Photosynthesis: Life from Light

Energy needs of life All life needs a constant input of energy Heterotrophs get their energy from “eating others” consumers of other organisms consume organic molecules Autotrophs get their energy from “self” get their energy from sunlight use light energy to synthesize organic molecules Heterotrophs consumers animals fungi most bacteria Autotrophs producers plants photosynthetic bacteria (blue-green algae)

making energy & organic molecules from ingesting organic molecules How are they connected? Heterotrophs making energy & organic molecules from ingesting organic molecules glucose + oxygen  carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP  + Autotrophs So, in effect, photosynthesis is respiration run backwards powered by light. Cellular Respiration oxidize C6H12O6  CO2 & produce H2O fall of electrons downhill to O2 exergonic Photosynthesis reduce CO2  C6H12O6 & produce O2 boost electrons uphill by splitting H2O endergonic making energy & organic molecules from light energy + water + energy  glucose + oxygen carbon dioxide 6CO2 6H2O C6H12O6 6O2 light energy  + 2005-2006

The Great Circle of Life! Energy cycle sun Photosynthesis glucose O2 H2O CO2 Cellular Respiration The Great Circle of Life! Where’s Mufasa? ATP

Pigments of photosynthesis Why does this structure make sense? chlorophyll & accessory pigments “photosystem” embedded in thylakoid membrane structure  function Orientation of chlorophyll molecule is due to polarity of membrane. 2005-2006

Light: absorption spectra Photosynthesis performs work only with absorbed wavelengths of light chlorophyll a — the dominant pigment — absorbs best in red & blue wavelengths & least in green other pigments with different structures have different absorption spectra

Photosystems Photosystems 2 photosystems in thylakoid membrane collections of chlorophyll molecules 2 photosystems in thylakoid membrane act as light-gathering “antenna complex” Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Don’t compete with each other, work synergistically using different wavelengths.

ETC of Photosynthesis ETC produces from light energy ATP & NADPH NADPH (stored energy) goes to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” at the REACTION CENTER. splits H2O (Photolysis!!) O2 released to atmosphere ATP is produced for later use

Cyclic photophosphorylation If PS I can’t pass electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH coordinates light reactions to Calvin cycle Calvin cycle uses more ATP than NADPH 2005-2006

Photorespiration Why? When the stomates are closed during hot-dry weather, the plant cannot get CO2 and Ribisco fixes O2 with RuBP. 2005-2006

Rubisco Enzyme which fixes carbon from atmosphere ribulose bisphosphate carboxylase the most important enzyme in the world! it makes life out of air! definitely the most abundant enzyme

Calvin cycle PGAL PGAL   important intermediate end product of Calvin cycle energy rich sugar 3 carbon compound “C3 photosynthesis” PGAL   important intermediate PGAL   glucose   carbohydrates   lipids   amino acids   nucleic acids

C4 Plants Ex: grass and corn They have mesophyll cells (like C3 plants) but their chloroplasts are in Bundle-sheath cells. Keep stomata closed most of the time b/c of hot weather to prevent water loss Pep carboxylase fixes CO2 to produce a 4 carbon molecule in mesophyll cells 2005-2006

C4 plants 4-carbon compound moves to the Bundle-sheath cells to make sugar. These plants only have PSI. 2005-2006

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CAM (Crassulacean acid metabolism) Ex: cacti and pineapples Close stomates during the day, open them at night. Take in CO2 during the night, but do not have ATP and NADPH. So incorporate the CO2 into a variety of organic acids. During the day, CO2 is released from the acids to be used to make sugars. 2005-2006

Photosynthesis summary Light reactions produced ATP produced NADPH consumed H2O produced O2 as byproduct Calvin cycle consumed CO2 produced PGAL regenerated ADP regenerated NADP 2005-2006