CHAPTER 7Where it Starts--Photosynthesis
SUSTAIN LIFE Obtain energy
AutotrophsHeterotrophs
Metabolism—biochemical processes release energyPhotosynthesisCellular Respiration
SUSTAIN LIFE Food energy stored in chemical bonds
Exergonic (cellular respiration)Endergonic (photosynthesis)
Energy transfers from endergonic to exergonic through ATP
PHOTOSYNTHESIS Chlorophyll
PlantsAlgaeSome bacteria
Transfer sun’s energy into chemical bonds
PHOTOSYNTHESIS Three stages
Light-capturingLight-dependentLight-independent
CO2 + H2O => C6H12O6 (glucose) + O2
PROPERTIES OF LIGHT Wavelength
Spectrum
PROPERTIES OF LIGHT Photons
Packets of particle-like lightFixed energy
Energy levelLow energy = long wavelength
Microwaves, radio wavesHigh energy = short wavelength
Gamma rays, x-rays
PROPERTIES OF LIGHT 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.
PIGMENTS Molecules that absorb photons of only a
particular wavelength Chlorophyll a
Absorbs red, blue, violet lightReflects green, yellow lightMajor pigment in almost all photoautotrophs
Chlorophyll bAbsorbs red-orange, some blueReflects green, some blue
PIGMENTS Carotenoids
Absorb blue-violet, blue-green lightReflect red, orange, yellow lightGive color to many flowers, fruits,
vegetablesColor leaves in Autumn
PIGMENTS Anthocyanins
Absorb green, yellow, some orange lightReflect red, purple lightCherries, many flowersColor leaves in Autumn
PhycobilinsAbsorb green, yellow, orange lightReflect red, blue-green lightSome algae & bacteria
ELECTRON ENERGY 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)
OVERVIEW 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
CHLOROPLASTS
LIGHT-DEPENDENT REACTIONS 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
LIGHT-DEPENDENT REACTIONS
H+ can only pass through channels inside ATP Synthase
Ion flow through channel makes protein turn, forcing Phosphate onto ADP
LIGHT-DEPENDENT REACTIONS
Electrons continue until bonding NADP+ to form NADPH
NADPH used in next part of cycle
LIGHT-DEPENDENT REACTIONS
LIGHT-INDEPENDENT REACTIONS
CO2 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
LIGHT-INDEPENDENT REACTIONS
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
LIGHT-INDEPENDENT REACTIONS
*(G3P = PGAL)
StomataClose when hot & dryKeeps water insidePrevents CO2 & O2 exchange
Basswood, beans, peas, evergreens 3-Carbon PGA is first stable intermediate
in Calvin-Benson cycle 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
C3 PLANTS
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 CO2 to Calvin-Benson cycle
This allows CO2 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
C4 PLANTS
C4 PLANTS
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 CO2 for C-B cycle
CAM PLANTS
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