bio 2, lecture 14
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BIO 2, Lecture 14. FIGHTING ENTROPY III: PHOTOSYNTHESIS. Autotrophs sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other inorganic molecules - PowerPoint PPT PresentationTRANSCRIPT
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BIO 2, Lecture 14BIO 2, Lecture 14FIGHTING ENTROPY III:FIGHTING ENTROPY III:
PHOTOSYNTHESISPHOTOSYNTHESIS
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• 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 complex organic molecules from H2O and CO2
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• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes
• These organisms feed not only themselves but also most of the living world
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(a) Plants
(c) Unicellular protist10 µm
1.5 µm
40 µm(d) Cyanobacteria
(e) Purple sulfur bacteria
(b) Multicellular alga
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• 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
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• In plants, the work of photosynthesis is done by organelles called chloroplasts
• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
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• In plants, leaves are the major locations of photosynthesis
• Their green color is from chlorophyll, the green pigment within chloroplasts
• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
• CO2 enters and O2 exits the leaf through microscopic pores called stomata
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• Chloroplasts are found mainly in cells of the mesophyll, an interior tissue of the leaf• A typical mesophyll cell has 30–40
chloroplasts
• The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
• Chloroplasts also contain stroma, a dense fluid
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5 µm
Mesophyll cell
Stomata CO2 O2
Chloroplast
Mesophyll
VeinLeaf cross section
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1 µm
Thylakoidspace
Chloroplast
GranumIntermembranespace
Innermembrane
Outermembrane
Stroma
Thylakoid
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• Photosynthesis can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
• Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
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• Photosynthesis is a redox process in which H2O is oxidized (to O2) and CO2 is reduced (to C6H12O6)
Reactants: 6 CO2
Products:
12 H2O
6 O26 H2OC6H12O6
Reduced CO2
Oxidized H2O
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Photosynthesis consists of two parts:
1.“Photo” part = light-dependent reactions• Require light; only occur in the daytime
2.“Synthesis” (of sugar) part = light independent reactions• Also called the Calvin cycle• Occur both in the daytime and at
night• Plant switches most energy to
Calvin Cycle at night
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• The light reactions– Occur in the thylakoid membrane and
thylakoid space (inside the thylakoid)– Split H2O into H+ and O2 (gas)– Reduce NADP+ to NADPH– Generate ATP from ADP and Pi
• The Calvin cycle • Occurs in the stroma• Forms sugar from CO2, using the ATP and
NADPH generated in the light reactions• Begins with carbon fixation,
incorporating CO2 into organic molecules
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Light
H2O
Chloroplast
LightReactions
NADP+
PADP
i+
ATP
NADPH
O2
CalvinCycle
CO2
[CH2O](sugar)
“Dark” (light-independent)
reactions (occur in the presence and absence of light)
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• Light is a form of electromagnetic energy, also called electromagnetic radiation
• Like other electromagnetic energy, light travels in rhythmic waves
• Wavelength is the distance between crests of waves• Wavelength determines the type of
electromagnetic energy• Shorter wavelength = higher energy
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• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
• Visible light consists of wavelengths (including those that drive photo-synthesis) that produce colors we can see
• Light also behaves as though it consists of discrete particles, called photons
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UV
Visible light
Infrared Micro-waves
RadiowavesX-raysGamma
rays
103 m1 m
(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm
380 450 500 550 600 650 700 750 nmLonger wavelengthLower energyHigher energy
Shorter wavelength
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• Pigments are substances that absorb visible light• Different pigments absorb different
wavelengths
• Wavelengths that are not absorbed are reflected back (and seen by observers)• Leaves appear green because chlorophyll
reflects green light back to our eyes • It is actually the combined wavelengths
not absorbed by chlorophyll that collectively appear green
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Reflectedlight
Absorbedlight
Light
Chloroplast
Transmittedlight
Granum
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• Chlorophyll a is the main photosynthetic pigment
• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
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• When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
• When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence
• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat
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(a) Excitation of isolated chlorophyll molecule
Heat
Excitedstate
(b) Fluorescence
Photon Groundstate
Photon(fluorescence)
Ener
gy o
f ele
ctro
n e–
Chlorophyllmolecule
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• Photosynthesis begins at photosystems located in the thylakoid membrane
• A photosystem consists of a reaction-center complex (comprised of several proteins) surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center
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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
• Photosystem I (PS I) functions second and is best at absorbing a wavelength of 700 nm
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• At the beginning of photosynthesis, a primary electron acceptor in the reaction center of Photosystem II accepts an electron from chlorophyll a that has been excited by a photon
• 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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• The chlorophyll molecule that is now missing an electron is a very strong oxidizing agent
• It grabs an electron from H2O (in the thylakoid space) and is reduced
• O2 is released as a by-product of this splitting of H2O
• H+ is also formed, which begins to build up in the thylakoid space (sound familiar?)
