Photosynthetic pigments

• There are two types of pigments in plants:

  • photosynthetic pigments
  • non-photosynthetic pigments

  photosynthetic pigments absorb light for energy for photosynthesis reactions

  non-photosynthetic pigments reduce exposure to excess uv to protect the leaves from uv damage
• different pigments absorb different wavelengths of light.

  having many pigments in the leaf increases the range of wavelengths that can be absorbed
• photosynthetic pigments don‘t dissolve in water so organic solvents are used for chromatography.
• TLC = thin layer chromatography. The stationary phase is a plastic sheet supporting a thin layer of silica gel
• Rf value = distance travelled by pigment / distance travelled by solvent
• to get a sample of pigment from the leaves for chromatography:

  • grind sample to disrupt the cells
  • dissolve in ethanol to damage membrane (dont heat, it damages the pigment)
• the Rf values can be compared to a database to identify the sample
• chloroplast structure:

  • stroma
  • double membrane
  • Intermembrane space
  • chloroplast DNA
  • ribosomes
  • starch storage granule + lipid droplets
  • lamella (flat pieces of thylakoid membrane)
  • granum (stacks of thylakoid membrane)
• pigments are non-polar and membrane bound.

  so we can use ethanol to dissolve the membrane and release the pigments
• method:

  • grind sample of leaf in pestle and mortar using sand
  • add non polar solvent and continue to grind
  • add a small spot of leaf extract to baseline on chromatography paper (drawn in pencil)
  • suspend chromatography paper in solvent and allow the solvent to run up the paper
  • mark the solvent front
  • mark the pigments on the resulting chromatogram
• the distance travelled in chromatography will be influenced by:

  • the solubility in the mobile phase
  • the mass
  • the polarity
  • the temperature
• if some pigments have similar/overlapping Rf values, repeat the chromatography with another solvent
• Stroma = where the light independent reaction occurs. contains enzymes that use ATP to convert co2 to glucose
• thylakoid: where the light dependent reaction occurs. phospholipid bilayer with many embedded chlorophyll molecules which absorb light energy
• granum = a stack of thylakoid membranes. stacking increases the efficiency of the light dependent reaction
• Excess carbohydrate produced in photosynthesis is stored in the starch grain / granule