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Cells Chapter 3 Cell Theory • Every organism is composed of one or more cells • Cell is smallest unit having properties of life • All cells come from pre-existing cells Cell • Smallest unit of life • Can survive on its own or has potential to do so • Is highly organized for metabolism • Senses and responds to environment • Has potential to reproduce Structure of Cells All start out life with: Two types: – Plasma membrane – Prokaryotic – Region where DNA is stored – Eukaryotic – Cytoplasm Why Are Cells so Small? • Surface-to-volume ratio • The bigger a cell is, the less surface area there is per unit volume • Above a certain size, material cannot be moved in or out of cell fast enough Lipid Bilayer • Main component of cell membranes • Gives membrane its fluid properties • Two layers of phospholipids – Hydrophilic heads face outward – Hydrophobic tails in center Animal Cell Components • • • • • • • • Plasma membrane Nucleus Ribosomes Endoplasmic reticulum Golgi body Vesicles Mitochondria Cytoskeleton Cytoskeleton • Basis for cell shape and internal organization • Enables organelle movement within cells and, in some cases, cell motility • Main elements are microtubules, microfilaments, and intermediate filaments Flagella and Cilia microtubule • Structures for cell motility • 9+2 internal structure • Arise from centrioles dynein Plasma Membrane • Extremely thin • Mosaic of proteins and lipids • Lipids give membrane its fluid quality • Proteins carry out most membrane functions Membrane Proteins • Transport proteins • Receptor proteins • Recognition proteins • Adhesion proteins Cytomembrane System • Group of related organelles • Assembles lipids • Modifies new polypeptide chains • Sorts and ships products to various destinations Endoplasmic Reticulum • In animal cells, continuous with nuclear membrane • Extends throughout cytoplasm • Two regions: rough and smooth Smooth ER • • • • A series of interconnected tubules No ribosomes on surface Lipids assembled inside tubules Smooth ER of liver inactivates wastes, drugs • Sarcoplasmic reticulum of muscle is a specialized form Rough ER • Arranged into flattened sacs • Ribosomes on surface give it a rough appearance • Some polypeptide chains enter and are modified • Most extensive in secretory cells Golgi Bodies • Put finishing touches on proteins and lipids that arrive from ER • Package finished material for shipment to final destinations • Material arrives and leaves in vesicles Vesicles • Membranous sacs that move through the cytoplasm • Lysosomes • Peroxisomes Diffusion • The net movement of like molecules or ions down a concentration gradient • Although molecules collide randomly, the net movement is away from the place with the most collisions (down gradient) Factors Affecting Diffusion Rate • Steepness of concentration gradient • Molecular size • Temperature • Electrical or pressure gradients Osmosis • Diffusion of water molecules across a selectively permeable membrane • Direction of net flow is determined by water-concentration gradient • Side with the most solute molecules has the lowest water concentration Tonicity Refers to relative solute concentration of two fluids Hypertonic - having more solutes Isotonic - having same amount Hypotonic - having fewer solutes Transport Proteins • Span the lipid bilayer • Interior is able to open to both sides • Change shape when they interact with solute • Play roles in active and passive transport Passive Transport • Flow of solutes through the interior of passive transport proteins down their concentration gradients • Passive transport proteins enable solutes to move both ways • Does not require any energy input Active Transport • Net diffusion of solute is against concentration gradient • Transport protein must be activated • ATP gives up phosphate to activate protein • Binding of ATP changes protein shape and affinity for solute Bulk Transport Exocytosis Endocytosis Cholera • Cholera exotoxin raises level of signaling molecule that causes cells to dump chloride into the intestine • Other solutes follow • Water moves into intestine by osmosis • Result is nearly clear diarrhea • Treated with oral rehydration therapy Functions of Nucleus • Keeps the DNA molecules of eukaryotic cells separated from metabolic machinery of cytoplasm • Makes it easier to organize DNA and to copy it before parent cells divide into daughter cells Components of Nucleus Nuclear envelope Nucleoplasm Nucleolus Chromatin • Cell’s collection of DNA and associated proteins • Chromosome is one DNA molecule and its associated proteins • Appearance changes as cell divides Mitochondria • ATP-producing powerhouses • Double-membrane system • Carry out the most efficient energyreleasing reactions • Reactions require oxygen Mitochondrial