Photosynthesis: Using Light to Make Food
 Energy

Autotrophs—self nourishing
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Obtain carbon from CO2
Obtain energy from light (photosynthesis) or chemical
reactions (chemosynthesis)
Heterotrophs—use others for energy source
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
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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


classification
& Energy
Enter life through photosynthesis (autotrophs)
Released through glycolysis & cellular respiration
(heterotrophs)
 Chlorophyll

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Plants
Algae
Some bacteria
 Transfer

sun’s energy into chemical bonds
Converts energy of photons to energy stored in
ATP
 Oxygen
production is a byproduct
 Three
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Light-capturing
Light-dependent


Convert light energy into chemical energy
Light-independent

Form organic compounds (glucose)
 CO2

stages
+ 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

Low energy = long wavelength


level
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 a



Absorbs red, blue, violet light
Reflects green, yellow light
Major pigment in almost all photoautotrophs
 Chlorophyll

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b
Absorbs red-orange, some blue
Reflects green, some blue
 Carotenoids

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Absorb blue-violet, blue-green light
Reflect red, orange, yellow light
Give color to many flowers, fruits, vegetables
Color leaves in Autumn
 Anthocyanins
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Absorb green, yellow, some orange light
Reflect red, purple light
Cherries, many flowers
Color leaves in Autumn
 Phycobilins
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
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



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
carbohydrates
 Occurs
in thylakoids
 Electrons transfer light energy in electron
transport chain in photosystems
 Photosystems—Clusters
of chlorophyll, pigments,
proteins



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
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 CalvinBenson)
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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 CO2 must be fixed and 12 PGAL must form
to produce one glucose molecule and keep
the cycle running
*(G3P = PGAL)
 Microscopic
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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
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