Download Photosynthesis

Ch 10 Photosynthesis--> To make with light!
LE 10-2
Photoautotrophs:
Self feeders, producers
Use light and inorganic
molecules to make own
organic molecules.
Plants
Unicellular protist
10 µm
Purple sulfur
bacteria
1.5 µm
Multicellular algae
Cyanobacteria
40 µm
• Heterotrophs (food from others):
-Consumers
-Obtain organic material from other organisms
-Dependent on photoautotrophs for food and oxygen
Photosynthesis: conversion of light energy into chemical energy
Reaction:
6CO2 + 12H2O + light --> C6H12O6 + 6O2 + 6H2O
glucose
Simplified rxn:
6CO2 + 6H2O + light --> C6H12O6 + 6O2
Simplest rxn:
carbohydrate
CO2 + H2O + light --> [CH2O]
+ O2
Enters through roots
LE 10-3
6CO2 + 6H2O + light --> C6H12O6 + 6O2
Organic molecule
for fuel or other
Gas enters through stomata
Leaf cross section
Vein
Stomata
CO2
O2
Mesophyll cell
Chloroplast
5 µm
Granum
Thylakoid
Thylakoid
space
or used
in respiration
Mesophyll
Stroma
Exits through
stomata
Outer
membrane
Intermembrane
space
Inner
membrane
1 µm
Two major reactions in photosynthesis
Light-dependent (in thylakoid)
Light-independent aka dark or Calvin cycle (in stroma)
LE 10-7
Chlorophyll in thylakoid
membranes
Light
Reflected
light
Chloroplast
Stroma
Absorbed
light
Granum
Transmitted
light
LE 10-10
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of
molecule; note
magnesium atom
at center
Hydrocarbon tail:
interacts with
hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
LE 10-9a
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400
500
600
Wavelength of light (nm)
Absorption spectra
700
How do we know that absorption of certain wavelengths of light
by plants stimulates a chemical reaction in plants?
Specifically how do we know that O2 is a product?
LE 10-9c
Aerobic bacteria
Filament
of algae
400
500
600
700
Engelmann’s experiment (1883): Action spectrum
What would be an important control experiment?
• Chlorophyll a:
– main photosynthetic pigment
• Accessory pigments
– chlorophyll b and carotenoids absorb excessive light that
would damage chlorophyll
– broaden the spectrum used for photosynthesis
Light-Induced Excitation:
• When a pigment absorbs light
– departs from a ground state to an excited state
--> unstable Draw
– excited electrons fall back to the ground state,
give off photons (glow)-->fluorescence
LE 10-11
e–
Excited
state
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
Excitation of isolated chlorophyll molecule
Fluorescence
LE 10-5_1
Light-dependent rxn: in thylakoid
H2O
Light
LIGHT
REACTIONS
Chloroplast
LE 10-5_2
H2O
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
LE 10-5_3
Calvin cycle: in stroma
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Photosynthesis as a Redox Process
• Water is oxidized (e- are removed).
• Carbon dioxide is reduced (e- are gained).
Two major reactions in photosynthesis
Light dependent (in thylakoid):
Creates ATP and an electron carrier, NADPH
Electrons supplied through splitting and
oxidation of H2O
Light -independent (aka dark or Calvin cycle)(in stroma):
Synthesis of organic molecules from CO2
Reduction reactions
Endergonic: requires ATP
Light Reaction:
Consists of 2 photosystems
Occurs at two different reaction centers
each surrounded by light harvesting complexes
Light harvesting complex
funnels energy to reaction center
LE 10-12
Thylakoid
Photosystem
Photon
Thylakoid membrane
Light-harvesting
complexes
Reaction
center
STROMA
Primary electron
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
LE 10-13_1
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
e–
Once P680 is oxidized
(gives up e-), is it functional?
How is it restored to functionality?
