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Overview: The Process That Feeds the Biosphere
Photosynthesis Is the process that converts solar energy into
chemical energy nourishes almost all the living world directly or
indirectlyAll organisms use organic compounds for
energy & for carbon skeletonsOrganisms obtain organic compounds by
one of two major modes: autotrophic nutrition or heterotrophic nutrition
Autotrophs
produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment
Autotrophs are the ultimate sources of organic compounds for all heterotrophic organisms
Autotrophs are the producers of the biosphere
Autotrophs
Photoautotrophs use light as a source of energy to synthesize organic compounds
Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes
Chemoautotrophs harvest energy from oxidizing inorganic substances, such as sulfur and ammonia
Chemoautotrophy is unique to prokaryotes
Photosynthesis
Occurs in plants, algae, certain other protists, and some prokaryotes
(a) Plants
(b) Multicellular algae
(c) Unicellular protist 10 m
40 m(d) Cyanobacteria
1.5 m(e) Pruple sulfurbacteria
Figure 10.2
BioFlix: PhotosynthesisBioFlix: Photosynthesis
Heterotrophs live on organic compounds produced by other organisms
These organisms are the consumers of the biosphere
The most obvious type of heterotrophs feeds on other organisms.
Animals feed this way
Other heterotrophs decompose and feed on dead organisms or on organic litter, like feces and fallen leaves.
Most fungi and many prokaryotes get their nourishment this way
Concept 10.1: Photosynthesis
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
All green parts of a plant have chloroplasts However, the leaves are the major site of
photosynthesis for most plants The color of a leaf comes from chlorophyll, the
green pigment in the chloroplasts
Vein
Leaf cross section
Figure 10.3
Mesophyll
CO2 O2Stomata
Chloroplasts: The Sites of Photosynthesis in PlantsChloroplasts are found mainly in
mesophyll cells forming the tissues in the interior of the leaf
O2 exits & CO2 enters the leaf through pores called stomata in the leaf
Chloroplast
Mesophyll
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
ThylakoidGranumStroma
1 µm
Chloroplastshave two membranes around a central
aqueous space, the stroma
In the stroma is an elaborate system of interconnected membranous sacs, the thylakoids
Prokaryotes
Photosynthetic prokaryotes lack chloroplasts
Their photosynthetic membranes arise from infolded regions of the plasma membranes, folded in a manner similar to the thylakoid
membranes of chloroplasts
Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
Notes: Water is input and output It’s really 2 G3P that are output, not glucose
The Splitting of Water
One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not
CO2
This was not shown until the 1950s.
C. B. van Niel suggested water split
In the bacteria that he was studying, hydrogen sulfide (H2S), not water, is used in photosynthesis
These bacteria produce yellow globules of sulfur as a waste, rather than oxygen
Van Niel proposed this chemical equation for photosynthesis in sulfur bacteria:
CO2 + 2H2S [CH2O] + H2O + 2S
He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis: CO2 + 2H2O [CH2O] + H2O + O2
In the 1950s…
Other scientists confirmed van Niel’s hypothesis twenty years later
They used 18O, a heavy isotope, as a tracer
They could label either C18O2 or H218O
They found that the 18O label only appeared in the oxygen produced in photosynthesis when
water was the source of the tracer
Photosynthesis as a Redox Process
Photosynthesis is a redox process Water is oxidized
Because electrons are more equally shared in O2 (potential energy has been lost)
Carbon dioxide is reduced Because it takes energy to move the electrons away
from oxygen and share equally in glucose (energy has been stored)
The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of:
1. The light reactions (the photo part)
2. The Calvin cycle (the synthesis part)
The Two Stages of Photosynthesis: A Preview The light reactions (in the thylakoids):
Split H2O
Release O2
Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation
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
An overview of photosynthesis
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
[CH2O](sugar)
NADPH
NADP
ADP
+ P
O2Figure 10.5
ATP
