Photosynthesis

• Autotrophs


  Organisms that obtain energy from non-living sources
• Types of autotrophs


  • Photoautotrophs
  • Chemoautotrophs
• Photoautotrophs


  • Organisms (like plants) that depend on photosynthesis for both energy and carbon compounds
  • Photosynthesis uses solar energy to convert carbon dioxide (CO2) into sugar (glucose C6H12O6)
• Products of photosynthesis


  • Cotton & other fibers
  • Wood
  • Grains, Veggies, & Fruit
  • Animal food – Meat, & dairy
  • Fossil fuels (oil & coal) formed millions of years ago from dead plants and animals
• Chemoautotrophs


  • Organisms that obtain energy by oxidizing inorganic substances (iron, sulfur, & other minerals) and use this energy to form sugars from carbon dioxide
  • All bacteria
  • Found in extreme environments – deep under ground & deep ocean, hot springs, volcanoes, etc.
  • Will take the place of plants as the basis of a food chain in environments in which plants can't exist
• Nutritional classification of organisms

  • Photoautotrophs (plants, algae, some bacteria)
  • Photoheterotrophs (some bacteria)
  • Chemoautotrophs (some bacteria)
  • Chemoheterotrophs (animals, fungi,some bacteria and other one-celled organisms)
• Light


  Vibrating electric & magnetic field
• Shorter waves of light have more energy, longer waves have less energy
• Visible light


  A particular range of wavelengths that causes reversible changes in molecules, allowing us to see
• Sunlight is a mixture of different wavelengths, many of which are visible to our eyes and make up the visible spectrum
• Pigments


  Light-absorbing particles
• Chlorophyll


  • The most important pigment in plants
  • Absorbs blue-violet and red light best
  • Reflects green light, which is why leaves are green
• Chloroplasts


  • Structures in plant cells that contain the pigments involved in photosynthesis
  • Site of photosynthesis
  • Contain thylakoids, grana, stroma
• Thylakoids


  • Membranes that form sacs, in which photosynthetic pigments are embedded (found in a chloroplast)
  • In bacterial cells, the thylakoids are found floating in the cytoplasm
• Grana


  The stacks of thylakoids that are formed
• Stroma


  • The fluid filled space inside the chloroplast surrounding the thylakoids
  • Enzymes are found in the stroma that catalyze the formation of sugar from carbon dioxide and water
  • Contains the chloroplast's DNA & RNA
• Chlorophyll a & b

  • The main pigments involved in photosynthesis
  • Absorb light in the blue/violet range and the red/orange range
  • Green light is not absorbed, it's reflected, that's why leaves are green
• Three major events of photosynthesis

  • Absorption of light energy
  • Conversion of light energy into chemical energy
  • Storage of chemical energy in the form of sugars
• Two groups of photosynthesis reactions

  • Light dependent reactions ("photo") – when thylakoids absorb light & convert it into chemical energy carried by short lived, energy-rich molecules
  • Calvin Cycle (Light-independent reactions) ("synthesis") – the energy of molecules produced in the thylakoids is used to produce 3-carbon sugars from carbon dioxide
• Calvin Cycle


  1. 3 CO2 + 3 H2O → C3H6O3 + 3 O2

  2. Raw materials – carbon dioxide & water

  3. CO2 = reduced

  4. H2O = oxidized

  5. Products – 3-carbon sugars (not glucose) and oxygen gas

  6. The two groups of reactions work together – products of light reactions = the raw materials of Calvin Cycle
• Photosynthesis


  The process by which plants and other organisms use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar
• The Light Reactions


  1. Conversion of visible light into chemical energy needed to produce sugars in the Calvin cycle

  2. Chlorophyll in the thylakoid absorb the light, water is split into hydrogen and oxygen, & light energy is converted into chemical energy
• The Calvin Cycle (Light Independent Reactions)


  1. Saves the chemical energy produced in the light reactions in the form of sugars

  2. Occurs in the stroma of the chloroplasts

  3. No light is needed

  4. Reactants: ATP and NADPH from light dependent reaction provides energy to CO2 absorbed from the atmosphere
• Photosystems


  • Light absorbing pigments form 1 of 2 kinds of clusters: Photosystem (PS II) and (PS I)
  • The PS absorbs light energy & this energy gets transferred to different molecules until it ends up at a reaction center
  • Reaction center – a specific chlorophyll a molecule
• Electron Transport System (ETS)


  1. Electrons from PS II replace electrons lost from PS I

  2. PS II receives replacements from an enzyme near its reaction center that splits water into protons, electrons, & oxygen
• Water is oxidized


  Oxygen is released as a gas & protons build up in the thylakoid
• NADPH formation


  1. When e- from H2O reach PS I, they receive an energy boost from the reaction center & this energy is used to reduce NADP+ into NADPH

  2. The protons & electrons of NADPH is then used later to reduce CO2 in the Calvin Cycle
• ATP synthesis


  1. The protons diffuse/move out of the thylakoid through an enzyme complex (ATP synthase) & as they pass through, they transfer energy to the ATP synthase

  2. ATP synthase uses the energy to synthesize ATP from ADP & a phosphate
• Carbon Fixation


  1. CO2 combines with a 5 carbon phosphate-sugar, called ribulose biphosphate (RuBP)

  2. This produces an unstable 6 carbon molecule, which instantly breaks down into two 3 carbon molecules called phosphoglyceric acid (PGA)
• PGAL formation


  1. Each molecule of PGA is reduced to the 3 carbon sugar-phosphate molecule, phosphoglyceraldehyde (PGAL)

  2. This requires 1 ATP and 1 NADPH (from the light reactions)
• RuBP regeneration


  1. A series of enzymes catalyzes the combination and rearrangement of the PGAL, producing a 5 carbon sugar-phosphate, ribulose 5-phosphate

  2. An ATP molecule is used to add a 2nd phosphate group to the Ribulose 5-phosphate, producing a molecule of the starting product, RuBP (completing the cycle)
• Three turns of the Calvin Cycle (which uses 3 CO2 molecules) results in 6 PGAL molecules
• 5 of those PGAL are used to regenerate RuBP & the last one is available to the organism to use for maintenance & growth
• The PGAL is removed from the Calvin Cycle to synthesize other compounds like complex carbohydrates or amino acids
• Examined the effect of varying colors of light, light intensity, and amount of carbon dioxide on the rate of photosynthesis
• Measured the number of bubbles (oxygen) after 30 seconds
• Factors Affecting Rate of Photosynthesis

  • Suitable temperature
  • Presence of chlorophyll
  • Light intensity
  • CO2
  • Water
• Photosynthesis


  1. Suitable temperature

  2. Presence of chlorophyll

  3. Light intensity

  4. CO2

  5. Water
• Factors that affect photosynthesis

  • Light intensity
  • Level of CO2
  • Temperature
• As the light intensity increases

  The rate of photosynthesis increases, and then levels off