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Chapter 5 The Working Cell
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MEMBRANE STRUCTURE AND FUNCTION
© 2012 Pearson Education, Inc. 2
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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
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Constituents of membrane
LE 5-10 Constituents of membrane Phospholipid + protein + carbohydrate Major components Outside of cell Cytoplasm
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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"
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Glycerol+2fatty acids+phosphate+alcohol
Hydrophilic head Phosphate group Phospholipid Glycerol+2fatty acids+phosphate+alcohol Alcohol: ethanolamine, serine, choline Phospholipid is a amphipathic molecule hydrophilic hydrophobic Symbol Hydrophobic tails
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In membranes, phospholipids form a bilayer Two-layer sheet
Phospholipid heads facing outward and tails facing inward Selectively permeable Nonolar lipid-soluble molecules pass through Polar molecules not soluble in lipids do not pass through
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LE 5-11b Water Hydrophilic heads Hydrophobic tails Water
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5.12 The membrane is a fluid mosaic of phospholipids and proteins
Membrane proteins and cholesterol are embedded in the phospholipid bilayer Membrane proteins: integral protein, peripheral protein Cholesterol keeps the membrane fluidity at low temperature and stabilizes the membrane at high temperature Carbohydrates are present as forms of glycoproteins and glycolipids -They function as identification tags
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LE 5-12 CYTOPLASM Enzymatic activity
Figure 5.1 CYTOPLASM Enzymatic activity Fibers of extracellular matrix (ECM) Phospholipid Cell-cell recognition Cholesterol Receptor Signaling molecule Intercellular junctions ATP Transport Glycoprotein Signal transduction Attachment to the cytoskeleton and extracellular matrix (ECM) Microfilaments of cytoskeleton CYTOPLASM
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Fluid mosaic model for the membrane structure
Lateral movement of phospholipid molecules and proteins is relatively free Movements of lipid molecules in the membrane Lateral diffusion Transbilayer diffusion (flip-flop diffusion) Acyl chain flexing
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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
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LE 5-13a Enzyme activity
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Messenger molecule Receptor Activated molecule Signal transduction
LE 5-13b Messenger molecule Receptor Activated molecule Signal transduction
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LE 5-13c ATP Transport
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5.14 Passive transport is diffusion across a membrane
High conc. low conc.로 solute 이동 Concentration gradient as an energy source 2. Active transport Low conc. high conc. Energy input is required
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Molecules of dye Membrane Pores Net diffusion Net diffusion
Figure 5.3A Molecules of dye Membrane Pores Net diffusion Net diffusion Equilibrium
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Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion
Figure 5.3B Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium
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Passive transport 1. Simple diffusion : Diffusion through the phospholipid bilayer Small molecule > large molecule Hydrophobic molecule > hydrophilic molecule e.g., O2, CO2 (o) Macromolecules, charged and polar molecules such as amino acids, sugars, ions (X) H2O: 농도가 높아서 느리지만 simple diffusion 가능 2. Facilitated diffusion Diffusion through channel- or carrier proteins
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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: channel protein Bind the substance, change shape, and then release the substance: carrier protein
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LE 5-15 Solute molecule channel protein
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Carrier protein
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5.16 Osmosis is the diffusion of water across a membrane
Diffusion of water across the selectively permeable membrane Hypotonic sol Hypertonic sol Water Isotonic solution : when two solutions has the same concentration of the total solutes, they are isotonic sol.
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Lower concentration of solute Higher concentration of solute
Figure 5.4 Lower concentration of solute Higher concentration of solute Equal concentrations of solute H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Osmosis
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5.17 Water balance between cells and their surroundings is crucial to organisms
Osmoregulation is the control of water balance Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution
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Shriveled (plasmolyzed)
Figure 5.5 Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O Animal cell Lysed Normal Shriveled H2O H2O Plasma membrane H2O Plant cell Turgid (normal) Flaccid Shriveled (plasmolyzed)
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5.18 Cells expend energy for active transport
1. Primary active transport ATP as an energy source Cation transport e.g., Na+-K+ pump (3Na+ outward/2K+ inward) 2. Secondary active transport Ion concentration gradient produce by the primary active transport energy source e.g., glucose-Na+ symport
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LE 5-18 Transport protein P P Protein changes shape Phosphate detaches
ATP Solute ADP Solute binding Phosphorylation Transport Protein reversion
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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): supply the membrane components to the plasma membrane Membranes may fold inward, enclosing material from the outside (endocytosis)
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LE 5-19a Fluid outside cell Vesicle Protein Cytoplasm
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Endocytosis can occur in three ways
Phagocytosis ("cell eating") Pinocytosis ("cell drinking") Receptor-mediated endocytosis
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Figure 5.9 Three kinds of endocytosis
Phagocytosis EXTRACELLULAR FLUID CYTOPLASM Food being ingested Pseudopodium “Food” or other particle Food vacuole Pinocytosis Plasma membrane Vesicle Figure 5.9 Three kinds of endocytosis Plasma membrane Receptor-mediated endocytosis Coat protein Receptor Coated vesicle Coated pit Coated pit Specific molecule Material bound to receptor proteins 35
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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
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Phagocytosis Ingestion of large particles (cells) Formation of the phagosome (food vacuole) e.g., feeding process by unicellular protists defense process by some white blood cells 2. Pinocytosis Uptake of small dissolved substances or fluids Formation of the vesicle
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Receptor-mediated endocytosis
