Chapter 5 Energy Flow in the Life of a Cell

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

5.1 What Is Energy? Types of energy –Kinetic: energy of movement –Potential: stored energy Fig. 5-1

5.1 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

5.1 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.

5.1 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 CO 2, it is less ordered and more random.

5.1 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.

5.1 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

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

energy released reactants products Exergonic reaction + + (a) 5.2 How Does Energy Flow In Chemical Reactions? Exergonic reaction Fig. 5-3a

5.2 How Does Energy Flow In Chemical Reactions? Endergonic reaction: a reaction that requires energy input from an outside source; the products contain more energy than the reactants

energy used products reactants Endergonic reaction + + (b) 5.2 How Does Energy Flow In Chemical Reactions? Endergonic reaction Fig. 5-3b

5.2 How Does Energy Flow In Chemical Reactions? Exergonic reactions release energy. –Example: sugar burned by a flame in the presence of oxygen produces carbon dioxide (CO 2 ) and water –Sugar and oxygen contain more energy than the molecules of CO 2 and water. –The extra energy is released as heat.

5.2 How Does Energy Flow In Chemical Reactions? Burning glucose releases energy. Fig. 5-4 energy released C 6 H 12 O 6 6 O 2 (glucose)(oxygen) + 6 CO 2 (carbon dioxide) 6 H 2 O (water) +

5.2 How Does Energy Flow In Chemical Reactions? Endergonic reactions require an input of energy. –Example: sunlight energy + CO 2 + water in photosynthesis produces sugar and oxygen –The sugar contains far more energy than the CO 2 and water used to form it.

5.2 How Does Energy Flow In Chemical Reactions? Photosynthesis requires energy. Fig. 5-5 C 6 H 12 O 6 6 O 2 (glucose)(oxygen) + 6 CO 2 (carbon dioxide) 6 H 2 O (water) + energy

high low progress of reaction energy content of molecules Activation energy needed to ignite glucose Energy level of reactants glucose + O 2 CO 2 + H 2 O glucose Activation energy captured from sunlight Energy level of reactants Burning glucose (sugar): an exergonic reactionPhotosynthesis: an endergonic reaction (a)(b) 5.2 How Does Energy Flow In Chemical Reactions? All reactions require an initial input of energy. –The initial energy input to a chemical reaction is called the activation energy. Fig. 5-6

5.2 How Does Energy Flow In Chemical Reactions? The source of activation energy is the kinetic energy of movement when molecules collide. Molecular collisions force electron shells of atoms to mingle and interact, resulting in chemical reactions.

5.2 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

5.3 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.

ADP ATP phosphate energy + AP P P APP P 5.3 How Is Energy Carried Between Coupled Reactions? ATP is made from ADP (adenosine diphosphate) and phosphate plus energy released from an exergonic reaction (e.g., glucose breakdown) in a cell. Fig. 5-7

5.3 How Is Energy Carried Between Coupled Reactions? 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.

phosphate ADP energy ATP + A APPP P P P 5.3 How Is Energy Carried Between Coupled Reactions? Breakdown of ATP releases energy. Fig. 5-8

5.3 How Is Energy Carried Between Coupled Reactions? 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.