What is the difference between an autotroph and heterotroph?
• Autotrophs: biotic producers; obtains organic food without eating other organisms
• Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their by-products
Photosynthesis: an overview
• 2 major steps:• light reactions
-light energy converted to cell energy (e- from chlorophyll used to make ATP & NADPH) (e- from water used to replace)
• Dark reaction (Calvin Cycle)- organic compounds made from inorganic (sugar made from CO2)
Photosystems
• Light harvesting units of the thylakoid membrane
• Composed mainly of protein and pigment antenna complexes
• Antenna pigment molecules are struck by photons
• Energy is passed to reaction centers (redox location)
• Excited e- from chlorophyll is trapped by a primary e- acceptor
Noncyclic electron flow
• Photosystem II (P680) and Photosystem I (P700) used
• Photosystem II (P680):– photons excite
chlorophyll e- – e- are replaced by
splitting of H2O – e-’s travel to Photosystem
I down an ETC (Pq~cytochromes~Pc)
– as e- fall, ADP ---> ATP (noncyclic)
• Photosystem I (P700):– Fallen e- absorbed by
chlorophyll e- in PI
– Photons boost e- in PI
– 2nd ETC ( Fd~NADP+ reductase)
– e- help form NADPH
Cyclic electron flow – (P700 complex)
• Alternative cycle• e- boosted by the light energy pass down
the 1st ETC (Fd Pq cytochrome complex Pc) and H+ gradient that is produced ATP
• e- return to the P700 complex• Why? The Calvin cycle consumes more
ATP than NADPH
The Calvin cycle
• 3 Phases:
• 1) - Carbon fixation - 3 CO2 are added to the cycle
• 2- Reduction - NADPH and ATP are used
• 3- Regeneration – RuBP is regenerated
Seven easy steps
1) each CO2 is attached to RuBP (5C) and forms Rubisco
3CO2 + 3RuBP 3Rubisco
- This is unstable so each Rubisco quickly splits in half to make 6 PGA (3C)
• 2) A Phosphate is added to each of the 6 PGA to form 6 DPGA– 6 ATP are used
• 3) 6 DPGA are converted to 6 PGAL– 6 NADPH are used
• 4) 1 PGAL is released (2 will form a glucose)
• 5) 5 PGAL continue around the cycle
• 6) these molecules reorganize to form 3 RuP (5C) molecules
• 7) an ATP is added to each RuP to make RuBP – 3 ATP are used
• The cycle starts over
??? Calvin Cycle ???
• How many NADH were used?
• How many ATP were used?
• Since non-cyclic electron flow makes equal amounts of ATP and NADPH What process help compensate for the additional ATP that are needed?
?????? Light Reaction ??????
• 2) In Cyclic electron flow where do the electrons come from to replace those that were boosted? What form of energy is made?
• The original e- return the photosystem, ATP
• 3) In non-cyclic electron flow where do the electrons come from to replace those lost by PII? By PI?
• PII – H2O; PI – ETC from PII
• 4) In non-cyclic electron flow what is the order of the electron acceptors on the 1st ETC? 2nd ETC?
• 1st - Pq cytochrome Pc
2nd Fd NADP+ reductase
• 5) In non-cyclic electron flow what is the energy made by the electrons that flow down the first ETC? the 2nd ETC?
• 1st ETC – ATP; 2nd ETC – NADPH
The problem with photorespiration
• On hot dry days the stomata close to avoid dehydration
• A limited amount of CO2 and an increases amount of O2
• Rubiso prefers O2 • The • Two Solutions…..• 1- C4 plants: 2 photosynthetic
cells, bundle-sheath & mesophyll; PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll; new 4C molecule releases CO2 (grasses)
Alternative carbon fixation methods, II
• 2- CAM plants: open stomata during night, close during day (crassulacean acid metabolism); cacti, pineapples, etc.