Energy Flow in the Life of a Cell Chapter 6 Energy Flow in the Life of a Cell

Chapter 6 At a Glance 6.1 What Is Energy? 6.2 How Does Energy Flow in Chemical Reactions? 6.3 How Is Energy Transported Within Cells? 6.4 How Do Enzymes Promote Biochemical Reactions? 6.5 How Do Cells Regulate Their Metabolic Reactions?

6.1 What Is Energy? Energy is the capacity to do work Work is a force acting on an object that causes the object to move

6.1 What Is Energy? Chemical energy is the energy that is contained in molecules and released by chemical reactions Molecules that provide chemical energy include sugar, glycogen, and fat Cells use specialized molecules such as ATP to accept and transfer energy from one chemical reaction to the next

There are two fundamental types of energy 6.1 What Is Energy? There are two fundamental types of energy Potential energy is stored energy For example, the chemical energy in bonds, the electrical charge in a battery, and a penguin poised to plunge Kinetic energy is the energy of movement For example, light, heat, electricity, and the movement of objects

Author Animation: Types of Energy

From Potential to Kinetic Energy Fig. 6-1

6.1 What Is Energy? The laws of thermodynamics describe the basic properties of energy The laws describe the quantity (the total amount) and the quality (the usefulness) of energy Energy can neither be created nor destroyed (the first law of thermodynamics), but can change form The first law is often called the law of conservation of energy The total amount of energy within a closed system remains constant unless energy is added or removed from the system

6.1 What Is Energy? The laws of thermodynamics describe the basic properties of energy (continued) The amount of useful energy decreases when energy is converted from one form to another (the second law of thermodynamics) Entropy (disorder) is the tendency to move toward a loss of complexity and of useful energy and toward an increase in randomness, disorder, and less-useful energy

6.1 What Is Energy? The laws of thermodynamics describe the basic properties of energy (continued) Useful energy tends to be stored in highly organized matter, and when energy is used in a closed system (such as the world in which we live), there is an overall increase in entropy For example, when gasoline is burned, the orderly arrangement of eight carbons bound together in a gasoline molecule are converted to eight randomly moving molecules of carbon dioxide

Energy Conversions Result in a Loss of Useful Energy Combustion by engine 100 units chemical energy (concentrated) 75 units heat energy  25 units kinetic energy (motion) Fig. 6-2

6.1 What Is Energy? Living things use the energy of sunlight to create the low-entropy conditions of life The highly organized low-entropy systems of life do not violate the second law of thermodynamics because they are achieved through a continuous influx of usable light energy from the sun In creating kinetic energy in the form of sunlight, the sun also produces vast entropy as heat

6.2 How Does Energy Flow in Chemical Reactions? A chemical reaction is a process that forms or breaks the chemical bonds that hold atoms together Chemical reactions convert one set of chemical substances, the reactants, into another set, the products All chemical reactions require a net input of energy Exergonic reactions release energy Endergonic reactions require an input of energy

Author Animation: Exergonic and Endergonic Reactions

An Exergonic Reaction energy + reactants + products Fig. 6-3

An Endergonic Reaction energy + products + reactants Fig. 6-4

6.2 How Does Energy Flow in Chemical Reactions? Exergonic reactions release energy Reactants contain more energy than products in exergonic reactions An example of an exergonic reaction is the burning of glucose As glucose is burned, the sugar (C6H12O6) combines with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O), releasing energy Because molecules of sugar contain more energy than do molecules of carbon dioxide and water, the reaction releases energy

Reactants and End Products of Burning Glucose energy C6H12O6 (glucose)  6 O2 (oxygen) 6 CO2 (carbon dioxide)  6 H2O (water) Fig. 6-5

Activation Energy in Exergonic Reactions Activation energy needed to ignite glucose high energy level of reactants glucose + O2 energy content of molecules CO2 + H2O low Fig. 6-6 progress of reaction

6.2 How Does Energy Flow in Chemical Reactions? Exergonic reactions release energy (continued) All chemical reactions require an initial energy input (activation energy) to get started The negatively charged electron shells of atoms repel one another and inhibit bond formation Molecules need to be moving with sufficient collision speed to overcome electronic repulsion and react Increasing the temperature will increase kinetic energy and, thus, the rate of reaction

