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What is the Structure of the Cell Membrane?. Constituents Phospholipids : molecule consists of two hydrocarbon chains ( hydrophobic ) and phosphate group ( hydrophilic ) Proteins
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What is the Structure of the Cell Membrane? • Constituents • Phospholipids: molecule consists of two hydrocarbon chains (hydrophobic) and phosphate group (hydrophilic) • Proteins • Integral proteins: embedded in lipids; some cross entire membrane, often act like gates (allow substances into/out of cell) • Peripheral proteins: along edge of membrane (inside or outside); often receptors for hormones; entire complex moves into cell • Cholesterol: maintains fluidity of membrane • Carbohydrate Chains: found on outside of cell; involved in cell to cell recognition and hormone reception • The Fluid-Mosaic Model: fluid bi-layer; lipids internalized in membrane
How do Substances Get Into or Out Of Cells? • Passive Transport • Diffusion: spread of substance along a concentration gradient (from high to low concentrations) • Only small, neutral molecules can pass through membrane (ex. O2, CO2) • Osmosis: diffusion of water across a cell membrane • Facilitated Transport • Molecule or ion crosses membrane via carrier/gate protein, following a concentration gradient (no energy expenditure required) • Active Transport • Molecule or ion crosses membrane via carrier/gate protein, against a concentration gradient (energy expenditure) • Endocytosis/Exocytosis (requires energy expenditure) • Endo: molecules enveloped by cell membrane vesicle into cell • Exo: molecules produced in cell excreted via vesicle
How is Energy Involved in Cellular Metabolism? • ATP and Metabolism • Energy required for many cellular reactions, including active transport, endo- and exocytosis, biosynthesis, and mechanical work/movement • Energy transferred from exothermic to endothermic reactions via adenosine triphosphate (ATP), the universal energy currency • Overview of Catabolic Processes • Stage I: Hydrolysis of Dietary Macromolecules into Small Subunits • Starch maltose glucose, catalyzed by amylase and maltase • Proteins denatured by stomach acid, digested by pepsin and various protease enzymes • Emulsion of fats by bile salts; hydrolysis by lipase • Stage II: Conversion of Monomers to Forms That Can Be Fully Oxidized • Monomers enter either glycolysis or the Krebs Cycle • Stage III: Complete Oxidation of Compounds and the Production of ATP
How are Enzymes Involved in Cellular Metabolism? • Importance of Shape and Structure with Proteins • Cellular functions related to shape and intact structures of proteins • Denaturation: loss of 3° and/or 4° structures; can be caused by excess temperature, pH changes, chemicals, or mechanical stress • Enzymes: biological catalysts; most are proteins • Increase rates of chemical reactions; reduce activation energy; each molecule recycled • IUPAC names derive from substrates and actions; end with –ase • Substrate(s) fit in active site(s); induced-fit model favored over lock-and-key model • Often require cofactors (metals, organics) and/or coenzymes
What Factors Affect Enzyme Function? • Effect of pH Levels • Enzymes are only active within narrow pH ranges, and work best at specific pH optima • Most cytoplasmic enzymes require pH of 7; pepsin works best at pH ~2 (in stomach acid) • Some bacteria have evolved to live at extreme pH levels • Effect of Temperature • Enzymes are only active within narrow temperature ranges, and work best at their temperature optima (humans ~37 C) • If too hot, can become denatured; if too cold, reaction rates can be reduced below critical rates • Enzyme Inhibition: compounds block active sites • Irreversible Inhibitors: include arsenic, snake venoms, nerve gases • Competitive Inhibitors: compounds are structural analogues to enzymes’ substrates (the dose makes the poison)
