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
The ability to capture sunlight energy and
convert it to chemical energy.
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6CO2 + 6H2O + light energy  C6H12O6 + 6O2
carbon water
dioxide
sunlight
glucose
(sugar)
oxygen


Plants, algae, and some prokaryotes
Are autotrophs (“self- feeders”)
uni-bielefeld.de


Are interconnected
Water, CO2, sugar, and O2 are used or
produced as byproducts in both processes

Leaves

Chloroplasts


Flattened leaf shape exposes large surface area
to catch sunlight
Epidermis


upper and lower leaf surfaces
Cuticle


waxy, waterproof outer surface
reduces water evaporation

Stomata


Mesophyll


adjustable pores allow for entry of air with CO2
inner cell layers that contain majority of chloroplasts
Vascular bundles (veins)

supply water and minerals to the leaf while carrying
sugars away from the leaf

Chloroplasts


Stroma


bounded by a double membrane composed of inner
and outer membranes
semi-fluid medium within the inner membrane
Thylakoids


disk-shaped sacs found within the stroma
in stacks called grana
two outer membranes
of chloroplast
stroma
part of thylakoid
membrane system:
thylakoid
compartment,
cutaway view
B Chloroplast structure. No matter how highly folded, its thylakoid
membrane system forms a single, continuous compartment in the stroma.
Fig. 7-5b, p. 111

2 sets of chemical
reactions occur in the:
1.
Thylakoid membranes
2.
Stroma

Pigment molecules (e.g. chlorophyll) of the
thylakoids capture sunlight energy

Sunlight energy is converted to the energy
carrier molecules ATP and NADPH

Oxygen is released as a by-product
sunlight
O2
CO2
H2O
CHLOROPLAST
lightdependent
reactions
NADPH, ATP
NADP+, ADP
lightindependent
reactions
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.
Fig. 7-5c, p. 111

Enzymes in stroma synthesize glucose
and other organic molecules using the
chemical energy stored in ATP and
NADPH

Sun radiates electromagnetic energy

Photons (basic unit of light)


packets of energy with different energy levels
 short-wavelength photons are very energetic
 longer-wavelength photons have lower
energies
Visible light is radiation falling between
400-750 nanometers of wavelength
Light Captured by Pigments

Absorption of certain wavelengths
 light is “trapped”

Reflection of certain wavelengths
 light bounces back

Transmission of certain wavelengths
 light passes through

Absorbed light drives biological processes when it is
converted to chemical energy

Pigments absorb visible light

Common pigments:

Chlorophyll a and b
 absorb violet, blue, and red light but reflect green light (hence
they appear green)

Carotenoids
 absorb blue and green light but reflect yellow, orange, or red
(hence they appear yellow-orange)
 Are accessory pigments
autumn-pictures.com

Chlorophyll breaks down
before carotenoids in dying
autumn leaves revealing yellow
colors

Red fall colors (anthocyanin
pigments) are synthesized by
some autumn leaves, producing
red colors

Photosystems within thylakoids
 Assemblies of proteins, chlorophyll, &
accessory pigments

Two Photosystems
 PSII (comes 1st) and PSI (comes 2nd)

Each Photosystem is associated with a
chain of electron carriers
Steps of the light reactions:
1.
Accessory pigments in Photosystems absorb light and
pass energy to reaction centers containing chlorophyll
2.
Reaction centers receive energized electrons…
3.
Energized electrons then passed down a series of
electron carrier molecules (Electron Transport Chain)
4.
Energy released from passed electrons used to
synthesize ATP from ADP and phosphate
5.
Energized electrons also used to make NADPH from
(NADP+) + (H+)
to second stage of
reactions
The Light-Dependent Reactions of Photosynthesis
light energy
photosystem II
electron
transfer chain
light energy
NADPH
ATP
ATP
synthase
ADP + Pi
photosystem I
NADP+
thylakoid
compartment
stroma
A Light energy drives
electrons out of
photosystem II.
C Electrons from
photosystem II enter an
electron transfer chain.
B Photosystem II pulls
replacement electrons
from water molecules,
which dissociate into
oxygen and hydrogen
ions (photolysis). The
oxygen leaves the cell
as O2.
D Energy lost by the
electrons as they
move through the
chain causes H+ to
be pumped from
the stroma into
the thylakoid
compartment. An H+
gradient forms across
the membrane.
E Light energy drives
electrons out of
photosystem I, which
accepts replacement
electrons from electron
transfer chains.
F Electrons from
photosystem I move
through a second
electron transfer chain,
then combine with
NADP+ and H+. NADPH
forms.
G Hydrogen ions in the
thylakoid compartment
are propelled through the
interior of ATP synthases
by their gradient across
the thylakoid membrane.
H H+ flow causes the ATP
synthases to attach
phosphate to ADP, so
ATP forms in the stroma.
Fig. 7-8, p. 113

Electrons from PSII flow one-way into PS I

PSII – produces ATP

PSI – produces NADPH

May be used by plant or released into
atmosphere



NADPH and ATP from light-dependent rxns
used to power glucose synthesis
Light not directly necessary for lightindependent rxns if ATP & NADPH available
Light-independent rxns called the CalvinBenson Cycle or C3 Cycle



6 CO2 molecules used to synthesize 1 glucose
(C6H12O6)
CO2 is captured and linked to a sugar called
ribulose bisphosphate (RuBP)
ATP and NADPH from light dependent rxns
used to power C3 reactions

“Photo”


“Synthesis”



capture of light energy (light dependent rxns)
glucose synthesis (light-independent rxns)
Light dependent rxns produce ATP and NADPH
which is used to drive light-independent rxns
Depleted carriers (ADP and NADP+) return to lightdependent rxns for recharging

The ideal leaf:


Large surface area
to intercept
sunlight
lowcarboneconomy.com
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Very porous to
allow for CO2
entry from air
forestry.about.com
sbs.utexas.edu

Problem:


Substantial leaf porosity leads to substantial water
evaporation, causing dehydration stress on the plant
Plants evolved waterproof coating and
adjustable pores (stomata) for CO2 entry


When stomata close, CO2 levels drop and O2 levels
rise
Photorespiration occurs

Carbon fixing enzyme combines O2 instead of CO2 with
RuBP

Photorespiration:
 O2 is used up as CO2 is generated
 No useful cellular energy made
 No glucose produced
 Photorespiration is unproductive and
wasteful

Hot, dry weather causes stomata to stay closed

O2 levels rise as CO2 levels fall inside leaf


Photorespiration very common under such
conditions
Plants may die from lack of glucose synthesis
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
“C4 plants” have chloroplasts in bundle sheath
cells and mesophyll cells

Bundle sheath cells surround vascular
bundles deep within mesophyll

C3 plants lack bundle sheath cell
chloroplasts

C4 plants utilize the C4 pathway

Two-stage carbon fixation pathway

Takes CO2 to chloroplasts in bundle
sheath cells



C4 pathway uses up more energy than C3
pathway
C3 plants thrive where water is abundant or if light
levels are low (cool, wet, and cloudy climates)
 Ex. : most trees, wheat, oats, rice, Kentucky
bluegrass
C4 plants thrive when light is abundant but water is
scarce (deserts and hot climates)
 Ex. : corn, sugarcane, sorghum, crabgrass, some
thistles