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• The electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
• Energy released by the fall is used by proteins in the electron transport chain to drive H+ ions from the stroma into the inner space of the thylakoid
• Thus, there are two sources of build-up of H+ ions in the inner space: from the splitting of water and from the electron transport chain
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• ATP synthase, embedded in the thylakoid membrane, coverts ADP + Pi to ATP using the energy generated by the rush of the H+ ions in the inner compartment out into the stroma (with their concentration gradient)
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• Meanwhile, the electrons passing through the electron transport chain of Photo-system II are eventually dumped off at Photosystem I• When photons hit chlorophyll molecules in
Photosystem I, electrons are kicked off chlorophyll to a second electron transport chain
• Chlorophyll molecules in Photosystem I (now strong oxidixing agents) grab electrons dumped dumped off from Photosystem II to return to their reduced state
• Water is not split at Photosystem I
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• Electrons excited by photons at Photosystem I “fall” down a second electron transport chain and are eventually dumped onto NADP+, reducing it to NADPH
• The electrons of NADPH are available for the reactions of the Calvin cycle (that drive the endergonic reactions that create sugar and starch)
• ATP produced through the proton motive force are also used in the Calvin Cycle
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Millmakes
ATP
e–
NADPH
Phot
on
e–e–
e–
e–
e–
Phot
on
ATP
Photosystem II Photosystem I
e–
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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 I4 H+
4 H+
Pq
Pc
Light NADP+
reductaseNADP+ + H+
NADPH
+2 H+
H2O O2
e–e–
1/21
2
3
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• 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, which is used to produce food
• Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities
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• In mitochondria, protons are pumped out of the inner space (matrix) and into the intermembrane space• ATP synthesis occurs as they diffuse back
into the mitochondrial matrix
• In chloroplasts, protons are pumped into the inner thylakoid space and out of the stroma• ATP synthesis occurs as they diffuse back
out of the inner thylakoid space (into the stroma)
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Key
Mitochondrion Chloroplast
CHLOROPLASTSTRUCTURE
MITOCHONDRIONSTRUCTURE
Intermembranespace
Innermembrane
Electrontransport
chain
H+ Diffusion
Matrix
Higher [H+]Lower [H+]
Stroma
ATPsynthase
ADP + PiH+
ATP
Thylakoidspace
Thylakoidmembrane
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• 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• The ATP and NADPH come from the light-
dependent reactions
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• 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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Ribulose bisphosphate(RuBP)
3-Phosphoglycerate
Short-livedintermediate
Phase 1: Carbon fixation
(Entering oneat a time)
Rubisco
InputCO2
P
3 6
3
3
P
PPP
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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 P61,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)Glucose andother organiccompounds
CalvinCycle
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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 P61,3-Bisphosphoglycerate
6
P
P6
66 NADP+
NADPH
i
Phase 2:Reduction
Glyceraldehyde-3-phosphate(G3P)
1 POutput G3P
(a sugar)Glucose andother organiccompounds
CalvinCycle
3
3 ADP
ATP
5 P
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
G3P
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LightReactions:
Photosystem II Electron transport chain
Photosystem I Electron transport chain
CO2
NADP+
ADPP i+
RuBP 3-PhosphoglycerateCalvinCycle
G3PATP
NADPH Starch(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2