Concept 10.2: The thylakoids convert solar energy to the chemical energy of ATP & NADPH
Light Is a form of electromagnetic energy, which
travels in rhythmic waves
Wavelength Is the distance between the crests of waves Determines the type of electromagnetic energy The shorter the wavelength, the higher the
energy
The electromagnetic spectrum
Gammarays X-rays UV Infrared
Micro-waves
Radiowaves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m
106 nm 103 m
380 450 500 550 600 650 700 750 nm
Visible light
Shorter wavelength
Higher energy
Longer wavelength
Lower energyFigure 10.6
The visible light spectrum
380 – 750 nm Includes the colors of light we can see Includes the wavelengths that drive photosynthesis
While light travels as a wave, many of its properties are those of a discrete particle, the photon
Photosynthetic Pigments: The Light Receptors When light meets matter, it may be:
Reflected Transmitted Absorbed
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or
transmitted Leaves appear green because chlorophyll
reflects and transmits green light
Animation: Light and PigmentsAnimation: Light and Pigments
Reflect light, which include the colors we see
Light
ReflectedLight
Chloroplast
Absorbedlight
Granum
Transmittedlight
Figure 10.7
The spectrophotometer
A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light
It beams narrow wavelengths of light through a solution containing the pigment & measures the fraction of light transmitted at each wavelength
An absorption spectrum
Is a graph plotting light absorption versus wavelength
Figure 10.8
Whitelight
Refractingprism
Chlorophyllsolution
Photoelectrictube
Galvanometer
Slit moves topass lightof selectedwavelength
Greenlight
The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
The low transmittance(high absorption) readingchlorophyll absorbs most blue light.
Bluelight
1
2 3
40 100
0 100
The absorption spectra of chloroplast pigments
Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis
Chlorophyll a, the dominant pigment, absorbs best in the red and violet-blue wavelengths and least in the green Other pigments with different structures have
different absorption spectra
The absorption spectra of three types of pigments in chloroplasts
Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.
EXPERIMENT
RESULTSA
bso
rptio
n o
f lig
ht
by
chlo
rop
last
pig
me
nts
Chlorophyll a
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.
Wavelength of light (nm)
Chlorophyll b
Carotenoids
Figure 10.9
Rat
e o
f ph
otos
ynth
esis
(mea
sure
d by
O2 r
elea
se)
Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.
(b)
The action spectrum of a pigment Profiles the relative effectiveness of different
wavelengths of radiation in driving photosynthesis
400 500 600 700
Aerobic bacteria
Filamentof alga
Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.
(c)
Light in the violet-blue and red portions of the spectrum are most effective in driving
photosynthesis.
CONCLUSION
The action spectrum for photosynthesisWas first demonstrated by Theodor W.
Engelmann in 1883
C
CH
CH2
CC
CC
C
CNNC
H3C
C
CC
C C
C
C
C
N
CC
C
C N
MgH
H3C
H
C CH2CH3
H
CH3C
HHCH2
CH2
CH2
H CH3
C O
O
O
O
O
CH3
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown
Figure 10.10
Structure of chlorophyll molecules
Chlorophyll a Is the main photosynthetic pigment
Chlorophyll b Is an accessory pigment
Only chlorophyll a participates directly in the light reaction, but accessory photosynthetic pigments absorb light & transfer energy to chlorophyll a
Carotenoids
Participate in photoprotection against excessive light
These compounds absorb & dissipate excessive light energy that would otherwise: damage chlorophyll or interact with oxygen to form reactive
oxidative molecules that could damage the cell
Similar pigment in human eyes (so eat your 5 fruits and veggies a day!)
Excitation of Chlorophyll by Light
When a pigment absorbs light It goes from a
ground state to an excited state, which is unstable
A particular compound absorbs Only photons of a
specific wavelength
Excitedstate
Ene
rgy
of e
lect
ion
Heat
Photon(fluorescence)
Chlorophyllmolecule
GroundstatePhoton
e–
Figure 10.11 A
If an isolated solution of chlorophyll is illuminated
It will fluoresce, giving off light and heat
Figure 10.11 B
A photosystem
Is composed of a reaction center surrounded by a number of light-harvesting complexes
What’s being reduced?
