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Autotrophs
• “self-feeders”• sustain themselves w/out eating
anything derived from other living things
• are the biosphere’s producers• photoautotrophs: plants, algae,
some protists, & some prokaryotes
Heterotrophs
• live on cpds produced by other organisms
• are the biosphere’s consumers• Includes: carnivores, omnivores,
decomposers
Origin of Photosynthesis
• Photosynthesis most likely originated in prokaryotes that had infolded regions of plasma membrane containing clusters of that included photosystems and photosynthetic enzymes
Chloroplasts
• All green parts of plants have chloroplasts but leaves are major sites of photosynthesis in most plants
• 1 mm² of top of a leaf contains .5 million chloroplasts!
Parts of a Leaf
• Mesophyll: leaf cells specialized for photosynthesis
• Stomata: microscopic pore surrounded by guard cells in epidermis of leaf & stems, allows gas exchange
• Veins: vascular bundle in a leaf
Chloroplast Structure
• ~2-4 μm by 4-7 μm• dbl membrane surrounding
dense fluid called the stroma• membrane w/in stroma made of
sacs called thylakoids • inside thylakoid: thylakoid space • Chlorophyll & photosystems
reside in thylakoid membrane
Photosynthesis Equation
• equation worked out in 19th century
• use glucose as product to simplify relationship with respiration but what actually happens….
Photosynthesis Equation
• direct product is 3-C sugar that can be used to make glucose
• light
• CO2 + H2O [CH2O] + O2
Photosynthesis Equation
• [CH2O] brackets mean what is inside is not actual sugar but represents general formula for carbohydrate (building a carbohydrate 1 carbon at a time)
Photosynthesis Equation
• originally, it was thought the O2 released came from CO2
• assumption challenged in 1930’s (van Niel, Stanford University)– Studied bacteria that use CO2 to
make carbohydrates but do not release O2
– Concluded that at least in the bacteria he studied CO2 was not split
Photosynthesis Equation
• Another group bacteria use H2S (hydrogen sulfide) instead of water releasing S:
• CO2 + 2H2S [CH2O] + H2O + 2S• vanNiel concluded the H2S split &
H was used with C to build sugars & inferred water must split in photosynthesis with H to sugars & O released
Photosynthesis Equation
• Van Niel’s hypothesis confirmed in 1950’s when O-18 used in water and traced thru photosynthesis
Photosynthesis Vocabulary
• Photophosphorylation: process of generating ATP from ADP + P by means of chemiosmosis using a proton-motive force generated across the thylakoid membrane
• Carbon Fixation: incorporation of C from CO2 into an organic cpd by an autotroph
Light
• Form of electromagnetic energy or radiation
• 380 nm – 750 nm = visible light• Photon: discrete particle of
energy– Amt nrg inversely proportional to
wavelength
Photosynthesis Pigments
• pigment substance that absorbs light– different pigments absorb different
wavelengths–we see color that is reflected (not
absorbed)– Chlorophyll absorbs violet-blue &
red
Spectrophotometer
• Measures the proportion of light of different wavelengths absorbed & transmitted by a pigment solution
Chlorophyll’s Absorption Spectrum
• gives clues to relative effectiveness of different wavelengths for driving photosynthesis (light can perform work in chloroplasts only if it is absorbed)
Pigments in Photosynthesis• chlorophyll a
– Violet-blue & red light works best– appear blue green
• chlorophyll b– violet blue– appear olive green
• carotenoids – violet & blue-green– appear various shades yellow &
orange
Carotenoids
• absorb wavelengths that chlorophylls do not
• more importantly they function as photoprotection: they absorb & dissipate light nrg that would otherwise damage chlorophyll or would interact with O2 yielding reactive oxidative products that would damage cell (act as antioxidants)
What Happens When Chlorophyll Absorbs Light ?
• colors from corresponding wavelengths that are absorbed disappear from the spectrum of transmitted & reflected light
• but nrg cannot disappear
What Happens When Chlorophyll Absorbs Light?
• nrg from photon absorbed by e- in pigment (more nrg .. e- moves to orbital further out from nucleus where it has more PE & less stability)
• ground state: when e- in normal orbital
• excited state: e- in orbital of higher nrg
Which Photons are Absorbed?
