chap 3 photosynthesis
TRANSCRIPT
BIO 150 Metabolism and cell division
Photosynthesis
By the end of this lecture you will be able to:
1. Understand that ENERGY can be transformed from one form to another.2. Know that energy exist in two forms; free energy - available for doing work
or as heat - a form unavailable for doing work.3. Appreciate that the Sun provides most of the energy needed for life on
Earth.4. Explain why photosynthesis is so important to energy and material flow for
life on earth.5. Know why plants tend to be green in appearance.6. Equate the organelle of photosynthesis in eukaryotes with the chloroplast. 7. Describe the organization of the chloroplast.8. Understand that photosynthesis is a two fold process composed of the
light-dependent reactions (i.e., light reactions) and the light independent reactions (i.e. Calvin Cycle or Dark Reactions).
9. Tell where the light reactions and the CO2 fixation reactions occur in the chloroplast.
10. Define chlorophylls giving their basic composition and structure.11. Draw the absorption spectrum of chlorophyll and compare it to the action
spectrum of photosynthesis.12. Define the Reaction Centers and Antennae and describe how it operates.13. Describe cyclic photophosphorylation of photosynthesis.14. Describe noncyclic photophosphorylation of photosynthesis.
LEARNING OBJECTIVES
Energy can be transformed from one form to another
FREE ENERGY (available for work)
vs.HEAT
(not available for work)
THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH
• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists– Autotrophs generate their own organic matter through
photosynthesis– Sunlight energy is transformed to energy stored in the
form of chemical bonds
(a) Mosses, ferns, andflowering plants
(b) Kelp
(c) Euglena (d) Cyanobacteria
THE BASICS OF PHOTOSYNTHESIS
Light Energy Harvested by Plants & Other Photosynthetic Autotrophs
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
Light use in photosynthesis
• Sun release energy in a broad spectrum of electromagnetic radiation.
• The energy of photon relates to its wavelength.
• Longer wavelength photons have low energy.• Short wavelength photons contain high
energy.• Photosynthesis use 2 wavelength – 450nm &
750nm
Electromagnetic Spectrum and Visible Light
Gammarays X-rays UV
Infrared & Microwaves Radio waves
Visible light
Wavelength (nm)
• When light strikes a leaf, 3 processes occur:• Light is absorbed - use to drive
photosynthesis.,• Light is reflected & light is transmitted – give
color to the object.
Why are plants green?
Reflected light
Transmitted light
Different wavelengths of visible light are seen by the human eye as different colors.
WHY ARE PLANTS GREEN?
Gammarays X-rays UV Infrared Micro-
wavesRadiowaves
Visible light
Wavelength (nm)
Pigments in chlorophyll
• Chlorophyll is the main photosynthetic pigment in tylakoid membranes.
• Chlorophyll a – a primary pigment absorbs blue violet and red wavelength.
• It initiates the light dependent reaction of photosynthesis.
• Chlorophyll b – an accessory pigment absorbs blue and red orange wavelength.
• Carotenoids – absorbs mainly blue green light and reflect mostly yellow, orange or red.
• Phycocyanin – absorbs green and yellow and reflect blue or purple wavelength.
• Chloroplasts absorb light energy and convert it to chemical energy
LightReflected
light
Absorbedlight
Transmittedlight
Chloroplast
THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED
• Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water
AN OVERVIEW OF PHOTOSYNTHESIS
Carbondioxide
Water Glucose Oxygengas
PHOTOSYNTHESIS
• The Calvin cycle makes sugar from carbon dioxide– ATP generated by the light
reactions provides the energy for sugar synthesis
– The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose
LightChloroplast
Lightreactions
Calvincycle
NADP
ADP+ P
• The light reactions convert solar energy to chemical energy– Produce ATP & NADPH
AN OVERVIEW OF PHOTOSYNTHESIS
Chloroplasts: Sites of Photosynthesis
• Photosynthesis– Occurs in chloroplasts, organelles in certain
plants– All green plant parts have chloroplasts and carry
out photosynthesis• The leaves have the most chloroplasts• The green color comes from chlorophyll in the
chloroplasts• The pigments absorb light energy
Photosynthesis occurs in chloroplasts
• In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts
• A chloroplast contains: – stroma, a fluid – grana, stacks of thylakoids
• The thylakoids contain chlorophyll– Chlorophyll is the green pigment that captures light
for photosynthesis
Mesophyll
Leaf cross sectionChloroplasts Vein
Stomata
Chloroplast Mesophyllcell
CO2 O2
20 m
Figure 10.4a
Outermembrane
Intermembranespace
Innermembrane
1 m
Thylakoidspace
ThylakoidGranumStroma
ChloroplastFigure 10.4b
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 (in the thylakoids)– Split H2O
– Release O2
– Reduce NADP+ to NADPH– Generate ATP from ADP by
photophosphorylation
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• The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
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Light
LightReactions
Chloroplast
NADP
ADP+ P i
H2OFigure 10.6-1
Light
LightReactions
Chloroplast
ATP
NADPH
NADP
ADP+ P i
H2O
O2
Figure 10.6-2
Light
LightReactions
CalvinCycle
Chloroplast
ATP
NADPH
NADP
ADP+ P i
H2O CO2
O2
Figure 10.6-3
Light
LightReactions
CalvinCycle
Chloroplast
[CH2O](sugar)
ATP
NADPH
NADP
ADP+ P i
H2O CO2
O2
Figure 10.6-4
Stages in photosynthesis
• Light reaction and dark reaction.• Light reactions requires direct energy of light
to make energy carrier molecules used in dark reaction.
