Chapter 5

1. Chapter 5 0 The Working Cell Energetics Enzymes Transport

2. ENERGY AND THE CELL 5.1 Energy is the capacity to perform work • All organisms require energy to do work • Kinetic energy is the energy of motion (heat, light) • Potential energy is stored energy (result of location or structure, molecular bonds) • Cells can convert chemical energy (potential energy in chemical bonds) to kinetic energy and perform work. Figure 5.1A–C

3. 5.2 Two laws govern energy transformations • Thermodynamics • Is the study of energy transformations • The First Law of Thermodynamics • The total amount of energy in the universe is constant • Energy can be changed from one form to another • Energy cannot be created or destroyed

4. The Second Law of Thermodynamics • Every energy change results in increased disorder, or increased entropy (when looking at the state of the energy throughout the system studied). • As energy is converted from one form to another, unusable energy is lost to the surrounding environment in the form of heat. • Biological systems function in much the same way; although some chemical energy may be channeled into useful work (protein synthesis), there is always an increase in disorder Heat Chemical reactions Carbon dioxide + Glucose + ATP ATP water Oxygen Energy for cellular work Figure 5.2B Reactants Products

5. Products Amount of energyrequired Energy required Potential energy of molecules Reactants • 5.3 Chemical reactions either store or release energy • Endergonic reactions • require an input of energy equal to the difference in the potential energy of the reactants and products, e.g., photosynthesis Figure 5.3A

6. Reactants Amount of energyreleased Energy released Potential energy of molecules Products • Exergonic reactions • result in an output of energy equal to the difference in the potential energy of the reactants and products, e.g., burning wood and cellular respiration Figure 5.3B

7. Cells carry out thousands of chemical reactions • Cellular metabolismis the sum total of all the endergonic and exergonic reactions in cells. • Energy coupling is the combination of an endergonic reaction with an exergonic reaction to obtain the desired products for the cell. energy output energy input energy output energy input

8. Phosphategroups Adenosine diphosphate Adenosine Triphosphate H2O + P Energy P P P P P + Hydrolysis Adenine Ribose ATP ADP 5.4 ATP shuttles chemical energy and drives cellular work • ATP powers nearly all forms of cellular work • Most endergonic cellular reactions require small amounts of energy, rather than the large amounts of energy available in food storage molecules. • The energy in an ATP molecule lies in the bonds between its phosphate groups • The hydrolysis of ATP to release some of its chemical energy (an exergonic reaction). ATP becomes ADP and an energy shuttle, the phosphate group. Figure 5.4A

9. ATP Mechanical work Chemical work Transport work Membraneprotein Solute + P Motorprotein P Reactants P P P Product P Molecule formed Protein moved Solute transported + ADP P • ATP drives endergonic reactions by phosphorylation • The phosphate group is one of the reactants and the energy source for an endergonic reaction. Transferring a phosphate group to make molecules more reactive and be able to do work 0 0 Figure 5.4B

10. ATP Phosphorylation Hydrolysis Energy fromexergonicreactions Energy forendergonicreactions ADP + P • Cellular work can be sustained • Because ATP is a renewable resource that cells regenerate • ATP is constantly being regenerated and used in a cycle involving endergonic dehydration synthesis and exergonic hydrolysis Figure 5.4C

11. HOW ENZYMES FUNCTION EA barrier Enzyme Reactants Products 1 2 • For a chemical reaction to begin • Reactants must absorb some energy, called the energy of activation 5.5 Enzymes speed up the cell’s chemical reactions by lowering energy barriers Figure 5.5A

12. EA withoutenzyme EA withenzyme Reactants Net changein energy Energy Products Progress of the reaction • Enzymes are large protein molecules that function as biological catalysts. • A catalyst is a chemical that speeds up the reaction without itself being consumed It can decrease the energy of activation (an “energy barrier”) needed to begin a reaction Figure 5.5B

