Download Ch. 10 2012

Chapter 10:
Photosynthesis
Photosynthesis in nature
• Autotrophs:
– Producers
– Make organic food from inorganics
• Photoautotrophs
• Chemoautotrophs
• Heterotrophs:
– Consumers
– Get organic food by eating organisms or their byproducts (decomposers)
The chloroplast
• Photosynthesis site
• Pigment: chlorophyll
• Plant cell: mesophyll
• Gas exchange: stomata
• Double membrane
• Thylakoids, grana, stroma
Photosynthesis: an overview
• Redox process
• H2O split (Photolysis)
• e- (w/ H+) given to CO2, reduces it to sugar
• 2 major steps:
– Light reactions (“photo”)
• NADP+ (electron acceptor) --> NADPH
• Photophosphorylation: ADP + P ---> ATP
– Calvin cycle (“synthesis”)
• Carbon fixation: carbon into organics
BioFlix: Photosynthesis
Photosystems
• Light harvesting units of thylakoid membrane
• Composed mainly of protein/antenna pigments
• Antenna pigment molecules struck by photons
• Energy passes to rxn centers
• Excited e- from chlorophyll is trapped by a primary eacceptor
• Used e- replaced by 1 from photolysis
Fig. 10-9
RESULTS
Chlorophyll a
Chlorophyll b
Carotenoids
(a) Absorption spectra
400
500
600
700
Wavelength of light (nm)
(b) Action spectrum
Aerobic bacteria
Filament
of alga
(c) Engelmann’s
experiment
400
500
600
700
Animation: Light and Pigments
Noncyclic electron flow
• Photosystem II (P680):
– 1st photosystem
– photons excite chlorophyll e- to an acceptor
– e- replaced by splitting H2O
• release of O2
– e-s fall down ETC to PSI
– as e- fall, ADP ---> ATP
• noncyclic photophosphorylation
Noncyclic electron flow cont.
• Photosystem I (P700):
– ‘fallen’ e- replace excited e- to primary eacceptor
– 2nd ETC transfers e- to NADP+ ---> NADPH
(...to Calvin cycle…)
• Equal amounts of ATP and NADPH produced
Cyclic electron flow
• Alternative cycle (due to low ATP)
• ONLY PS I used (not II)
• Produces ATP only
• Why?
– Calvin cycle uses more ATP than NADPH
• Cyclic photophosphorylation
The Calvin cycle
• 3 molecules CO2 are ‘fixed’ into 1 G3P
• Phases:
– Carbon fixation
• CO2 attached to each RuBP (rubisco enzyme)
– Reduction
• E-s from NADPH reduce 6C to G3P, ATP used
– Regeneration
• G3P rearranged to RuBP , ATP used, cycle cont.
Calvin Cycle, net synthesis
• For each G3P (and for 3 CO2)…….
– Consumption of 9 ATP’s & 6 NADPH
– light rxns regenerate these
• G3P used by plant to make glucose & other
organic compounds
A review of photosynthesis
You should now be able to:
1. Describe the structure of a chloroplast
2. Describe the relationship between an action
spectrum and an absorption spectrum
3. Trace the movement of electrons in linear electron
flow
4. Trace the movement of electrons in cyclic electron
flow
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
5. Describe the similarities and differences between
oxidative phosphorylation in mitochondria and
photophosphorylation in chloroplasts
6. Describe the role of ATP and NADPH in the Calvin
cycle
7. Describe the major consequences of
photorespiration
8. Describe two important photosynthetic adaptations
that minimize photorespiration
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings