Energy-Acquiring Pathways

AEROBIC RESPIRATIONmain energy-releasing process

Usable energy released when glucose broken down.Released energy coupled to electron transfers to produce 36 ATP molecules.

Sunlight energy converted to chemical bond energy in ATP.NADPH also forms.ATP and NADPH used in rxns. that form glucose

PHOTOSYNTHESISmain energy-acquiring process

O2

released

O2

required

CO2

+ H2Oreleased

CO2

+ H2Orequired

Photosynthesis has 2 stages:

• Light-dependent stage: trapping of sunlight energy

• Light-independent stage: formation of glucose

Photosynthesis equation =Photosynthesis equation =

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2sunlight

glucosechlorophyll a

= biosynthetic pathway

Photosynthesis occurs in the chloroplast• Grana (= thylakoid membrane): the inner

membrane system folded into a series of fattened disks extending through the semi-fluid interior. The site of light-dependent reactions.– Contain clusters of proteins + 200-300

pigments = photosystems

• Stroma: the semi-fluid interior. The site of the light-independent reactions.

thylakoid membrane system

stroma

Light-dependent Reactions

• Occur in Thylakoid MembraneOccur in Thylakoid Membrane• Sunlight energy excites electrons in Sunlight energy excites electrons in

Photosystems (clusters of light-trapping Photosystems (clusters of light-trapping pigments and proteins) pigments and proteins)

• Photosystems give up electrons to electron Photosystems give up electrons to electron transport systemstransport systems

• Energy drives ATP (and NADPH) formationEnergy drives ATP (and NADPH) formation

Electron Transport Systems

most energy

least energy

e-

e-

e-e-

energy released used for ATP synthesis

Two different light-dependent pathways• Cyclic pathway of ATP formation

– involves a Type I photosystem– electrons are recycled back to photsystem

• Non-cyclic pathway of ATP formation– involves a Type II and Type I photosystem– electrons are replaced by less excited ones– oxygen is produced as a waste product

Cyclic Photophosphorylation

Non-cyclic Photophosphorylation

• Transfers electrons thru Transfers electrons thru twotwo photosystems photosystems (II and I) and (II and I) and twotwo electron transport systems electron transport systems in the thylakoid membranein the thylakoid membrane

• Starts with photosystem II Starts with photosystem II • Incoming photons excite electrons which Incoming photons excite electrons which

enter electron transport systementer electron transport system

Non-cyclic Photophosphorylation

• photolysis breaks down Hphotolysis breaks down H22O into electrons, O into electrons, HH++, and oxygen, and oxygen– tthe electrons replace those of photosystem II that he electrons replace those of photosystem II that

entered electron transport systementered electron transport system– HH++ remain inside thylakoid membrane (involved remain inside thylakoid membrane (involved

in chemiosmosis and ATP formation)in chemiosmosis and ATP formation)– oxygen released as by-product and diffuses out of oxygen released as by-product and diffuses out of

the systemthe system

Non-cyclic Photophosphorylation

• Photosystem I’s chlorophyll P700 absorbs photon Photosystem I’s chlorophyll P700 absorbs photon energy and electrons from its reaction center are energy and electrons from its reaction center are excited and enter electron transport systemexcited and enter electron transport system

• The electrons that left photosystem II arrive at The electrons that left photosystem II arrive at photosystem I and replace those that just leftphotosystem I and replace those that just left

• In the second electron transport system, NADPIn the second electron transport system, NADP++ accepts two electrons plus an H+ ion from accepts two electrons plus an H+ ion from photolysis to become NADPH.photolysis to become NADPH.

• Both NADPH and ATP enter the light-Both NADPH and ATP enter the light-independent stage of photosynthesis.independent stage of photosynthesis.

ATP Formation in Chloroplasts• HH++ accumulation sets up strong concentration accumulation sets up strong concentration

and electrical gradientand electrical gradient

• HH++ follow gradient across thylakoid follow gradient across thylakoid membrane into stroma by passing thru membrane into stroma by passing thru interior of transport proteins called ATP interior of transport proteins called ATP synthases;synthases;--> flow drives ADP + P--> flow drives ADP + Pii --> ATP --> ATP

= = chemiosomosischemiosomosis

The light-independent stage of photosynthesis

Photosynthesis equation =Photosynthesis equation =

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2sunlight

glucosechlorophyll a

For each glucose molecule formed during photosynthesis, a total of 6 CO2 moleculesmust be fixed.

