Energy is the capacity to do work.

Energy is the capacity to do work.
What Is Energy? Energy is the capacity to do work. Synthesizing molecules Moving objects Generating heat and light

What Is Energy? Types of energy (Jumping Penguins or log in a hearth)
Kinetic: energy of movement Potential: stored energy Fig. 5-1

First Law of Thermodynamics
What Is Energy? First Law of Thermodynamics “Energy cannot be created nor destroyed, but it can change its form.” Example: potential energy in gasoline can be converted to kinetic energy in a car, but the energy is not lost

Second Law of Thermodynamics
What Is Energy? Second Law of Thermodynamics “When energy is converted from one form to another, the amount of useful energy decreases.” No process is 100% efficient. Example: more potential energy is in the gasoline than is transferred to the kinetic energy of the car moving Where is the rest of the energy? It is released in a less useful form as heat—the total energy is maintained.

Matter tends to become less organized.
What Is Energy? Matter tends to become less organized. There is a continual decrease in useful energy, and a build up of heat and other non-useful forms of energy. Entropy: the spontaneous reduction in ordered forms of energy, and an increase in randomness and disorder as reactions proceed Example: gasoline is made up of an eight-carbon molecule that is highly ordered When broken down to single carbons in CO2, it is less ordered and more random.

What Is Energy? In order to keep useful energy flowing in ecosystems where the plants and animals produce more random forms of energy, new energy must be brought in.

What Is Energy? Sunlight provides an unending supply of new energy to power all plant and animal reactions, leading to increased entropy. Fig. 5-2

How Does Energy Flow In Chemical Reactions?
Chemical reaction: the conversion of one set of chemical substances (reactants) into another (products) Exergonic reaction: a reaction that releases energy; the products contain less energy than the reactants

Exergonic reaction energy released + reactants + products (a) Exergonic reaction Fig. 5-3a

Burning glucose releases energy. energy released C6H12O6 + 6 O2 (glucose) (oxygen) 6 CO2 + 6 H2O (carbon dioxide) (water) Fig. 5-4

Exergonic reactions release energy. Example: sugar burned by a flame in the presence of oxygen produces carbon dioxide (CO2) and water Sugar and oxygen contain more energy than the molecules of CO2 and water. The extra energy is released as heat.

Endergonic reaction: a reaction that requires energy input from an outside source; the products contain more energy than the reactants

Endergonic reaction energy used + products + reactants (b) Endergonic reaction Fig. 5-3b

Photosynthesis requires energy. energy C6H12O6 + 6 O2 (glucose) (oxygen) 6 CO2 + 6 H2O (carbon dioxide) (water) Fig. 5-5

Endergonic reactions require an input of energy. Example: sunlight energy + CO2 + water in photosynthesis produces sugar and oxygen The sugar contains far more energy than the CO2 and water used to form it.

All reactions require an initial input of energy. The initial energy input to a chemical reaction is called the activation energy. Activation energy needed to ignite glucose Activation energy captured from sunlight high glucose Energy level of reactants glucose + O2 energy content of molecules CO2 + H2O CO2 + H2O Energy level of reactants low progress of reaction progress of reaction (a) Burning glucose (sugar): an exergonic reaction (b) Photosynthesis: an endergonic reaction Fig. 5-6

The source of activation energy is the kinetic energy of movement when molecules collide. The match provided the activation energy in the log example.

How Does Energy Flow in Chemical Reactions?
Exergonic reactions may be linked with endergonic reactions. Endergonic reactions obtain energy from energy-releasing exergonic reactions in coupled reactions. Example: the exergonic reaction of burning gasoline in a car provides the endergonic reaction of moving the car Example: exergonic reactions in the sun release light energy used to drive endergonic sugar-making reactions in plants

How Is Energy Carried Between Coupled Reactions?
The job of transferring energy from one place in a cell to another is done by energy-carrier molecules. ATP (adenosine triphosphate) is the main energy carrier molecule in cells, and provides energy for many endergonic reactions.

ATP is made from ADP (adenosine diphosphate) and phosphate plus energy released from an exergonic reaction (e.g., glucose breakdown) in a cell. energy A P P P ATP A P P + P ADP phosphate Fig. 5-7

ATP is the principal energy carrier in cells. ATP stores energy in its phosphate bonds and carries the energy to various sites in the cell where energy-requiring reactions occur. ATP’s phosphate bonds then break yielding ADP, phosphate, and energy. This energy is then transferred to the energy-requiring reaction.

Breakdown of ATP releases energy. energy A P P P ATP A P P + P ADP phosphate Fig. 5-8

To summarize: Exergonic reactions (e.g., glucose breakdown) drive endergonic reactions (e.g., the conversion of ADP to ATP). ATP moves to different parts of the cell and is broken down exergonically to liberate its energy to drive endergonic reactions.

glucose A P P P exergonic (glucose breakdown) protein endergonic (ATP synthesis) exergonic (ATP breakdown) endergonic (protein synthesis) CO2 + H2O + heat A P P + P amino acids Fig. 5-9

A biological example of coupled reactions Muscle contraction (an endergonic reaction) is powered by the exergonic breakdown of ATP. During energy transfer in this coupled reaction, heat is given off, with overall loss of usable energy.

