Where it Starts--Photosynthesis

 Obtain energy  Autotrophs  Heterotrophs  Metabolism—biochemical processes release energy  Photosynthesis  Cellular Respiration

 Food energy stored in chemical bonds  Exergonic (cellular respiration)  Endergonic (photosynthesis)  Energy transfers from endergonic to exergonic through ATP

 Chlorophyll  Plants  Algae  Some bacteria  Transfer sun’s energy into chemical bonds

 Three stages  Light-capturing  Light-dependent  Light-independent  CO 2 + H 2 O => C 6 H 12 O 6 (glucose) + O 2

 Wavelength  Spectrum

 Photons  Packets of particle-like light  Fixed energy  Energy level  Low energy = long wavelength Microwaves, radio waves  High energy = short wavelength Gamma rays, x-rays

 The light that you see is REFLECTED, not absorbed.  Therefore, a green plant is reflecting the green part of the spectrum (and photons of that energy), not absorbing them.

 Molecules that absorb photons of only a particular wavelength  Chlorophyll a  Absorbs red, blue, violet light  Reflects green, yellow light  Major pigment in almost all photoautotrophs  Chlorophyll b  Absorbs red-orange, some blue  Reflects green, some blue

 Carotenoids  Absorb blue-violet, blue-green light  Reflect red, orange, yellow light  Give color to many flowers, fruits, vegetables  Color leaves in Autumn

 Anthocyanins  Absorb green, yellow, some orange light  Reflect red, purple light  Cherries, many flowers  Color leaves in Autumn  Phycobilins  Absorb green, yellow, orange light  Reflect red, blue-green light  Some algae & bacteria

 Pigment absorbs light of specific wavelentgh  Corresponds to energy of photon  Electron absorbs energy from photon  Energy boosts electron to higher level  Electron then returns to original level  When it returns, emits some energy (heat or photon)

 Stage 1 (Light-Dependent)  Light energy converted to bond energy of ATP  Water molecules split, helping to form NADPH  Oxygen atoms escape  Stage 2 (Light-Independent)  ATP energy used to synthesize glucose & other carbohydrates

 Occurs in thylakoids  Electrons transfer light energy in electron transport chain

 Electron transfers pump H + into inner thylakoid compartment  Repeats, building up concentration and electric gradients

 H + can only pass through channels inside ATP Synthase  Ion flow through channel makes protein turn, forcing Phosphate onto ADP

 Electrons continue until bonding NADP+ to form NADPH  NADPH used in next part of cycle

 CO 2 in air attaches to rubisco (RuBP)  Splits to form PGA  PGA gets phosphate from ATP, then H + and electrons from NADPH  Forms PGAL  Two PGAL combine to form glucose plus phosphate group

 Some PGAL recycles to form more RuBP  Takes 6 “turns” of cycle to form one glucose molecule  6 CO 2 must be fixed and 12 PGAL must form to produce one glucose molecule and keep the cycle running

*(G3P = PGAL)

 Stomata  Close when hot & dry  Keeps water inside  Prevents CO 2 & O 2 exchange

 Basswood, beans, peas, evergreens  3-Carbon PGA is first stable intermediate in Calvin-Benson cycle  Stomata close, O 2 builds up  Increased O 2 levels compete w/ CO 2 in cycle  Rubisco attaches oxygen, NOT carbon to RuBP  This yields 1 PGA rather than 2  Lowers sugar production & growth of plant  12 “turns” rather than 6 to make sugars  Better adapted to cold & wet

 Corn, tropical plants  Also close stomata on hot, dry days  Pumps carbon through cycles in 2 cells  Mesophyll cells: create 4-carbon molecule (oxaloacetate)  Bundle-sheath cells: take 4-carbon molecule (malate), releases CO 2 to Calvin-Benson cycle  This allows CO 2 to remain high for C-B cycle  Requires 1 more ATP than C3, but less water lost & more sugar produced  Adapted to higher light & temp, lower water

 Desert plants (cactus)  Crassulcean Acid Metabolism  Opens stomata at night, uses C4 cycle  Cells store malate & organic acids  During day when stomata close, malate releases CO 2 for C-B cycle