LEC6: PHOTOSYNTHESIS

• • PHOTOAUTOTROPHS organisms that use light energy to drive the synthesis of organic molecules from carbon dioxide and water
• PHOTOSYNTHESIS

  CHLOROPLAST

  • Site of photosynthesis
  • CO2 enters & O2 exits the leaf (stomata)
  • Leaves → mesophyll tissue
  • Mesophyll cell ～30-40 chloroplasts
  • Green color - chlorophyll (thylakoid membrane, grana)
  • Chloroplasts contain stroma (dense fluid)
  • Chlorophyll absorbs light energy → synthesis of organic molecules in the chloroplast
• Photosynthesis can be summarized as the following:

  6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
• THE SPLITTING OF WATER

  Chloroplasts split H2O into hydrogen & oxygen, where hydrogen is incorporated into sugar molecules while oxygen is the one given off by the plants.
• 2 Stages of Photosynthesis: A Preview

  1. Photo part – Light Reactions

  •  in the thylakoids
  • light absorption, split water, release O2, produce ATP, & form NADPH

  1. Synthesis part – Calvin Cycle

  • in the stroma - forms sugar from CO2, using ATP & NADPH
  • begins with carbon fixation, incorporating CO2 into organic molecules
• The light reactions convert solar energy to the chemical energy of ATP & NADPH
• Chloroplasts – solar powered chemical factories

  Thylakoids transform light energy into the chemical energy of ATP & NADPH
• 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
• 3 Types of Pigments in Chloroplast

  Chlorophyll a

  Chlorophyll b

  Xanthophylls, carotenoids
• Chlorophyll a

  Main photosynthetic pigment
• Chlorophyll b

  Accessory

  Broaden the spectrum used for photosynthesis
• Xanthophylls, carotenoids

  Accessory (photoprotection)

  Absorb excessive light that would damage chlorophyll
• Excitation of chlorophyll by light 

  • When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
  • Some pigments like chlorophyll emit light and heat after absorbing photons
• A Photosystem: 

  A reaction center associated with light-harvesting complexes

  • photosystem consists of a reaction center surrounded by light-harvesting complexes (LHC) 
  • light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center
• 2 types of photosystem in thylakoid membrane

  Photosystem II

  • functions first* (order of discovery);
  • Chl a absorbs wavelength of 680 nm

  Photosystem I

  • Chl a absorbs wavelength of 700 nm

  The two photosystems work together to use light energy to generate ATP and NADPH
• Photochemical Reactions

  • During the light reactions, there are two possible routes for electron flow: 
  • CYCLIC
  • NON-CYCLIC or LINEAR
• NON CYCLIC or LINEAR ELECTRON FLOW (PHOTOPHOSPHORYLATION)

  • process of making ATP from ADP & Pi using energy derived from light (photon)
  • primary pathway (both photosystems & produces ATP & NADPH)
• When a photon of light strikes a pigment molecule in the Light Harvesting Complex (LHC), it is relayed to other pigment molecules until it reaches one of the 2 P680 chlorophyll a molecules in the Photosystem II (PSII) reaction center
• The photon excites one of the P680 electrons of chlorophyll a to a higher energy state, which is then captured by the primary electron acceptor
• Photolysis is the splitting of H2O into 2H & O atom, supplying electrons one by one to P680, each replacing an electron lost to the primary electron acceptor; the O atom combines with another O atom, forming O2
• In the Electron Transport Chain (ETC), photoexcited electrons pass from the 1st electron acceptor of PS II to PS I via ETC components like Plastoquinone (Pq), cytochrome complex, and plastocyanin (Pc)
• Chemiosmosis involves the exergonic “fall” of electrons to a lower energy level, providing energy for ATP synthesis
• Transfer of light energy via LHC to PS I excites an electron to one of the 2 P700 chlorophyll a molecules; the photoexcited electron is captured by PS I’s primary electron acceptor, creating an electron “hole” in P700, which is filled by an electron reaching the bottom of the ETC from PS II
• In the 2nd ETC, photoexcited electrons from PS I’s primary electron acceptor pass down a second ETC through the protein ferredoxin (Fd)
• Electrons are transferred from Fd to NADP+ by NADP+ reductase, with 2 electrons required for the reduction to NADPH
• CYCLIC ELECTRON FLOW:

  2nd Photophosphorylation Sequence

  • Cyclic electron flow uses only PS I and produces only ATP
  • No NADPH is produced
  • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle
• 3 Phases: CALVIN CYCLE (C3 Pathway)

  1. Carbon fixation (catalyzed by rubisco)

  2. Reduction

  3. Regeneration of the CO2

  The function of the pathway is to produce a single molecule of glucose
• 1. CARBON FIXATION

  • involves carboxylation: CO2 combines with RuBP to produce PGA
  • Ribulose 5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the merging of CO2 & RuBP
  • **(1 turn - 3 CO2 combine with 3 RuBP to produce 6 PGA)

  2. REDUCTION

  • ATP & NADPH are incorporated into G3P, making it very energy-rich
  • ADP, Pi, NADP+ are released & re-energized in noncyclic photophosphorylation
  • **(6 ATP & 6 NADPH are used to convert 6 PGA to 6 G3P)

  3. REGENERATION

  • regenerating the 3 RuBP originally used to combine
  • with 3 CO2
  • allows the cycle to repeat
  • **(3 ATPs are used to convert 5 G3P to 3 RuBP)
• Calvin cycle (like citric acid cycle), regenerates its starting material after molecules enter and leave the cycle

  2 cycles to produce glucose
• Summary of Calvin Cycle

  The cycle takes CO2 from the atmosphere & the energy

  in ATP & NADPH to create 1 glucose molecule (2 TURNS)

  6 CO2 + 18 ATP + 12 NADPH → 1 glucose + 18 ADP + 18 Pi + 12 NADP+ + 12 H+
• Alternative mechanisms of carbon fixation have evolved in hot, arid climates

  C4 Plants

  (sugarcane, corn, grass family, other 19 plant families)

  • special “add-on” feature to C3 pathway
  • minimize the cost of photorespiration
  • mesophyll cells - incorporates CO2 into 4-carbon compounds (before Calvin Cycle)
  • 4C compounds are exported to bundle-sheath cells, they release CO2 that is then used in the Calvin cycle
  • CO2 concentration is maintained in the bundle sheath, favoring photosynthesis over respiration
• CAM (Crassulacean Acid Metabolism) Plants

  • another special “add-on” feature to C3 pathway
  • NIGHT stomata - open & incorporates CO2 into organic acids

  DAY stomata - close & CO2 is released from organic acids & used in the Calvin cycle succulent plants, many cacti, pineapple