SM1501: The Cell as a Factory 1

Building Molecules of Life

• The main questions are, what are the most important molecules of life and how do cells generate energy?

Chemical Structures and Functions of Carbohydrates

• Carbohydrates have the general formula Cn(H2O)n
• Primary functions of carbohydrates include source of stored energy and transport stored energy
• Carbohydrates act as Carbon skeletons for many other molecules
• Monosaccharides are simple sugars
• Disaccharides are two simple sugars linked by covalent bonds
• Oligosaccharides contain from three to 20 monosaccharides
• Polysaccharides contain hundreds or thousands of monosaccharides like starch, glycogen, and cellulose
• All cells use glucose (monosaccharide) as an energy source.
• Glucose exists as a straight chain or ring form
• Ring form exists as a- or B-glucose
• a- and B-glucose can interconvert
• Monosaccharides have different numbers of carbons
  • Hexoses have six carbons and are structural isomers
  • Pentoses have five carbons
• Monosaccharides bind together in condensation reactions to form glycosidic linkages

Disaccharides Formation

• Disaccharides examples are sucrose, and maltose
• Sucrose is formed from a-D-glucose and Fructose, creating α-1,2 glycosidic linkage
• Maltose is formed form a-D-glucose and B-D-glucose, creating α-1,4 glycosidic linkage
• Oligosaccharides may include other functional groups
• They are often covalently bonded to proteins and lipids on cell surfaces and act as recognition signals
• Human blood groups get specificity from oligosaccharide chains
• Polysaccharides are giant polymers of monosaccharides
• Starch is used for storage of glucose in plants
• Glycogen is used for storage of glucose in animals
• Cellulose is very stable and is good for structural components

Chemical Structures and Functions of Lipids

• Lipids are nonpolar hydrocarbons
• When sufficiently close together, weak but additive van der Waals forces hold them together
• Lipids are not polymers in the strict sense, because they are not covalently bonded
• Fats and oils store energy
• Phospholipids have a structural role in cell membranes
• Carotenoids and chlorophylls capture light energy in plants
• Steroids and modified fatty acids act as hormones and vitamins
• Fats and oils are triglycerides (simple lipids) composed of fatty acids and glycerol
• Glycerol contains 3 -OH groups and is an alcohol
• Fatty acids are nonpolar hydrocarbons with a polar carboxyl group
• Carboxyls bond with hydroxyls of glycerol in an ester linkage
• Saturated fatty acids have no double bonds between carbons; saturated with H atoms
• Unsaturated fatty acids have some double bonds in carbon chain
  • Monounsaturated fats have one double bond
  • Polyunsaturated fats have more than one double bond
• Fatty acids are amphipathic and have opposing chemical properties
• The carboxyl group ionizes to COO- and is strongly hydrophilic, the other end is hydrophobic
• Phospholipids are fatty acids bound to glycerol where a phosphate group replaces one fatty acid
• The phosphate group is hydrophilic and forms the "head"
• The fatty acid chains form "tails" that are hydrophobic
• Phospholipids are amphipathic

Role of ATP in Biochemical Energetics

• ATP (adenosine triphosphate) captures and transfers free energy
• ATP releases a large amount of energy when hydrolyzed
• ATP can phosphorylate, or donate phosphate groups to other molecules
• ATP is a nucleotide, not a nucleoside
• Hydrolysis of ATP yields free energy
  • Hydrolysis equation: ATP + H2O → ADP + P + free energy; AG = -7.3 to -14 kcal/mol and is exergonic
• Bioluminescence is an endergonic reaction driven by ATP hydrolysis
• Bioluminescence equation: luciferin + O2 + ATP – luciferase → oxylucifer in + AMP + PP + light
• The formation of ATP is endergonic
  • Equation: ADP + P + free energy → ATP + H2O
• Formation and hydrolysis of ATP couples exergonic and endergonic reactions

Pathways that Harvest Chemical Energy

• The main questions are, how does glucose oxidation release chemical energy, what are the aerobic pathways of glucose metabolism, how does oxidative phosphorylation form ATP, how is energy harvested from glucose in the absence of oxygen, and how are metabolic pathways interrelated and regulated?
• Fuels are molecules whose stored energy can be released for use
• The most common fuel in organisms is glucose; other molecules are first converted into glucose or other intermediate compounds
• Complex chemical transformations occur in a series of reactions
• Each reaction is catalyzed by a specific enzyme
• Metabolic pathways are similar in all organisms
• In eukaryotes, metabolic pathways are compartmentalized in organelles
• Each pathway is regulated by key enzymes
• Burning or metabolism of glucose yields: C6H12O6 +602 → 6CO2 + 6H2O + free energy
• Glucose metabolism pathway traps the free energy in ATP: ADP + P + free energy → ATP
• AG is the change in free energy and the AG from complete combustion of glucose = -686 kcal/mol
• Three metabolic pathways are involved in harvesting the energy of glucose
  • Glycolysis is the conversion of glucose to pyruvate
  • Cellular respiration uses aerobic pathways which converts pyruvate into H₂O, CO₂, and ATP
  • Fermentation uses anaerobic pathways and converts pyruvate into lactic acid or ethanol, CO2, and ATP
• If O₂ is present (aerobic) glycolysis is followed by three pathways of cellular respiration
  • Pyruvate oxidation
  • Citric acid cycle
  • Electron transport chain
• If O₂ is not present, pyruvate from glycolysis is metabolized by fermentation
• Oxidation and reduction always occur together
• The reactant that becomes reduced is the oxidizing agent
• The reactant that becomes oxidized is the reducing agent
• In glucose combustion, glucose is the reducing agent, O₂ is the oxidizing agent
• Energy is transferred in a redox reaction
• Energy in the reducing agent (glucose) is transferred to the reduced product
• Coenzyme NAD+ is a key electron carrier in redox reactions
  • NAD+ (oxidized)
  • NADH (reduced)
• Oxygen accepts electrons from NADH
  • NADH + H+ + O2 → NAD+ + H₂O; exergonic with AG = -52.4 kcal/mol
• Oxidizing agent is molecular oxygen-O₂