Light
P680
Photosystem II
(PS II)
LE 10-13_2
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Energy of electrons
Primary
acceptor
2
H+
1/ 2
+
O2
Light
H2O
e–
e–
e–
P680
Photosystem II
(PS II)
Splitting of H2O yields ethat fill e-”hole” in oxidized
P680
LE 10-13_3
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
Pq
2 H+
+
1/ 2 O 2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
Photosystem II
(PS II)
LE 10-13_4
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Energy of electrons
Pq
2
H+
1/ 2
+
O2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
LE 10-13_5
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Pq
Energy of electrons
2
H+
e–
H2O
Cytochrome
complex
+
1/2 O2
Light
Fd
e–
e–
NADP+
reductase
Pc
e–
e–
NADPH
+ H+
P700
P680
Light
ATP
Photosystem II
(PS II)
NADP+
+ 2 H+
Photosystem I
(PS I)
After P700 is oxidized by light energy in PS I
are its missing electrons replaced?
If so what is the electron source?
What would be the effect on photosynthesis if P700 were not
reduced to its original state i.e. if the e- hole were not filled?
Electron Flow
• Noncyclic electron flow
– involves both photosystems (II & I)
– produces ATP and NADPH
LE 10-14
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
Cyclic Electron Flow
- Uses only photosystem I
- Produces only ATP, no NADPH
- Generates surplus ATP
– to satisfy demand in the Calvin cycle
LE 10-15
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
How is ATP made?
By chemiosmosis
LE 10-17
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Light
2
Photosystem I
Light
NADP+
reductase
H+
NADP+ + 2H+
Fd
NADPH + H+
Pq
H2O
THYLAKOID SPACE
(High H+ concentration)
1/2
Pc
O2
+2 H+
2 H+
To
Calvin
cycle
Thylakoid
membrane
STROMA
(Low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
• Current chemiosmotic model:
– H+ (protons) accumulate in thylakoid space
• 1. Through splitting of water
• 2. By translocation into thylakoid when e- are transported
• 3. By removal of H+ from stroma due to bonding with NADPH
– H+ diffuses from thylakoid space --> stroma
through membrane enzyme, ATP synthase
– Movement activates ATP synthase
– ATP synthesized on stromal face where the
Calvin cycle takes place
Products from light reactions power Calvin cycle!
What are the light reaction products?
ATP: energy carrier
NADPH: electron carrier
What is the product of the Calvin cycle?
Glucose (fuel)
What additional molecule must enter
the Calvin cycle to make sugar?
CO2
• Calvin cycle
– Three phases:
• Carbon fixation (catalyzed by rubisco)
• Reduction
Regeneration of the CO2 acceptor (RuBP)
LE 10-18_1
H2 O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
Short-lived
intermediate
P
P
6
3-Phosphoglycerate
3 P
P
Ribulose bisphosphate
(RuBP)
6
6 ADP
CALVIN
CYCLE
ATP
LE 10-18_2
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
CALVIN
CYCLE
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Phase 2:
Reduction
Glucose and
other organic
compounds
LE 10-18_3
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
CALVIN
CYCLE
6 P
ATP
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
I had no idea I
could do these
things!
Alternative mechanisms of carbon fixation in hot,
dry climates
• How to avoid dehydration during day?
close stomata
Consequences? Positive & Negative
conserves water
but also blocks CO2 uptake
Overall: reduces rate of photosynthesis
LE 10-20
CAM: Crassulacean acid metabolism
Sugarcane
Pineapple
CAM
C4
CO2
Mesophyll
cell
Organic acid
Bundlesheath
cell
CO2
CO2 incorporated
into four-carbon Organic acid
organic acids
(carbon fixation)
CO2
CALVIN
CYCLE
Sugar
Spatial separation of steps
CO2
Organic acids
release CO2 to
Calvin cycle
Night
Day
CALVIN
CYCLE
Sugar
Temporal separation of steps
CAM Plants
• CAM plants open stomata at night,
incorporating CO2 into organic acids
• Stomata closed during the day
• CO2 released from organic acids and used in
the Calvin cycle
• Photosynthesis can occur during day!
The Importance of
Photosynthesis: A Review
• sunlight stored as chemical energy in organic
compounds by chloroplasts
• Sugar supplies chemical energy and carbon
skeletons to synthesize other organic molecules
• Production of food and atmospheric oxygen
LE 10-21
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+ Pi
RuBP
Photosystem II
Electron transport
chain
Photosystem I
ATP
NADPH
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
O2
Sucrose (export)