PHOTOSYNTHESIS
CH 10
I. Overview
 Synthesis of glucose using sun’s
NRG
 Is anabolic (endergonic)
 Occurs in some bacteria, algae and
plants (in their leaves) = autotrophs
 Occurs in 2 stages:
o The light dependent reaction
(LDR) uses the energy from the
sun to make ATP and NADPH
o The Calvin cycle uses the ATP
and NADPH from the LDR to
make a precursor to glucose
 The chemical reaction is:
6CO2 + 6H2O + light → C6H12O6 + 6O2
II. The chloroplast
 structurally similar to
photosynthetic prokaryotes and
most likely evolved from them
 The LDR takes place in the
membranes of the thylakoids
 The Calvin cycle takes place in the
stroma
III. The Light Dependent Reaction
(LDR)
A. Role of light
 composed of different wavelengths
 also behaves like it has particle
called photons
 Why does a plant appear green?
 Chlorophylls absorb the photons of
energy from RO IV
 Chlorophylls found in plants, algae,
and cyanobacteria
 Cyanobacteria are ancestor of
chloroplasts
 Other photosynthesizing bacteria
have bacteriorhodopsin which
absorbs green wavelengths
B. The components of the LDR
1. Photosystem I and II
 They have:
 Antenna complex: contains
chlorophyll and absorbs photons
 Reaction center: uses energy from
photons to boost electrons.
2. Electron Transport Chain (ETC)
 series of proteins that transfer
electrons by redox reactions
3. Proton pump
 uses the energy in the electron to
pump protons (H+) across the
membrane.
4. NADP reductase
 uses the energy from the boosted
electron to make NADPH
5. ATP synthase
 uses energy from the proton
gradient to make ATP
C. How the light dependent reaction
works
 There are 2 reactions, noncyclic
phosphorylation and cyclic
phosphorylation
1. Noncyclic phosphorylation:
 Used to make BOTH ATP and
NADPH
 Photosystem I and II are used
 How:
o Photon from sun is captured by
chlorophyll in photosystem II and is
transferred to the reaction center
where e- is boosted
o The boosted e- is IMMEDIATELY
captured by the ETC
o The lost e- from the reaction center
is replaced by an e- from water
2H2O →4e- + O2 +4H+
o the energy from boosted e- is used
to actively transport H+ ions across
the thylakoid membrane forming
electrochemical gradient
o The H+ diffuse DOWN their
concentration gradient thru ATP
synthase using the stored energy in
the proton gradient to make ATP
o A photon from the sun is captured
by chlorophyll in photosystem I.
o This energy is transferred to the
reaction center where an e- is
boosted
o The e- is IMMEDIATELY
captured by the ETC
o The e- and its energy is transferred
to NADP reductase which then
makes NADPH
In essence, NRG transfer is:
eH+
ATP
Solar
NRG
eNADPH
http://faculty.nl.edu/jste/noncyclic_pho
tophosphorylation.htm
http://highered.mcgrawhill.com/sites/0072437316/student_view
0/
2. Cyclic phosphorylation:
 Only activated when leaf cell needs
more ATP
 Only Photosystem II is working.
Photosystem I is bypassed
IV. The Light Independent Reaction
A. The Calvin Cycle = C3
photosynthesis
 It uses ATP and NADPH from the
LDR as well as CO2 from the
atmosphere to make a precursor to
glucose (PGAL)
 It occurs in the stroma of the
chloroplast
 RUBISCO is the enzyme that fixes
CO2 to start the Calvin cycle
http://glencoe.mcgrawhill.com/sites/9834092339/student_view
0/chapter39/calvin_cycle.html
B. Photorespiration
 When Rubisco uses O2 instead
of CO2
 Causes calvin cycle to shut
 Can occur in C3 plants
 Factors that can cause
photorespiration
o High levels of O2 compared
to CO2 (O2 acts like
competitive inhibitor)
o High temps (lowers
RUBISCO’s specificity
C. Evolutionary pathways that avoid
photorespiration
1. C4 photosynthesis
 CO2 is fixed by another enzyme,
that can work with low CO2 levels
 C4 compound gives CO2 to
RUBISCO
2. CAM photosynthesis
 It is primarily the C4 pathway, but
plants keep their stomata open at
night when the atmospheric O2
levels decrease
http://www.uic.edu/classes/bios/bios10
0/lectures/c4.htm
V. How the LDR and Calvin cycle
interact