Chapter 10 Photosynthesis

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

What is the equation for photosynthesis 6CO2 + 6H2O  C6H12O6 + 6O2

The chloroplast Sites of photosynthesis Pigment: chlorophyll Draw and Label a chloroplast

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

Primary e- Acceptor Primary e- Acceptor H2O Photosystem I P700 Photosystem II P680

These photosystems produce equal amounts of ATP and NADPH which will be used in the Calvin cycle

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

Cyclic electron flow

Primary e- Acceptor Primary e- Acceptor H2O Photosystem I P700 Photosystem II P680

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

6) In cyclic electron flow what is the order of electron acceptors? Fd  cytochrome complex  Pc

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.

A review of photosynthesis