Chapter 5 Ground Rules of Metabolism Sections 6-10

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1 Chapter 5 Ground Rules of Metabolism Sections 6-10

2 5.6 Cofactors in Metabolic Pathways Most enzymes require cofactors Energy in ATP drives many endergonic reactions

3 Table 5-1 p86

4 Cofactors and Coenzymes Cofactors Atoms or molecules (other than proteins) that are necessary for enzyme function Example: Iron atoms in catalase Coenzymes Organic cofactors such as vitamins May become modified during a reaction

5 Catalase and Cofactors Catalase is an antioxidant that neutralizes free radicals (atoms or molecules with unpaired electrons that attack biological molecules) Catalase has four hemes (small organic compound with an iron atom at its center) Catalase works by holding a substrate molecule close to one of its iron atoms (cofactors) Iron pulls on the substrate s electrons, bringing on the transition state

6 Heme

7 ATP A Special Coenzyme ATP (adenosine triphosphate) A nucleotide with three phosphate groups Transfers a phosphate group and energy to other molecules Phosphorylation A phosphate-group transfer ADP binds phosphate in an endergonic reaction to replenish ATP (ATP/ADP cycle)

8 adenine three phosphate groups ribose A adenine ribose AMP ADP ATP B C energy in ADP + phosphate energy out Figure 5-18 p87

9 Coupled Reactions

10 Take-Home Message: How do cofactors work? Cofactors associate with enzymes and assist their function. Metal ions stabilize the structure of many enzymes. They also participate in some enzymatic reactions by donating or accepting electrons Many coenzymes carry chemical groups, atoms, or electrons from one reaction to another The formation of ATP from ADP is an endergonic reaction; ADP forms again when a phosphate group is transferred from ATP to another molecule energy from such transfers drives cellular work

11 5.7 A Closer Look at Cell Membranes A membrane is a continuous, selectively permeable barrier A cell membrane is organized as a lipid bilayer with many proteins embedded in it and attached to its surfaces

12 Membrane Lipids Phospholipid molecules in the plasma membrane have two parts Hydrophilic heads interact with water molecules Hydrophobic tails interact with each other, forming a barrier to hydrophilic molecules

13 The Fluid Mosaic Model Fluid mosaic model Describes the organization of cell membranes Phospholipids drift and move like a fluid The bilayer is a mosaic mixture of phospholipids, steroids, proteins, and other molecules

14 Cell Membrane Organization one layer of lipids one layer of lipids

15 Membrane Proteins Cell membrane function begins with the many proteins associated with the lipid bilayer Peripheral membrane proteins temporarily attach to the lipid bilayer s surfaces by interactions with lipids or other proteins Integral membrane proteins permanently attach to a bilayer

16 Types of Membrane Proteins Each type of protein in a membrane has a special function Adhesion proteins Recognition proteins Receptor proteins Enzymes Transport proteins (active and passive)

17 Types of Membrane Proteins B Recognition proteins such as this MHC molecule tag a cell as belonging to one s own body. c Receptor proteins such as this B cell receptor bind substances outside the cell. B cell receptors help the body eliminate toxins and infectious agents. D Transport proteins bind to molecules on one side of the membrane, and release them on the other side. This one transports glucose. E This transport protein, an ATP synthase, makes ATP when hydrogen ions flow through its interior. Extracellular Fluid Lipid bilayer Cytoplasm

18 ANIMATED FIGURE: Cell membranes To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

19 Table 5-2 p88

20 Take-Home Message: What is a cell membrane? The structural foundation of all cell membranes is the lipid bilayer Adhesion proteins, recognition proteins, transport proteins, receptors, and enzymes embedded in or associated with the lipid bilayer impart functionality to a cell membrane

21 ANIMATION: Lipid bilayer organization To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

22 5.8 Diffusion and Membranes Ions and molecules tend to move spontaneously from regions of higher to lower concentration Water diffuses across cell membranes by osmosis

23 Diffusion Diffusion The net movement of molecules down a concentration gradient Moves substances into, through, and out of cells A substance diffuses in a direction set by its own concentration gradient, not by the gradients of other solutes around it

24 Diffusion

25 The Rate of Diffusion Rate of diffusion depends on five factors Size Temperature Steepness of the concentration gradient Charge Pressure

26 Concentration Gradients Concentration The number of molecules (or ions) of substance per unit volume of fluid Concentration gradient The difference in concentration between two adjacent regions Molecules move from a region of higher concentration to one of lower concentration

27 Tonicity Tonicity The relative concentrations of solutes in two fluids separated by a selectively permeable membrane For two fluids separated by a semipermeable membrane, the one with lower solute concentration is hypotonic, and the one with higher solute concentration is hypertonic Isotonic fluids have the same solute concentration

28 Osmosis Osmosis The movement of water down its concentration gradient through a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration

29 Osmosis selectively permeable membrane

30 Membrane Permeability Selective permeability The ability of a cell membrane to control which substances and how much of them enter or leave the cell Allows the cell to maintain a difference between its internal environment and extracellular fluid Supplies the cell with nutrients, removes wastes, and maintains volume and ph