6.2 How Does Energy Flow in Chemical Reactions? Endergonic reactions require a net input of energy The reactants in endergonic reactions contain less energy than the products An example of an endergonic reaction is photosynthesis In photosynthesis, green plants add the energy of sunlight to the lower-energy reactants water and carbon dioxide to produce the higher-energy product sugar

Photosynthesis energy C6H12O6 (glucose)  6 O2 (oxygen) 6 CO2 (carbon dioxide)  6 H2O (water) Fig. 6-7

6.3 How Is Energy Transported Within Cells? Most organisms are powered by the breakdown of glucose Energy in glucose cannot be used directly to fuel endergonic reactions Energy released by glucose breakdown is first transferred to an energy-carrier molecule

6.3 How Is Energy Transported Within Cells? Energy-carrier molecules are high-energy, unstable molecules that are synthesized at the site of an exergonic reaction, capturing some of the released energy These high-energy molecules then transfer the energy to an endergonic reaction elsewhere in the cell

6.3 How Is Energy Transported Within Cells? ATP is the principal energy carrier in cells Adenosine triphosphate (ATP) is the most common energy-carrying molecule ATP is composed of the nitrogen-containing base adenine, the sugar ribose, and three phosphates

The Interconversion of ADP and ATP energy P P P P P  P ATP phosphate ADP (a) ATP synthesis: Energy is stored in ATP energy P P P ATP P P  P phosphate ADP Fig. 6-8 (b) ATP breakdown: Energy is released

6.3 How Is Energy Transported Within Cells? ATP is the principal energy carrier in cells (continued) Energy is released in cells during glucose breakdown and is used to combine the relatively low-energy molecules adenosine diphosphate (ADP) and phosphate (P) into ATP Energy is stored in the high-energy phosphate bonds of ATP The formation of ATP is an endergonic reaction

6.3 How Is Energy Transported Within Cells? ATP is the principal energy carrier in cells (continued) At sites in the cell where energy is needed, ATP is broken down into ADP + P and its stored energy is released This energy is then transferred to endergonic reactions through coupling Unlike glycogen and fat, ATP stores energy very briefly before being broken down

6.3 How Is Energy Transported Within Cells? Electron carriers also transport energy within cells ATP is not the only energy-carrier molecule in cells Energy can be transferred to electrons in glucose metabolism and photosynthesis Electron carriers like nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) transport high-energy electrons Electron carriers donate their high-energy electrons to other molecules, often leading to ATP synthesis

6.3 How Is Energy Transported Within Cells? Coupled reactions link exergonic with endergonic reactions In a coupled reaction, an exergonic reaction provides the energy needed to drive an endergonic reaction Sunlight energy stored in glucose by plants is transferred to other organisms by the exergonic breakdown of the sugar and its use in endergonic processes such as protein synthesis The two reactions may occur in different parts of the cell, so energy-carrier molecules carry the energy from one to the other

Author Animation: Coupled Reactions

Coupled Reactions Within Living Cells high-energy reactants (glucose) high-energy products (protein) ATP exergonic (glucose breakdown) endergonic (protein synthesis) low-energy products (CO2, H2O) ADP  P low-energy reactants (amino acids) Fig. 6-9

6.4 How Do Enzymes Promote Biochemical Reactions? Enzyme structures allow biochemical reactions to catalyze specific reactions Enzymes, like all catalysts, lower activation energy Enzymes control the rate of energy release and capture some energy in ATP

6.4 How Do Enzymes Promote Biochemical Reactions? At body temperatures, spontaneous reactions proceed too slowly to sustain life Reaction speed is generally determined by the activation energy required; that is, how much energy is required to start the process Reactions with low activation energies proceed rapidly at body temperature Reactions with high activation energies (e.g., sugar breakdown) move very slowly at body temperature, even if exergonic overall

6.4 How Do Enzymes Promote Biochemical Reactions? At body temperatures, spontaneous reactions proceed too slowly to sustain life (continued) Enzymes are employed to catalyze (speed up) chemical reactions in cells by lowering the activation energy needed to start the reaction Enzymes are biological catalysts and regulate all the reactions in living cells

