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Where It Starts Photosynthesis
Chapter 7 Part 1
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7.1 Sunlight as an Energy Source
Photosynthetic organisms use pigments to
capture the energy of sunlight
Photosynthesis
The synthesis of organic molecules from
inorganic molecules using the energy of light
Chlorophyll has to be present as an enzyme
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Properties of Light
Visible light is part of an electromagnetic
spectrum of energy radiating from the sun
Travels in waves Organized into photons
Wavelength
The distance between the crests of two
successive waves of light (nm)
Shorter wavelengths are more energy filled
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Electromagnetic Spectrum
of Radiant Energy
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The Rainbow Catchers
Different wavelengths form colors of the rainbow Photosynthesis uses wavelengths of 380-750 nm
Thats the range of visible light for us
Pigment An organic molecule that selectively absorbs light of
specific wavelengths
Chlorophyll a The most common photosynthetic pigment
Absorbs violet and red light (appears green-reflected to us)
Are other photosynthetic pigments (Chlorophyll b, etc.)
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Photosynthetic Pigments
Collectively, chlorophyll and accessory pigments
absorb most wavelengths of visible light
Certain electrons in pigment molecules absorb
photons of light energy, boosting electrons to a
higher energy level
Energy is captured and used for photosynthesis
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Beautiful fall colors they were obscured by chlorophyll before
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7.2 Exploring the Rainbow
Engelmann identified major colors of light that
drive photosynthesis (violet and red) by using a
prism to divide light into colors
Algae using these WL gave off the most oxygen
This attracted oxygen-seeking organisms in water
An absorption spectrum shows which
wavelengths a pigment absorbs best
Organisms in different environments use different
pigments
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10/54Fig. 7-4c, p. 110
phycoerythrobilin
100 chlorophyll b phycocyanobilin
80
-carotenechlorophyll a
60
40
20Lightabsorption(%)
0
Wavelength (nanometers)
C Absorption spectra of a few photosynthetic pigments. Line
color indicates the characteristic color of each pigment.
400 500 600 700
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7.1-7.2 Key Concepts:
The Rainbow Catchers
The flow of energy through the biosphere startswhen chlorophylls and other photosynthetic
pigments absorb the energy of visible light
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7.3 Overview of Photosynthesis
Chloroplast
An organelle that specializes in photosynthesis in plants
and many protists
Made by plant as needed; has active chlorophyll
Stroma and thylakoid of the chloroplast
Sunlight energy is captured in inner thylakoid membrane
Stroma is a semifluid matrix surrounded by the two outermembranes of the chloroplast
Sugars are built in the stroma using energy from thylakoid
Result is C6H12O6, - glucose
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Overview of Photosynthesis
Thylakoid membrane
Folded membrane that make up thylakoids
Contains clusters of light-harvesting pigmentsthat absorb photons of different energies
Photosystems (type I and type II)
Groups of molecules that work as a unit to begin
the reactions of photosynthesis
Convert light energy into chemical energy
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Overview of Photosynthesis
Light-dependent reactions (gotta have sun)
Light energy is transferred to ATP and NADPH
Water molecules are split, releasing O2 Electrons are released from water split and they
are used to supercharge the energy storage
Light-independent reaction (sun not needed)
Energy in ATP and NADPH drives synthesis of
glucose and other carbohydrates from CO2 and
water
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Photosynthesis Equation
12H2O + 6CO2 sun / chlorophyll C6H12O6 + 6O2 + 6H2O
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Sites of Photosynthesis
in Plants
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18/54Fig. 7-5c, p. 111
sunlight O2 H2O CO2
CHLOROPLAST
light-
dependentreactions
NADPH, ATP
NADP+, ADP
light-
independentreactions
sugars
CYTOPLASM
C In chloroplasts, ATP and NADPH form in the light-dependent stage of
photosynthesis, which occurs at the thylakoid membrane. The second
stage, which produces sugars and other carbohydrates, proceeds in the
stroma.