A process by which animal cells capture specific macromolecules from the outside Endocytosis site: coated pit Receptor proteins are required e.g., the uptake of cholesterol by most mammalian cells High conc. of cholesterol high risk of heart disease Liver cells remove excess cholesterol from the blood by receptor-mediated endocytosis Cholesterol in the blood is present in the form of Low density lipoproteins Hypercholesterolemia
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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
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LE 5-20 Phospholipid outer layer LDL particle Vesicle Cholesterol
Protein Plasma membrane Receptor protein Cytoplasm
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ENERGY AND THE CELL © 2012 Pearson Education, Inc. 41
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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 All organisms require energy to stay alive Energy makes change possible Unit of energy Calorie (cal): water 1 g 을 1C 올리는데 필요한 energy Joule (J): 1 cal = J
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Kinetic energy : energy of movement
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
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5.2 Two laws govern energy transformations
Thermodynamics is the study of energy transformations system surrounding Closed system: surrounding과 물질과 에너지 교환 x Open system:
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The First Law of Thermodynamics
Energy can be changed from one form to another but cannot be created or destroyed
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The Second Law of Thermodynamics
Energy transformations increase disorder, or entropy, and some energy is lost as heat Entropy: the amount of disorder in a system Total energy = usable energy + unusable energy H = G + TS H: enthalpy, G: free energy, T: absolute temperature, S: entropy
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Kinetic energy of movement
Figure 5.10 Fuel Energy conversion Waste products Heat energy Carbon dioxide Gasoline Combustion Kinetic energy of movement Oxygen Water Energy conversion in a car Heat energy Cellular respiration Glucose Carbon dioxide ATP ATP Oxygen Energy for cellular work Water Energy conversion in a cell
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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 Reactants Products Energy
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Amount of energy required
LE 5-3a Products Amount of energy required Energy required Potential energy of molecules Reactants
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Yield products that contain less potential energy than their reactants
Exergonic reactions Release energy Yield products that contain less potential energy than their reactants Examples: cellular respiration, burning Reactants Products Energy
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Amount of energy released
LE 5-3b Reactants Amount of energy released Energy released Potential energy of molecules Products
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Cells carry out thousands of chemical reactions, which constitute cellular metabolism
Energy coupling uses energy released from exergonic reactions to drive endergonic reactions Anabolism Simple molecule complex molecule Endergonic reaction e.g., synthesis of a protein from amino acids 2. Catabolism Complex molecule simple molecule Exergonic reaction
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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
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Adenosine diphosphate
LE 5-4a Adenosine Triphosphate Adenosine diphosphate Phosphate group H2O P P P P P + P + Energy Hydrolysis Adenine Ribose ATP ADP 12 kcal/mol under physiological conditions
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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
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Protein filament moved Solute transported
Figure 5.12B Chemical work Mechanical work Transport work ATP ATP ATP Solute P Motor protein P P Reactants Membrane protein P P P Product Molecule formed Protein filament moved Solute transported ADP P ADP P ADP P
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Cellular work can be sustained, because ATP is a renewable resource that cells regenerate
The ATP cycle involves continual phosphorylation and hydrolysis
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Energy from exergonic reactions Energy for endergonic reactions
Figure 5.12C ATP Phosphorylation Hydrolysis Energy from exergonic reactions Energy for endergonic reactions ADP P
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HOW ENZYMES FUNCTION © 2012 Pearson Education, Inc. 62
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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)
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Enzymes A protein (RNA) molecule that serves as a biological catalyst
Increase the rate of a reaction without itself being changed Speed up a reaction by lowering the activation energy Do not change chemical equilibrium Reactants Products Chemical reaction In principle, chemical reactions can run both forward and backward The rate and direction of a reaction depend on [reactants] and [products] At equilibrium, the rate of the forward reaction = the rate of the backward reaction
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Activation energy barrier
Figure 5.13A Activation energy barrier Enzyme Activation energy barrier reduced by enzyme Reactant Reactant Energy Energy Figure 5.13A The effect of an enzyme in lowering EA Products Products Without enzyme With enzyme 65
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LE 5-5b enzyme enzyme EA without EA with Reactants Energy Net change
in energy Products Progress of the reaction
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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
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A single enzyme may act on thousands or millions of substrate molecules per second
H-bond, ionic bond, hydrophobic interaction The enzyme is unchanged and can repeat the process
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Enzyme available with empty active site
Figure 5.14_s4 1 Enzyme available with empty active site Active site Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Enzyme (sucrase) Glucose Fructose H2O Figure 5.14_s4 The catalytic cycle of an enzyme (step 4) 4 Products are released 3 Substrate is converted to products 69
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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
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Temperature Increase in temperature increase in the rate of contact between the substrate and the enzyme Above the optimal temperature: denaturation of the enzyme
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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
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Normal binding of substrate
Figure 5.15A Substrate Active site Enzyme Allosteric site Normal binding of substrate Competitive inhibitor Noncompetitive inhibitor Enzyme inhibition
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Competitive inhibitor
Resemble the enzyme’s normal substrate Bind to the active site of the enzyme, thereby blocking the enzyme-substrate interaction As [substrate] increases, the inhibition is overcome
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Feedback inhibition in a metabolic pathway
Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting molecule Product
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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
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