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 eacceptor
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