• only those photons whose wavelength’s nrg is exactly = to the nrg difference between the ground & excited states
• explains why each pigment has unique absorption spectrum
Electrons in the Excited State
• unstable when excited• if isolated: chlorophyll molecules
absorb photons the e- immediately drop back to ground state releasing nrg as heat or light (fluorescence) & heat
Photosystems
• chlorophyll molecules in native state found in thylakoid membrane organized into complexes called photosystems
Photosystem• composed of:1. Reaction-Center Complex
surrounded by:2. Light-Harvesting Complex– consists of various pigments bound
to proteins:– chlorophyll a– chlorophyll b– carotenoids
Reaction-Center Complex
• contains molecule (the primary e- acceptor) capable of accepting e- & becoming reduced
1st Step of Light Reactions
• solar-powered transfer of e- from reaction-center chlorophyll a primary e- acceptor
• redox reaction
2 Types of Photosystems• PS II– It’s reaction-center complex known
as P680– 680 wavelength in red part of
spectrum
• PS I– It’s reaction-center complex known
as P700– 700 wavelength far red part of
spectrum
Linear e- Flow Figure 10.14
• 1. photon strikes PS II chlorophyll a & 1 e- jumps to higher nrg level -> this e- falls back to its ground state transferring its nrg to nearby chlorophyll molecule so 1 of its e- jump to excited state …..continues like this until it reaches P680 pair of chlorophyll a molecules in reaction-center & a pair of e- jumps to excited state
Linear Electron Flow:Figure 10.14
Step 2: e- transferred from excited P680 primary e- acceptor making P680 P680+
Linear Electron Flow:Figure 10.14
Step 3: enzyme splits 2water 4 e- + 4 H+ + O2
2 e- replace those that left P680 so P680+ returns to P680
one of strongest biological oxidizing agents
H+ released into thylakoid spaceO2 diffuses out of chloroplast out
of cell
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesishttp://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html
http://bcs.whfreeman.com/thelifewire/content/chp08/0802002.html
Linear Electron Flow:Figure 10.14
Step 4: excited e- from PSII PS I thru e- transport chain
between PS II and PS I e- carrier plastoquinone (Pq), a cytochrome complex, & a protein called plastocyanin (Pc)
Linear Electron Flow:Figure 10.14
Step 5: as e- pass thru cytochrome complex, H+ are pumped into the thylakoid lumen, building H+ gradient (later used in chemiosmosis)
Linear Electron Flow:Figure 10.14
Step 6: also happening: photons absorbed by PS I reaction center complex exciting e- of P700 pair of chlorophyll a molecules
photoexcited e- PS I e- acceptor creating “e- hole” in P700+
P700+ gets e- from bottom of e- transport chain that started @ PS II
Linear Electron Flow:Figure 10.14
Step 7: photoexcited e- from PS I passed thru series of redox reactions down 2nd e- transport chain thru the protein ferrodoxin (Fd)
this 2nd chain does not add to H+ gradient & does not make ATP
Linear Electron Flow:Figure 10.14
Step 8: enzyme NADP+ reductase catalyzes transfer of 2 e- from Fd NADP+ + H+ (from stroma) NADPH
Cyclic Electron Flow
• Only uses PS I• e- cycle back from Fd
Cytochrome complex P700• no NADPH• no release of O2
• + generation of ATP (supplements supply of ATP)
Cyclic Electron Flow
• used by some photosynthetic bacteria
• only have PS I• evolutionary biologists: these
bacteria descendants of bacteria in which photosynthesis 1st evolved– ex: purple sulfur bacteria
Cyclic Electron Flow• also can occur in photosynthetic
species with both PS II and PS I• ex: cyanobacteria, eukaryotic
photosynthetic species• at least 1 benefit: mutant plants
that cannot carry out cyclic e- flow grow well only in low light; do not grow well in intense light …. So cyclic e- flow must have a photoprotective role
Chemiosmosis• exact same basic mechanism used
in chloroplasts & mitochondria:e- transport chain in a membrane actively pumps protons (H+) across it while e- move thru series of carriers 1 more electronegative than the last 1
ATP synthase also in membrane using PE of proton gradient that couples the diffusion of H+ with phosphorylation of ADP
Chemiosmosis
• e- carriers & ATP synthase similar in chloroplasts & mitochondria
• Differences: –Mitochondria: oxidative
phosphorylation• e- moving thru from organic molecules• driven by nrg from food
– Chloroplasts: photophosphorylation• e- moving thru from water • driven by nrg from photons
Proton Gradient across Thylakoid Membrane
• can think of it as pH gradient: w/light can drop pH of thylakoid space to 5 as pH in stroma rises to 8
• difference in pH corresponds to a thousandfold difference in [H+]
• take away the light and pH gradient is lost
The Calvin Cycle
• anabolic • endergonic: uses ATP as nrg
source & NADPH as reducing power
• C added as CO2 sugar which is a 3-C sugar: glyceraldehyde 3-phosphate (G3P)
The Calvin Cycle
• goes around 3x to add 3 C from CO2 to make G3P
• carbon fixation: incorporation of CO2 into organic material
The Calvin Cycle• Phase 1: Carbon Fixation• incorporates 1 C @ time by attaching
it to a 5-carbon sugar: ribulose biphosphate (RuBP)
