Ch 10 Photosynthesis--> To make with light!
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Transcript of Ch 10 Photosynthesis--> To make with light!
Ch 10 Photosynthesis--> To make with light!
LE 10-2
Plants
Unicellular protist
Multicellular algae Cyanobacteria
Purple sulfurbacteria
10 µm
1.5 µm
40 µm
Photoautotrophs:
Self feeders, producers
Use light and inorganic
molecules to make own
organic molecules.
• 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
Simplified rxn:
6CO2 + 6H2O + light --> C6H12O6 + 6O2
Reaction:
6CO2 + 12H2O + light --> C6H12O6 + 6O2 + 6H2Oglucose
Simplest rxn:
CO2 + H2O + light --> [CH2O] + O2
carbohydrate
LE 10-3
Leaf cross sectionVein
Mesophyll
Stomata CO2 O2
Mesophyll cellChloroplast
5 µm
Outermembrane
IntermembranespaceInner
membrane
Thylakoidspace
ThylakoidGranumStroma
1 µm
6CO2 + 6H2O + light --> C6H12O6 + 6O2
Gas enters through stomata
Enters through roots
Exits throughstomata
or used
in respiration
Organic moleculefor fuel or other
Two major reactions in photosynthesis
Light-dependent (in thylakoid)
Light-independent aka dark or Calvin cycle (in stroma)
LE 10-7
Chloroplast
Light
Reflected light
Absorbed light
Transmitted light
Granum
Stroma
Chlorophyll in thylakoid membranes
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 hydrophobicregions of proteins insidethylakoid membranes of chloroplasts; H atoms not shown
LE 10-9a
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
Absorption spectra
Ab
sorp
tio
n o
f lig
ht
by
chlo
rop
last
pig
men
ts
400 500 600 700
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
Engelmann’s experiment (1883): Action spectrum
400 500 600 700
Aerobic bacteria
Filament of algae
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
• 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
Light-Induced Excitation:
LE 10-11
Excitedstate
Heat
Photon(fluorescence)
GroundstateChlorophyll
molecule
Photon
Excitation of isolated chlorophyll molecule Fluorescence
En
erg
y o
f el
ectr
on
e–
LE 10-5_1
H2O
LIGHTREACTIONS
Chloroplast
Light
Light-dependent rxn: in thylakoid
LE 10-5_2
H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
LE 10-5_3
H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
NADP+
CO2
ADPP+ i
CALVINCYCLE
[CH2O](sugar)
Calvin cycle: in stroma
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 centerseach surrounded by light harvesting complexes
Light harvesting complex funnels energy to reaction center
LE 10-12
Thylakoid
Photon
Light-harvestingcomplexes
Photosystem
Reactioncenter
STROMA
Primary electronacceptor
e–
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Th
ylak
oid
mem
bra
ne
LE 10-13_1
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
ATP
ADP
CALVINCYCLE
LIGHTREACTIONS
NADP+
Light
H2O CO2
En
erg
y o
f el
ectr
on
sO2
Once P680 is oxidized(gives up e-), is it functional?
How is it restored to functionality?
LE 10-13_2
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
ATP
ADP
CALVINCYCLE
LIGHTREACTIONS
NADP+
Light
H2O CO2
En
erg
y o
f el
ectr
on
sO2
e–
e–
+2 H+
H2O
O21/2
Splitting of H2O yields e- that fill e-”hole” in oxidized P680
LE 10-13_3
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
ATP
ADP
CALVINCYCLE
LIGHTREACTIONS
NADP+
Light
H2O CO2
En
erg
y o
f el
ectr
on
sO2
e–
e–
+2 H+
H2O
O21/2
Pq
Cytochromecomplex
Electron transport chain
Pc
ATP
LE 10-13_4
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
ATP
ADP
CALVINCYCLE
LIGHTREACTIONS
NADP+
Light
H2O CO2
En
erg
y o
f el
ectr
on
s
O2
e–
e–
+2 H+
H2O
O21/2
Pq
Cytochromecomplex
Electron transport chain
Pc
ATP
P700
e–
Primaryacceptor
Photosystem I(PS I)
Light
LE 10-13_5
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
ATP
ADPCALVINCYCLE
LIGHTREACTIONS
NADP+
Light
H2O CO2E
ner
gy
of
elec
tro
ns
O2
e–
e–
+2 H+
H2O
O21/2
Pq
Cytochromecomplex
Electron transport chain
Pc
ATP
P700
e–
Primaryacceptor
Photosystem I(PS I)
e–e–
ElectronTransportchain
NADP+
reductase
Fd
NADP+
NADPH
+ H+
+ 2 H+
Light
After P700 is oxidized by light energy in PS Iare 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
ATP
Photosystem II
e–
e–
e–e–
MillmakesATP
e–
e–
e–
Ph
oto
n
Photosystem I
Ph
oto
n
NADPH
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
Photosystem I
Photosystem II ATP
Pc
Fd
Cytochromecomplex
Pq
Primaryacceptor
Fd
NADP+
reductase
NADP+
NADPH
Primaryacceptor
How is ATP made?
By chemiosmosis
LE 10-17
STROMA(Low H+ concentration)
Light
Photosystem IICytochrome
complex
2 H+
Light
Photosystem I
NADP+
reductase
Fd
PcPq
H2O O2
+2 H+
1/2
2 H+
NADP+ + 2H+
+ H+NADPH
ToCalvincycle
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane ATP
synthase
ATP
ADP+P
H+i
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light
• 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 carrierNADPH: 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
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
LightInput
3
CO2
(Entering oneat a time)
Rubisco
3 P P
Short-livedintermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVINCYCLE
3 P P
Ribulose bisphosphate(RuBP)
LE 10-18_2
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light Input
CO2
(Entering oneat a time)
Rubisco
3 P P
Short-livedintermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVINCYCLE
3
P P
Ribulose bisphosphate(RuBP)
3
6 NADP+
6
6 NADPH
P i
6 P
1,3-BisphosphoglycerateP
6 P
Glyceraldehyde-3-phosphate(G3P)
P1
G3P(a sugar)Output
Phase 2:Reduction
Glucose andother organiccompounds
LE 10-18_3
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light Input
CO2
(Entering oneat a time)
Rubisco
3 P P
Short-livedintermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVINCYCLE
3
P P
Ribulose bisphosphate(RuBP)
3
6 NADP+
6
6 NADPH
P i
6 P
1,3-BisphosphoglycerateP
6 P
Glyceraldehyde-3-phosphate(G3P)
P1
G3P(a sugar)Output
Phase 2:Reduction
Glucose andother organiccompounds
3
3 ADP
ATP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5
G3P
I had no idea Icould do these things!
Alternative mechanisms of carbon fixation in hot, dry climates
• How to avoid dehydration during day?
close stomata
conserves water
but also blocks CO2 uptake
Consequences? Positive & Negative
Overall: reduces rate of photosynthesis
LE 10-20
Bundle-sheathcell
Mesophyllcell Organic acid
C4
CO2
CO2
CALVINCYCLE
Sugarcane Pineapple
Organic acidsrelease CO2 toCalvin cycle
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Organic acid
CAM
CO2
CO2
CALVINCYCLE
Sugar
Spatial separation of steps Temporal separation of steps
Sugar
Day
Night
CAM: Crassulacean acid metabolism
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
CO2H2O
Light reactions Calvin cycle
NADP+
RuBP
G3PATP
Photosystem IIElectron transport
chainPhotosystem I
O2
Chloroplast
NADPH
ADP+ P i
3-Phosphoglycerate
Starch(storage)
Amino acidsFatty acids
Sucrose (export)