• Light reaction occur in the grana and dark reaction take place in the stroma.
A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes
• A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center
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Figure 10.13
(b) Structure of photosystem II(a) How a photosystem harvests light
Thyl
akoi
d m
embr
ane
Thyl
akoi
d m
embr
ane
PhotonPhotosystem STROMA
Light-harvestingcomplexes
Reaction-centercomplex
Primaryelectronacceptor
Transferof energy
Special pair ofchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Chlorophyll STROMA
Proteinsubunits THYLAKOID
SPACE
e
Figure 10.13a
(a) How a photosystem harvests light
Thyl
akoi
d m
embr
ane
PhotonPhotosystem STROMA
Light-harvestingcomplexes
Reaction-centercomplex
Primaryelectronacceptor
Transferof energy
Special pair ofchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
e
• A primary electron acceptor in the reaction center accepts excited electron and is reduced as a result
• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
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• There are two types of photosystems in the thylakoid membrane
• Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm
• The reaction-center chlorophyll a of PS II is called P680
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• Photosystem I (PS I) is best at absorbing a wavelength of 700 nm
• The reaction-center chlorophyll a of PS I is called P700
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Linear Electron Flow
• During the light reactions, there are two possible routes for electron flow: cyclic and linear
• Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
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• A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
• An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)
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Figure 10.14-1
Primaryacceptor
P680
Light
Pigmentmolecules
Photosystem II(PS II)
1
2e
• P680+ is a very strong oxidizing agent• H2O is split by enzymes, and the electrons are
transferred from the hydrogen atoms to P680+, thus reducing it to P680
• O2 is released as a by-product of this reaction
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Figure 10.14-2
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
1
2
3
e
e
e
• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
• The electron transport chain consists of plastoquinone (Pq), cytochrome complex and plastocyanin (Pc).
• Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
• Diffusion of H+ (protons) across the membrane drives ATP synthesis
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Photophosphorylation
• The released energy is harnessed by the thylakoids membrane to produce ATP.
• The ATP synthesis is called photophosporylation because it is driven by light energy.
Figure 10.14-3
Cytochromecomplex
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
Pq
Pc
ATP
1
2
3
5
Electron transport chaine
e
e
4
• In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor
• P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain
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Figure 10.14-4
Cytochromecomplex
Primaryacceptor
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
Photosystem I(PS I)
Pq
Pc
ATP
1
2
3
5
6
Electron transport chain
P700Light
e
e
4
e
e
• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
• The electrons are then transferred to NADP+ and reduce it to NADPH by NADP+ reductase.
• The electrons of NADPH are available for the reactions of the Calvin cycle
• This process also removes an H+ from the stroma
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Figure 10.14-5
Cytochromecomplex
Primaryacceptor
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
Photosystem I(PS I)
Pq
Pc
ATP
1
2
3
5
6
7
8
Electron transport chain
Electron transport
chain
P700Light
+ HNADP
NADPH
NADP
reductase
Fd
e
e
e
e
4
e
e
Photosystem II Photosystem I
Millmakes
ATP
ATP
NADPH
e
e
e
ee
e
e
Phot
on
Phot
on
Figure 10.15
Cyclic Electron Flow
• Cyclic electron flow uses only photosystem I (P700) and produces only ATP, but not NADPH.
• Occurs when cells require additional ATP and thre is no NADP+ to reduce to NADPH.