13. 1 Enzyme availablewith empty activesite Substrate binds to enzyme with induced fit 2 4 Products arereleased 3 Substrate is converted to products 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 Active site Substrate(sucrose) Enzyme(sucrase) Glucose Fructose H2O Figure 5.6

14. Enzymes catalyze many reactions in a cell. There are hundreds of different enzymes in a cell — each with a unique three-dimensional shape. Why do cells have so many different enzymes? • Each enzyme molecule can only be used once. • The shape of enzyme’s active site generally fits a specific substrate. • The substrate molecules react with enzymes to create new enzymes. • Enzymes are randomly produced. With thousands of different shapes — one is likely to work.

15. 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 • Many vitamins are coenzymes

16. Active site Substrate Enzyme Normal binding of substrate Competitiveinhibitor Noncompetitiveinhibitor Enzyme inhibition 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 (Allosteric) • Alters an enzyme’s function by changing its shape Figure 5.8

17. CONNECTION • 5.9 Many poisons, pesticides, and drugs are enzyme inhibitors For example, cyanide inhibits the production of ATP during respiration and the nerve gas sarin inhibits the enzyme acetylcholinesterase. The pesticide malathion also inhibits the enzyme acetylcholinesterase but is used at doses too low to be harmful to humans. The antibiotic penicillin interferes with an enzyme that helps build bacterial cell walls. Pain killers such as aspirin and ibuprofen inhibit the enzyme used to induce pain. Other therapeutic drugs used to combat HIV and cancer are also enzyme inhibitors.

18. MEMBRANE STRUCTURE AND FUNCTION Outside of cell Cytoplasm TEM 200,000  5.10 Membranes organize the chemical activities of cells • Membranes provide structural order for metabolism • is selectively permeable • controls the passage of molecules from one side of the membrane to the other. • In eukaryotes, membranes form subcellular organelles • Membranes provide reaction surfaces, and organize enzymes and their substrates.

19. CH3 Hydrophilic head + N CH2 CH3 CH3 CH2 Phosphategroup O P O– O O CH CH2 CH2 O O O O C C CH2 CH2 CH2 CH2 Water Hydrophilicheads CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 Symbol Hydrophobictails CH CH2 CH CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 Water CH3 Hydrophobic tails • 5.11 Membrane phospholipids form a bilayer • Phospholipids are like fats, with two nonpolar fatty acid “tails” that are hydrophobic and one polar phosphate “head” attached to the glycerol that is hydrophilic Outside of a cell O2 O2 O2 O2 O2 O2 CO2 CO2 CO2 CO2 CO2 CO2 Inside of a cell

20. Water Hydrophilicheads Hydrophobictails Water • Phospholipids form a two-layer sheet • Called a phospholipid bilayer. In water, thousands of individual molecules form a stable bilayer, aiming their polar heads out, toward the water, and their nonpolar tails in, away from the water • The hydrophobic interior of this bilayer offers an effective barrier to the flow of most hydrophilic molecules but allows the passage of hydrophobic molecules Figure 5.11B

21. But, membranes are not just a phospholipid bilayer Fibers of the extracellular matrix Carbohydrate(of glycoprotein) Glycoprotein Glycolipid Phospholipid Proteins Cholesterol Microfilamentsof cytoskeleton Cytoplasm Plasma Membrane Figure 5.12

22. 5.12 The membrane is a fluid mosaic of phospholipids and proteins • It is a mosaic because the proteins form a “tiled pattern” in a rather thin bi-layer of phospholipids. • It is fluid (like soap bubbles) because the individual molecules are more or less free to move about laterally (sideway). • The two sides of the membrane usually incorporate different sets of proteins and lipids: glycoproteins and glycolipids. • Some proteins extend through both sides of the bilayer and bind to the cytoskeleton and/or the extracellular matrix. Cholesterol is a common constituent of animal cell membranes and stabilizes membrane fluidity at different temperatures

