Chapter 8: Photosynthesis

  • Photosynthesis  

    • Process in which carbon and energy become carbohydrates 

    • 60% of visible light from sun energy is unavailable for photosynthesis

    • 8% transmitted or reflect 

    • 20% lost to photorespiration

    • 4% yield into carbohydrates 

    • O2 is lost

    • 8% is converted to heat 

    • Occurs in autotrophs 

  • Equation

    • CO2 + H20 → C6H12O6 + O2

    • CO2 → C6H12O6 (reduction) 

    • H2O → O2 (oxidation) 

  • In photosynthesis, H2O is the electron donor, and NAP+ is the final electron acceptor it is then reduced to NADPH and ATP combines with NADPH to form carbohydrates from Co2 

  • Calvin cycle 

    • H20 will donate electrons to the photosynthetic electron transport chain and O2 will be released as a by-product 

    • After NADPH and ATP have been reduced they go into the Calvin cycle 

    • Calvin cycle is a light-independent reaction (no sunlight required)

    • CO2 will be reduced to form higher-order carbohydrates 

  • Chloroplasts 

    • Outer membrane 

    • Inner membrane

    • Thylakoid 

      • Light-dependent reactions occur

    • Grana 

      • Stacks of thylakoid 

    • Lumen 

      • Inside the fluid compartment of thylakoid

    • Stroma 

      • The outside part that surrounds the thylakoid 

Calvin's fixation takes place

  •  Chlorophyll

    • A major component of photosynthesis 

    • Leaves appear green because chlorophyll reflects green wavelength and is poor at absorbing it  

    • Not free to move in chloroplast 

    • Positioned in the integral membrane proteins 

    • Chlorophyll a

      • Methyl group (CH3)

      • Takes part in photosynthesis 

    • Chlorophyll b

      • Aldehyde group (COH)

      •  Does not take part in photosynthesis 

    • Hydocrabon tail which associates it self with hydrophobic regions in the mesophyll layer and anchors it self

    • Prophyin ring

      • Chlorophyll contains a MG senter which attracst suns rays and hots c-c single bonds to bexicted to C=C double bonds. 

      • Contains alternating C-C and C=C bonds. 

  • Calvin cycle 

    • 3-step process that uses CO2 to synthesize carbohydrates

  1. Carboxylation:  co2 is added to a 5-carbon compound, RUBP is catalyzed by an enzyme called Rubisco 

  2. Reduction: the energy from ATP and NADPH are used to form a compound called triose phosphate 

  3. Regeneration of RUBP: 3-carbon compound rearranges and combines to form RUBP

  • 1 turn of calvin cycle = 1 carbon 

  • Cycle must turn 6 times for it to produce glucose 


  • Accessory pigments 

    • Yellow and orange carotenoids that absorb wavelengths that chlorophyll does not absorb 

    • Procetc the photosynthetic electron chain from damage

  • Photoexcitaion 

    • Photon strikes the chlorophyll molecule and the electron is a move to a higher potential energy level (excited state) from low potential energy level (ground state)

  • Fluorescence 

    • Photon strikes a chlorophyll molecule and the electron becomes excited but electrons don't like to state excited because at that state they are unstable so they come back down to ground state while releasing heat and light 

  • Lab + chlorophyll molecule 

    • Photon strikes the chlorophyll molecule and the electron become excited, the electron does not like to stay excited so it returns to ground states releasing heat and light (fluorescence)

  • In plants 

    • Photon strikes the chlorophyll molecule, electron becomes excited and the energy can be transferred to adjacent molecule 

  • Antenna complexes 

    • Absorb light energy and transfer energy from on pigment to another until it reaches the reaction center chlorophyll a 

  • Reaction center 

    • Reaction center becomes reduced as it accepts the electron 

    • Electron acceptor will take teh electron and oxide the reaction center and become reduced. 

    • The electrons then drive light-dependent reactions to lead to the formation of NADPH

    • After the reaction center gives the electron to the election acceptor it needs an electron donor to donate another electron, water acts as an electron donor to replenish the reaction center with electrons. 

  • Photosyetm II 

    • Takes electrons from an electron donor (H2O) and passes them to photosystem I

  • Photosystem I

    • Uses the electrons and the energy to make NADPH

    • Energy increases as electrons are passed from photosystem II to NADPH

    • Energy decreases as electrons are passed between photosystem II and photosystem I

  • Photosynthetic electron transport chain 

    • Photon strikes Photosystem II → (H20 added and H+ released)  primary electron acceptor → plastoquinone → Cytochrome b6-f complex (h+ released into lumen) → plastocyanin → photosystem I → primary electron acceptor → Ferrodoxin → NADP+ reductase (NADP+ → NADPH)

    • Accumulation of protons in the lume drives the synthesis of ATP 

  • Cyclic electron flow 

    • Produces ATP and H+

    • Production of H+ in lumen increases atp synthase production 

    • Does not produced NAPH

    • Photosyetsm I → primary electron acceptor → Ferrodixoin → plastaquinon → cytochrome b6-f complex → plastocyanin → photosuystem I

  • Challange 1 of photosynthetic ETC 

    • Excessive light energy → so much NADPH is produced than NADP+ and therefore NADPH passes its electrons to an O2. when it passes it to an O2 (electron acceptor), reactive oxygen species are formed (ROS) 

    • ROS → damages cells and important structures 

    • Rescue: 

      • Antozidants → neutralize ROS

      • Xanthoypulls → slow down the production of ROS

  • Challenge 2: photorespiration 

    • Rubsico can catalyze o2 and co2, when o2 is catalyzed then it oxygenases, 3-phosphoglycerate and 2-phosphoglycolate. However, 2-phosphoglycerate cannot process in the Calvin cycle therefore it has to be converted back into 3-phosphoglycerate. As a result, ATP is added and oxidized to ADP and CO2 gets released 

    • Loss in CO2 and Loss in energy  

  • Evolution of photosynthesis 

    • First organisms only contained 1 photosystem, therefore, they used other oxidized compounds to gain electrons

    • Did not use water, therefore, no oxygen produced 

  • Step 1: the creation of 2 photosystems 

    • Cyanobacteria 

      • First organisms to have 2 photosystems

      • They used water to gain electrons therefore released oxygen

    • Why did cyanobacteria have 2 photosystems and other organisms only had 1?

      • Hypothesis #1: horizontal gene transfer 

        • Photosyewtm 2 from 1 cell transferred its genes to another cell that potentially contained photosystem 1 and results in photosystem 2 and 1 in the same cell 

      • Hypothesis 2: gene duplication and divergence 

        • Photosystem 2 and duplicate it which results in two photosystems 2 and then through mutations and genetic divergence photosystem 1 and photosystem 2 happened. 

  • Step #2: Endosymbios 

    • Overtime, cyanobacterium lost the ability to survive outside the host cell and evolved to be inserted inside the cytoplasm

    • One cell takes up the residence in another cell 

  • Conjunction 

    • Pila connects the donor and the recpietnt otgethe and pulls them together, the dna is then transferred throuhg a small opening  

  • Transformation 

    • The dead donor cell releases dna into the envirment and the reciepntet takes it in

  • Transduction 

    • Viruses are on the donor cell and thent he virsues transfer the dna by attaching onto the recipient and injecting the dna inside