• No oxygen is released during this reaction.• Cyclic electron flow generates surplus ATP,
satisfying the higher demand in the Calvin cycle
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• Non-cyclic electron flow produces ATP and NADPH roughly in equal quantities but Calvin cycle consumes more ATP than NADPH.
• If ATP runs low, the Calvin cycle will slow down and NADPH will accumulate.
• The rise of NADPH will stimulate a temporary shift from noncyclic to cyclic electron flow until ATP supply is enough.
Figure 10.16
Photosystem I
Primaryacceptor
Cytochromecomplex
Fd
Pc
ATP
Primaryacceptor
Pq
Fd
NADPH
NADP
reductase
NADP
+ H
Photosystem II
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
• Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy
• Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP
• Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities
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• In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix
• In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma
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Mitochondrion Chloroplast
MITOCHONDRIONSTRUCTURE
CHLOROPLASTSTRUCTURE
Intermembranespace
Innermembrane
Matrix
Thylakoidspace
Thylakoidmembrane
Stroma
Electrontransport
chain
H Diffusion
ATPsynthase
H
ADP P iKey Higher [H ]
Lower [H ]
ATP
Figure 10.17
• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place
• In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH
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Figure 10.18
STROMA(low H concentration)
STROMA(low H concentration)
THYLAKOID SPACE(high H concentration)
LightPhotosystem II
Cytochromecomplex Photosystem ILight
NADP
reductase
NADP + H
ToCalvinCycle
ATPsynthase
Thylakoidmembrane
21
3
NADPH
Fd
Pc
Pq
4 H+
4 H++2 H+
H+
ADP+P i
ATP
1/2
H2O O2
Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
• The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
• The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
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• Dark reaction occurs in stroma of the chloroplast and involves many reaction.
• Each reaction are catalyzed by different enzymes.
• This reaction do not require light.• Both light reaction and dark reaction occur at
the same time in chloroplasts during daylight.• Each dependent upon each other.
• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)
• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2
• The Calvin cycle has three phases– Carbon fixation (catalyzed by rubisco)– Reduction– Regeneration of the CO2 acceptor (RuBP)
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Input3 (Entering one
at a time)CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P6
Short-livedintermediate
3-Phosphoglycerate3 P P
Ribulose bisphosphate(RuBP)
Figure 10.19-1
Input3 (Entering one
at a time)CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P6
Short-livedintermediate
3-Phosphoglycerate6
6 ADP
ATP
6 P P1,3-Bisphosphoglycerate
CalvinCycle
6 NADPH
6 NADP
6 P i
6 PPhase 2: Reduction
Glyceraldehyde 3-phosphate(G3P)
3 P PRibulose bisphosphate
(RuBP)
1 PG3P
(a sugar)Output
Glucose andother organiccompounds
Figure 10.19-2
Input3 (Entering one
at a time)CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P6
Short-livedintermediate
3-Phosphoglycerate6
6 ADP
ATP
6 P P1,3-Bisphosphoglycerate
CalvinCycle
6 NADPH
6 NADP
6 P i
6 PPhase 2: Reduction
Glyceraldehyde 3-phosphate(G3P)
P5G3P
ATP
3 ADP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
3 P PRibulose bisphosphate
(RuBP)
1 PG3P
(a sugar)Output
Glucose andother organiccompounds
3
Figure 10.19-3
Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates
• Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
• On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis
• The closing of stomata reduces access to CO2
and causes O2 to build up• These conditions favor an apparently wasteful
process called photorespiration© 2011 Pearson Education, Inc.
Photorespiration: An Evolutionary Relic?
• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)
• In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound
• Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
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• Rubisco adds O2 to the Calvin cycle instead of CO2
• The product split and two carbon compound formed called glycolic acid and exported from the chloroplast
• Mitochondria and perosixomes then break the glycolic acid to CO2.
• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
• Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle
• In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle
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C4 Plants
• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
• This step requires the enzyme PEP carboxylase• PEP carboxylase has a higher affinity for CO2 than
rubisco does; it can fix CO2 even when CO2 concentrations are low
• These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle
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• CO2 bound to the 3 carbon molecule known as phospoenol pyruvate (PEP) to form 4 carbon compounds called oxaloacetate
• This reaction catalyzed by enzyme PEP carboxylase
• Oxaloacetate then converted to the intermediate malate which was then sent to adjacent bundle sheath through plasmodesmata.
• Malate then is decarboxylated and dehydrogenated to form pvruvate and NADPH and CO2 is released.