• enzyme that catalyzes reaction is RuBP carboxylase (rubisco), the most abundant protein in chloroplasts (maybe on Earth)
• product is 6-C intermediate so unstable it immediately splits in half 2 3-phosphoglycerate for each CO2
The Calvin Cycle
• Step 2: Reduction• each 3-phosphoglycerate recieves
1 more phosphate group (from ATP) 1,3-biphosphoglycerate
• NADPH hands over pair of e- & phosphate group is lost G3P
• for every 3 CO2 that enter cycle 6 G3P formed but only 1 is a net gain which leaves cycle to be used by plant cell
The Calvin Cycle
• Phase 3: Regeneration of RuBP:• carbon skeletons of 5 molecules
of G3P rearranged by last steps of Calvin Cycle into 3 molecules of RuBp
• costs 3 ATP• RuBP starts cycle over
G3P from Calvin Cycle
• becomes starting material for metabolic pathways that synthesize other organic cpds
Alternatives Mechanisms for Carbon Fixation
• Evolutionary History– 475 million yrs ago plants moved on
land– adaptations to land came with
trade-offs: need to keep water in and yet photosynthesis uses water
Water Homeostasis in Land Plants
• dry, hot weather most plants close their stomata to conserve water …. reduces photosynthesis output because less CO2 entering cell … CO2 concentrations decrease in air spaces in leaf and O2 released from light reactions increases
Photorespiration
• a metabolic pathway that consumes oxygen & ATP, releases CO2 & decreases photosynthetic output
• generally occurs on hot, dry, bright days, when stomata close & the O2/CO2 ratio in leaf increases, favoring the binding of O2 rather than CO2 by rubisco
C3 Plants• most plants: initial fixation of C
occurs via rubisco (enzyme that adds CO2 to ribulose phosphate)
• these plants called C3 because the 1st organic cpd of C fixation is a 3-C cpd, 3-phosphoglycerate
• ex: soybeans, rice, wheat
C3 Plants
• stomata close on hot, dry days…produce less sugar because of declining CO2 levels … rubisco binds to O2 ….product splits & a 2-C cpd leaves chloroplast
Photorespiration in C3 Plants
• Photo because occurs in light • Respiration because it consumes
O2 while releasing CO2
• *no ATP generated; no sugar made
• ATP consumed
Potato plants
Photorespiration in C3 Plants
• Overall decreases photosynthetic output by siphoning organic material from Calvin Cycle & releasing CO2 that would otherwise be fixed
?? Big Question ??
• Why is there a metabolic process that seems to be counterproductive for the plant?
• 1 hypothesis:–Evolutionary relic from long
ago when atmosphere had less O2 & more CO2
– sugar beets
Problems with Photorespiration
• drains as much as 50% of carbon fixed by Calvin Cycle in a significant # of crop plants
• if scientists could reduce photorespiration in crop plants photosynthetic productivity crop yields & food supplies would increase
Photosynthetic Adaptations
• In some plant species alternate modes of carbon fixation have evolved that minimize photorespiration & optimize the Calvin Cycle, even in hot arid climates:
1. C 4 Photosynthesis2. CAM (crassulacean acid
metabolism)
C 4 Plants• C”4” because Calvin Cycle is
prefaced with alternate mode of carbon fixation so Cycle starts with a 4 C cpd as its 1st product
• ex: sugar cane, corn, grasses
C 4 Plants
• have 2 distinct types of photosynthetic cells:
1. bundle-sheath Cells2. mesophyll Cells
Bundle-Sheath Cells
• arranged around veins of the leaf
• Calvin Cycle limited to chloroplasts of bundle-sheath cells
Mesophyll Cells• Surround bundle-sheath cells• Incorporate CO2 into organic cpds • 3 steps in C 4 pathway:1. In mesophyll cells (only place
have enzyme: PEP carboxylase) CO2 added to PEP 4 C oxaloacetate … works even in hot, dry conditions
C4 Pathway: Step 3
• In bundle-sheath cells the 4 C cpd releases CO2 used in Calvin Cycle & releases pyruvate which mesophyll cells where ATP used to convert pyruvate to PEP so cycle can continue
C4 Pathway Advantage• Mesophyll cells of C4 plants keep
[CO2] hi enough in bundle-sheath cells for rubisco to bind CO2 not O2
• cyclic series of reactions involving PEP carboxylase & regeneration of PEP can be thought of as a CO2-concentrating pump powered by ATP
Effects of Increasing CO2 in Atmosphere
on C 3 Plants• rising [CO2]
should lower amount of photorespiration but rising average temperatures will have opposite effect
On C 4 Plants• rising [CO2] or
temperatures should have no affect
CAM Plants
• 2nd photosynthetic adaptation to arid conditions
• in many succulents• ex: cacti, pineapple
CAM Plants• open their stomata during night
& close them during day• conserves water and prevents
CO2 from entering leaf• CO2 enters leaf during night
where it is incorporated into a variety of organic acids
CAM Plants• this mode of carbon fixation called:
Crassulacean Acid Metabolism• organic acids made with CO2
during the night stored in vacuoles in mesophyll cells until daylight when stomata close light reactions start…. Calvin Cycle has CO2 coming from the stored organic acids so sugars made as usual during day
Comparing C4 & CAM Photosynthesis
Importance of Photosynthesis• responsible for presence of O2
in atmosphere• makes an estimated 160 billion
metric tons of carbohydrate per year
• No other process as important to welfare of life on Earth