BSCI 230 Today, 3/01/01

  • Fund Raising Team: please see Dr. Know-It-All
  • Exams returned next Tuesday in lecture

    • -You must be present to win!

    Summary of Photosynthesis Lectures to Date:
    From the Light Reactions
    NADPH and ATP
    2:3 ratio
    Dark Reactions: Calvin Cycle
    Reduce atmospheric C as CO2 into (CH2O)

    The Key Enzyme:
    Ribulose bisphosphate
    carboxylase/oxygenase
    RUBISCO

    (3) Ribulose 1,5 bisphosphate + 3 CO2 + 3 H2O
    3 (5 C) = 15 C
    (6) 3-phosphoglycerate

    6 (3 C) = 18 C
    Problem: at low CO2 : O2 ratios
    Rubisco functions as an oxygenase using O2 to form phosphoglycolate and
    3-phosphoglycerate
    Photorespiration
    Composition of air:
    CO2 = 0.035 %
      O2 = 21%
    Thus 1 of every 3 (3C) molecules formed is phosphoglycolate



     
     

    Strategy 1: Glycolate Pathway

    Returns phosphoglycolate to Calvin Cycle but requires ATP
     

    Strategy 2: Hatch-Slack Cycle

  • In hot tropical climates, plants have lower dissolved O2 and CO2
  • This means an even lower ratio of CO2/O2
  • So they initially fix C form CO2 into a 4C molecule:
    • CO2 + phopsphoenolpyruvte (3C)--->
    Oxaloacetate (4C)

    Two types of Plants based on Photosynthesic metabolism:
    C3 and C4 Plants

    Hatch-Slack Cycle Occurs in mesophyll cells of C4 Cells


    Hatch-Slack Cycle

  • Occurs in mesophyll cells
  • Oxaloacetate reduced to malate
  • 4C malate molecules diffuse into Bundle Sheath Cells
  • Malate decarboxylated and released CO2 is then put into Calvin Cycle
    • -Works due to high [CO2] in Bundle Sheath Cells
    And now, Cell Membranes
    Chapter 7
    Lipids
    Fatty Acids
     
  • Write as: CH3(CH2)nCOOH
  • Polarity????
  • Has both polar and non-polar regions or both hydrophyllic and hydrophobic regions:amphipathic


    • Fats and Oils

  • 3 Carbon molecule - glycerol
  • one, two, or three fatty acids
  • mono-, di-, or triglyceride
  • Fat is solid @ room temperature
  • Oil is a liquid @ room temperature
    • Making a monoglyceride
    Making a monoglyceride:
    another dehydration reaction
    In a similar fashion,
    di- and triglycerides
    Drawing a lipid:
    Floating Lipids on Water?
    Lipids mixed in water:


    Micelle:

    Lipids in water:


    Bilayer:

    Table 7-2

     
    Floating Lipids on Water?

     

    Phospholipids: Fig 7-6

    What holds a bilayer together?
  • Hydrophobic interactions (Entropy wins!)
  • van der Waals interactions
    • -electron clouds
  • Both of above depend on fatty acid carbon chain length and close contact among chains
    • -longer chains and closer spacing makes for stronger links

     

    Saturated and Unsaturated
    Fatty Acids

  • No double bonds in carbon chains = saturated (straight)
  • One double bond = mono-unsaturated (bend in chain)
  • Two or more double bonds = polyunsaturated (bent chain)
    • How do we know cell membranes are a bilayer ?
    Early Electron micrographs - Unit Membrane

    Experiment by Gorter and Grendel, 1925

  • Extracted lipids from red blood cells (erythroctes)
    • -Knew the size and the number
    -Calculated surface area
  • Floated the lipids on water
  • Measured the area they occupied


    • Gorter and Grendel, 1925

  • Calculated a ratio of area of lipids to area of rbc membranes to be 2:1
  • Thus they concluded a bilayer
  • Two errors:
    • -Failed to extract all the lipids
    -Assumed rbc was a sphere
  • Two errors canceled out!

    • Phospholipid: See structure above

    Membrane Lipids


     
  • Phospholipid composition varies with source of membrane (Fig 7-7)
  • Asymetric distribution of phospholipids in halves of the bilayer


    • Membrane Lipids

  • Flip-flop or transverse diffusion across bilayer is rare!
    • -There are phopholipid translocators or flippases
  • Lateral diffusion occurs freely
  • Phospholipids create local environment for proteins


    • Look at history of cell membrane
    structure discovery
    Fig 7-3