant molecules absorb from their surroundings

How Enzymes Lower the EA Barrier

       Enzymes catalyze reactions by lowering the EA barrier

       Enzymes do not affect the change in free energy (G); instead, they hasten reactions that would occur eventually

Substrate Specificity of Enzymes

       The reactant that an enzyme acts on is called the enzyme’s substrate

       The enzyme binds to its substrate, forming an enzyme-substrate complex

       The active site is the region on the enzyme where the substrate binds

       Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction

Catalysis in the Enzyme’s Active Site

       In an enzymatic reaction, the substrate binds to the active site of the enzyme

       The active site can lower an EA barrier by

    Orienting substrates correctly

    Straining substrate bonds

    Providing a favorable microenvironment

    Covalently bonding to the substrate

Effects of Local Conditions on Enzyme Activity

       An enzyme’s activity can be affected by

    General environmental factors, such as temperature and pH

    Chemicals that specifically influence the enzyme

Effects of Temperature and pH

       Each enzyme has an optimal temperature in which it can function

       Each enzyme has an optimal pH in which it can function

       Optimal conditions favor the most active shape for the enzyme molecule

Cofactors

       Cofactors are nonprotein enzyme helpers

       Cofactors may be inorganic (such as a metal in ionic form) or organic

       An organic cofactor is called a coenzyme

       Coenzymes include vitamins

Enzyme Inhibitors

       Competitive inhibitors bind to the active site of an enzyme, competing with the substrate

       Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective

       Examples of inhibitors include toxins, poisons, pesticides, and antibiotics

The Evolution of Enzymes

        Enzymes are proteins encoded by genes

        Changes (mutations) in genes lead to changes in amino acid composition of an enzyme

        Altered amino acids in enzymes may alter their substrate specificity

        Under new environmental conditions a novel form of an enzyme might be favored

Concept 8.5: Regulation of enzyme activity helps control metabolism

       Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated

       A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes

Allosteric Regulation of Enzymes

       Allosteric regulation may either inhibit or stimulate an enzyme’s activity

       Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site

Allosteric Activation and Inhibition

       Most allosterically regulated enzymes are made from polypeptide subunits

       Each enzyme has active and inactive forms

       The binding of an activator stabilizes the active form of the enzyme

       The binding of an inhibitor stabilizes the inactive form of the enzyme

       Cooperativity is a form of allosteric regulation that can amplify enzyme activity

       One substrate molecule primes an enzyme to act on additional substrate molecules more readily

       Cooperativity is allosteric because binding by a substrate to one active site affects catalysis in a different active site

Identification of Allosteric Regulators

       Allosteric regulators are attractive drug candidates for enzyme regulation because of their specificity

       Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses

Feedback Inhibition

       In feedback inhibition, the end product of a metabolic pathway shuts down the pathway

       Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed

Specific Localization of Enzymes Within the Cell

       Structures within the cell help bring order to metabolic pathways

       Some enzymes act as structural components of membranes

       In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria