CHAPTER 5 The Working Cell Modules 5.1 – 5.4

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

CHAPTER 5 The Working Cell Modules 5.5 – 5.9

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

CHAPTER 5 The Working Cell Modules 5.10 – 5.21

5.10 Membranes organize the chemical activities of cells MEMBRANE STRUCTURE AND FUNCTION 5.10 Membranes organize the chemical activities of cells Membranes organize the chemical reactions making up metabolism   Cytoplasm Figure 5.10

Membranes are selectively permeable They control the flow of substances into and out of a cell Membranes can hold teams of enzymes that function in metabolism

5.11 Membrane phospholipids form a bilayer Phospholipids are the main structural components of membranes They each have a hydrophilic head and two hydrophobic tails Head Symbol Tails Figure 5.11A

In water, phospholipids form a stable bilayer The heads face outward and the tails face inward Water Hydrophilic heads Hydrophobic tails Water Figure 5.11B

5.12 The membrane is a fluid mosaic of phospholipids and proteins Phospholipid molecules form a flexible bilayer Cholesterol and protein molecules are embedded in it Carbohydrates act as cell identification tags

The plasma membrane of an animal cell Glycoprotein Carbohydrate (of glycoprotein) Fibers of the extracellular matrix Glycolipid Phospholipid Cholesterol Microfilaments of the cytoskeleton Proteins CYTOPLASM Figure 5.12

5.13 Proteins make the membrane a mosaic of function Some membrane proteins form cell junctions Others transport substances across the membrane Figure 5.13 Transport

Many membrane proteins are enzymes Some proteins function as receptors for chemical messages from other cells The binding of a messenger to a receptor may trigger signal transduction Messenger molecule Receptor Activated molecule Figure 5.13 Enzyme activity Signal transduction

5.14 Passive transport is diffusion across a membrane In passive transport, substances diffuse through membranes without work by the cell They spread from areas of high concentration to areas of lower concentration Molecule of dye Membrane EQUILIBRIUM EQUILIBRIUM Figure 5.14A & B

5.15 Osmosis is the passive transport of water Hypotonic solution Hypertonic solution In osmosis, water travels from an area of lower solute concentration to an area of higher solute concentration Selectively permeable membrane Solute molecule HYPOTONIC SOLUTION HYPERTONIC SOLUTION Water molecule Selectively permeable membrane Solute molecule with cluster of water molecules NET FLOW OF WATER Figure 5.15

5.16 Water balance between cells and their surroundings is crucial to organisms Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution The control of water balance (osmoregulation) is essential for organisms ISOTONIC SOLUTION HYPOTONIC SOLUTION HYPERTONIC SOLUTION ANIMAL CELL (1) Normal (2) Lysing (3) Shriveled Plasma membrane PLANT CELL Figure 5.16 (4) Flaccid (5) Turgid (6) Shriveled

5.17 Transport proteins facilitate diffusion across membranes Small nonpolar molecules diffuse freely through the phospholipid bilayer Many other kinds of molecules pass through selective protein pores by facilitated diffusion Solute molecule Transport protein Figure 5.17

5.18 Cells expend energy for active transport Transport proteins can move solutes across a membrane against a concentration gradient This is called active transport Active transport requires ATP

Active transport in two solutes across a membrane FLUID OUTSIDE CELL Phosphorylated transport protein Active transport in two solutes across a membrane Transport protein First solute 1 First solute, inside cell, binds to protein 2 ATP transfers phosphate to protein 3 Protein releases solute outside cell Second solute 4 Second solute binds to protein 5 Phosphate detaches from protein 6 Protein releases second solute into cell Figure 5.18

5.19 Exocytosis and endocytosis transport large molecules To move large molecules or particles through a membrane a vesicle may fuse with the membrane and expel its contents (exocytosis) FLUID OUTSIDE CELL CYTOPLASM Figure 5.19A

or the membrane may fold inward, trapping material from the outside (endocytosis) Figure 5.19B

Material bound to receptor proteins Three kinds of endocytosis Pseudopod of amoeba Food being ingested Plasma membrane Material bound to receptor proteins PIT Cytoplasm Figure 5.19C

5.20 Connection: Faulty membranes can overload the blood with cholesterol Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors Phospholipid outer layer LDL PARTICLE Receptor protein Protein Cholesterol Plasma membrane Vesicle CYTOPLASM Figure 5.20

5.21 Chloroplasts and mitochondria make energy available for cellular work Enzymes and membranes are central to the processes that make energy available to the cell Chloroplasts carry out photosynthesis, using solar energy to produce glucose and oxygen from carbon dioxide and water Mitochondria consume oxygen in cellular respiration, using the energy stored in glucose to make ATP

Chemicals recycle among living organisms and their environment Sunlight energy Nearly all the chemical energy that organisms use comes ultimately from sunlight Chloroplasts, site of photosynthesis CO2 + H2O Glucose + O2 Mitochondria sites of cellular respiration Chemicals recycle among living organisms and their environment (for cellular work) Heat energy Figure 5.21

CHAPTER 5 Extra Photographs

Kinetic energy Figure 5.1x1

Potential and kinetic energy Figure 5.1x2

Potential and kinetic energy Figure 5.1x3

Forest fire Figure 5.3x

ATP, molecular model Figure 5.4Ax