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. ENERGY AND THE CELL • 5.1 Energy is the capacity to perform work • Energy is defined as the capacity to do work • Life depends on the fact that energy can be converted from one form to another • Kinetic energy is the energy of motion • Heat and light are forms of kinetic energy

5. Potential energy is stored energy that is dependent on an object's location or structure • The most important potential energy for living things is the chemical energy stored in molecules • Potential energy can be converted to kinetic energy Animation: Energy Concepts

9. 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 • The Second Law of Thermodynamics • Energy transformations increase disorder, or entropy, and some energy is lost as heat

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

12. 5.3 Chemical reactions either store or release energy • Endergonic reactions • Require an input of energy from the surroundings • Yield products rich in potential energy • Example: photosynthesis

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

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

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

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

17. 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

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

19. 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

20. 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

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

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

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

25. Enzymes • Proteins that function as biological catalysts • Increase the rate of a reaction without themselves being changed • An enzyme can decrease the energy of activation needed to begin a reaction

26. LE 5-5b EAwithout enzyme EAwith enzyme Reactants Energy Net change in energy Products Progress of the reaction

27. 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

28. 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

29. LE 5-6 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

30. 5.7 The cellular environment affects enzyme activity • Physical factors influence enzyme activity • Temperature, salt concentration, pH • Some enzymes require nonprotein cofactors • Metal ions, organic molecules called coenzymes

31. 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 • In feedback inhibition, enzyme activity is blocked by a product of the reaction catalyzed by the enzyme

32. LE 5-8 Substrate Active site Enzyme Normal binding of substrate Competitive inhibitor Noncompetitive inhibitor Enzyme inhibition

33. CONNECTION • 5.9 Many poisons, pesticides, and drugs are enzyme inhibitors • Cyanide inhibits an enzyme involved with ATP production during cellular respiration • Some pesticides irreversibly inhibit an enzyme crucial for insect muscle function • Many antibiotics inhibit enzymes essential for disease-causing bacteria • Ibuprofen and aspirin inhibit enzymes involved in inducing pain

34. MEMBRANE STRUCTURE AND FUNCTION • 5.10 Membranes organize the chemical activities of cells • Membranes provide structural order for metabolism • Form most of the cell's organelles • Compartmentalize chemical reactions • The plasma membrane forms a boundary between a living cell and its surroundings • Exhibits selective permeability • Controls traffic of molecules in and out

35. LE 5-10 Outside of cell Cytoplasm

36. 5.11 Membrane phospholipids form a bilayer • Phospholipids are the main structural components of membranes • Two nonpolar hydrophobic fatty acid "tails" • One phosphate group attached to the hydrophilic glycerol "head"

37. LE 5-11a Hydrophilic head Phosphate group Symbol Hydrophobic tails

38. In membranes, phospholipids form a bilayer • Two-layer sheet • Phospholipid heads facing outward and tails facing inward • Selectively permeable • Polar lipid-soluble molecules pass through • Nonpolar molecules not soluble in lipids do not pass through

39. LE 5-11b Water Hydrophilic heads Hydrophobic tails Water

40. 5.12 The membrane is a fluid mosaic of phospholipids and proteins • A membrane is a mosaic • Proteins and other molecules are embedded in a framework of phospholipids • A membrane is fluid • Most protein and phospholipid molecules can move laterally • Membrane glycoproteins and glycolipids function in cell identification

41. LE 5-12 Extracellular matrix Glycoprotein Carbohydrate Glycolipid Plasma membrane Phospholipid Proteins Microfilaments of cytoskeleton Cholesterol Cytoplasm

42. 5.13 Proteins make the membrane a mosaic of function • Proteins perform most membrane functions • Identification tags • Junctions between adjacent cells • Enzymes • Receptors of chemical messages from other cells (signal transduction) • Transporters of substances across the membrane

43. LE 5-13a Enzyme activity

44. LE 5-13b Messenger molecule Receptor Activated molecule Signal transduction

45. LE 5-13c ATP Transport

46. Animation: Membrane Selectivity

47. 5.14 Passive transport is diffusion across a membrane • Diffusion is the tendency for particles to spread out evenly in an available space • From an area of high concentration to an area of low concentration • Passive transport across membranes occurs when a molecule diffuses down a concentration gradient • Small nonpolar molecules such as O2 and CO2 diffuse easily across the phospholipid bilayer of a membrane

48. LE 5-14a Molecules of dye Membrane Equilibrium

49. LE 5-14b Equilibrium

50. Animation: Diffusion