CHAPTER 5 The Working Cell: Energy and Enzymes Modules 5.10-16

Cool “Fires” Attract Mates and Meals Fireflies use light, instead of chemical signals, to send signals to potential mates Females can also use light flashes to attract males of other firefly species — as meals, not mates

The light comes from a set of chemical reactions, the luciferin-luciferase system Fireflies make light energy from chemical energy Life is dependent on energy conversions

ENERGY AND THE CELL Living cells are compartmentalized by membranes Membranes are sites where chemical reactions can occur in an orderly manner Living cells process energy by means of enzyme-controlled chemical reactions

5.1 Energy is the capacity to perform work Energy is defined as the capacity to do work All organisms require energy to stay alive Energy makes change possible

Kinetic energy is energy that is actually doing work Figure 5.1A Potential energy is stored energy Figure 5.1B

5.2 Two laws govern energy conversion First law of thermodynamics Energy can be changed from one form to another However, energy cannot be created or destroyed Figure 5.2A

Second law of thermodynamics Energy changes are not 100% efficient Energy conversions increase disorder, or entropy Some energy is always lost as heat Figure 5.2B

5.3 Chemical reactions either store or release energy Cells carry out thousands of chemical reactions The sum of these reactions constitutes cellular metabolism

Potential energy of molecules There are two types of chemical reactions: Endergonic reactions absorb energy and yield products rich in potential energy Products Amount of energy INPUT Potential energy of molecules Reactants Figure 5.3A

Amount of energy OUTPUT Potential energy of molecules Exergonic reactions release energy and yield products that contain less potential energy than their reactants Reactants Amount of energy OUTPUT Potential energy of molecules Products Figure 5.3B

5.4 ATP shuttles chemical energy within the cell In cellular respiration, some energy is stored in ATP molecules ATP powers nearly all forms of cellular work ATP molecules are the key to energy coupling

Adenosine triphosphate Adenosine diphosphate (ADP) When the bond joining a phosphate group to the rest of an ATP molecule is broken by hydrolysis, the reaction supplies energy for cellular work Adenine Phosphate groups Hydrolysis Energy Ribose Adenosine triphosphate Adenosine diphosphate (ADP) Figure 5.4A

Potential energy of molecules How ATP powers cellular work Reactants Products Potential energy of molecules Protein Work Figure 5.4B

Dehydration synthesis The ATP cycle Hydrolysis Dehydration synthesis Energy from exergonic reactions Energy for endergonic reactions Figure 5.4C

For a chemical reaction to begin, reactants must absorb some energy HOW ENZYMES WORK 5.5 Enzymes speed up the cell’s chemical reactions by lowering energy barriers For a chemical reaction to begin, reactants must absorb some energy This energy is called the energy of activation (EA) This represents the energy barrier that prevents molecules from breaking down spontaneously

A protein catalyst called an enzyme can decrease the energy barrier EA barrier Enzyme Reactants 1 Products 2 Figure 5.5A

EA without enzyme EA with enzyme Reactants Net change in energy Products Figure 5.5B

5.6 A specific enzyme catalyzes each cellular reaction Enzymes are selective This selectivity determines which chemical reactions occur in a cell

The enzyme is unchanged and can repeat the process How an enzyme works Active site Enzyme (sucrase) Substrate (sucrose) Glucose Fructose 1 4 Enzyme available with empty active site Products are released 3 2 Substrate is converted to products Substrate binds to enzyme with induced fit Figure 5.6 The enzyme is unchanged and can repeat the process

5.7 The cellular environment affects enzyme activity Enzyme activity is influenced by temperature salt concentration pH Some enzymes require nonprotein cofactors Some cofactors are organic molecules called coenzymes

5.8 Enzyme inhibitors block enzyme action Inhibitors interfere with enzymes A competitive inhibitor takes the place of a substrate in the active site A noncompetitive inhibitor alters an enzyme’s function by changing its shape Substrate Active site Enzyme NORMAL BINDING OF SUBSTRATE Competitive inhibitor Noncompetitive inhibitor ENZYME INHIBITION Figure 5.8

5.9 Connection: Some pesticides and antibiotics inhibit enzymes Certain pesticides are toxic to insects because they inhibit key enzymes in the nervous system Many antibiotics inhibit enzymes that are essential to the survival of disease-causing bacteria Penicillin inhibits an enzyme that bacteria use in making cell walls