Pigments and Light

• Photosynthesis

  • Use of sunlight energy to manufacture sugar from carbon dioxide and water
  • Opposite of cellular respiration, which animals and plants do - animals as heterotrophs take in energy from food, don’t make it
  • Autographs: multicellular plants, seaweeds, unicellular eukaryotes, prokaryotes - not all of these plants photosynthesize with the main formula
• Light-independent reactions (Calvin cycle)

  • Produce sugar from carbon dioxide
  • Electrons and ATP are used to reduce carbon dioxide
• Light-dependent reactions

  • Water splits to form oxygen
  • Electrons from water are excited by light energy
  • High-energy electrons are transferred to the electron carrier NADP+, forming NADPH
  • ATP is produced
• Purple Sulfur Bacteria

  • Anaerobic bacteria that uses sulphur instead of water as their reducing agent
  • Found in stratified water environments (stagnant water, hot springs) and intertidal zones as microbial mats
  • Since water isn’t used, oxygen cannot be produced
  • Can use sulphur in the form of sulphide or thiosulphate
• Chloroplast

  • Site of photosynthesis in eukaryotes
  • Can be 40-50 per cell
  • Each has three membrane layers - outer membrane, inner membrane, flattened membranes inside called thylakoid membranes (looks like discs or stack of coins) - one stack of these is called granum
  • Between granum is space with aqueous fluid called stroma
  • Photosynthetic prokaryotes don’t have chloroplasts, but have thylakoid membranes
  • Chlorophyll located within thylakoid membrane, arranged in light harvesting complexes
• Electromagnetic Spectrum

  • Light is a type of electromagnetic radiation energy
  • Light behaves as a wave and particle
  • Wavelength - distance between two wave crests
  • Particle - light exists in discrete packets called photons
  • Shorter wavelengths have more energy
• Pigments

  • Two major classes of plant pigments:
  • Chlorophylls (chlorophyll a and b)
  • Absorb red and blue light
  • Reflect and transmit green light
  • Main photosynthetic pigments
  • Carotenoids
  • Absorb blue and green light
  • Reflect and transmit yellow, orange, and red light
• Chlorophyll

  • Chlorophyll a & b have similar structure. Difference responsible for their different light wavelength absorption
  • Chlorophyll a = primary electron donor
  • Chlorophyll b = accessory pigment
• Carotenoids and Xanthophylls

  • Accessory pigments found in chloroplasts
  • Absorb light and pass energy to chlorophyll a, the primary electron donor
  • They extend the range of wavelengths of light energy capture
  • Protect chlorophyll fro damage by absorbing some of the light energy and stabilizing free radicals
  • Provide colour to flowers and fruits (attract pollinators and animal to spread seed)
• Autumn Leaf Colour Change

  • Chlorophyll masks other pigments
  • Preparation for winter:
  • Chlorophyll production stops
  • Chlorophyll is broken down & components stored in stem & roots for use in spring
  • Prevents loss from plant of green leaves dropped to ground
  • Maple trees: glucose trapped in leaves after photosynthesis stops is turned into red anthocyanins
• Energy Levels

  • Excited electrons are unstable
  • If excited electron falls back to ground state energy is released as heat or light (fluorescence)
  • About 2% of photons in chlorophyll produce fluorescence, 98% drive photosynthesis