6.4 How Do Enzymes Promote Biochemical Reactions? Catalysts reduce activation energy Catalysts speed up the rate of a chemical reaction without themselves being used up All catalysts have three important properties: They speed up reactions by lowering the activation energy required for the reaction to begin They speed up only exergonic reactions They are not consumed or changed by the reactions they promote

6.4 How Do Enzymes Promote Biochemical Reactions? Catalysts reduce activation energy (continued) Catalytic converters in cars facilitate the conversion of carbon monoxide (CO) to carbon dioxide (CO2) 2 CO + O2  2 CO2 + heat energy

Author Animation: Activation Energy

Catalysts Such As Enzymes Lower Activation Energy high Activation energy without catalyst Activation energy with catalyst energy content of molecules reactants products low Fig. 6-10 progress of reaction

6.4 How Do Enzymes Promote Biochemical Reactions? Enzymes are biological catalysts Enzymes are composed primarily of protein synthesized by living organisms and may require small nonprotein helper molecules called coenzymes in order to function Many water-soluble vitamins (certain B vitamins) are essential to humans because they are used by the body to synthesize coenzymes Enzymes orient, distort, and reconfigure molecules in the process of lowering activation energy

6.4 How Do Enzymes Promote Biochemical Reactions? Enzymes are biological catalysts (continued) Enzymes (proteins) have two attributes that set them apart from nonbiological catalysts: Enzymes are very specific for the reactions they catalyze Enzyme activity is regulated

6.4 How Do Enzymes Promote Biochemical Reactions? Enzyme structures allow them to catalyze specific reactions Each enzyme has a pocket called an active site into which one or more reactant molecules, called substrates, can enter The amino acid sequence of the enzyme protein and the way the protein chains are folded create in the active site a distinctive shape and distribution of electrical charge The distinctive shape of the active site is both complementary and specific to the substrate Active site amino acids bind to the substrate and distort bonds to facilitate a reaction

6.4 How Do Enzymes Promote Biochemical Reactions? Enzyme structures allow them to catalyze specific reactions (continued) There are three steps of enzyme catalysis Both the shape and the charge of the active site allow substrates to enter the enzyme only in specific orientations Upon binding, the substrates and active site change shape to promote a reaction When the reaction between the substrates is finished, the product(s) no longer properly fit into the active site and drift away

Author Animation: Enzymes and Substrates

The Cycle of Enzyme-Substrate Interactions substrates active site of enzyme enzyme 1 Substrates enter the active site in a specific orientation 3 The substrates, bonded together, leave the enzyme; the enzyme is ready for a new set of substrates The substrates and active site change shape, promoting a reaction between the substrates 2 Fig. 6-11

6.4 How Do Enzymes Promote Biochemical Reactions? Enzymes, like all catalysts, lower activation energy The breakdown or synthesis of a molecule within a cell usually occurs in many small steps, each catalyzed by a different enzyme Each of the enzymes lowers the activation energy for its particular reaction, allowing the reaction to occur readily at body temperature

6.4 How Do Enzymes Promote Biochemical Reactions? Enzymes control the rate of energy release and capture some energy in ATP Thanks to a series of chemical transformations, each catalyzed by a different enzyme, the energy stored in sugar is released gradually during its breakdown Some energy is lost as heat, while some is harnessed to power endergonic reactions that lead to ATP synthesis

6.5 How Do Cells Regulate Their Metabolic Reactions? The sum of all the chemical reactions inside a cell is its metabolism Many cellular reactions are linked through metabolic pathways In metabolic pathways, an initial reactant molecule is modified by an enzyme, creating a slightly different intermediate molecule, which is modified by another enzyme, and so on, until a final product is produced

Simplified Metabolic Pathways 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. 6-12

6.5 How Do Cells Regulate Their Metabolic Reactions? Reaction rates tend to increase as substrate or enzyme levels increase For a given amount of enzyme, as substrate levels increase, the reaction rate will increase until the active sites of all the enzyme molecules are being continuously occupied by new substrate molecules

