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Chapter 10—Photosynthesis
From photons to food…
I. Photosynthesis in Nature
• Autotrophs—producers
(plants, algae, some protists, some
bacteria)
• Heterotrophs—consumers
(animals, fungi, bacteria)
Locating Photosynthesis
in a Plant
Leaves are the major organs of
photosynthesis
Chloroplasts are the organelle
responsible
Stomata—pores that allow gas
exchange
Mesophyll—interior tissue of the
leaf
Chlorophyll—green pigment in
thylakoid membrane, absorbs
light energy
II. The Pathways of Photosynthesis
• Tracking Atoms through Photosynthesis
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
Respiration Photosynthesis
Energy is released from
sugar
Electrons from hydrogen in
oxidized food lose potential
energy through ETC,
forming water at end
Energy is used to make ATP
Energy is stored in sugar
Electrons from water gain
potential energy as they
reduce CO2 to sugar
Energy is provided by light
Overview of Photosynthesis
Overview of Photosynthesis
Overview of Photosynthesis
Light reactions = “photo” Calvin Cycle = “synthesis”
Electromagnetic Spectrum
Visible light is the radiation (energy) that drives photosynthesis
Light meets the chloroplast—why leaves are
green
Pigment—molecule
that absorbs visible
light
Ex. Chlorophyll
(absorbs red and
blue, reflects green)
Spectrophotometer--
Measures the
proportions of light of
different wavelengths
absorbed or transmitted
by a pigment solution
Using a spectrophotometer to find an
absorption spectrum
Which colors work best for
photosynthesis?
Absorption spectrum vs. Action
spectrum
Structure of Chlorophyll
Where does the light energy
absorbed by chlorophyll go?
Excitation of Chlorophyll by light
The energy of an absorbed photon is converted to the potential energy
of an e- raised to an excited state, (higher orbital = unstable)
Returning to ground state releases heat and light (fluorescence)
Chlorophyll
solution
excited
with UV
light
Photosystems—Light Harvesting Complexes
The electron acceptor traps the high-energy e- before it can return to
ground state in the chlorophyll
• Photosystem =
– complex of proteins & other molecules
– includes an ―antenna‖ of 100s of pigment molecules
– has a specific reaction center (primary e-
acceptor next to reaction center chlorophyll) • Photosystem I
– Reaction center—P 700 (absorbs best at λ of 700 nm)
• Photosystem II – Reaction center—P 680 (absorbs best at λ of 680 nm)
Electromagnetic Spectrum
Noncyclic electron flow during the light
rxns generates ATP & NADPH
ATP synthesis = noncyclic photophosphorylation
Uses both Photosystem I & II, & two ETCs
Solar powered light reactions → make ATP (chemical energy) & NADPH
(reducing power) → used in Calvin Cycle (sugar-making reactions)
Cyclic Electron Flow—alternative path for e-’s
Uses Photosystem I, but not Photosystem II
Generates ATP = cyclic photophosphorylation
but not NADPH or O2
ATP produced is used
in Calvin Cycle
Comparing chemiosmosis in chloroplasts &
mitochondria
ETCs in both organelles
pump H+ across a
membrane against the
[gradient]
H+ diffuses back across
lending the energy for
ATP synthesis
Light Reactions & Chemiosmosis
Calvin Cycle—converts CO2 to sugar using
ATP & NADPH
Phase 1—Carbon Fixation
Incorporating CO2 into an organic compound
Enzyme = Rubisco (most abundant enzyme on earth)
Phase 2—Reduction
Phosphate is added from
ATP
Electrons are added from
NADPH
1 3-carbon sugar (G3P)
leaves the cycle for every 3
CO2 added
Phase 3—Regeneration
of CO2 acceptor (RuBP)
ATP is used to
rearrange carbon
compounds back into
RuBP and cycle begins
again
Net Input:
9 ATP & 6 NADPH
Net Output:
1 G3P (3-carbon sugar)
(becomes glucose and
other carbs)
It’s gettin’ hot (& dry) in here!
• On hot, dry days—stomata close to conserve water (therefore O2,↑, CO2 ↓, photosyn. rate goes down)
• Photorespiration—metabolic pathway that decreases photosynthetic output by pulling organic compounds out of the Calvin cycle
• Rubisco adds O2 to the Calvin Cycle instead of CO2
• consumes O2, releases CO2, and makes no ATP or food
– Done by C3 plants (1st product of Calvin Cycle is a
3-C compound)
• Examples: rice, wheat, soybeans
– Can drain as much as 50% of the carbon fixed by the Calvin cycle
C4 plants are adapted to hot, dry
climates
C4 plants—(first product of Calvin cycle
is a 4-C compound)
Examples: sugarcane, corn
C4 & CAM photosynthesis—minimize
photorespiration (in hot, dry climates)
Both involve:
1. CO2 incorporated into
4-C organic acids
(carbon fixation)
2. Organic acids release
CO2 to the Calvin cycle
C4 plants—uses spatial
separation (different
types of cells)
CAM plants—uses
temporal separation
(same cells, different
times)
&
cacti
Light Reactions Calvin Cycle
Reactions
Carried out by
molecules in the
thylakoid membrane
Convert light energy to
the chemical energy of
ATP & NADPH
Split H2O and release O2
to the atmosphere
Take place in the
stroma
Use ATP & NADPH to
convert CO2 to the
sugar G3P
Return ADP, inorganic
phosphate, and NADP+
to the light reactions
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