The light-dependent reactions in plants Flashcards
Where do light-dependent reactions occur?
-In the membranes
In Photosynthetic bacteria what is their photosynthetic membrane?
-The plasma membrane itself
Why is the thylakoid membrane important?
-Bc it contains structures that are involved in light-dependent reactions which is they are called Thylakoid reactions
What are the 4 stages of thylakoid reactions?
- Primary photoevent
- Charge separation
- Electron transport
- Chemiosmosis
What is the Primary Photoevent stage of Thylakoid reactions?
-This is where a photon of light is captured by a pigment & excites an electron within the pigment
What is the Charge Separation stage of Thylakoid reactions?
-The excitation energy from the electron is transferred to the reaction center by transferring an energetic electron to an acceptor molecule= initiates electron transport
What is the Electron Transport stage of Thylakoid reactions?
- This is where the excited electrons are shuttled along a series of electron carrier molecules embedded within the photosynthetic membrane
- Some of these molecules react by transporting membrane protons across the membrane= generates a proton gradient
- The electrons are then reduced to NADPH
What is the Chemiosmosis stage of Thylakoid reactions?
-The protons that accumulate on one side of the membrane now flow back across the membrane through ATP synthase= where chemiosmotic synthesis of ATP takes place
What are the 2 stages of the Light-dependant reactions?
- The first stage is the capturing energy from light which makes up stage 1-3 for the thylakoid reactions
- The 2nd stage is the generation of ATP which is the 4th stage of the thylakoid reactions
What is Cyclic Photophosphorylation?
- That a single photosystem in purple & green bacteria is used to generate ATP via electron transport & that process returns the electrons back to the reaction center
- These systems are Anoxygenic
What is the peak absorption for purple nonsulfur bacteria?
- It occurs at a wavelength of 870nm= not visible to the human eye
- which is why they use the pigment P870 in the reaction center
What happens when P870 reaction center absorbs a photon?
-The excited electron is passed to an electron transport chain that passed the electrons back to the reaction center= generates a proton gradient for ATP synthesis
What is the peak absoprtion for the green sulfur bacteria?
-It occurs at a wavelength of 840nm
What happens to the green sulfur photosystem?
- Excited electrons can either be passed to NADPH or returned to chlorophyll by an electron transporter chain (like the one in purple bacteria)
- Then use electrons from hydrogen sulfide to replaced those passed to NADPH
What do the proteins for the purple bacterial photosystem appear to be homologous with?
-The proteins are homologous to the proteins in the modern photosystem II
What do the proteins for the green bacterial photosystem appear to be homologous with?
-The proteins are homologus to the proteins in the modern photosystem I
What do the photosystem of purple & green bacteria have in common?
- They both don’t generate sufficient oxidizing power to oxidize H2O
- They are both anoxygenic & anaerobic
Why is having 2 linked photosystems better than having the cyclic photophosorylation?
- The linked systems aren’t as limiting like the cyclic one
- It provides an alternative source of electrons from the oxidation of water which also generates O2= oxygenic photosynthesis
- The transfer of electrons also produces NADPH= can be used in biosynthesis of carbs
What are the 2 photosystems for plants?
- Photosystem I
- Photosystem II
What is the absoption peak of Photosystem I?
- The peak is 700nm
- So its reaction center pigment is P700
What is Photosystem I?
- It can pass electrons to NADP+ which produces NADPH
- The electrons lost are replaced by electrons from Photosystem II
What is the absoprtion peak of Photosystem II?
- The peak is 680nm
- So its reaction center pigment is P680
What is Photosystem II?
-It can generate an oxidation potential high enough to oxidize water to replace the electrons transferred to Photosystem I
What happens when Photosystem I & II work together?
-They carry out a transfer of electrons that generate both ATP & NADPH
How is Photosystem I & II connected?
-They are connected by the electron carriers= Cytochrome/b6-f complex
What is the Cytochrome/b6-f complex?
- It connects photosystem I & II together
- This complex can use energy from the passage of electrons to move protons across the thylakoid membrane to generate the proton gradient used by ATP synthase enzyme
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