Photosynthesis

Campbell and ReeceChapter 10

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

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

Chloroplast Structure

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 as Redox Reactions

• photosynthesis equation: C becomes reduced & O oxidized

Summary of 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 Reactions

Light • Form of electromagnetic energy

or radiation• 380 nm – 750 nm = visible light• Photon: discrete particle of

energy– Amt nrg inversely proportional to

wavelength

Electromagnetic Spectrum

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

Absorption Spectrum• graph plotting a pigment’s light

absorption versus wavelength

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

Chlorophyll a/b

Rate of Photosynthesis vs. Wavelength of Light

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)

Phytochemicals • carotenoids in health food

products • promoted 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

Excitation of Isolated Chlorophyll Molecule

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

Photosystems• Light-Harvesting Complex acts

like “antennae” for the Reaction-Center Complex

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

P680 & P700 both Chlorophyll a

Linear Electron Flow

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

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

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

Linear Electron Flow: Figure 10.14

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

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

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

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

Linear Electron Flow:Figure 10.14

Step 8: enzyme NADP+ reductase catalyzes transfer of 2 e- from Fd NADP+ + H+ (from stroma) NADPH

NADPH provides reducing power in the Calvin Cycle

NADP+ 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

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 Light Reactions Summarized

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

Cost of Calvin Cycle• for synthesis of 1 G3P:– 9 ATP– 6 NADPH

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

Stomata • On underside of leaves, allows

for– CO2 enters plant– transpiration of 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

Photorespiration

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

Rice/Wheat/ Soy Beans

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 unique leaf anatomy:

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

C 4 Pathway

C4 Pathway: Step 2• Mesophyll cell exports the 4 C

product bundle-sheath cells via plasmodesmata

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

C 4 Pathway

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

C4 Pathway

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

CAM Photosynthesis

Comparing C4 & CAM Photosynthesis

Comparisons

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

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