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Topic 3: The Chemistry of Life


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Topic 3 of SL Biology. IB students testing out of SL Biology need to know these key points from the syllabus.

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Topic 3: The Chemistry of Life

  1. 1. Topic 3The Chemistry of Life
  2. 2. Polarity of Water• Hydrogen pole is positive• Oxygen pole is negative• This makes water molecules polar
  3. 3. Hydrogen Bonding in Water• Hydrogen bond: bond that can formbetween the positive pole of one watermolecule and the negative pole of another• Many bonds form in liquid water whichmakes it useful for living organisms
  4. 4. The Properties of WaterProperty name Outline Use in living organismsCohesion Hydrogen bonds let watermolecules stick to each otherUsed as a transportmedium in plantsSolvent properties Water’s polarity allows it todissolve many differentsubstancesWater is the medium formetabolic reactionsHeat capacity Large amounts of energy areneeded to raise water’stemperatureWater in blood cantransfer heatBoiling point Water has a high boiling point;large amounts of energy areneeded to break hydrogen bondsto turn water into a gasWater is the medium formetabolic reaction when itis a liquidCooling effect ofevaporationWater can evaporate at tempsbelow boiling point; heat energy istaken from breaking H bondsWater can act as a coolant(i.e. sweat)
  5. 5. Elements in Living Organisms• Four most common elements:1. Carbon2. Hydrogen3. Oxygen4. Nitrogen• Other important elements:– Sulfur, Calcium, Phosphorus, Iron, Sodium
  6. 6. Organic and Inorganic Compounds• Organic compounds: containing carbon andfound in living organisms• All compounds that do not contain carbonare inorganic• Few compounds that do contain carbon areinorganic (Example: Carbon dioxide)• Living organisms generally have three typesof organic compounds:– Carbohydrates– Lipids– Proteins
  7. 7. Subunits of OrganicMacromolecules• Many organic compounds are made up oflarge molecules called macromolecules• They are built up using subunits
  8. 8. Condensation Reactions• Two molecules are joined together andwater is also formed in the reaction• Two amino acids in a condensationreaction create a peptide bond• Two monosaccharides in a condensationreaction create a disaccharide• Fatty acids link to glyceral in acondesnation reaction and createsgylcerides
  9. 9. Hydrolysis Reactions• Hydrolysis: breaks down large moleculesinto smaller molecules with water• The reverse of condensation reactions
  10. 10. Examples of CarbohydratesName Example In animals In plantsMonosaccharide •Glucose•Fructose•GalactoseGlucose is carriedby blood totransport energyFructose makesfruit sweet toattract animals todisperse seedsDisaccharide •Lactose•Maltose•SucroseLactose is thesugar in milk thatprovides energySucrosetransports energythrough thephloemPolysaccharide •Starch•Gycogen•CulluloseGlycogen is usedfor short-termenergy storage inthe liver/musclesCellulose makesstrong fiber toconstruct the plantcell wall
  11. 11. Functions of Lipids• Energy storage– Create fat in humans– Create oil in plants• Heat insulation– Layer of fat under skin reduces heat loss• Buoyancy– Lipids are less dense than water so they canhelp animals float
  12. 12. Carbohydrates and Lipids inEnergy StorageAdvantages of lipids Advantages of carbohydratesContain more energy per gram thancarbohydrates, making stores lighterMore easily digested than lipids,making energy release fasterInsoluble in water and so do not effectosmosis in cellsSoluble in water and so are easier totransportUsually used for long term energystorageUsually used for short term energystorage
  13. 13. Nucleotide Subunits of DNA• DNA is made of nucleotides• Each nucleotide contains a sugar,phosphate group, and a base• There are four different bases found innucleotides:– Adenine– Thymine– Guanine– Cytosine
  14. 14. Building DNA Molecules• Two DNA nucleotides can be linkedtogether by a covalent bond– Bond forms between the sugar of onenucleotide and the phosphate of another• DNA consists of two strands of nucleotidesin a double helix
  15. 15. Complementary Base Pairing• Certain nucleotide bases will only bondwith one other kind of nucleotide base– Adenine bonds with Guanine– Thymine bonds with Cytosine• This is called complementary base pairing
  16. 16. DNA Replication• Semi-conservative: each molecule formed byreplication contains one strand of the originalDNA and one new strand• Stages of replication:1. DNA uncoils and hydrogen bonds are broken bythe enzyme Helicase2. Single strand acts as a template upon whichfree nucleotides bond to; caused by the enzymePolymerase3. New strands of DNA form double helix
