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 II P680

Photosystem I P700

• 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 II P680

Photosystem I P700

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