Primary electionacceptor
Photon
Thylakoid
Light-harvestingcomplexes
Reactioncenter
Photosystem
STROMA
Thy
lako
id m
embr
ane
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Figure 10.12
e–
In the photosystems...
The light-harvesting complexes Consist of pigment molecules bound to
particular proteins Funnel the energy of photons of light to the
reaction center
When a reaction-center chlorophyll molecule absorbs energy One of its electrons gets bumped up to a
primary electron acceptor
Two types of photosystems
Photosystem II Discovered second, but comes first Most effectively absorbs light at 680 nm (P680)
Photosystem I Discovered first, but comes second Most effectively absorbs light at 700 nm (P700)
Use identical chlorophyll a but are associated with different proteins in the thylakoid membrane
Linear electron flow
Is the primary pathway of energy transformation in the light reactions
Also the most evolved, requiring both photosystem II and I to work together
Inputs: H20 and light/photons
Outputs: NADPH, ATP, and oxygen
Figure 10.13Photosystem II
(PS II)
Photosystem-I(PS I)
ATP
NADPH
NADP+
ADP
CALVINCYCLE
CO2H2O
O2 [CH2O] (sugar)
LIGHTREACTIONS
Light
Primaryacceptor
Pq
Cytochromecomplex
PC
e
P680
e–
e–
O2
+
H2O
2 H+
Light
ATP
Primaryacceptor
Fd
ee–
NADP+
reductase
ElectronTransportchain
Electron transport chain
P700
Light
NADPH
NADP+
+ 2 H+
+ H+
1
5
7
2
3
4
6
8
A mechanical analogy for the light reactions
MillmakesATP
ATP
e–
e–e–
e–
e–
Pho
ton
Photosystem II Photosystem I
e–
e–
NADPH
Pho
ton
Figure 10.14
Cyclic Electron Flow
• Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle
Fig. 10-15
ATPPhotosystem II
Photosystem I
Primary acceptor
Pq
Cytochromecomplex
Fd
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
NADP+
+ H+
Some organisms such as purple sulfur bacteria have PS I but not PS II
Cyclic electron flow is thought to have evolved before linear electron flow
Cyclic electron flow may protect cells from light-induced damage
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Chloroplasts and mitochondria Generate ATP by the same basic mechanism:
chemiosmosis But use different sources of energy to
accomplish this (organic molecules vs. light energy)
Fig. 10-16
Key
Mitochondrion Chloroplast
CHLOROPLASTSTRUCTURE
MITOCHONDRIONSTRUCTURE
Intermembranespace
Innermembrane
Electrontransport
chain
H+ Diffusion
Matrix
Higher [H+]Lower [H+]
Stroma
ATPsynthase
ADP + P i
H+ATP
Thylakoidspace
Thylakoidmembrane
Chemiosmosis
In both organelles Redox reactions of electron transport chains
generate a H+ gradient across a membrane
ATP synthase Uses this proton-motive force to make ATP
Some of the electron carriers, including the cytochromes, are very similar in chloroplasts & mitochondria
The proton gradient
or pH gradient, across the thylakoid membrane is substantial
When chloroplasts are illuminated the pH in the thylakoid space drops to about 5 the pH in the stroma increases to about 8 a thousandfold difference in H+ concentration
Light
Fd
Cytochromecomplex
ADP +
i H+
ATPP
ATPsynthase
ToCalvinCycle
STROMA(low H+ concentration)
Thylakoidmembrane
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
LightNADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2OO2
e–
e–
1/21
2
3
Concept 10.3: The Calvin cycle uses ATP & NADPH to convert CO2 to sugar
The Calvin cycle Is similar to the citric acid cycle Occurs in the stroma
The Calvin cycle has three phases Carbon fixation (making G3P) Reduction Regeneration of the CO2 acceptor
The Calvin Cycle – What to Know
Know all the inputs & outputsKnow which molecules are the same in the
citric acid cycle & the Calvin cycleWhat is so special about G3P & Rubisco?Ribulose biphosphate (RuBP) is the CO2
acceptorWhat the complimentary compound to
RuBP in the citric acid cycle?
