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Chapter 15 - Photosynthesis
• Photosynthesis: a process that converts atmospheric CO2
and H2O to carbohydrates
•Light reactions:
Solar energy is converted into chemical energy
ATP and NADPH
•Dark reactions
ATP and NADPH are used to convert CO2 to hexose
phosphates
Both processes can occur simultaneously
Modern euk absolutely depend on O2 by photo (and as food source)
Chloroplasts: specialized organelles in algae and plants
where photosynthesis occurs
Thylakoid membrane: highly folded continuous membrane network, site of the
light-dependent reactions that produce NADPH and ATP
Lumen: aqueous space within the thylakoid membrane
Stroma: aqueous matrix of the chloroplast which surrounds the thylakoid
membrane
Pump protons into lumen--produce ATP/NADPH in stroma for biosyn (dark rxns)
Photosynthesis in purple bacteria
(a PSII system).
Photo system complex in membrane
chlorophylls and accessory pigments
Absorb light energy and pump H+ produce ATP/ NADPH
Chlorophyll and bacteriochlorophyll
Absorb in red range (reflect green)
Absorb light: excite/transfer e-
• Chlorophylls - usually most abundant and most important pigments
in light harvesting
• Contain tetrapyrrole ring (chlorin) similar to heme, but contains
Mg2+
• Chlorophylls a (Chl a) and b (Chl b) in plants
• Bacteriochlorophylls a (BChla) and b (BChlb) are major pigments
in bacteria
Absorption maxima depends on
structure and micro-environment
within the pigment protein complex
Photo system I in bacteria
Light gathering part of complex (proteins, pigments and cofactors)
96 chlorophylls and 22 accessory pigments (carotenoids)
One special pair (P700)
Reaction centers of the photosystems
• PSI and PSII each contain a reaction center
(site of the photochemical reaction)
• Special pair: two chlorophylls in each reaction
center that are energized by light (2 e- lost)
•In PSI special pair is: P700
(absorb light maximally at 700nm)
•In PSII the special pair is: P680
(absorb light maximally at 680nm)
(Notice PSII in purple bacteria it is P870)
Remaining chlo act as: antenna molecules
A reaction center
Capture light energy and transfer to special pair
Resonance energy transfer
Special pair: only two chlo mol that give up e- to begin e- transfer chain
Different states of Chlorophyll (special pair)
Special pair--identified as pigments that absorb at specific wavelength
Two special chlo mol that actually give up ee- from low-energy level
promoted to higher-energy
molecular orbital
reduced
Excited state
oxidized
Photo system I in bacteria
96 chlorophylls and 22 accessory pigments (carotenoids)
One special pair (P700)
Accessory pigments
Absorb light/transfer energy to
adjacent chl
Conjugated double bonds
allow light absorption
Absorb in blue: appear red/yellow
Fall colors
Broaden the range of system
Stabilize adjacent chl
Prevent loss of e-
Evolution of photosystems
Bacteria: simple system
Photosystem II: purple bacteria
Photosystem I: green sulfur bacteria
Cyanobacteria: more complex (coupled II/I)
Most abundant class of photosyn bacteria
Plants and algae: coupled II/I
PSII
Photosynthesis in purple bacteria
(a PSII system).
(Antenna pigment mol not shown)
Light energy excites P870
release of ee- transferred one at a time
only down the right branch
Tightly bound quinone
transfer via Fe mol to
Loosely bound Q on left
Diffuse in membrane
Two photons of light:
2 e- transferred
2H+ from cyto to QH2
Photosynthesis in purple bacteria
(a PSII system).
2 photons:
4 H+ released
Generate ATP
Cyclic process
No outside source
of e- needed
Photosynthesis in green sulfur bacteria
(PS I)
Photosynthesis in
green sulfur bacteria
(PS I)
Pigment centers are chlorophylls
Both branches are active
Three Fe-S clusters
Terminal e- acceptor:
ferredoxin
Primarily
Leads to NADPH formation
Non cyclic e- transfer process
Photosynthesis in green sulfur bacteria
(PS I)
Usually non-cyclic
NADPH formation
Cyto c reduced by
sulfer compounds
Allows reduction of
P700+
cyclic
If need ATP
Can pass e- to
quinone
Evolution of photosystems
Bacteria: simple system
Photosystem II: purple bacteria
Photosystem I: green sulfur bacteria
Cyanobacteria: more complex (coupled)
Most abundant class of photosyn bacteria
Generated O2
environment
poison
Plants and algae: coupled II/I
Photosynthesis in cyanobacteria (PS I - PSII)
Allow light energy to form ATP and NADPH
Mobile quinone: plastoquinone (PQ)
Transfer e- to Cbf complex
Terminal e- acceptor (plastocyanin)
passes on to PSI
feulle = leaf (french)
Oxygen evolving complex splits water
And reduces P680+
(Most important biochem event
in history of life)
Cyclic electron flow pathway
H+
(Not to NADP+ but back to the PQ pool via a specialized cytochrome.)
• For each CO2 reduced to (CH2O) in carbohydrate synthesis (dark
cycle), 2 NADPH and 3 ATP are required
• Cyclic electron transport yields ATP but not NADPH, thus
balancing the need for 3 ATP for every 2 NADPH,
(Cyclic flow increases the protonmotive force without
NADPH synthesis.)
