Chapter 5 The Working Cell

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

5.1 Energy is the capacity to perform work 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

Animation: Energy Concepts 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

Proteins that function as biological catalysts 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

LE 5-5b enzyme enzyme EA without EA with Reactants Energy Net change in energy Products Progress of the reaction

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

Animation: How Enzymes Work 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

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

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

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

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

5.9 Many poisons, pesticides, and drugs are enzyme inhibitors 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

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

LE 5-10 Outside of cell Cytoplasm

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"

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

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

LE 5-11b Water Hydrophilic heads Hydrophobic tails Water

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

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

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

LE 5-13a Enzyme activity

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

LE 5-13c ATP Transport

Animation: Membrane Selectivity

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

LE 5-14a Molecules of dye Membrane Equilibrium

LE 5-14b Equilibrium Animation: Diffusion

5.15 Transport proteins may facilitate diffusion across membranes In facilitated diffusion Transport proteins that span the membrane bilayer help substances diffuse down a concentration gradient To transport the substance, a transport protein may Provide a pore for passage Bind the substance, change shape, and then release the substance

LE 5-15 Solute molecule Transport protein

5.16 Osmosis is the diffusion of water across a membrane In osmosis water, molecules diffuse across a selectively permeable membrane From an area of low solute concentration To an area of high solute concentration Until the solution is equally concentrated on both sides of the membrane The direction of movement is determined by the difference in total solute concentration Not by the nature of the solutes Animation: Osmosis

cluster of water molecules Lower concentration of solute Higher concentration of solute Equal concentration of solute H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water

5.17 Water balance between cells and their surroundings is crucial to organisms Osmoregulation is the control of water balance Tonicity is the tendency of a cell to lose or gain water in solution Isotonic solution: solute concentration is the same in the cell and in the solution No osmosis occurs Animal cell volume remains constant; plant cell becomes flaccid

Hypotonic solution: solute concentration is greater in the cell than in the solution Cell gains water through osmosis Animal cell lyses; plant cell becomes turgid Hypertonic solution: solute concentration is lower in the cell than in the solution Cell loses water through osmosis Animal cell shrivels; plant cell plasmolyzes

LE 5-17 Isotonic solution Hypotonic solution Hypertonic solution H2O Animal cell (1) Normal (2) Lysed (3) Shriveled H2O H2O H2O Plasma membrane H2O Plant cell (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed) Video: Plasmolysis Video: Turgid Elodea

Animation: Active Transport 5.18 Cells expend energy for active transport Active transport requires energy to move solutes against a concentration gradient ATP supplies the energy Transport proteins move solute molecules across the membrane Animation: Active Transport

LE 5-18 Transport protein P P Protein changes shape Phosphate detaches ATP Solute ADP Solute binding Phosphorylation Transport Protein reversion

5.19 Exocytosis and endocytosis transport large molecules To move large molecules or particles through a cell membrane A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis) Membranes may fold inward, enclosing material from the outside (endocytosis)

LE 5-19a Fluid outside cell Vesicle Protein Cytoplasm

LE 5-19b Vesicle forming

Endocytosis can occur in three ways Phagocytosis ("cell eating") Pinocytosis ("cell drinking") Receptor-mediated endocytosis

Receptor-mediated endocytosis LE 5-19c Pseudopodium of amoeba Food being ingested LM 230 Phagocytosis Plasma membrane Material bound to receptor proteins PIT TEM 54,000 TEM 96,500 Cytoplasm Pinocytosis Receptor-mediated endocytosis

Animation: Receptor-Mediated Endocytosis Animation: Exocytosis and Endocytosis Introduction Animation: Exocytosis Animation: Pinocytosis Animation: Phagocytosis

5.20 Faulty membranes can overload the blood with cholesterol CONNECTION 5.20 Faulty membranes can overload the blood with cholesterol Cholesterol is carried in the blood by low-density lipoprotein (LDL) particles Normally, body cells take up LDLs by receptor-mediated endocytosis Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors People with hypercholesterolemia have more than twice the normal level of blood cholesterol

LE 5-20 Phospholipid outer layer LDL particle Vesicle Cholesterol Protein Plasma membrane Receptor protein Cytoplasm