notes photosynthesis 2010
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Cellular Energy
Photosynthesis
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How do plants make energy & food?
Plants use the energy from the sun
to make ATP energy to make sugars
glucose, sucrose, cellulose, starch, & more
sun
ATP
sugars
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Overview of Photosynthesis
Photosynthesis involves 2
energy conversions: Conversion of light energy into
chemical energy (The LightReactions)
Storage of chemical energy in
the form of sugars (The Calvin
Cycle)
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Chloroplasts organelles ofphotosynthesis
Leaves
Chloroplastscontain
Chlorophyll
Chloroplastsin cell
sun
Pigments in chloroplast absorb
sunlight. (All colors?)
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Chloroplasts- organelles ofphotosynthesis
Thylakoids:
Membranes containing
pigments
Granum: a stack of
thylakoids
Stroma: Fluid
around Grana
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Chloroplasts- organelles of
photosynthesis
Pigments light absorbing
substances. Found in thylakoidmembranes of chloroplasts. Absorbwaves of visible light, reflect thecolor seen by your eye
Chlorophyll a and b Major pigments
involved in photosynthesis. Greenpigments, absorb all colors exceptgreen.
Accessory pigments absorb green.
Visible in autumn, when chlorophyll
breaks down.
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Any Questions?
H2Osugars
ATP
enzymes
CO2
sun
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Photosynthesis: 2 Stages
Light Reactions: in
thylakoid membranes Sunlight ATP +
NADPH
Calvin Cycle: in
stroma
NADPH + ATP Sugar
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The Light Reactions
Converts visible light into chemical energy carried by
electrons in high-energy molecules (ATP and NADPH) Splits an H2O. Uses H, releases O.
Next, we zoom in to look more
closely at the thylakoids!
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1. Light strikes the first photosystem (PSI), causing it to transfer
excited e- to the primary electron acceptor. These e- are replaced
by splitting H2O, which releases O2 as a product.
1
2
3
4
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1. Light strikes the first photosystem
(PSI), causing it to transfer excited e-
to the primary electron acceptor. These
e- are replaced by splitting H2O, which
releases O2 as a product.
2. The excited e- travel down and e-transport
chain. This process pumps H+ ions across the
membrane into the thylakoid.
1
2
3
4
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1. Light strikes the first photosystem
(PSI), causing it to transfer excited e-
to the primary electron acceptor. These
e- are replaced by splitting H2O, which
releases O2 as a product.
2. The excited e- travel down
and e-transport chain. This
process pumps H+ ions across
the membrane into the
thylakoid.
3. Light-excited e-
in PSII are
transferred to
NADP+. These e-are replaced by
those coming from
e- transport chain.
1
2
3
4
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1. Light strikes the first photosystem
(PSI), causing it to transfer excited e-
to the primary electron acceptor. These
e- are replaced by splitting H2O, which
releases O2 as a product.
2. The excited e- travel down
and e-transport chain. This
process pumps H+ ions across
the membrane into the
thylakoid.
3. Light-excited e- in PSII are
transferred to NADP+. These e- are
replaced by those coming from e-
transport chain.
4. Thebackflow of
hydrogen ions
out of the
thylakoid pass
through ATP
synthase,powering ATP
production.
1
2
3
4
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The Light Reactions Another View
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The Light Reactions - Summary
Light absorbed into thylakoidmembrane by pigments in
photosystems PSI and PSII (see figure4.10)
Light energy is transferred to thereaction center within thephotosystem.
H2O splits apart at PSII (2H2O 4H+ +
4e- + O2) Oxygen diffuses out of the plant
Protons (H+): Transported to the thylakoid
Electrons (e-): electron transport chain toPSI
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The Light Reactions Summary cont.
Formation of NADPH (use e- from H2O splitting)
Electrons reach PSI
Used to join NADP+ with H+ NADPH
This high-energy molecule will be used later for the Calvin
cycle.
Build-up of H+ ions
Energy electrons received from reaction center is used inactive transport to pump protons into thylakoid
H+ build up inside thylakoid, result in potential energy
difference (like a battery)
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Review: Light Reaction
H2O + sunlight O2 + ATP +
NADPH To see this in action, check out the
YouTube video:Light Dependent
Reactions
2 Unstablecompounds
formed. Must be
converted to
C6
H12
O6
for
storage!
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The Calvin Cycle (Dark Reactions)
Occurs in stroma
of chloroplast. Energy from ATP
and NADPHmoleculesconverted tochemical bondswithin glucose forstorage.
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The Calvin Cycle
1
2
3
1. Carbon Fixation:
CO2 taken in
through leaves.
Combines with
RuBP, a 5-Carbon
sugar-phosphate,
by enzyme rubisco.
CO2 gas
is fixed
into anorganic
molecule.
6-Carbon
sugar
formed,
immediate
ly splits
into two
3-C PGA
molecules
4
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The Calvin Cycle
1
2
3
1. Carbon Fixation:
CO2 taken in through
leaves. Combines with
RuBP, a 5-Carbon
sugar-phosphate, by
enzyme rubisco.
CO2 gas is
fixed into
an organic
molecule.
6-Carbon
sugar
formed,
immediately
splits into
two 3-C PGA
molecules2. ATP and
NADPH
used torearrange 6
PGA
molecules
into 6 G3P
(PGAL).
4
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The Calvin Cycle
1
2
3
1. Carbon Fixation:
CO2 taken in through
leaves. Combines with
RuBP, a 5-Carbon
sugar-phosphate, by
enzyme rubisco.
CO2 gas is
fixed into
an organic
molecule.
6-Carbon
sugar
formed,
immediately
splits into
two 3-C PGA
molecules2. ATP and NADPH
used to rearrange 6
PGA molecules into6 G3P (PGAL).