Electron carriers also transport energy within cells. Besides ATP, other carrier molecules transport energy within a cell. Electron carriers capture energetic electrons transferred by some exergonic reaction. Energized electron carriers then donate these energy-containing electrons to endergonic reactions.

Common electron carriers are NAD+ and FAD. high-energy reactants energized NADH e– e– high-energy products depleted NAD+ + H+ low-energy products low-energy reactants Fig. 5-11

How Do Cells Control Their Metabolic Reactions?
Cell metabolism: the multitude of chemical reactions going on at any specific time in a cell Metabolic pathways: the sequence of cellular reactions (e.g., photosynthesis and glycolysis) Initial reactant Intermediates Final products PATHWAY 1 A B C D E enzyme 1 enzyme 2 enzyme 3 enzyme 4 PATHWAY 2 F G enzyme 5 enzyme 6 Fig. 5-12

At body temperature, many spontaneous reactions proceed too slowly to sustain life. A reaction can be controlled by controlling its activation energy (the energy needed to start the reaction). At body temperature, reactions occur too slowly because their activation energies are too high. Molecules called catalysts are able to gain access to energy that is not produced spontaneously.

Catalysts reduce activation energy. Catalysts are molecules that speed up a reaction without being used up or permanently altered. They speed up the reaction by reducing the activation energy. high Activation energy without catalyst Activation energy with catalyst energy content of molecules reactants products low progress of reaction Fig. 5-13

Three important principles about all catalysts Catalysts speed up a reaction. They speed up reactions that would occur anyway, if their activation energy could be surmounted. Catalysts are not altered by the reaction.

Enzymes are biological catalysts. Almost all enzymes are proteins. Enzymes are highly specialized, generally catalyzing only a single reaction. In metabolic pathways involving multiple reactions, each reaction is catalyzed by a different enzyme.

The structure of enzymes allows them to catalyze specific reactions. Enzymes have an active site where the reactant molecules, called substrates, enter and undergo a chemical change as a result. The specificity of an enzyme reaction is due to the distinctive shape of the active site, which only allows proper substrate molecules to enter.

How does an enzyme catalyze a reaction? Both substrates enter the enzyme’s active site. Substrates enter an enzyme’s active site, changing both of their shapes. The chemical bonds are altered in the substrates, promoting the reaction. The substrates change into a new form that will not fit the active site, and so are released.

The cycle of enzyme–substrate interactions substrates active site of enzyme enzyme Substrates enter the active site in a specific orientation 1 The substrates, bonded together, leave the enzyme; the enzyme is ready for a new set of substrates 3 The substrates and active site change shape, promoting a reaction between the substrates 2 Fig. 5-14

6.5 How Are Enzymes Regulated?
Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) Enzyme activity may be controlled by competitive or noncompetitive inhibition In competitive inhibition, a substance that is not the enzyme’s normal substrate binds to the active site of the enzyme, competing with the substrate for the active site In noncompetitive inhibition, a molecule binds to a site on the enzyme distinct from the active site

Figure 6-13a A substrate binding to an enzyme
active site enzyme noncompetitive inhibitor site A substrate binding to an enzyme 37

Competitive inhibition can be temporary or permanent. Some regulatory molecules temporarily bind directly to an enzyme’s active site, preventing the substrate molecules from binding. These molecules compete with the substrate for access to the active site, and control the enzyme by competitive inhibition.

Figure 6-13b Competitive inhibition
A competitive inhibitor molecule occupies the active site and blocks entry of the substrate Competitive inhibition 39

Figure 6-13c Noncompetitive inhibition
The active site changes shape so the substrate no longer fits when a noncompetitive inhibitor molecule binds the enzyme noncompetitive inhibitor molecule Noncompetitive inhibition 40

Poisons, drugs, and environmental conditions influence enzyme activity Drugs and poisons often inhibit enzymes by competing with the natural substrate for the active site This process occurs either by competitive or by noncompetitive inhibition Environmental conditions can denature enzymes, distorting the three-dimensional structure crucial for their function

Poisons, drugs, and environmental conditions influence enzyme activity (continued) Some poisons and drugs are competitive or noncompetitive inhibitors of enzymes Competitive inhibitors of enzymes, including some nerve gases and insecticides, permanently block the active site of acetylcholinesterase Arsenic, mercury, and lead bind permanently to the nonactive sites of various enzymes, inactivating them

Poisons, drugs, and environmental conditions influence enzyme activity (continued) The activity of an enzyme is influenced by the environment The three-dimensional structure of an enzyme is sensitive to pH, salts, temperature, and the presence of coenzymes

The activity of an enzyme is influenced by the environment (continued) Enzyme structure is distorted (denatured) and function is destroyed when pH is too high or low Salts in an enzyme’s environment can also destroy function by altering structure Salt ions can bind with key amino acids in enzymes, influencing three-dimensional structure and destroying function

The activity of an enzyme is influenced by the environment (continued) Temperature also affects enzyme activity Low temperatures slow down molecular movement High temperatures cause enzyme shape to be altered, destroying function Most enzymes function optimally only within a very narrow range of these conditions

Energy is the capacity to do work.

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