Chapter 5 The Working Cell

Cool “Fires” Attract Mates and Meals Fireflies use light to send signals to potential mates Instead of using chemical signals like most other insects

The light comes from a set of chemical reactions That 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 are then eaten by the female

5.1 Energy is the capacity to perform work ENERGY AND THE CELL 5.1 Energy is the capacity to perform work All organisms require energy Which is defined as the capacity to do work

Kinetic energy is the energy of motion Potential energy is stored energy And can be converted to kinetic energy Figure 5.1A–C

5.2 Two laws govern energy transformations Thermodynamics Is the study of energy transformations

The First Law of Thermodynamics According to the first law of thermodynamics Energy can be changed from one form to another Energy cannot be created or destroyed Figure 5.2A

Energy for cellular work The Second Law of Thermodynamics The second law of thermodynamics States that energy transformations increase disorder or entropy, and some energy is lost as heat Heat Chemical reactions Carbon dioxide + Glucose + ATP ATP water Oxygen Energy for cellular work Figure 5.2B

5.3 Chemical reactions either store or release energy Endergonic reactions Absorb energy and yield products rich in potential energy Potential energy of molecules Reactants Energy required Products Amount of energy required Figure 5.3A

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

Cells carry out thousands of chemical reactions The sum of which constitutes cellular metabolism Energy coupling Uses exergonic reactions to fuel endergonic reactions

5.4 ATP shuttles chemical energy and drives cellular work ATP powers nearly all forms of cellular work

The energy in an ATP molecule Lies in the bonds between its phosphate groups Phosphate groups ATP Energy P Hydrolysis Adenine Ribose H2O Adenosine diphosphate Adenosine Triphosphate + ADP Figure 5.4A

ATP drives endergonic reactions by phosphorylation Transferring a phosphate group to make molecules more reactive ATP Chemical work Mechanical work Transport work P Molecule formed Protein moved Solute transported ADP + Product Reactants Motor protein Membrane protein Solute Figure 5.4B

Cellular work can be sustained Because ATP is a renewable resource that cells regenerate ATP ADP + P Energy for endergonic reactions Energy from exergonic reactions Phosphorylation Hydrolysis Figure 5.4C

HOW ENZYMES FUNCTION 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, called the energy of activation EA barrier Reactants Products 1 2 Enzyme Figure 5.5A

Progress of the reaction A protein catalyst called an enzyme Can decrease the energy of activation needed to begin a reaction Reactants EA without enzyme EA with enzyme Net change in energy Products Energy Progress of the reaction Figure 5.5B

5.6 A specific enzyme catalyzes each cellular reaction Enzymes have unique three-dimensional shapes That determine which chemical reactions occur in a cell

The catalytic cycle of an enzyme 1 Enzyme available with empty active site Active site Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Enzyme (sucrase) Glucose Fructose H2O 4 Products are released 3 Substrate is converted to products Figure 5.6

5.7 The cellular environment affects enzyme activity Temperature, salt concentration, and pH influence enzyme activity Some enzymes require nonprotein cofactors Such as metal ions or organic molecules called coenzymes

5.8 Enzyme inhibitors block enzyme action Inhibitors interfere with an enzyme’s activity

Normal binding of substrate 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 Enzyme Active site Normal binding of substrate Enzyme inhibition Noncompetitive inhibitor Competitive inhibitor Figure 5.8

CONNECTION 5.9 Many poisons, pesticides, and drugs are enzyme inhibitors

MEMBRANE STRUCTURE AND FUNCTION 5.10 Membranes organize the chemical activities of cells Membranes Provide structural order for metabolism

The plasma membrane of the cell is selectively permeable Controlling the flow of substances into or out of the cell Cytoplasm Outside of cell TEM 200,000  Figure 5.10

5.11 Membrane phospholipids form a bilayer Have a hydrophilic head and two hydrophobic tails Are the main structural components of membranes CH2 CH3 CH N + O O– P C Phosphate group Symbol Hydrophilic head Hydrophobic tails Figure 5.11A

Phospholipids form a two-layer sheet Called a phospholipid bilayer, with the heads facing outward and the tails facing inward Water Hydrophilic heads Hydrophobic tails Figure 5.11B

5.12 The membrane is a fluid mosaic of phospholipids and proteins A membrane is a fluid mosaic With proteins and other molecules embedded in a phospholipid bilayer Fibers of the extracellular matrix Carbohydrate (of glycoprotein) Glycoprotein Microfilaments of cytoskeleton Phospholipid Cholesterol Proteins Plasma membrane Glycolipid Cytoplasm Figure 5.12

5.13 Proteins make the membrane a mosaic of function Many membrane proteins Function as enzymes Figure 5.13A

Other membrane proteins Function as receptors for chemical messages from other cells Messenger molecule Receptor Activated molecule Figure 5.13B

Membrane proteins also function in transport Moving substances across the membrane ATP Figure 5.13C

5.14 Passive transport is diffusion across a membrane In passive transport, substances diffuse through membranes without work by the cell Spreading from areas of high concentration to areas of low concentration Equilibrium Membrane Molecules of dye Figure 5.14A Figure 5.14B

Small nonpolar molecules such as O2 and CO2 Diffuse easily across the phospholipid bilayer of a membrane

5.15 Transport proteins may facilitate diffusion across membranes Many kinds of molecules Do not diffuse freely across membranes For these molecules, transport proteins Provide passage across membranes through a process called facilitated diffusion Solute molecule Transport protein Figure 5.15

5.16 Osmosis is the diffusion of water across a membrane In osmosis Water travels from a solution of lower solute concentration to one of higher solute concentration 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 Figure 5.16

5.17 Water balance between cells and their surroundings is crucial to organisms Osmosis causes cells to shrink in hypertonic solutions And swell in hypotonic solutions In isotonic solutions Animal cells are normal, but plant cells are limp Plant cell H2O Plasma membrane (1) Normal (2) Lysed (3) Shriveled (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed) Isotonic solution Hypotonic solution Hypertonic solution Animal cell Figure 5.17

The control of water balance Is called osmoregulation

5.18 Cells expend energy for active transport Transport proteins can move solutes against a concentration gradient Through active transport, which requires ATP P Protein changes shape Phosphate detaches ATP ADP Solute Transport protein Solute binding 1 Phosphorylation 2 Transport 3 Protein reversion 4 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 Protein Vesicle Figure 5.19A

Membranes may fold inward Enclosing material from the outside (endocytosis) Vesicle forming Figure 5.19B

Endocytosis can occur in three ways Phagocytosis Pinocytosis Receptor-mediated endocytosis Pseudopodium of amoeba Food being ingested Phagocytosis Pinocytosis Receptor-mediated endocytosis Material bound to receptor proteins PIT Cytoplasm Plasma membrane TEM 54,000 TEM 96,500  LM 230 Figure 5.19C

5.20 Faulty membranes can overload the blood with cholesterol CONNECTION 5.20 Faulty membranes can overload the blood with cholesterol Harmful levels of cholesterol Can accumulate in the blood if membranes lack cholesterol receptors LDL particle Protein Phospholipid outer layer Cytoplasm Receptor protein Plasma membrane Vesicle Cholesterol Figure 5.20

5.21 Chloroplasts and mitochondria make energy available for cellular work Enzymes 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