Catabolic
and
Anabolic
Pathways
depend on
ATP / ADP
shuttle
Figure 10.0 Sunbeams
Question:
•Where does
photosynthesis take
place?
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Plants
• Autotrophs: self-producers.
• Location:
1. Leaves
a. stoma
b. mesophyll cells
Mesophyll
Cell
Chloroplast
Stoma
Stomata (stoma)
• Pores in a plant’s cuticle through which water
and gases are exchanged between the plant
and the atmosphere.
Oxygen
(O2)
Carbon Dioxide
(CO2)
Guard Cell
Guard Cell
Mesophyll Cell
Nucleus
Cell Wall
Chloroplast
Central Vacuole
Chloroplast
• Organelle where photosynthesis takes place.
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Granum
Thylakoid
Thylakoid Membrane
Granum
Thylakoid Space
Chloroplast
• Chlorophyll – green pigment located
inside chloroplasts
• Found in mesophyll (interior of leaf)
• Double membrane
• Enclose stroma (dense liquid)
containing grana made up of
thylakoid membranes which contain
chlorophyll
Photosynthesis
+
H2 O
CO2
Energy
Which splits
water
ATP and
NADPH2
Light is Adsorbed
By
Chlorophyll
ADP
NADP
Chloroplast
O2
Light Reaction
Calvin Cycle
Used Energy and is
recycled.
+
C6H12O6
Dark Reaction
Photosynthesis
• An anabolic, endergonic, carbon dioxide
(CO2) requiring process that uses light energy
(photons) and water (H2O) to produce organic
macromolecules (glucose).
SUN
photons
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
Question:
• Why are plants green?
Chlorophyll Molecules
• Located in the thylakoid membranes.
• Chlorophyll have Mg+ in the center.
• Chlorophyll pigments harvest energy (photons)
by absorbing certain wavelengths (blue-420
nm and red-660 nm are most important).
• Plants are green because the green
wavelength is reflected, not absorbed.
Light Absorption Spectrum
Absorption of
Chlorophyll
Absorption
violet
blue
green yellow
wavelength
orange
red
Question:
• During the fall, what
causes the leaves to
change colors?
Fall Colors
• In addition to the chlorophyll pigments, there
are other accessory pigments present.
• During the fall, the green chlorophyll
pigments are greatly reduced revealing the
other pigments.
• Carotenoids are pigments that are either red
or yellow.
Chromatography
• Pigments can be separated out using the
process of CHROMATOGRAPHY.
Redox Reaction
• The transfer of one or more electrons from
one reactant to another.
• Two types:
1. Oxidation
2. Reduction
Oxidation Reaction
• The loss of electrons from a substance.
• Or the gain of oxygen.
Oxidation
6CO2 + 6H2O 
C6H12O6 + 6O2
glucose
Reduction Reaction
• The gain of electrons to a substance.
• Or the loss of oxygen.
Reduction
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
Breakdown of
Photosynthesis
• Two main parts (reactions).
1. Light Reaction or
Light Dependent Reaction
Produces energy from solar power (photons)
in the form of ATP and NADPH.
Breakdown of
Photosynthesis
2.
Calvin Cycle or
Light Independent Reaction or
Carbon Fixation or
C3 Fixation
Uses energy (ATP and NADPH) from light rxn
to make sugar (glucose).
Chlorophyll
• Two different types : Chlorophyll a and b
• “a” – main pigment in photosynthesis
• “b” – accessory pigment
– Both have similar structure but absorb
different wavelengths of light
• Chlorophyll absorbs light of a
particular wavelength, electrons are
excited and jump to higher energy
level, drops back down and gives off
heat
• This energy is passed along until it
finds Chlorophyll a, which, when
excited, passes electron to primary
electron acceptor…powering light
dependent reactions.
How a photosystem harvests
light..
1. Light Dependent Reaction
• Occurs in the Thylakoid membranes
• During the light reaction, there are two
possible routes for electron flow.
A.
Photosystem II
B.
Photosystem I
*Pigments of the thylakoid space organize
themselves into groups called photosystems
Chemiosmosis
• Powers ATP synthesis.
• Located in the thylakoid membranes.
• Uses ETC and ATP synthase (enzyme) to
make ATP.
• Photophosphorylation: addition of phosphate
to ADP to make ATP.
Chemiosmosis…Making ATP
SUN
H+ H+
Thylakoid
(Proton Pumping)
E
T
PS II
PS I
C
H+
H+ H+
H+ H+
H+
ADP + P
H+
H+
high H+
concentration
ATP Synthase
ATP
Thylakoid
Space
low H+
concentration
Things to Remember….
Light Reactions…
•
•
•
•
Occur in the Thylakoid Membrane
Inputs are water and light
Products : ATP, NADPH and O2
Oxygen produced comes from the
water, not Carbon Dioxide!