The Z-scheme
• Z-scheme: path of electron flow and reduction potentials of the
components in photosynthesis
• Absorption of light energy converts P680 and P700 (poor reducing
agents) to excited molecules (good reducing agents)
• Light energy drives the electron flow uphill
• NADP+ is ultimately reduced to NADPH
Cyanobacteria
internal structure
Chloroplasts
Evolved into
Thylakoid membrane: site of the light-dependent reactions
Lumen: aqueous space within the thylakoid membrane
Stroma: aqueous matrix of the chloroplast which surrounds the thylakoid
membrane
Pump protons into lumen (from stroma)--
produce ATP/NADPH in stroma for biosyn
photosynthesis plant membrane systems
Interior
Inner/
Outer memb
• Light is captured by antenna complexes
• Light energy drives the transport of electrons from PSII through
cytochrome bf complex to PSI and ferridoxin and then to NADPH
• The proton gradient generated is used to drive ATP production
• For 2 H2O oxidized to O2, 2 NADP+ are reduced to 2 NADPH
The Dark Reactions
• Reductive conversion of CO2 into carbohydrates
• Process is powered by ATP and NADPH (formed
during the light reactions of photosynthesis)
Occurs in light (inhibited in dark): need ATP/NADPH
Produces Starch (provide energy during the night)
and sucrose (mobile form of carbo)
The CO2 fixation pathway has several names:
The Calvin cycle.
CO2 enters the plant through pores on the leaf
surface called stomata,
The Calvin Cycle
3 stages:
(1) Carboxylation :catalyzed by Rubisco
(2) Reduction: 3-phosphoglycerate converted to glyceraldehyde 3phosphate (G3P)
(3) Regeneration: most of the G3P is
converted to ribulose 1,5-bisphosphate
3CO2 + 9ATP + 6NADPH + 5H2O
9ADP + 8 Pi + 6NADP+ + G3P
ribulose 1,5-bisphosphate
Glc
main product is G3P
Glc
3CO2 + 9ATP + 6NADPH + 5H2O
9ADP + 8 Pi + 6NADP+ + G3P
ribulose 1,5-bisphosphate
3CO2 needed before one C3 unit (G3P) can be removed without
diminishing metabolic pools
1
2
The Calvin Cycle.
3
Pentose phosphate
pathway
The Calvin Cycle
Sucrose
Starch
Gluconeogenesis
Carboxylation
3ADP
3ATP
Reduction
Pentose Phosphate
Regeneration Cycle
Carboxylation: Ribulose 1,5-Bisphosphate
Carboxylase-Oxygenase (Rubisco)
• Gaseous CO2 and the 5-carbon sugar ribulose
1,5-bisphosphate form two molecules of
3-phosphoglycerate
• Reaction is metabolically irreversible
• Rubisco makes up about 50% of the soluble
protein in plant leaves, and is one of the most
abundant enzymes in nature
Not very efficient so a lot needed
The Rubisco Reaction
Regulation of the Rubisco Reaction
• Rubisco cycles between an active form (in the
light) and an inactive form (in the dark)
• Activation requires light, CO2, Mg2+ and
correct stromal pH (H+ gradient)
• At night 2-carboxyarabinitol 1-phosphate
(synthesized in plants) inhibits Rubisco
Why shut down Rubisco at night?
Need photosyn for ATP/NADPH
Need to turn off calvin cycle
Prevent inefficient accumulation of 3PG
and oxygenation rxn
Oxygenation of Ribulose 1,5-Bisphosphate
CO2
ATP and NADPH
3PG
Photorespiration
Calvin C
(3PG)
O2 and CO2 compete for same active site
Limits crop yields
No CO2 fixation (O2 utilization).
consumes ATP and NADPH without hexose production.
But…..no Rubisco mutants found without
this process (i.e. with only CO2 fixation and no oxygenation).
Oxygenation of Ribulose 1,5-Bisphosphate
CO2
ATP and NADPH
3PG
Photorespiration
Calvin C
(3PG)
Normally carboxylation 3-4 fold higher than oxygenation rate
High temp and light conditions:
inc oxygenation rate and also lose water
Other plants get around problem: increase local CO2 concentration
by using a secondary pathway to fix carbon
The C4 Pathway
Avoid photorespiration loss
Rubisco
No Rubisco
(A CO2 shuttle, present in
corn, sorghum, sugar cane,
many weeds)
Bundle sheath cells are impermeable to gases (both CO2 and O2).
Concentrates CO2 for Rubisco
Allow growth at high temp/light
High temp favors oxygenation of Rubisco
Crassulacean acid metabolism (CAM)
Occurs in succulent plants.
H2O can be lost during CO2
fixation due to open stomata,
succulent plants reduce this by
fixing CO2 at night (when it is
cooler)
and in the day the stomata remain
closed.
Malate is stored in large vacuole
and released during the day
decarboxylated and CO2 utilized
Closed stomata in day: keeps H2O in
O2 out and allows high CO2 buildup
Temporally separation : PEP carboxylase inhibited in day (malate)
No competition between PEP carb and Rubisco for CO2