  17. 17. Differences Between DNA and RNADNA RNATwo strands of nucleotides to form adouble helixOne strand onlyDeoxyribose sugar Ribose sugarUse bases Adenine, Guanine,Cytosine, and ThymineUses bases Adenine, Guanine,Cytosine, and Uracil (instead ofThymine)
  18. 18. Genes and Polypeptides• Polypeptides are made up of amino acids• Information for making polypeptides arestored in a coded form in the genes of theamino acids• The sequence of bases in a gene codesfor the sequence of amino acids in apolypeptide
  19. 19. Transcription• Transcription: Copying of a base sequence of DNAby making RNA• Uses complementary base pairing; replacingThymine with Uracil when bonded to Adenine• Stages:1. DNA uncoils and separates2. Free RNA nucleotides assemble using one strand asa template3. RNA nucleotides link together4. The RNA strand then separates from DNA; it is nowmessenger RNA (mRNA)5. DNA reforms a double helix
  20. 20. Translation• Translation: Translates the RNA genetic code(which are in groups of codons) into an amino acidchain• Stages:1. mRNA binds to the small subunit a of ribosome;mRNA contains codons2. Free transfer RNA (tRNA) molecules haveanticodons that are complementary to certain mRNAcodons; tRNA also carries amino acids3. tRNA bonds to the ribosome if it is complementary tothe mRNA codon; these codons and anticodonsform hydrogen bonds4. The amino acids carried by the bonding tRNAmolecules bond together into peptides
  21. 21. One Gene-One Polypeptide Hypothesis• There is almost always a single gene tocode for a polypeptide that does not codefor any other polypeptide
  22. 22. Introducing Enzymes• Enzymes: globular proteins that catalyzechemical reactions• By making only certain enzymes, cells cancontrol what chemical reactions take place• Denaturation: changing the structure of anenzyme so it can no longer carry out itsfunction• Substrates: the reactants in enzymereactions
  23. 23. Enzyme-Substrate Specificity• Most enzymes are specific and only catalyzecertain reactions with certain substrates• Substrates bond to active site of an enzyme• Active site: region on the surface of anenzyme to which substrates bind to catalyzea chemical reaction with the substrate– Only certain substrates can fitthe shape (like a key fitting intoa lock)
  24. 24. Factors Affecting Enzyme Activity• Temperature– As temp increases, so does enzyme activity– At a certain temp enzyme activity will drop• pH– Optimum pH is 7– pH levels closest to 7 have highest activity• Substrate concentration– The greater the amount of substrates available tobind to, the greater the activity– Enzyme activity eventually plateaus
  25. 25. Lactase and Lactose-Free Milk• Lactose: natural sugar in milk• Lactase: converts lactose into glucose andgalactose• Biotechnology can extract lactase toprevent possible negative effects causedby lactose
  26. 26. Energy and Cells• All living cells need continuous energy• Cell respiration: controlled release of ATPenergy from organic compounds in cells• Can be aerobic (with oxygen) oranaerobic (without oxygen)
  27. 27. Use of Glucose in Respiration• Cell respiration often uses glucose• Glucose is broken down into pyruvate (asimpler ogranic compound)• This produces a small amount of ATPenergy that is released by glucose
  28. 28. Anaerobic Cell Respiration• If oxygen is unavailable, pyruvate isconverted into a waste product– Waste product is either lactate (in humans); orethanol or carbon dioxide (in yeast)• No ATP is produced
  29. 29. Aerobic Cell Respiration• If oxygen is available, the pyruvate isabsorbed into the mitochondria and isbroken down into Carbon Dioxide andWater• ATP is produced
  30. 30. Introducing Photosynthesis• Photosynthesis: process used by plants toproduce organic substances from lightenergy and inorganic substances• Light energy is converted to chemicalenergy• Chlorophyll is the main pigment thatabsorbs light• Some of the absorbed energy makes ATPand other split into water molecules
  31. 31. Measuring Rates of Photosynthesis• Can measure the rate of photosynthesiswith:– Production of Oxygen– Uptake of Carbon Dioxide– Increase in Biomass
  32. 32. Factors Affecting Photosynthesis• Light intensity– As light intensity increases, the rate of photosynthesisincreases– Eventually plateaus• Carbon Dioxide– As CO2 concentration increases, the rate ofphotosynthesis increases– Eventually plateaus• Temperature– As temperature increases, the rate of photosynthesisincreases steeply– Reaches an optimum temperature then the rate drops