Chapter 5

1. Chapter 5 The Working Cell

2. Cool "Fires" Attract Mates and Meals • Living cells put energy to work by means of enzyme-controlled reactions • The firefly's use of light to signal mates results from a set of such reactions • The reactions occur in light-producing organs at the rear of the insect • Females of some species produce a light pattern that attracts males of other species, which the female eats

4. 5.1 Energy is the capacity to perform work • Energy is defined as the capacity to do work • Work is a force acting on an object that causes the object to move • Life depends on the fact that energy can be converted from one form to another

5. The two fundamental types of energy • Kinetic energyis the energy of movement • e.g. light, heat, electricity, moving objects • Potential energyis stored energy that is dependent on an object's location or structure • e.g. chemical energy in bonds, electrical charge in a battery, a rock at the top of a hill • The most important potential energy for living things is the chemical energy stored in molecules

6. Chemical energy is the energy that powers life • The objects that move are electrons, which reposition during chemical reactions • Potential energy can be converted to kinetic energy

10. 5.2 Two laws govern energy transformations • Thermodynamics is the study of energy transformations • The First Law of Thermodynamics • Energy can be changed from one form to another but cannot be created or destroyed

11. The Second Law of Thermodynamics • Energy doesn't tend to stay concentrated in a small space; it tends to flow toward becoming dispersed if it can • Ex. electricity in a battery, power line or lightning, wind from a high pressure weather system, air compressed in a tire, all heated objects, loud sounds, or boulders that are high up on a mountain. • Energy transformations increase disorder, or entropy, and some energy is lost as heat

12. All these different kinds of energy spread out if there's a way they can do so.

13. The Laws of Thermodynamics Availability and usefulness of energy: • The amount of useful energy decreases when energy is converted from one form to another (second law of thermodynamics) • Entropy (disorder) increases

15. LE 5-2b Heat Chemical reactions Carbon dioxide Glucose ATP ATP Water Oxygen Energy for cellular work

16. Energy of Sunlight Living things must gain external energy in order to counteract the increase in their entropy

17. 5.3 Chemical reactions either store or release energy • Endergonic reactions (stores energy) • Require an input of energy from the surroundings • Yield products rich in potential energy • Example:

18. LE 5-3a Products Amount of energy required Energy required Potential energy of molecules Reactants

19. Exergonic reactions • Release energy • Yield products that contain less potential energy than their reactants • Examples: cellular respiration, burning

20. LE 5-3b Reactants Amount of energy released Energy released Potential energy of molecules Products

21. Cellular respiration An “Exergonic Reaction” Heat Chemical reactions Carbon dioxide Glucose ATP ATP Water Oxygen Energy for cellular work

23. Coupled Reactions • Cells carry out thousands of chemical reactions, which constitute cellular metabolism • Energy coupling uses energy released from exergonic reactions to drive endergonic reactions

24. The exergonic and endergonic parts of coupled reactions often occur at different places within the cell Energy-carrier molecules are used to transfer the energy within cells

25. ATP Adenosine triphosphate (ATP) is the most common energy carrying molecule

26. 5.4 ATP shuttles chemical energy and drives cellular work • ATP (adenosine triphosphate) powers nearly all forms of cellular work • ATP is composed of one adenine, one ribose, and three negatively charged phosphates • The energy in an ATP molecule lies in the bonds between its phosphate groups

27. LE 5-4a Adenosine Triphosphate Adenosine diphosphate Phosphate group H2O P P P P P + P + Energy Hydrolysis Adenine Ribose High energy bond ADP ATP

28. ATP powers cellular work through coupled reactions • The bonds connecting the phosphate groups are broken by hydrolysis, an exergonic reaction • Hydrolysis is coupled to an endergonic reaction through phosphorylation • A phosphate group is transferred from ATP to another molecule

29. LE 5-4b ATP Chemical work Mechanical work Transport work Membrane protein Solute P Motor protein P Reactants P P P Product P Solute transported Protein moved Molecule formed ADP  P

30. Cellular work can be sustained, because ATP is a renewable resource that cells regenerate • The ATP cycle involves continual phosphorylation and hydrolysis

31. LE 5-4c ATP Phosphoylation Hydrolysis Energy from exergonic reactions Energy for endergonic reactions ADP + P

32. Spontaneous Reactions At body temperatures, spontaneous reactions proceed too slowly to sustain life

33. Spontaneous Reactions Reaction speed is generally determined by the activation energy required • 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

34. Enzyme molecules (proteins) are employed to catalyze (speed up) chemical reactions in cells

35. Catalysts Reduce Activation Energy Catalysts speed up the rate of a chemical reaction without themselves being used up

36. 5.5 Enzymes speed up the cell's chemical reactions by lowering energy barriers HOW ENZYMES FUNCTION • Energy of activation= Amount of energy that must be input before an exergonic reaction will proceed (the energy barrier) link: activation energy

38. EAwithout enzyme EAwith enzyme Reactants Energy Net change in energy Products Progress of the reaction

39. Link: activation energy animation

40. 5.6 A specific enzyme catalyzes each cellular reaction • Each enzyme has a unique three-dimensional shape that determines which chemical reaction it catalyzes • Substrate: a specific reactant that an enzyme acts on • Active site: A pocket on the enzyme surface that the substrate fits into

41. Induced fit: The way the active site changes shape to "embrace" the substrate • A single enzyme may act on thousands or millions of substrate molecules per second Animation: How Enzymes Work

42. Enzyme available with empty active site Active site Substrate (sucrose) Substrate binds to enzyme with induced fit Enzyme (sucrase) Glucose Fructose H2O Products are released Substrate is converted to products

43. Environmental Conditions Some enzymes require helper non protein cofactors molecules to function (e.g. certain B vitamins, Metal ions, organic molecules called coenzymes) Metal ions Vitamin B12

44. 5.7 The cellular environment affects enzyme activity Three-dimensional structure of an enzyme is sensitive to pH, salts, temperature, and presence of coenzymes

45. Environmental Conditions Enzyme structure is distorted and function is destroyed when pH is too high or low Salts in an enzyme’s environment can also destroy function by altering structure

47. Environmental Conditions Temperature also affects enzyme activity • Low temperatures slow down molecular movement • High temperatures cause enzyme shape to be altered, destroying function

49. Environmental Conditions Most enzymes function optimally only within a very narrow range of these conditions

50. 5.8 Enzyme inhibitors block enzyme action • Inhibitors interfere with an enzyme's activity • 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