31 Selective Permeability of Lipid Bilayers lipid bilayer gases water glucose and other polar molecules; ions

32 Effects of Fluid Pressure Hydrostatic pressure (turgor) The pressure exerted by a volume of fluid against a surrounding structure (membrane, tube, or cell wall) which resists volume change Osmotic pressure The amount of hydrostatic pressure that can stop water from diffusing into cytoplasmic fluid or other hypertonic solutions

33 Effects of Fluid Pressure

34 Take-Home Message: What influences the movement of ions and molecules? Molecules or ions tend to diffuse into an adjoining region of fluid in which they are not as concentrated he steepness of a concentration gradient as well as temperature, molecular size, charge, and pressure affect the rate of diffusion Osmosis is a net diffusion of water between two fluids that differ in water concentration and are separated by a selectively permeable membrane Fluid pressure that a solution exerts against a membrane or wall influences the osmotic movement of water

35 3D ANIMATION: Osmosis

36 5.9 Membrane Transport Mechanisms Many types of molecules and ions can cross a lipid bilayer only with the help of transport proteins

37 How Substances Cross Membranes Gases and nonpolar molecules diffuse freely across a lipid bilayer Ions and large polar molecules require other mechanisms to cross the cell membrane Passive transport Active transport Endocytosis and exocytosis

38 Passive Transport Passive transport (facilitated diffusion) Requires no energy input A passive transport protein allows a specific solute (such as glucose) to follow its concentration gradient across a membrane A gated passive transporter changes shape when a specific molecule binds to it

39 Passive Transport of Glucose

40 Active Transport Active transport Requires energy input (usually ATP) Moves a solute against its concentration gradient, to the concentrated side of the membrane Calcium pumps Active transporters move calcium ions across muscle cell membranes into the sarcoplasmic reticulum

41 Active Transport: Calcium Pump Extracellular Fluid Cytoplasm

42 Active Transport: Calcium Pump

43 Active Transport: Calcium Pump

44 ANIMATED FIGURE: Active transport To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

45 Cotransport Cotransporter An active transport protein that moves two substances across a membrane at the same time Example: The sodium-potassium pump moves Na + out of the cell and K + into the cell

46 Cotransport: Sodium-Potassium Pump Extracellular Fluid ADP Cytoplasm

47 Take-Home Message: How do molecules or ions cross a cell membrane? Transport proteins help specific molecules or ions to cross cell membranes In passive transport, a solute binds to a protein that releases it on the opposite side of the membrane; he movement is driven by a concentration gradient In active transport, a transport protein pumps a solute across a membrane, against its concentration gradient; the movement is driven by an energy input, such as ATP

48 ANIMATED FIGURE: Passive transport To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

49 5.10 Membrane Trafficking By processes of endocytosis and exocytosis, cells take in and expel particles that are too big for transport proteins, as well as substances in bulk Requires formation and movement of vesicles formed from membranes, involving motor proteins and ATP

50 Exocytosis and Endocytosis Exocytosis The fusion of a vesicle with the cell membrane, releasing its contents to the surroundings Endocytosis The formation of a vesicle from cell membrane, enclosing materials near the cell surface and bringing them into the cell

51 A Molecules get concentrated inside coated pits at the plasma membrane. Endocytosis Exocytosis B The pits sink inward and become endocytic vesicles. coated pit D Many of the sorted molecules cycle to the plasma membrane. C Vesicle contents are sorted. E Some vesicles are routed to the nuclear envelope or ER membrane. Others fuse with Golgi bodies. F Some vesicles and their contents are delivered to lysosomes. lysosome Golgi Stepped Art Figure 5-27 p94

52 ANIMATED FIGURE: Membrane cycling To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

53 plasma membrane Figure 5-28a p94

54 aggregates of lipoproteins Figure 5-28b p94

55 Three Pathways of Endocytosis Receptor-mediated endocytosis Specific molecules bind to surface receptors, which are then enclosed in an endocytic vesicle Phagocytosis Larger target particles such as microbes or cellular debris are engulfed by pseudopods which merge as a vesicle, which fuses with a lysosome in the cell Pinocytosis A less selective endocytic pathway that brings materials in bulk into the cell

56 A Pseudopods of a white blood cell surround Tuberculosis bacteria (red). Figure 5-29a p95

57 ANIMATED FIGURE: Phagocytosis To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

58 Phagocytosis B Endocytic vesicle forms. c Lysosome fuses with vesicle; enzymes digest pathogen. D Cell uses the digested material or expels it.

59 Membrane Cycling Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles that fuse with plasma membrane

60 Forming New Plasma Membrane

61 Take-Home Message: How do cells take in large particles and bulk substances? Exocytosis and endocytosis move materials in bulk across plasma membranes In exocytosis, a cytoplasmic vesicle fuses with the plasma membrane and releases its contents to the outside of the cell In endocytosis, a patch of plasma membrane sinks inward and forms a vesicle in the cytoplasm Phagocytosis is an endocytic pathway by which cells engulf particles such as microorganisms

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