Upload
dorothy-suzanna-howard
View
227
Download
0
Embed Size (px)
Citation preview
Photosynthesis: Using Light to Make Food
Energy classification Autotrophs—self nourishing
Obtain carbon from CO2
Obtain energy from light (photosynthesis) or chemical reactions (chemosynthesis)
Heterotrophs—use others for energy source Obtain carbon from autotrophs Obtain energy from autotrophs Even if ingest other heterotrophs, at some point the
original carbon & energy came from an autotroph
Carbon & Energy Enter life through photosynthesis (autotrophs) Released through glycolysis & cellular respiration
(heterotrophs)
ChlorophyllPlantsAlgaeSome bacteria
Transfer sun’s energy into chemical bondsConverts energy of photons to energy
stored in ATP Oxygen production is a byproduct
Three stagesLight-capturingLight-dependent
Convert light energy into chemical energyLight-independent
Form organic compounds (glucose)
CO2 + H2O => C6H12O6 (glucose) + O2
Remember that this is the opposite direction but the same basic reaction as cellular respiration.
Wavelength
Spectrum
Photons Packets of particle-like light Fixed energy (each photon a specific energy
wavelength) Think of them as bundles of energy, like an
electrified rubber ball Energy level
Low energy = long wavelength Microwaves, radio waves
High energy = short wavelength Gamma rays, x-rays
Only a small part of spectrum (400-750 nm) is used for vision & photosynthesis
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; it absorbs all parts of the spectrum except green.
Molecules that absorb photons of only a particular wavelength
Chlorophyll aAbsorbs red, blue, violet lightReflects green, yellow lightMajor pigment in almost all photoautotrophs
Chlorophyll bAbsorbs red-orange, some blueReflects green, some blue
CarotenoidsAbsorb blue-violet, blue-green lightReflect red, orange, yellow lightGive color to many flowers, fruits,
vegetablesColor leaves in Autumn
AnthocyaninsAbsorb green, yellow, some orange lightReflect red, purple lightCherries, many flowersColor leaves in Autumn
PhycobilinsAbsorb green, yellow, orange lightReflect red, blue-green lightSome algae & bacteria
Pigment absorbs light of specific wavelentghCorresponds 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
ATPWater molecules split, helping to form
NADPHOxygen 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 in photosystems
Photosystems—Clusters of chlorophyll, pigments, proteinsLight-gathering “antennae”Photosystem I (P680)—absorbs red light at 680nmPhotosystem II (P700)—absorbs far-red light at
700nm
Electrons transfer from photosystems Electron transfers pump H+ into inner
thylakoid compartment Repeats, building up concentration and
electric gradients Chemiosmosis!
H+ can only pass through channels inside ATP Synthase
Ion flow through channel makes protein turn, forcing Phosphate onto ADP
Phosphorylation!
Electrons continue until bonding NADP+ to form NADPH
NADPH used in next part of cycle Process is very similar to cellular
respiration!!!!Oxidative phosphorylation
ATP provides energy for bond formation NADPH provides hydrogen & electrons CO2 provides carbon & oxygen
CO2 in air diffuses into stroma CO2 attaches to rubisco (RuBP) Enters Calvin cycle (also called Calvin-
Benson)RuBP splits to form PGAPGA gets phosphate from ATP, then H+ and
electrons from NADPHForms PGALTwo 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 CO2 must be fixed and 12 PGAL must
form to produce one glucose molecule and keep the cycle running
*(G3P = PGAL)
Microscopic openings in leavesClose when hot & dryKeeps water insidePrevents CO2 & O2 exchange
Basswood, beans, peas, evergreens 3-Carbon PGA is first stable intermediate
in Calvincycle Stomata close, O2 builds up Increased O2 levels compete w/ CO2 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, sugar cane, tropical plants Adapted to hot, dry climates Close stomata to conserve water
This limits CO2 entry and allows O2 to accumulate
This allows CO2 to remain high for Calvin cycle
Carbon stored in special cells, can be donated to Calvin cycle later
Requires 1 more ATP than C3, but less water lost & more sugar produced
Desert plants (cactus) Crassulcean Acid Metabolism (CAM) Opens stomata at night, uses C4 cycle Cells store malate & organic acids During day when stomata close, malate
releases CO2 for Calvin cycle