23. Fibers of the extracellular matrix Carbohydrate(of glycoprotein) Glycoprotein Glycolipid Phospholipid Proteins Cholesterol Microfilamentsof cytoskeleton Cytoplasm The Fluid Mosaic Model of Phospholipid Membranes Plasma Membrane Figure 5.12

24. Messenger molecule Receptor ATP Activatedmolecule • 5.13 Proteins make the membrane a mosaic of function • Enzymes: catalyzing intracellular and extracellular reactions. • Receptors: triggering cell activity when a messenger molecule attaches. • Signal transduction is a message-transfer process that causes the cell to respond to the external message that bound to the receptor. • Cell junctions: either attachments to other cells or the internal cytoskeleton. • Transporters: of hydrophilic molecules.

25. Molecules of dye Membrane Equilibrium Equilibrium • 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 • Vitally important -- Small nonpolar molecules such as O2 and CO2 diffuse easily across the phospholipid bilayer of a membrane Figure 5.14A Figure 5.14B

26. Hypertonic solution Hypotonic solution Isotonic solution H2O H2O H2O H2O Animalcell (3) Shriveled (1) Normal (2) Lysed Plasmamembrane H2O H2O H2O H2O Plantcell (6) Shriveled (plasmolyzed) (5) Turgid (4) Flaccid • 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 0.9% salt solution 2 % Figure 5.17 0.1% • The control of water balance is calledosmoregulation

27. Solutemolecule Transportprotein • 5.15 Transport proteins may facilitate diffusion across membranes • Many kinds of molecules do not diffuse freely across membranes • For these molecules, transport proteins (like channels or tunnels) provide passage across membranes through a process called facilitated diffusion • The cell does not expend energy • Water is a polar molecule and, therefore, needs the assistance of transport proteins when crossing membranes. A good example of this is the aquaporins (water transport proteins) in the collecting ducts of the kidneys.

28. Transportprotein P P P Phosphatedetaches Proteinchanges shape ATP Solute ADP Transport 1 Solute binding 2 Phosphorylation 3 4 Protein reversion • 5.18 Cells expend energy for active transport • involves the assistance of a transport protein when moving a solute against a concentration gradient. • Energy expenditure in the form of ATP is required to help the protein change its structure and, thus, move the solute molecule. Figure 5.18

29. Fluid outside cell Vesicle Protein Cytoplasm • 5.19 Exocytosisand endocytosis transport large molecules • To move large molecules or particles through a membrane-bound vesicle. • In exocytosis, membrane-bounded vesicles containing large molecules fuse with the plasma membrane and release their contents outside the cell (secretion)

30. Vesicle forming • Membranes may fold inward • In endocytosis, the plasma membrane surrounds materials outside the cell, closes around the materials, and forms membrane-bounded vesicles containing the materials Figure 5.19B

31. Plasma membrane Food being ingested Pseudopodium of amoeba Material bound to receptor proteins PIT TEM 96,500  Cytoplasm TEM 54,000 LM 230 Phagocytosis Receptor-mediated endocytosis Pinocytosis • Endocytosis can occur in three ways • Phagocytosis: amoeba, white blood cells • Pinocytosis: virtually all cells • Receptor-mediated endocytosis: cholesterol intake by cells Figure 5.19C

32. Phospholipid outer layer LDL particle Vesicle Cholesterol Protein Plasmamembrane Receptorprotein Cytoplasm • 5.20 Faulty membranes can overload the blood with cholesterol • Cholesterol is carried in the blood by low-density lipoprotein (LDL) particles. • In people with normal cholesterol metabolism, excess LDL-bound cholesterol in the blood is eliminated by receptor-mediated endocytosis by liver cells. • In people with a genetic condition that results in increased levels of cholesterol (hypercholesterolemia), fewer or no such receptor sites exist, and the people accumulate LDL-bound cholesterol. These people are at high risk for developing heart disease. Figure 5.20