The Calvin-Benson Cycle

• Each CO2 molecule goes through a number of steps beginning with Carbon Fixation = inorganic CO2 is converted into organic carbon– carbon from the air diffuses into cell– the enzyme rubisco attaches the inorganic carbon

to a 5-carbon RuBP molecule, resulting in the formation of a 6-carbon intermediate that is unstable

– the unstable intermediate splits into two 3-carbon stable, organic PGA molecules

Carbon Fixation• RuBP (5C) + CORuBP (5C) + CO22 --> 2 “3PG” (3C) --> 2 “3PG” (3C)

– ribulose 1, 5-bisphosphate --> 3-phosphoglycerateribulose 1, 5-bisphosphate --> 3-phosphoglycerate

= = carbon fixationcarbon fixation• Enzyme = Rubisco (ribulose bisphosphate Enzyme = Rubisco (ribulose bisphosphate

carboxylase/oxygenase)carboxylase/oxygenase)- 2 subunits form mature enzyme- 2 subunits form mature enzyme- - rbcLrbcL gene (large subunit) in chloroplast genome gene (large subunit) in chloroplast genome- - rbcSrbcS in nuclear genome in nuclear genome

The Calvin-Benson Cycle• ATP provides the energy and attaches a phosphate to each

PGA molecule and NADPH donates H+ and electrons to form two PGAL molecules

• Since we started with 6 CO2 molecules, we end up with 12 PGAL molecules

• 2 of these PGAL molecules combine to become a sugar phosphate = glucose: Pi - C6H12O6

• The 10 remaining PGAL molecules are rearranged to recycle the 6 RuBP molecules needed to fix the next 6 CO2 molecules

Calvin-Benson Cycle

6 CO6 CO2 2 + 6 RuBP + 6 RuBP 6 unstable 6-carbon 6 unstable 6-carbon intermediates intermediates 12 stable 3-carbon 12 stable 3-carbon PGA PGA 1 glucose + 10 recycled PGA 1 glucose + 10 recycled PGA

At a cost of:At a cost of:12ATP --> 12ADP + P12ATP --> 12ADP + Pii

12NADPH --> 12NADP12NADPH --> 12NADP++

Calvin-Benson Cycle• Glucose --> celluloseGlucose --> cellulose• Glucose --> sucrose (most easily transported)Glucose --> sucrose (most easily transported)

--> starch --> starch• Starch is main storage form, but can be Starch is main storage form, but can be

readily converted to sucrose for transportreadily converted to sucrose for transport

Photosynthesis Types

• Most plants are CMost plants are C33 photosynthesizers: photosynthesizers:

- first product of Calvin-Benson cycle is a - first product of Calvin-Benson cycle is a 3-carbon molecule (3PG)3-carbon molecule (3PG)

Photorespiration• Normal conditions CONormal conditions CO22 and O and O22 pass through openings in pass through openings in

leaves called stomataleaves called stomata• ProblemProblem: Rubisco can't distinguish between CO: Rubisco can't distinguish between CO22 and O and O22 • When COWhen CO22 can't get in (hot dry climates), O can't get in (hot dry climates), O22 that is that is

building up in cells gets attached to Rubisco = building up in cells gets attached to Rubisco = PhotorespirationPhotorespiration (plants become unhappy) (plants become unhappy)

- RuBP + ORuBP + O2 2 --> 1 3PG + glycolate --> 1 3PG + glycolate

glycolate --> glycineglycolate --> glycine2 glycine --> serine + CO2 glycine --> serine + CO22

net effect = less carbohydrate formed!!net effect = less carbohydrate formed!!

Coping Strategies:• Two separate mechanisms have evolved to

adapt to hot dry environments – C4 plants: (fix carbon as the 4-carbon

oxaloacetate). Examples include crabgrass, sugarcane, etc.

– CAM plants: (Crassulacean Acid Metabolism) Examples include cacti, pineapple, and other desert succulents.

C4 plants• C4 plants have 2 types of photosynthetic

cells. – Mesophyll cells use CO2 to produce a 4-carbon

intermediate called oxaloacetate, which is transferred to bundle sheath cells around leaf veins.

– In the bundle sheath cells, CO2 is released and fixed again in the Calvin-Benson cycle.

• Carbon is therefore fixed twice. • Catch: uses energy!

C4 Photosynthesis• Mesophyll cells have PEP (phosphoenolpyruvate) Mesophyll cells have PEP (phosphoenolpyruvate)

carboxylase in place of Rubiscocarboxylase in place of Rubisco• COCO22 + PEP --> oxaloacetate (4C molecule) + PEP --> oxaloacetate (4C molecule)• Bundle sheath cells have Rubisco for reaction:Bundle sheath cells have Rubisco for reaction:

- RuBP + CO- RuBP + CO22 (released from oxaloacetate (released from oxaloacetate --> 2, --> 2, 3PG3PG

• PEP carboxylasePEP carboxylase- does not have oxygenase activity- does not have oxygenase activity- fixes CO- fixes CO22 at very low levels at very low levels

CAM plants• Fix CO2 only at night and store intermediate

in their central vacuoles until daytime. • Carbon is therefore fixed at 2 different

times. • Catch: limited utility • advantageous only in water limiting

environments

CC44 plant events plant eventsseparated in spaceseparated in space

CAM plant eventsCAM plant eventsseparated in timeseparated in time