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7.4 Light-Dependent Reactions
In the first stage of photosynthesis, light energy
drives electrons out of photosystems
The electrons may be used in a noncyclic or
cyclic pathway of ATP formation
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Capturing Energy for Photosynthesis
Photons boost electrons in pigments to higher
energy levels thus storing more sun energy
Light-harvesting complexes absorb the energy
Electrons are released from special pairs of
chlorophyll a molecules in photosystems
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The Thylakoid Membrane
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ADP + Pi
NADP+ Light-dependent reactions(noncyclic pathway)
H2O O2
NADPH
ATP
ADP + Pi Light-dependent reactions
(cyclic pathway)
ATP
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Replacing Lost Electrons
Electrons lost from photosystem II are replaced by
photolysis of water molecules, which dissociate
into hydrogen ions and oxygen
Photolysis
Process by which light energy breaks down a
molecule such as water
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Electron Flow In Noncyclic Pathway
Electrons lost from a photosystem enter an
electron transfer chain in the thylakoid
membrane
Electron transfer chains
Organized arrays of enzymes, coenzymes, and
other proteins that accept and donate electrons ina series
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Harvesting Electron Energy
Light energy is converted to chemical energy Entry of electrons from a photosystem into the
electron transfer chain is the first step in light-
dependent reactions
ATP forms in the stroma
Electron energy is used to build up a H+ gradient
across the membrane H+ flows through ATP synthase, which attaches a
phosphate group to ADP
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Noncyclic Pathway Of Photosynthesis
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Electron Flow In Cyclic Pathway
When NADPH accumulates in the stroma, the
noncyclic pathway stalls
A cyclic pathway runs in type I photosystems to
make ATP; electrons are cycled back to
photosystem I and NADPH does not form
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7.5 Energy Flow In Photosynthesis
Energy flow in the light-dependent reactions isan example of how organisms harvest energy
from their environment
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Photophosphorylation
Photophosphorylation A light-driven reaction that attaches a phosphate
group to a molecule
Cyclic photophosphorylation
Electrons cycle within photosystem I
Noncyclic photophosphorylation
Electrons move from water to photosystem II, to
photosystem I, to NADPH
E Fl I
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Energy Flow In
Light-Dependent Reactions
7 3 7 5 K C t
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7.3-7.5 Key Concepts:
Making ATP And NADPH
Photosynthesis proceeds through two stages in
the chloroplasts of plants and many types of
protists
In the first stage, sunlight energy is converted to
the chemical bond energy of ATP
The coenzyme NADPH forms in a pathway that
also releases oxygen
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Where It Starts Photosynthesis
Chapter 7 Part 2
7 6 Light Independent Reactions:
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7.6 Light-Independent Reactions:
The Sugar Factory
The cyclic, light-independent reactions of the
Calvin-Benson cycle are the synthesis part of
photosynthesis (where sugar is put together)
Calvin-Benson cycle (the dark side)
Enzyme-mediated reactions that build sugars in
the stroma of chloroplasts Use energy from radiant energy captured by the
chloroplast in light dependent stage
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Carbon Fixation
Carbon fixation
Extraction of carbon atoms from inorganicsources (atmosphere) and incorporating them
into an organic molecule
Builds glucose from CO2 plus energy bonds
Uses bond energy of molecules formed in light-dependent reactions (stored ATP, NADPH)
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The Calvin-Benson Cycle
Enzyme rubisco attaches C from CO2 to RuBP
Forms two 3-carbon PGA molecules
PGAL is formed PGAs receive a phosphate group from ATP, and
hydrogen and electrons from NADPH
Two PGAL combine to form a 6-carbon sugar
Rubisco is regenerated with each turn of wheel
But takes 6 wheel turns to complete sugar for
normal growing conditions, with one carbon
added to growing glucose with each turn
Inputs And Outputs Of
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Inputs And Outputs Of
Calvin-Benson Cycle
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Calvin-Benson Cycle Summary
Each turn adds 1 carbon to sugar and rebuilds 5 carbon RuBP
7 7 Adaptations:
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7.7 Adaptations:
Different Carbon-Fixing Pathways
Environments differ, and so do details ofphotosynthesis
C3 plants most plants, with wilting when water