• Bundle sheath cells are impermeable to CO2, result with a high concentration of CO2 within the bundle sheath cells
• The increase of CO2 concentration minimizes photorespiration and increase sugar production.
• The pyruvate goes back to the mesophyll cell and reacts with ATP to regenerate PEP and ready to accept another CO2
Figure 10.20
C4 leaf anatomy The C4 pathway
Photosyntheticcells of C4 plant leaf
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Stoma
Mesophyll cell PEP carboxylase CO2
Oxaloacetate (4C) PEP (3C)
Malate (4C)
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
ADP
ATP
Figure 10.20a
C4 leaf anatomy
Photosyntheticcells of C4 plant leaf
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Stoma
Figure 10.20bThe C4 pathway
Mesophyll cell PEP carboxylase CO2
Oxaloacetate (4C) PEP (3C)
Malate (4C)
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
ADPATP
• In the last 150 years since the Industrial Revolution, CO2 levels have risen gratly
• Increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing relative abundance of these species
• The effects of such changes are unpredictable and a cause for concern
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Advantages of C4 pathway functions in addition to C3 pathway
• Photosynthesis can progress even at a very low CO2 concentration.
• It be able to store CO2 temporarily so that it can be used later.
• Be able to avoid photorespiration. Photorespiration is a wasteful process and reduces their photosynthetic efficiency.
CAM Plants
• Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
• CAM plants open their stomata at night, incorporating CO2 into organic acids
• Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
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• Develop a special carbon fixation pathway known as CAM.
• At night when the stomata open, CAM plants use the enzyme PEP carboxylase to fix CO2 and turn to oxaloacetate.
• The oxaloacetate then converted to malate and stored in cell vacuole.
• During day time, exchange of gas cannot occur because of the closing stomata, CO2 then taken from malate by decarboxylation process.
Sugarcane
Mesophyllcell
Bundle-sheathcell
C4 CO2
Organic acid
CO2
CalvinCycle
Sugar(a) Spatial separation of steps (b) Temporal separation of steps
CO2
Organic acid
CO2
CalvinCycle
Sugar
Day
Night
CAM
Pineapple
CO2 incorporated(carbon fixation)
CO2 releasedto the Calvincycle
2
1
Figure 10.21
Figure 10.21a
Sugarcane
Figure 10.21b
Pineapple
Features C3 plant C4 plant CAM plant
Representative spp (e.g)
Rice, wheat, soybean Maize, sugarcane Cactus, pineapple
Efficiency Less than C4 and CAM More than C3 More than C3
Carbon fixation Once Twice Twice
Site for carbon (C) fixation and C.cycle
Mesophyll cells Initial carbon fixation – mesophyll cells Calvin cycle – bundle sheath cells
Both occur in mesophyll cells but at different period of time
CO2 acceptor RuBP RuBP, PEP RuBP, PEP
CO2 fixing enzyme Rubisco Rubisco, PEP carboxylase
Rubisco, PEP carboxylase
Rate of photorespiration
High Low Low
Rate of photosynthesis High High High
Yields Low High High
Factors affecting photosynthesis process
• Light intensity• Concentration of carbon dioxide • Temperature • Concentration of chlorophyll• Concentration of oxygen• Water • Chemical compounds
Autotrophic prokaryotes
• Prokaryotes consist of bacteria and the cynobacteria
• Certain prokaryotes are autotrophic – able to make their own complex organic compounds as food.
• Cynobacteria – photosynthetic organisms. They use CO2, water and energy from sun to produce sugar and other substance without the presence of chloroplast but use the photosynthetic pigment located in membrane system in the cytoplasm.
• Autotrophic bacteria• Photosynthetic bacteria• Chemosynthetic bacteria
Photosynthetic bacteria• Use pigment known as bacteriochlorophyll to
capture light• This pigment is similar as chlorophyll found in plants• The source of hydrogen to reduce CO2 does not
come from water but from other molecule.• Purple sulphur bacteria use hydrogen sulphide as a
source of hydogen.• Brown or red bacteria use organic compound such
as ethanoic acid as the source of hydrogen
Chemosynthetic bacteria• This type of bacteria produce complex organic
compound from CO2 and water but the chemical energy is not coming from light but from specific chemical reaction which they able to catalyze.
• Nitrifying bacteria like Nitrosomonas and Nitrococcus get its energy from oxidation of ammonia to nitrite in the nitrification process in nitrogen cycle.
• Other chemical such as H2S, NH3 (ammonia), Fe2+.
It's not that It's not that easy bein' easy bein' green… but it green… but it is essential for is essential for life on earth!life on earth!