PHOTOSYNTHESIS* A.***PHASE*1.*

PHOTOSYNTHESIS* ! A.***PHASE*1.*

Light'Dependent!Reactions!(phase&of&photosynthesis&where&light&energy&is&converted&to&chemical& energy&in&the&form&of&ATP;&results&in&the&splitting&of&water&and&the&release&of&oxygen)! ! 1. Sunlight!strikes!the!chlorophyll!molecules!in!the!thylakoid!membrane;!the!energy!in!the!light!

is!transferred!to!electrons.! ! These!highly!energized,!or!excited,!electrons!are!passed!from!chlorophyll!to!an!electron! transport!chain!(series&of&proteins&embedded&in&a&membrane&along&which&energized&electrons& are&transported;&as&electrons&are&passed&from&molecule&to&molecule,&energy&is&released),!a! series!of!proteins!embedded!in!the!thylakoid!membrane.!

a. Each!protein!in!the!chain!passes!energized!electrons!along!from!protein!to!protein,! similar!to!a!bucket!brigade!in!which!a!line!of!people!pass!a!bucket!of!water!from! person!to!person!to!fight!a!fire.!!At!each!step!along!the!transport!chain,!the!electron! loses!energy,!just!as!some!of!the!water!might!be!spilled!from!buckets!in!the!fire' fighting!chain.!

b. The!electron!transport!chain!allows!small!amounts!of!the!electron's!energy!to!be! released!at!a!time.!!This!energy!is!used!to!form!ATP!from!ADP,!or!to!pump!from!ADP,! or!to!pump!hydrogen!ions!into!the!center!of!the!thylakoid!disc.! !

! ! After!the!electron!has!traveled!down!the!first!electron!transport!chain,!it!is!passed!down!a! second!electron!transport!chain.! ! 2. Following!the!second!electron!transport!chain,!the!electron!is!still!very!energized.!!So!that! this!energy!is!not!wasted,!the!electron!is!transferred!to!the!stroma!of!the!chloroplast.!!To!do! this,!an!electron!carrier!molecule!called!NADP+!(nicotinamide!adenine!dinucleotide! phosphate,!electron&carrier&molecule;&when&carrying&excited&electrons&it&becomes&NADPH)!is! used.! ! 3. When!carrying!the!excited!electron,!NADP+!combines!with!a!hydrogen!ion,!H+,!and!becomes! NADPH.!!NADPH!will!play!an!important!role!in!the!formation!of!carbohydrates!later.! !

! !

Restoring!Elections!To!Chlorophyll! 4. Although!some!of!the!light'energized!electrons!may!be!returned!to!chlorophyll!after!they've!

moved!through!the!electron!transport!chain,!many!leave!with!NADPH!for!the!light' independent!reactions.!!If!these!electrons!are!not!replaced,!the!chlorophyll!will!be!unable!to! absorb!light!and!the!light'dependent!reactions!will!stop,!as!will!the!production!of!ATP.! ! To!replace!the!lost!electrons,!molecules!of!water!are!split.! ! 5. Each!molecule!of!water!produces!one'half!molecule!of!oxygen!gas,!two!hydrogen!ions,!and! two!electrons.!!This!process!is!called!photolysis!(reaction&taking&place&in&the&thylakoid& membranes&of&a&chloroplast&during&lightBdependent&reactions&where&two&molecules&of&water& are&split&to&form&oxygen,&hydrogen&ions,&and&electrons).!!The!oxygen!of!photolysis!supplies!the! oxygen!we!breath.! !

! !

PHOTOSYNTHESIS* * B.***PHASE*2.*

Light'Independent!Reactions!(phase&of&photosynthesis&where&energy&from&lightBdependent& reactions&is&used&to&produce&glucose)!

!

This!phase!of!photosynthesis!does!not!require!light.!

!

It!takes!place!in!the!stroma!of!the!chloroplast.!

! 6. The!Calvin!cycle!(series&of&reactions&during&the&lightBindependent&phase&of&photosynthesis&in& which&simple&sugars&are&formed&from&carbon&dioxide&using&ATP)!

a. We!begin!this!process!with!carbon!dioxide!reacting!with!ribulose!biphosphate!(RuBP)!

to!produce!3'phosphoglyceric!acid!(PGA).!

! !!!?!!+!!?'?'?'?'?!!!!!?'?'?'?'?'?!!!!!?'?'?!!+!!?'?'?!

&(CO2)&&&&&&&&&&&&(RuBP)&&&&&&&&&&&&&(unstable&intermediate)&&&&&&&(PGA)&&&&&&&&&&&&&(PGA)&

!

b. PGA!molecules!are!converted!into!molecules!of!phosphoglyceraldehyde!(PGAL).!!This!

requires!ATP!and!NADPH!from!the!light'dependent!reactions.!

! ?'?'?!!+!!ATP!!+!!NADPH!!!!!?'?'?!!+!!(ADP!+!P)!!+!NADP+! !!(PGA)&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&(PGAL)&

!

c. After!several!rounds!of!the!Calvin!cycle,!molecules!of!PGAL!leave!the!cycle!to!form!

glucose!(C6H12O6).!

! ?'?'?!!!!!?'?'?'?'?'?!

&(PGAL)&&&&&&&&&&&&&&&&&&&&(Glucose)&

!

Each!sugar!molecule!made!by!the!Calvin!cycle!contains!six!carbon!atoms,!and!because!

only!one!molecule!of!CO2!is!added!to!the!cycle!each!time,!it!takes!a!total!of!six!rounds!

of!the!cycle!to!form!one!sugar.!

!

!

d. In!order!for!the!cycle!to!continue!incorporating!carbon!dioxide,!RuBP!must!be!

generated.!!PGAL!molecules!are!used!to!form!RuBP.!!This!process!requires!ATP.!

! ?B?B?&&!&&?B?B?B?B?&

&&(PGAL)&&&&&&&&&&&&&&&&&&(RuBP)&

!

e. Once!the!RuBP!is!formed,!it!is!available!to!unite!with!CO2!molecules!from!the!

atmosphere!and!enter!another!turn!of!the!cycle.!

!

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