Lecture 12

• Photosynthesis is the process that converts solar energy into chemical energy
• Directly or indirectly, photosynthesis nourishes the living world
• Autotrophs sustain themselves without eating anything derived from other organisms
• Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other inorganic molecules
• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2
• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes

  • These organisms feed not only themselves but also most of the living world

  
• Cyanobacteria are thought to be the originators of photosynthesis on our planet
• Heterotrophs obtain their organic material from other organisms
• Heterotrophs are the consumers of the biosphere
• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
• Photosynthesis converts light energy to the chemical energy of food
• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
• The structural organization of photosynthetic cells allows for the chemical reactions of photosynthesis
• Leaves are the major locations of photosynthesis
• Their green color is from chlorophyll, the green pigment within chloroplasts
• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
• CO2 enters and O2 exits the leaf through microscopic pores called stomata
• • Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
  • A typical mesophyll cell has 30–40 chloroplasts
  • The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
  • Chloroplasts also contain stroma, a dense fluid

  
• Photosynthesis can be summarized as the following equation:

  • 6 CO2 + 12 H2O + Light energy --> C6H12O6 + 6 O2 + 6 H2O
• Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

  
• Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

  • Energy + 6CO2 + 6H2O --> C6H12O6 + 6O2 is an endergonic reaction!
• Photosynthesis consists of:

  • the light reactions (the photo part)
  • and the Calvin cycle (the synthesis part)
• The light reactions (in the thylakoids):

  • Split H2O
  • Release O2
  • Reduce NADP+ to NADPH
  • Generate ATP from ADP by photophosphorylation
• The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

  
• Photosynthesis

  A) Light Reactions

  B) Calvin Cycle

  
• The light reactions convert light energy to chemical energy

  
• Electron excitation leads to the production of NADPH

  
• Chloroplasts are solar-powered chemical factories
• Their thylakoids transform light energy into the chemical energy of ATP and NADPH
• Light is a form of electromagnetic energy, also called electromagnetic radiation
• Like other electromagnetic energy, light travels in rhythmic waves
• Wavelength is the distance between crests of waves
• Wavelength determines the type of electromagnetic energy
• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
• Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see (380–750 nm)
• Light also behaves as though it consists of discrete particles, called photons
• Electromagnetic Spectrum

  
• Photosynthetic Pigments: The Light Receptors

  • Pigments are substances that absorb visible light
  • Different pigments absorb different wavelengths
  • Wavelengths that are not absorbed are reflected or transmitted
  • Leaves appear green because chlorophyll reflects and transmits green light
• A spectrophotometer measures a pigment’s ability to absorb various wavelengths