Biology Topic 2

• Gas exchange


  • Rate of gas exchange by diffusion is increased by:
  • Increased surface area exchanged across
  • Decreased diffusion distance
  • More steep diffusion gradient
• Fick's Law


  Used to determine the rate of diffusion, states that the larger the surface area, difference in concentration and shorter the diffusion distance the quicker the rate
• Lungs


  • Adapted for rapid gas exchange in the following ways:
  • Large surface area due to many alveoli
  • Good supply of circulating blood to the lungs maintains steep concentration gradient
  • Short diffusion distance as alveoli are one cell thick
• Cell membrane


  Partially permeable membrane composed of a sea of phospholipids with protein molecules between, controlling the movement of substances in and out of the cell/organelle
• Cell membrane


  • Contains receptors for other molecules, enables adjacent cells to stick together

  Fluid mosaic model - fluidity and mosaic arrangement of proteins
• Movement of molecules through cell membrane

  Diffusion - passive movement of small, non-polar, lipid-soluble molecules down concentration gradient

  Facilitated diffusion - requires channel protein to transport polar, charged and water-soluble molecules

  Osmosis - movement of water molecules from low to high solute concentration

  Active transport - can transport all types of molecules against concentration gradient, requires ATP

  Endocytosis/Exocytosis - transport of large particles
• DNA mononucleotide


  Bases: Purines (adenine, guanine) and Pyrimidines (cytosine, thymine)

  Pairing: A-T, C-G

  Sugar: Deoxyribose

  Bonding: Phosphodiester bonds, hydrogen bonds
• DNA structure


  Double-stranded, alpha double helix with sugar-phosphate backbone on each strand
• mRNA


  Bases: Purines (adenine, guanine), Pyrimidines (cytosine, uracil)

  Pairing: A-U, C-G

  Sugar: Ribose

  Bonding: Same as DNA

  Structure: Single-stranded, carries codons
• tRNA


  Bases, pairing, sugar, bonding: Same as mRNA

  Structure: Single-stranded, folded into specific pattern, carries anticodons complementary to mRNA codons
• Protein synthesis


  Transcription - DNA to mRNA in nucleus

  Translation - mRNA to polypeptide chain at ribosomes
• Transcription


  DNA uncoils and strands separate

  2. One DNA strand used as template to make mRNA

  3. mRNA moves out of nucleus to ribosome
• Translation


  mRNA attaches to ribosome

  tRNA binds specific amino acids and anticodons bind to mRNA codons

  Ribosome joins amino acids by peptide bonds, tRNA detaches

  Repeats until stop codon reached
• Genetic code


  Gene is a series of bases on DNA coding for a sequence of amino acids in a polypeptide chain

  Genetic code consists of triplets of bases, each triplet codes for a particular amino acid

  Amino acids joined by peptide bonds to form polypeptide chain
• Codons


  Held in place by hydrogen bonds
• Ribosome joining amino acids

  1. Attaches tRNA molecules by peptide bond

  2. tRNA molecules detach from amino acids

  3. Process repeated leading to polypeptide chain formation

  4. Until stop codon reached on mRNA
• Gene


  Series of bases on a DNA molecule which codes for a series of amino acids in a polypeptide chain
• Genetic code


  Order of bases on DNA, consists of triplets of bases where each triplet codes for a particular amino acid
• Genetic code


  • Non-overlapping
  • Degenerate
  • Triplet code
  • Contains start and stop codons
• Amino acids


  Monomers from which proteins are made, contain amino group, carboxyl group, and variable R group
• Peptide bonds


  Bonds formed between amino acids in condensation reactions
• Dipeptide


  Contains two amino acids
• Polypeptide


  Contains three or more amino acids
• Levels of protein structure

  • Primary structure (sequence of amino acids)
  • Secondary structure (2D arrangement - alpha helix or beta pleated sheet)
  • Tertiary structure (3D folding of secondary structure)
  • Quaternary structure (3D arrangement of more than one polypeptide)
• Fibrous proteins


  • Long parallel polypeptides
  • Very little tertiary/quaternary structure
  • Occasional cross-linkages
  • Insoluble
  • Used for structural purposes
• Globular proteins


  • Complex tertiary/quaternary structures
  • Form colloids in water
  • Many uses e.g. hormones, antibodies, carrier proteins
• Collagen


  Fibrous protein with high tensile strength due to hydrogen and covalent bonds, forms structure of bones, cartilage, connective tissue, and tendons
• Haemoglobin


  Globular protein that carries oxygen in the blood, consists of four beta polypeptide chains and a haem group
• Enzymes


  Biological catalysts that increase the rate of reactions by lowering the activation energy
• Active site


  Area of enzyme where substrate binds, enzymes are specific to substrates
• Induced fit model


  When enzyme and substrate form a complex, the structure of the enzyme is distorted so the active site fits around the substrate
• Initial rate of reaction

  Measured by calculating the gradient of a concentration-time graph
• Factors affecting rate of enzyme-controlled reactions

  • Enzyme concentration
  • Substrate concentration
  • Temperature
  • pH
• Semi-conservative replication

  Ensures genetic continuity between generations of cells, where genetic information is passed on from one generation to the next
• Meselson-Stahl experiment


  Proved semi-conservative replication, where one strand of DNA contains 15-N and the other 14-N
• Semi-conservative replication of DNA

  1. Double helix unwinds

  2. Hydrogen bonds between complementary bases break, catalysed by DNA helicase

  3. One strand used as template

  4. Complementary base pairing occurs between template strand and free nucleotides

  5. Adjacent nucleotides joined by phosphodiester bonds formed in condensation reactions, catalysed by DNA polymerase
• Meselson-Stahl experiments


  Provided evidence for semi-conservative replication (as opposed to conservative or dispersive replication)
• Meselson and Stahl originally grew DNA in a culture containing N15 (an isotope of nitrogen) for several generations, then grew it in a culture of N14 for one generation. After this generation, the DNA contained one strand with 15-N and one strand with 14-N. After another generation, half the DNA molecules were the same as in generation one, and the other half contained entirely 14-N.
• This provides evidence for the semi-conservative model of DNA replication
• Mutation


  Permanent change in the DNA of an organism