Structure • Outer membrane faces cytoplasm • Inner membrane folds back on itself • Membranes form two distinct compartments • ATP-making machinery is embedded in the inner mitochondrial membrane ATP Universal Energy Currency • ATP is earned in reactions that yield energy, and spent in reactions that require it adenine P P P ribose Metabolic Pathways large energy-rich molecules ADP + Pi Anabolic Pathways Catabolic Pathways ATP energy-poor products Energy input simple organic compounds Participants in Metabolic Pathways • Substrates • Energy Carriers • Intermediates • Enzymes • End Products • Cofactors Enzyme Structure and Function • Enzymes speed the rate at which certain reactions occur • Nearly all are proteins • An enzyme recognizes and binds to only certain substrates • Reactions do not alter or use up enzyme molecules Induced-Fit Model active sight • Enzyme-substrate complex is short lived • Enzyme resumes its prebinding shape as a product molecule is released Factors Influencing Enzyme Activity Temperature pH Salt concentration Allosteric regulators Coenzymes and cofactors Overview of Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose carbon oxygen dioxide water Glycolysis • Occurs in cytoplasm • Reactions are catalyzed by enzymes Glucose (six carbons) 2 Pyruvate (three carbons) Glycolysis Occurs in Two Stages • Energy-requiring steps – ATP energy activates glucose and its six-carbon derivatives • Energy-releasing steps – The products of the first part are split into three- carbon pyruvate molecules – ATP and NADH form Net Energy Yield from Glycolysis Energy-requiring steps: 2 ATP invested Energy-releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH Mitochondrial Reactions • Mitochondrial membranes form two distinct compartments • ATP-making machinery is embedded in the inner mitochondrial membrane • Reactions begin when pyruvate enters a mitochondrion Two Parts of Second Stage • Preparatory reactions – Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide – NAD+ is reduced • Krebs cycle – The acetyl units are oxidized to carbon dioxide – NAD+ and FAD are reduced The Krebs Cycle Overall Reactants Overall Products • • • • • • • • • Acetyl-CoA 3 NAD+ FAD ADP and Pi Coenzyme A 2 CO2 3 NADH FADH2 ATP Results of the Second Stage • All the carbon molecules in pyruvate end up in carbon dioxide • Coenzymes are reduced (they pick up electrons and hydrogen) • One molecule of ATP forms • Four-carbon oxaloacetate is regenerated Coenzyme Reductions during First Two Stages • Glycolysis • Preparatory reactions • Krebs cycle 2 NADH 2 FADH2 + 6 NADH • Total 2 FADH2 + 10 NADH 2 NADH Electron Transport • Coenzymes give up electrons to electron transport system • Electrons are transported through the system • The final electron acceptor is oxygen • H+ is moved from inner to outer compartment • H+ flow back across membrane drives ATP synthesis Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT Making ATP ATP INNER COMPARTMENT ADP + Pi Importance of Oxygen • Operation of the electron transport system requires oxygen • Oxygen withdraws spent electrons from the electron transport system, then combines with H+ to form water Summary of Energy Harvest (per molecule of glucose) • Glycolysis – 2 ATP formed by substrate-level phosphorylation • Krebs cycle and preparatory reactions – 2 ATP formed by substrate-level phosphorylation • Electron transport phosphorylation – 32 ATP formed Carbohydrate Breakdown and Storage • Glucose is absorbed into blood • Pancreas releases insulin • Insulin stimulates glucose uptake by cells • Cells convert glucose to glucose-6-phosphate • This traps glucose in cytoplasm where it can be used for glycolysis Making Glycogen • If glucose intake is high, ATP-making machinery goes into high gear • When ATP levels rise high enough, glucose6-phosphate is diverted into glycogen synthesis (mainly in liver and muscle) • Glycogen is the main storage polysaccharide in animals Using Glycogen • When blood levels of glucose decline, pancreas releases glucagon • Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood • (Muscle cells do not release their stored glycogen) Energy Reserves • Glycogen makes up only about 1 percent of the body’s energy reserves • Proteins make up 21 percent of energy reserves • Fat makes up the bulk of reserves (78 percent) Energy from Fats • Most stored fats are triglycerides • Triglycerides are broken down to glycerol and fatty acids • Glycerol is converted to PGAL, an intermediate of glycolysis • Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle Energy from Proteins • Proteins are broken down to amino acids • Amino acids are broken apart • Amino group is removed; ammonia forms, is converted to urea, and is excreted • Carbon backbones can enter the Krebs cycle or its preparatory reactions