6.5 How Do Cells Regulate Their Metabolic Reactions? Reaction rates tend to increase as substrate or enzyme levels increase (continued) Metabolic pathways are controlled in several ways Control of enzyme synthesis, which regulates availability Control of enzyme activity

6.5 How Do Cells Regulate Their Metabolic Reactions? Cells regulate enzyme synthesis Genes that code for specific proteins are turned on and off according to metabolic need An increase in substrate can trigger increased enzyme production, leading to decreased substrate levels

6.5 How Do Cells Regulate Their Metabolic Reactions? Cells regulate enzyme activity Some enzymes are synthesized in inactive form For example, the protein-digesting enzymes pepsin and trypsin are inactive when synthesized, but become activated in the stomach under acidic conditions (pepsin) or in the small intestine under alkaline conditions (trypsin)

6.5 How Do Cells Regulate Their Metabolic Reactions? Cells regulate enzyme activity (continued) Some enzymes are inhibited 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

Competitive and Noncompetitive Enzyme Inhibition substrate active site enzyme noncompetitive inhibitor site (a) A substrate binding to an enzyme Fig. 6-13a

Competitive and Noncompetitive Enzyme Inhibition A competitive inhibitor molecule occupies the active site and blocks entry of the substrate (b) Competitive inhibition Fig. 6-13b

Competitive and Noncompetitive Enzyme Inhibition A noncompetitive inhibitor molecule causes the active site to change shape, so the substrate no longer fits noncompetitive inhibitor molecule (c) Noncompetitive inhibition Fig. 6-13c

6.5 How Do Cells Regulate Their Metabolic Reactions? Cells regulate enzyme activity (continued) Small regulator molecules can bind to enzymes and enhance or inhibit activity by allosteric regulation Enzymes that undergo allosteric regulation have a special regulatory binding site on the enzyme that is distinct from the enzyme’s active site and similar to a noncompetitive inhibitor site Allosteric regulation can either increase or decrease enzyme activity, whereas noncompetitive inhibition only reduces activity allosteric regulation 變構調節：變構調節是指某些調節物能與酶的調節部位結合使酶分子的構象發生改變，從而改變酶的活性，稱酶的變構調節

6.5 How Do Cells Regulate Their Metabolic Reactions? Cells regulate enzyme activity (continued) Feedback inhibition is a negative feedback type of allosteric inhibition that causes a metabolic pathway to stop producing its product when quantities reach an optimum level An enzyme near the beginning of a metabolic pathway is inhibited allosterically by the end product of the pathway

Allosteric Regulation of an Enzyme by Feedback Inhibition intermediates A B C D enzyme 1 enzyme 2 enzyme 3 enzyme 4 enzyme 5 As levels of isoleucine rise, it binds to the regulatory site on enzyme 1, inhibiting it threonine (initial reactant) enzyme 1 isoleucine (end product) isoleucine Fig. 6-14

6.5 How Do Cells Regulate Their Metabolic Reactions? 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

6.5 How Do Cells Regulate Their Metabolic Reactions? Poisons, drugs, and environmental conditions influence enzyme activity (continued) Some inhibitors bind permanently to the enzyme Some nerve gases and insecticides permanently block the active site of acetylcholinesterase Arsenic, mercury, and lead bind permanently to the non-active sites of various enzymes, inactivating them

6.5 How Do Cells Regulate Their Metabolic Reactions? 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

6.5 How Do Cells Regulate Their Metabolic Reactions? 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

6.5 How Do Cells Regulate Their Metabolic Reactions? 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

Human Enzymes Function Best Within Narrow Ranges of pH and Temperature For pepsin, maximum activity occurs at about pH 2 For trypsin, maximum activity occurs at about pH 8 fast For most cellular enzymes, maximum activity occurs at about pH 7.4 rate of reaction slow Fig. 6-15a (a) Effect of pH on enzyme activity

Human Enzymes Function Best Within Narrow Ranges of pH and Temperature fast For most human enzymes, maximum activity occurs at about 98.6°F (37°C) rate of reaction slow (b) Effect of temperature on enzyme activity Fig. 6-15b