electron transport chain

Photosynthesis: Using Light to Make Food
 Energy
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
 Chlorophyll
Some bacteria
 Transfer
sun’s energy into chemical bonds
Converts energy of photons to energy stored in
 Oxygen
production is a byproduct
 Three
Convert light energy into chemical energy
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
Think of them as bundles of energy, like an
electrified rubber ball
 Energy
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
 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 a
Absorbs red, blue, violet light
Reflects green, yellow light
Major pigment in almost all photoautotrophs
 Chlorophyll
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
 Stage
Light energy converted to bond energy of ATP
Water molecules split, helping to form NADPH
Oxygen atoms escape
 Stage
1 (Light-Dependent)
2 (Light-Independent)
ATP energy used to synthesize glucose & other
 Occurs
in thylakoids
 Electrons transfer light energy in electron
transport chain in photosystems
 Photosystems—Clusters
of chlorophyll, pigments,
Light-gathering “antennae”
Photosystem I (P680)—absorbs red light at 680nm
Photosystem 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
 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
Oxidative phosphorylation
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 CalvinBenson)
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
 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 leaves
Close when hot & dry
Keeps water inside
Prevents CO2 & O2 exchange
 Basswood,
beans, peas, evergreens
 3-Carbon PGA is first stable intermediate in
 Stomata close, O2 builds up
 Increased O2 levels compete w/ CO2 in cycle
 Rubisco attaches oxygen, NOT carbon to
 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
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