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Georgia Tech School of Biology
Photosynthesis
• Context: role of photosynthesis in energetics and
biogeochemistry.
• Light as energy
– EM spectrum
– Pigments and light capture
• Light reactions
• Carbon fixation reactions
– Calvin-Benson cycle
– Rubisco
– C3 vs C4 photosynthesis
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.02
Georgia Tech School of Biology
Energy Flow Through an Ecosystem
Biology 1510/1511 Fig. 55.3 in Purves et al. (2004) Life: The Science of Biology Fall 2008
Georgia Tech School of Biology
Atmospheric CO2
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 55.21
Georgia Tech School of Biology
Antarctic Ice Core Record
Climate Change 2001, Fig 2.2: Variations of T, CH4, and CO2 concentrations
derived from air trapped within ice cores from Antarctica.
Biology 1510/1511 http://www.grida.no/climate/ipcc_tar/wg1/fig2-22.htm Fall 2008
Georgia Tech School of Biology
Stoichiometry of Photosynthesis
• Photosynthesis:
• Reverse of respiration:
Biology 1510/1511 Fall 2008
Georgia Tech School of Biology
Photosynthesis: Tracking Atoms
• Overall reaction:
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.04
Georgia Tech School of Biology
Chloroplasts
Biology 1510/1511 Campbell & Reece Figs. 10.02 & 10.03 Fall 2008
Georgia Tech School of Biology
Photosynthesis Overview
Biology 1510/1511 Left: Campbell & Reece Fig. 10.05 Fall 2008
Georgia Tech School of Biology
Light is both particle and wave:
Energy of light varies with frequency: E = h
and wavelength: E = hc/
h = Planck’s constant, c = speed of light in a vacuum
Biology 1510/1511 Figure from Purves et al., 7th ed. Fall 2008
Georgia Tech School of Biology
Visible light is a portion of the EM spectrum
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.06
Georgia Tech School of Biology
Absorption and Action
Spectra
Biology 1510/1511 Campbell & Reece Figs. 10.08 & 10.09 Fall 2008
Georgia Tech School of Biology
Chlorophyll & PS
• Chlorophyll absorbs energy from photons or neighboring excited
chlorophyll molecules (Chl*).
• Chl* is a good reducing agent (electron donor).
Biology 1510/1511 Fall 2008
Campbell & Reece Figs. 10.10 & 10.12
Georgia Tech School of Biology
Noncyclic electron flow analogy
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.14
Georgia Tech School of Biology
Noncyclic photophosphorylation (Z-scheme)
Light + H2O + NADP+ + ADP + Pi NADPH + H+ + ATP + 1/2 O2
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.13
Georgia Tech School of Biology
PSII & PSI in the Thylakoid
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.17
Georgia Tech School of Biology
Chemiosmosis & ATP Synthesis
Biology 1510/1511 Campbell & Reece Fig. 10.16 Fall 2008
Georgia Tech School of Biology
Non-Cyclic Photophosphorylation Summary
Property PSII PSI
Reaction center: P680 P700
Gets e- from: H2O ETS, Pc
Gives e - to: Pq, ETS Fd
Generates: O2, ATP NADPH
Biology 1510/1511 Fall 2008
Georgia Tech School of Biology
A Stoichiometric Problem …
• C fixation by the Calvin Benson cycle requires ATP
and NADPH in a 3:2 ratio, but …
• Noncyclic photophosphorylation via the Z-scheme
produces ATP and NADPH at a ratio < 3:2
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.13
Georgia Tech School of Biology
Cyclic Photophosphorylation (PSI)
• Fd(red) can reduce NADP+, or PQ (redox chain)
• Cyclic photorespiration produces ATP but not NADPH
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.15
Georgia Tech School of Biology
Calvin and Benson
(1948)
14CO was injected into culture of Chlorella
2
(green alga). After ≥30 s, algal cells were
boiled in ethanol and extract analyzed by
chromatography and autoradiography.
Biology 1510/1511 Figures from Purves et al., 7th ed. Fall 2008
Georgia Tech School of Biology
Calvin & Benson Observations
• 3PG (3-phosphoglycerate: PGA) is the first
stable molecule to be labeled by 14CO2.
• 14C first appears in the carboxyl group of 3PG.
• Other carbon atoms of 3PG become labeled
with time => a cyclic process.
• No 2-carbon compound could be found as
acceptor for CO2
• Hypothesize that 3PG reduced to G3P
(glyceraldehyde-3-phosphate) using ATP
Biology 1510/1511 Fall 2008
Georgia Tech School of Biology
Ribulose bisphosphate (RuBP) is the
CO2 acceptor molecule
3-Phosphoglycerate
(3PG)
CO2 + RuBP [6-carbon intermediate] 2 x 3PG
Rubisco (RuBP carboxylase/oxygenase) may be the most abundant
enzyme on Earth!
Biology 1510/1511 Fall 2008
Georgia Tech School of Biology
Calvin-Benson Cycle
2 turns per glucose
(as drawn) This is a poorly designed figure!
A total of 6 C have to be fixed to make
one glucose. This means SIX passes
through the Calvin-Benson Cycle or 2
6 CO2 runs as shown (3 C per run).
one glucose molecule
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.18
Georgia Tech School of Biology
Calvin-Benson Cycle
2 turns per glucose
(as drawn)
ATP is also required for
regeneration of RuBP Equal quantities of ATP
and NADPH are required
for the reductions
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.18
Georgia Tech School of Biology
Energy Required to Make Glucose
• 18 ATP + 12 NADPH (from PSI & PSII)
– Each ATP is worth ~12 kcal/mol (in vivo)
– Each NADPH is worth ~50 kcal/mol
• Net: ~816 kcal/mol
• Theoretical maximum efficiency of
photosynthesis ≈ 25%
– Typical efficiency ≈ 3-6%
Biology 1510/1511 Fall 2008
Georgia Tech School of Biology
Photosynthesis Overview
Biology 1510/1511 Fall 2008
Georgia Tech School of Biology
Rubisco and Photorespiration
• Rubisco is the primary source of organic carbon
on Earth, but it’s a very inefficient enzyme.
– Requires high CO2 to work at high rates.
• Photorespiration: Rubisco can add O2 to RuBP
– Add O2 to RuBP to glycolate (2-carbon compound)
– O2 competes with CO2, especially at higher
temperature (>28 deg. C), low CO2, high O2
– Photorespiration undoes carbon fixation and doesn’t
produce ATP.
Biology 1510/1511 Fall 2008
Georgia Tech School of Biology
C4 Anatomy and Pathway
• PEP carboxylase fixes CO2 efficiently in
mesophyll cells.
• Malate is translocated to bundle sheath cells,
where it’s decarboxylated to pyruvate + CO2
and fed into the Calvin Cycle
• Pyruvate is phosphorylated by ATP to reform
PEP.
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.19
Georgia Tech School of Biology
C4 & CAM
• C4: Spatial separation
of C-fixation and
Calvin-Benson cycle.
• CAM: Temporal
separation of C-fixation
and Calvin-Benson
cycle..
Biology 1510/1511 Fall 2008
Campbell & Reece Fig. 10.20
