chapter 5

• Sunlight powers photosynthesis
• Cyanobacteria carry out photosynthesis, mainly in the water. Eukaryotes such as algae and plants carry out photosynthesis in the water and on land. No Archaea are known to carry out photosynthesis at all
• Photosynthesis produces oxygen and sugar
• Photosynthetic organisms


  • Cyanobacteria, algae, plants
• Photons are packets of light energy captured by plants and other photosynthesizers
• The sun emits energy in waves. Shorter wavelengths have higher energy than longer wavelengths
• Gas exchange occurs at leaf pores called stomata
• Plant pigments do not absorb green light. Green light is reflected, which is why leaves appear green
• Chlorophyll a


  Main photosynthetic pigment in plants
• Ingredients needed for photosynthesis

  • Sunlight
  • Carbon dioxide
  • Water
• Life depends on photosynthesis
• Pigments

  Molecules that capture energy from light
• Being able to carry out photosynthesis

  Allows organisms to make their own food
• Pigments of Photosynthesis

  • Chlorophyll a (Blue-green) in Plants, algae, cyanobacteria
  • Chlorophyll b (Yellow-green) in Plants, green algae
  • Carotenoids (carotenes and xanthophylls) (Red, orange, yellow) in Plants, algae, bacteria
• Photosynthesis

  Converting light (kinetic) energy into chemical (stored) energy
• Visible light is a fraction of the sun’s energy. Only some wavelengths are visible to us, perceived as colors
• Autotrophs (producers) carry out photosynthesis
• Why are leaves green?

  Plant pigments absorb almost every wavelength except for green
• Chloroplasts are the organelles of photosynthesis
• PSI produces NADPH

  1. Electrons arrive at the second photosystem (PSI)

  2. When light hits PSI, chlorophyll transfers light energy to the electrons

  3. Electrons move to a 2nd electron transport chain

  4. Electrons reduce to NADPH
• How do the light reactions work?

  1. Light strikes the first photosystem (PSII)

  2. Chlorophyll absorbs the light energy and transfers it to electrons, which come from water molecules

  3. Electrons move to an electron transport chain
• Plant pigments reflect green light
• Photosynthesis occurs in leaf mesophyll cells
• Photosynthesis

  1. Photosynthesis is a series of chemical reactions

  2. Overall, the reactants are light, water, and carbon dioxide

  3. Overall, the products are oxygen and sugar

  4. Photosynthesis occurs in two stages: Light reactions where light energy is captured and converted to chemical energy, and Carbon reactions where energy is used to produce sugar
• Light reactions

  1. The light reactions occur in the thylakoids and require water and light

  2. ATP and NADPH are produced to carry stored energy

  3. Oxygen gas (O2) is formed as a byproduct
• PSII produces ATP

  1. Electrons from PSII move down an electron transport chain

  2. H+ is pumped into the thylakoid

  3. H+ leaves through ATP synthase

  4. ATP is produced by ATP synthetase through chemiosmotic phosphorylation
• PSI produces NADPH


  Electrons arrive at the second photosystem (PSI). When light hits PSI, chlorophyll transfers light energy to the electrons. Electrons move to a 2nd electron transport chain. Electrons reduce to NADPH
• Plants, C3 plants, and CAM plants each do photosynthesis, using different pathways for fixing carbon. Each has its own advantages and disadvantages, depending on the environment the plant is in
• ATP and NADPH power the carbon reactions


  ATP and NADPH produced during light reactions carry the stored chemical energy derived from sunlight. In the carbon reactions this energy is used to break up molecules of CO2 and build molecules of sugar from it
• C3 plants do well in cool, moist weather and CAM plants are adapted to hot, dry weather
• Photorespiration


  Occurs due to oxygen buildup inside the leaf, decreasing photosynthesis
• Light capture occurs in pigment molecules in the thylakoid membranes
• Light reactions

  1. Light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to synthesize ATP and NADPH

  2. Light absorbed by chlorophyll or other pigments drives a transfer of electrons and hydrogen from water to an acceptor called NADP+, reducing it to NADPH by adding a pair of electrons and a single proton (hydrogen nucleus)

  3. Water is split in the process, producing waste oxygen
• Carbon fixation pathways as plant adaptations

  The Calvin cycle occurs in mesophyll cells during the daytime, leading to RuBP, then PGA (3 carbons) to produce sugar. In bundle sheath cells during the daytime, the Calvin cycle occurs, leading to Oxaloacetate (4 carbons) to produce sugar. During both day and night, CO2 leads to a 4-carbon molecule that leads to CO2 to produce sugar
• Carbon reactions requiring Rubisco

  The cycle begins with carbon fixation where carbon dioxide is added to RuBP, creating an unstable molecule. Then, PGA synthesis occurs where the unstable intermediate splits to form PGA. The PGA molecules are combined to form glucose, which is used to form starch, sucrose, and other organic molecules. The regeneration of RuBP follows by rearranging the remaining molecules. Rubisco enzyme adds CO2 onto a molecule of RuBP during carbon fixation, producing unstable 6-carbon organic molecules. Small organic molecules are formed during PGA synthesis. ATP and NADPH provide the chemical energy needed to keep the cycle running. ATP and NADPH are built up during the light-dependent reaction
• Carbon reactions in photosynthesis

  In chloroplast, there are two reactions: light reactions and carbon reactions. In the light reaction, water and oxygen are involved, and NADPH is transformed into NADP plus. In the carbon reaction, carbon dioxide is used to produce sugars, and ATP is transformed into ADP. The process of light reaction is highlighted