Energy Flow in the Life of a Cell 6 Energy Flow in the Life of a Cell 1

Chapter 6 At a Glance 6.1 What Is Energy? 6.2 How Is Energy Transformed During Chemical Reactions? 6.3 How Is Energy Transported Within Cells?

6.1 What Is Energy? Energy is the capacity to do work Work is a transfer of energy to an object, which 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

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

Figure 6-1 Converting potential energy to kinetic energy

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

Figure 6-2 Energy conversions result in a loss of useful energy Combustion by engine gas 100 units chemical energy 80 units heat energy  20 units kinetic energy 10

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 Is Energy Transformed During 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 either release energy or require a net input of energy Exergonic reactions release energy Endergonic reactions require an input of energy

Figure 6-3 An exergonic reaction energy reactants products 13

Figure 6-4 An endergonic reaction energy products reactants 14

6.2 How Is Energy Transformed During 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

Figure 6-5 Reactants and products of burning glucose energy C6H12O6  6 O2 (glucose) (oxygen) 6 CO2  6 H2O (carbon dioxide) (water) 16

6.2 How Is Energy Transformed During 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

Figure 6-6 Photosynthesis energy C6H12O6  6 O2 (glucose) (oxygen) 6 CO2  6 H2O (carbon dioxide) (water) 18

6.2 How Is Energy Transformed During Chemical Reactions? Endergonic reactions require a net input of 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

Figure 6-7 Activation energy in an exergonic reaction Activation energy required to start the reaction high energy level of reactants reactants energy content of molecules energy level of products products low progress of reaction An exergonic reaction Sparks ignite gas 20

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 Energy-carrier molecules are high-energy, unstable molecules that are synthesized at the site of an exergonic reaction, capturing some of the released energy

6.3 How Is Energy Transported Within Cells? ATP and electron carriers transport energy within 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 ATP is sometimes called the “energy currency” of cells

6.3 How Is Energy Transported Within Cells? ATP and electron carriers transport energy within 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

Figure 6-8 The interconversion of ADP and ATP energy ATP phosphate ADP ATP synthesis: Energy is stored in ATP energy ATP phosphate ADP ATP breakdown: Energy is released 24

6.3 How Is Energy Transported Within Cells? ATP and electron carriers transport energy within 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? ATP and electron carriers transport energy within cells (continued) ATP is not the only energy-carrier molecule in cells Energy can be transferred to electrons in glucose metabolism and photosynthesis Electron carriers such as 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

Animation: Coupled Reactions

Figure 6-9 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  Pi low-energy reactants (amino acids) 29