becomes in short supply, closing stomata
C4 plants drought resistant plants that can
store carbon in two places and resist wilting
longer
CAM plants desert plants, require little waterdue to ability to fix carbon only at night and thus
keep stomata closed during day
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Stomata
Stomata (not stroma)
Small openings through the waxy cuticle covering
epidermal surfaces of leaves and green stems
especially on lower surfaces
Allow CO2 in and O2 and water out
Close on dry days to minimize water loss; this
stops carbon attainment by preventing intake ofCO2 from air and leads to wilting
There are C3, C4, and CAM plants based upon
ability to deal with drought conditions
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C3 Plants
C3 plants
Plants that use only the CalvinBenson cycle to
fix carbon
Most plants are C3
Forms 3-carbon PGA in mesophyll cells
Used by most plants, but inefficient in dry weather
when stomata are closed Examples: most flowering and vegetable plants
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Photorespiration
When stomata are closed, CO2 needed for light-
independent reactions cant enter, O2 produced
by light-dependent reactions cant leave
Photorespiration
At high O2 levels, rubisco attaches to oxygen
instead of carbon
CO2 is produced rather than fixed
So efficiency lost and more turns needed to
make sugar product
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C4 Plants
C4 plants
Plants that have an additional set of reactions forsugar production on dry days when stomata are
closed; compensates for inefficiency of rubisco
Forms 4-carbon oxaloacetate in mesophyll cells,
then bundle-sheath cells make sugar Examples: Corn, switchgrass, bamboo
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C3 And C4 Plant Leaves
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CAM Plants
CAM plants (Crassulacean Acid Metabolism)
Plants with an alternative carbon-fixing pathwaythat allows them to conserve water in climates
where days are hot
Forms 4-carbon oxaloacetate at night, which is
later broken down to CO2 for sugar production Example: succulents, cactuses
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A CAM Plant Many People Grow
Jade plant (Crassula argentea)
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C3, C4, And CAM Reaction Summary
7 6-7 7 Key Concepts:
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7.6-7.7 Key Concepts:
Making Sugar By Photosynthesis
The second stage is the synthesis part of
photosynthesis, in which sugars are assembled
from CO2
The reactions use ATP and NADPH that form in
the first stage of photosynthesis as radiant
energy is captured from sunlight
Details of the reactions vary among organisms
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7.8 Photosynthesis And Atmosphere
The evolution of photosynthesis dramatically andpermanently changed Earths atmosphere
Oxygen comes only from photosynthesis and
prior to development in plants the atmosphere
did not contain oxygen
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Food Sources And Consumers
Autotrophs
Organisms that make their own food using
energy from the environment and inorganic
carbon
Heterotrophs
Organisms that get energy and carbon fromorganic molecules assembled by other organisms
that are autotrophs or feed on autotrophs
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Two Kinds Of Autotrophs
Chemoautotrophs
Extract energy and carbon from simple molecules
in the environment (hydrogen sulfide, methane)
Used before the atmosphere contained oxygen
Photoautotrophs
Use photosynthesis to make food from CO2 and
water, releasing O2
Allowed oxygen to accumulate in the atmosphere
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Effects of Atmospheric Oxygen
Selection pressure on evolution of life
Oxygen radicals
Development of ATP-forming reactions
Aerobic respiration
Formation of ozone (O3) layer
Protection from UV radiation
7.8 Key Concepts:
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7.8 Key Concepts:
Evolution And Photosynthesis
The evolution of photosynthesis changed the
composition of Earths atmosphere
New pathways that detoxified the oxygen by-
product of photosynthesis evolved
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The Carbon Cycle
Photosynthesis locks CO2 from the atmosphere
in organic molecules; aerobic respiration returns
CO2
to the atmosphere
A balanced cycle of the biosphere is the result
Humans burn wood and fossil fuels for energy,
releasing locked carbon into the atmosphere
Contributes to global warming, disrupting
biological systems as it is not balanced by Nature
7.9 Key Concepts:
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7.9 Key Concepts:
Global Warming?
Photosynthesis by autotrophs removes CO2from
the atmosphere; metabolism by all organismsputs it back into atmosphere
Human activities have disrupted this balance,
and contribute to global warming by releasinggreater amounts of CO
2than what can be
balanced by natural processes