• Two different pathways…..
Cyclic vs. Noncyclic
Pathways…
• Noncyclic Light Reaction
– Electrons taken from Chlorophyll a are not
recycled back down to the ground state –
therefore do not make it back to the
chlorophyll molecule when the reaction is
complete. Electrons end up on NADPH
• Cyclic Light Reaction
– Uses ONLY Photosystem I, sunlight hits P700
– Energy given off down ETC, only produces
ATP
…Because of the Calvin
Cycle
• Calvin Cycle uses more ATP than NADPH
• This is eventually a problem because the
Light Reaction produces equal amounts of
ATP and NADPH
• Plants compensate for this disparity by
dropping into the cyclic phase when ATP is
needed to keep the Light Independent
Reaction from coming to a grinding halt!!!
Calvin Cycle
Light-Independent Reaction takes
place in Stroma
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Granum
Chloroplast
Begins Synthesis Phase of Photosynthesis
Meet Mr. Melvin Calvin
Calvin Cycle
• The light reactions provide ATP and
NADPH for the Calvin cycle
• Process of carbon fixation – CO2 from air
incorporated into organic molecules
• Fixed carbon reduced to form a
carbohydrate (powered by NADPH
electrons and energy from ATP)
• Enzyme “Rubisco” assists in reduction
• Product is actually glyceraldehyde-3phosphate (G3P or GAP) – a 3-carbon
molecule
Calvin Cycle
• For 1 molecule of GAP, the cycle must take
place 3 times
• GAP can be used to make carbohydrates
(like glucose), amino acids, lipids, nucleic
acids, and other needed molecules.
• The Calvin cycle begins and ends with
ribulose-1,5-biphosphate (RuBP)
• At the end of the cycle, 3 CO2 molecules
have formed 6 GAP molecules.
• One GAP molecule is used by the plant cell,
the other 5 are used to regenerate RuBP
Calvin Cycle
Things to remember about Calvin
Cycle (C3 Fixation)…
•
•
•
•
Occurs in Stroma of Chloroplast
Inputs: NADPH, ATP and CO2
Products: NADP+, ADP and a sugar.
More ATP used than NADPH = need for
Cyclic Photophosphorylation
• Carbon of sugar comes from CO2
• To produce glucose: it takes 6 turns and
uses 18 ATP and 12 NADPH.
• Some plants must make changes to the
system in order to successfully use light to
produce energy.
• Transpiration = water loss
Guard
Cell
Guard
Cell
• Stomata can be closed to prevent water
loss, however they experience a shortage
of CO2 and the O2 produced in
photosynthesis is unable to leave plant.
Photorespiration
• Occurs on hot, dry, bright days.
• Stomates close.
• Fixation of O2 instead of CO2.
• Produces 2-C molecules instead of 3-C sugar
molecules.
• Produces no sugar molecules or no ATP.
• Result = plants have lowered capacity for
growth
Photorespiration
• Because of photorespiration: Plants have
special adaptations to limit the effect of
photorespiration.
1. C4 plants
2. CAM plants
C3 Plants
• Examples: rice, wheat, bluegrass, and
soybeans
• The cyclic series of reactions whereby CO2
is fixed into a 3-carbon carbohydrate during
the Calvin cycle.
• During hot, dry days, these plants must keep
their stomata closed to prevent water loss.
• O2 build up and CO2 cannot be replenished.
• Uses photorespiration to convert O2 to CO2.
• Produces no ATP nor food – actually
decreases photosynthetic output.
C4 Plants
• Examples: tropical and desert plants, as well
as sugar cane, and corn. (15% of plants)
• The series of reactions that efficiently fixes CO2
into 4- carbon organic acids for later release
and incorporation into the C3 (Calvin) cycle.
• The 4-carbon molecule (oxaloacetate) can
easily regenerate CO2 that reenters the Calvin
cycle.
• The reaction requires lots of ATP and is only
advantageous when lots of light and little water
are available.
C4 Plants vs. C3 Plants
• Has 2 different types of photosynthetic
cells: Mesophyll Cells and Bundle
Sheath Cells
• Divides photosynthesis spatially.
– Light rxn - mesophyll cells.
– Calvin cycle - bundle sheath cells.
• C4 uses PEP Carboxylase to drive
photosynthesis by fixing carbon.
C4 Leaf Anatomy and the C4 Pathway
CAM Plants
• Examples: succulents including many cacti and
pineapples that live in very arid climates (5%)
• Open stomata at night and close them during the
day.
• The mesophyll cells of CAM plants store the CO2 (at
night) in the form of acids until needed the next day.
• During the day when stomata are closed, CO2 is
released from the organic acids and enters the
Calvin cycle.
• This must occur during the day instead of night
because the light reactions must supply ATP and
NADPH for the Calvin cycle.
C4 vs. CAM Plants
A Review of Photosynthesis