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 a seemingly wasteful
process called photorespiration
cuticle
Internal leaf structure of a C3 plant
upperepidermis
mesophyllcells
lowerepidermis
chloroplasts
bundle sheathvascular bundle(vein)
stoma
C3 Plants & Photorespiration
C3 plants – Calvin cycles initial steps form a 3-carbon compound
C3 plants include rice, wheat & soybeans
On dry hot days photorespiration increases O2 substitutes for CO2 in the active site of the
enzyme rubisco
Photorespiration consumes ATP
Photorespiration: An Evolutionary Relic?The photosynthetic rate is reduced
sometimes as much as 50%This metabolic pathway seems
counterproductive for plantsIs it, or do we just not understand enough?If it is, how could it have evolved?Maybe on a planet with less O2, there was
no penalty for having an enzyme (rubisco) that sometimes processes O2 instead of CO2
C4 Plants
C4 plants minimize the cost of photorespiration By incorporating CO2 into a four carbon
compound (oxaloacetate) in mesophyll cells Using PEP carboxylase
These 4-carbon compounds Are exported to bundle sheath cells, where they
release CO2 used in the Calvin cycle
C4 Plants - Adaptation
PEP carboxylase has no affinity for O2 unlike rubisco, so no photorespiration in mesophyll cells
CO2 is kept high in bundle-sheath cells so rubisco doesn’t process O2
This adaptation is especially useful in hot, dry climates
Corn, sugarcane and some invasive & weedy plants (like crabgrass) have this adaptation
C4 leaf anatomy and the C4 pathway
CO2
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Photosyntheticcells of C4 plantleaf
Stoma
Mesophyllcell
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C) PEP (3 C)
Malate (4 C)
ADP
ATP
Bundle-Sheathcell CO2
Pyruate (3 C)
CALVINCYCLE
Sugar
Vasculartissue
Figure 10.19
CO2
CAM (Crassulacean acid metabolism) Plants
From the plant family Crassulaceae (succulents like sedums)
During the night, the stomata open and they store CO2 in organic acids until day...
During the day, the stomata close And the CO2 is released from the organic acids
for use in the Calvin cycle
CAM Plants - Adaptations
Similar to C4, but the separation is temporal instead of spatial
Another adaptation to hot, dry climatesJade (and similar) plants, many cacti and
pineapples are some examples
Both C4 and CAM also conserve water
CO2
Sugarcane
Mesophyllcell
CO2
C4
Bundle-sheathcell
Organic acidsrelease CO2 to Calvin cycle
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Pineapple
Night
Day
CAM
SugarSugar
CalvinCycle
CalvinCycle
Organic acid Organic acid
(a) Spatial separation of steps (b) Temporal separation of steps
CO2 CO2
1
2
The Importance of Photosynthesis: A Review
Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere
Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions
O2
CO2H2O
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP 3-Phosphoglycerate
Amino acidsFatty acids
Starch(storage)
Sucrose (export)
G3P
Photosystem IIElectron transport chain
Photosystem I
Chloroplast
Figure 10.21
On a global scale...
Photosynthesis is the most important process on Earth.
It is responsible for the presence of oxygen in our atmosphere.
Each year, photosynthesis synthesizes 160 billion metric tons of carbohydrate.
No process is more important than photosynthesis to the welfare of life on Earth.
You should now be able to:
1. Describe the structure of a chloroplast
2. Describe the relationship between an action spectrum and an absorption spectrum
3. Trace the movement of electrons in linear electron flow
4. Trace the movement of electrons in cyclic electron flow
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
5. Describe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts
6. Describe the role of ATP and NADPH in the Calvin cycle
7. Describe the major consequences of photorespiration
8. Describe two important photosynthetic adaptations that minimize photorespiration
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings