Overview: The Process That Feeds the Biosphere
• Photosynthesis is the process that converts solar energy into chemical energy
• Directly or indirectly, photosynthesis nourishes almost the entire living world
• Autotrophs sustain themselves without eating anything derived from other organisms
• Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules
• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2
Autotrophs
(a) Plants
(c) Unicellular protist10 µm
1.5 µm
40 µm(d) Cyanobacteria
(e) Purple sulfur bacteria
(b) Multicellular alga
Photosynthesis occurs in plants,
algae, certain other protists, and some prokaryotes
• Heterotrophs obtain their organic material from other organisms
• Heterotrophs are the consumers of the biosphere
• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
Heterotrophs
The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
• The “light” reactions are in the thylakoids
• The Calvin cycle is in the stroma
1 µm
Thylakoidspace
Chloroplast
GranumIntermembranespace
Innermembrane
Outermembrane
Stroma
Thylakoid
• The “light” reactions overview
– Capture light energy using chlorophyll
– Split H2O to electrons, protons and oxygen
– Release O2
– Reduce NADP+ to NADPH
– Generate ATP from ADP by photophosphorylation
• The Calvin cycle overview
- begins with carbon fixation, incorporating CO2 into organic molecules
- forms sugars from these organic molecules, using ATP and NADPH (from light reactions)
Light
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
Overview of Photosynthesis 1
Light
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
Overview of Photosynthesis 2
Light
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
Overview of Photosynthesis 3
Light
H2O
Chloroplast
LightReactions
NADP+
P
ADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
[CH2O]
(sugar)Overview of Photosynthesis 4
Chlorophyll is the light-capturing substance
• Anchored to thylakoid membranes by proteins
Surrounded by other pigment
molecules, enzymes and
electron carriers
A “photosystem”
(a) Excitation of isolated chlorophyll molecule
Heat
Excitedstate
(b) Fluorescence
Photon Groundstate
Photon(fluorescence)
En
erg
y o
f el
ectr
on
e–
Chlorophyllmolecule
If illuminated, a solution of chlorophyll will fluoresce, giving off light and heat
Illustration of important properties of chlorophyll
PhotosyntheticEnergy Capture
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
STROMA
e–
Pigments-chlorophylland accessory
Photon
Transferof energy
Special pair ofchlorophyll amolecules
Th
yla
koid
me
mb
ran
e
Photosystem
Primaryelectronacceptor
Reaction-centercomplex
Light-harvestingcomplexes
Pigmentmolecules
Light
P680
e–
Primaryacceptor
2
1
e–
e–
2 H+
O2
+3
H2O
1/2
4
Pq
Pc
Cytochromecomplex
Electron transport chain
5
ATP
Photosystem I(PS I)
Light
Primaryacceptor
e–
P700
6
Fd
Electron transport chain
NADP+
reductase
NADP+
+ H+
NADPH
8
7
e–
e–
6
“Z-Diagram” of photosynthesis
Photosystem II(PS II)
This illustrates “non-cyclic” electron flow- electrons enter with water and leave with NADPH
ATPPhotosystem II
Photosystem I
Primary acceptor
Pq
Cytochromecomplex
Fd
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
NADP+
+ H+
Cyclic electron flow uses only photosystem I and
produces ATP, but not NADPH
• “Light” reactions of PSI and PSII generate ATP and “reducing power” in the form of NADPH
• Reducing power allows the conversion of carbon dioxide to carbohydrate
• CO2 to CH2O requires NADPH reducing power
Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)
Glucose andother organiccompounds
CalvinCycle3
3 ADP
ATP
5 P
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
G3P
Carbon fixation conversion of carbon dioxide to carbohydrate
Rubisco is the key
enzyme of the cycle
Plants that use this cycle
exclusively are C3 plants
• Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
• Plants have evolved two alternate types of carbon fixation
• C4 and Crassulean Acid Metabolism (CAM) fixation
LightReactions:
Photosystem II Electron transport chain
Photosystem I Electron transport chain
CO2
NADP+
ADP
P i+
RuBP 3-Phosphoglycerate
CalvinCycle
G3PATP
NADPHStarch(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2
C3 Photosynthesis: a review
Note Card
What is the “reducing power” of NADPH and how does it link the light reactions with the Calvin
Cycle?