3. One PGAL released,
remaining 5 stay in Calvin
cycle.
2 PGAL 1 Glucose
4
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The Calvin Cycle
1
2
3
1. Carbon Fixation:
CO2 taken in through
leaves. Combines with
RuBP, a 5-Carbon
sugar-phosphate, by
enzyme rubisco.
CO2 gas is
fixed into
an organic
molecule.
6-Carbon
sugar
formed,
immediately
splits into
two 3-C PGA
molecules2. ATP and NADPH
used to rearrange 6
PGA molecules into6 G3P (PGAL).
3. One PGAL released, remaining 5 stay in
Calvin cycle.
2 PGAL 1 Glucose
4. Remaining 5
PGAL
rearranged,
turned backinto 3 RuBP
molecules.
Calvin cycle
complete, can
start overagain. 4
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Calvin Cycle - Summary
ATP + NADPH PGAL or Glucose
Carbon fixation by rubisco
Rearrange molecules to produce a PGAL and get back to
starting molecule (RuBP)
YouTube video: The Calvin Cycle by Prentice
Hall
Any Questions??
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Quizzam!! Working with your neighbor, write a 1-sentence
summary explaining The importance of the light reactions
The importance of the Calvin cycle
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Oxidation and Reduction (Redox)Reactions
Oxidation: Removal of electrons from a molecule(decomposition)
Reduction: Addition of electrons (synthesis)
In photosynthesis: Light energy is used to split water.The water molecule looses an electron (it is oxidized).The 1st protein in electron transport chain gains the e-
(is reduced).
oxidation
2H2O O2 + 4H+ + 4e
reduction
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Redox Reactions in Photosynthesis
The oxidation of water is accompanied by a
reduction reaction resulting in the formationNADPH.
NADP+ + H20 NADPH + H+ + O
(oxidized form) (reduced form) (oxygen)
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Redox Reactions in Photosynthesis
Addition of a phosphate group (labeled, as Pi)
to ADP during the light reaction is calledphotophosphorylation.
ADP + Pi ATP
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How are products of photosynthesis
used?
PGAL produced in Calvin cycle can be used in manyways: Synthesized into larger carbohydrates: glucose, sucrose
Modified to make amino acids, glycerol
sugars used by leaf cell, or transported to other cellsin plant.
This occurs in chloroplasts, cell, or other organisms ifplant is eaten.
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Rate of Photosynthesis
Measured by CO2 consumed per unit time or
by O2 produced. Bromthymol Blue indicator. Turns yellow in the
presence of CO2.
An acid-base indicator: H2O + CO2 H2CO3 (l)
In water, CO2 creates carbonic acid.
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Photorespiration
O2 enters Calvin Cycle
Enzyme Rubisco fixes CO2 in Calvin Cycle. Due toshape, rubisco can bind with either O2 or CO2.
CO2 binds to rubisco 2 PGA molecules
O2 binds to rubisco 1 PGA and 1 2-C acid (glycolate).
Plant loses fixed carbons instead of gaining.
Benefits unknown.
Uses O2 and liberates CO2, which is why it is called
photorespiration.
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Photorespiration
Weather and Photosyntheis
In hot, dry weather, plants close leaf openings calledstomates. Helps reduce water loss. No CO2 enters.Photorespiration is favored.
Light reactions release oxygen high light intensities and
high temperatures (above ~ 30C)favor photorespiration.
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Photorespiration: C3,C4 and CAM
plants
All plants carry out photosynthesis byadding CO2 to a 5-carbon sugar.
Reaction is catalyzed by the enzymeRUBISCO.
Results in production of PGA (startingmolecule for glucose)
The process is called the Calvin cycle
and the pathway is called the C3pathway. PGA has 3 carbons. C3
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Photorespiration - C3 Plants
C3 Plants Plants that use onlytheCalvin Cycle to fix CO
2
. Make up 90% of plants on Earth
Wheat, rice, soy
Most vulnerable to high O2concentrations.
Some plants have evolved strategiesfor increasing photosynthesis, andreducing photorespiration: C4 andCAM plants.
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C4 Plants Limit Photorespiration
The C4 cycle: Structural changes in
leaf anatomy
- C4 and C3 pathways are
separated in different parts of the
leaf
- RUBISCO sequestered where the
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C4 Plants Limit Photorespiration
C4 plants are well adapted to habitats with
(1) high daytime temperatures and (2)intense sunlight.
Some examples:
crabgrass
corn (maize)
sugarcane
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CAM Plants Limit Photorespiration
How are
C4 and
CAMDifferent?
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CAM Plants Limit Photorespiration
CAM plants separate their C3 and C4 cycles by time. *Note: C4 plants separate C3 and C4 cycles by
location. CAM plants separate cycles by time of day. At night,
CAM plants take in CO2 through their stomata
CO2 is fixed into a 4-carbon molecule that accumulates in thecentral vacuole of the cells.
In the morning, the stomata close conserving moisture
The accumulated 4-C molecule leaves the vacuole andis broken down to release CO2.
The CO2 is taken up into the Calvin (C3) cycle.
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CAM Plants Limit Photorespiration
CAM plants well adapted to conditions of
(1) high daytime temperatures (2) intense sunlight, and
(3) low soil moisture.
Examples: cacti, aloe, pineapple, bryophyllum
CAM least efficient system. These plantsgrow slowly.
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Evolution ofC4 and CAM Plants
When photosynthetic organisms first evolved,the atmosphere contained no oxygen. Rubisco did not need to differentiate.
More photosynthesis occurring more O2 inthe air.
Plants adapted by developing other pathwaysto fix carbon C
4cells in C
3plants
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Photosynthesis Summary
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