AP Biology
Ms. Haut

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Photosynthesis is the process that converts solar energy into chemical energy
Directly or indirectly, photosynthesis nourishes almost the entire living world
Photosynthesis —process in which some of the solar energy is captured by plants (producers) and transformed into glucose molecules used by other organisms (consumers).
6CO
2
+ 6H
2
Light energy
O C enzymes
6
H
12
O
6
+ 6O
2

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
Glucose is the main source of energy for all life.
The energy is stored in the chemical bonds.
Cellular Respiration — process in which a cell breaks down the glucose so that energy can be released. This energy will enable a cell to carry out its activities.
C
6
H
12
O
6
+ 6O
2
6CO
2
+ 6H
2
O + energy enzymes


Autotroph —organisms that synthesize organic molecules from inorganic materials (a.k.a. producers)
–Photoautotrophs —use light as an energy source
(plants, algae, some prokaryotes)
–Chemoautotrophs —use the oxidation of inorganic substances (some bacteria)
• Heterotroph —organisms that acquire organic molecules from compounds produced by other organisms (a.k.a. consumers)

Thylakoids trap sunlight
Sunlight = electromagnetic energy
•Wavelike properties
•Particlelike properties
(photon)
Light may be reflected, transmitted, or absorbed when it contacts matter

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
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Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or transmitted
Leaves appear green because chlorophyll reflects and transmits green light

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

Absorb light of varying wavelengths and transfer the energy to chlorophyll a
Chlorophyll b -yellow-green pigment
Carotenoids -yellow and orange pigments

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
An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
Chlorophyll a
Chlorophyll b
Carotenoids
400 500 600
Wavelength of light (nm)
Absorption spectra
700


Endergonic redox process; energy is required to reduce CO

2
Light is the energy source that boosts potential energy of electrons (e-) as they are moved from water to CO
2

When water is split, e- are transformed from the water to CO
2
, reducing it to sugar

reduction
6CO
2
+ 6H
2
O oxidation
C
6
H
12
O
6
+ 6O
2

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


Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
Light reactions (in the thylakoids) split water, release O
2
, produce ATP, and form NADPH
Calvin cycle (in the stroma) forms sugar from
CO
2
, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO
2 into organic molecules


Light reactions —convert light energy to chemical bond energy in ATP and NADPH




Occurs in thylakoids in chloroplasts
NADP+ reduced to NADPH—temporary energy storage (transferred from water)
Give off O
2 as a by-product
Generates ATP by phosphorylating ADP


Calvin Cycle —carbon fixation reactions assimilate CO2 and then reduce it to a carbohydrate
 Occur in the stroma of the chloroplast
 Do not require light directly, but requires products of the light reactions
 Incorporates into existing organic molecules and then reduces fixed carbon into carbohydrate
 NADPH provides the reducing power
 ATP provides chemical energy

Interdependent
Reactions
Light reactions produce:
ATP and NADPH that are used by the Calvin cycle;
O
2 released
Calvin Cycle produces:
ADP and NADP+ that are used by the light reactions; glucose produced

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

Photosystem : assemblies of several hundred chlorophyll a, chlorophyll b, and carotenoid molecules in the thylakoid membrane form a light gathering antennae that absorb photons and pass energy from molecule to molecule
Photosystem I —specialized chlorophyll a molecule, P700

Photosystem II —specialized chlorophyll a molecule, P680

• Light drives the light reactions to synthesize
NADPH and ATP
• Includes cooperation of both photosystems, in which e- pass continuously from water to
NADP+

1. When photosystem II absorbs light an e- is excited in the reaction center chlorophyll (P680) and gets captured by the primary e- acceptor.
• This leaves a hole in the P680

2. To fill the hole left in P680, an enzyme extracts e- from water and supplies them to the reaction center
• A water molecule is split into 2 H+ ions and an oxygen atom, which immediately combines with another oxygen to form O2

3. Each photoexcited e- passes from primary e- acceptor to photosystem I via an electron transport chain.
• e- are transferred to plastoquinone (Pq) and plastocyanin (Pc) (e- carriers)

4. As e- cascade down the e- transport chain, energy is released and harnessed by the thylakoid membrane to produce ATP (PHOTOPHOSPHORYLATION)
• This ATP is used to make glucose during Calvin cycle

5. When e- reach the bottom of e- transport chain, it fills the hole in the reaction center P700 of photosystem I.
• Pre-existing hole was left by former e- that was excited

6. When photosystem I absorbs light an e- is excited in the reaction center chlorophyll (P700) and gets captured by the primary eacceptor.
• e- are transferred to ferredoxin (Fd) (e- carrier)
• NADP+ reductase transfers e- from Fd to NADP+, storing energy in NADPH (reduction reaction)
• NADPH provides reducing power for making glucose in Calvin cycle

 Only photosystem I is used
 Only ATP is produced

 Energy released from e- transport chain is used to pump H+ ions (from the split water) from the stroma across the thylakoid membrane to the interior of the thylakoid.
 Creates concentration gradient across thylakoid membrane
 Process provides energy for chemisomostic production of ATP

 The light reactions and chemiosmosis: the organization of the thylakoid membrane
H
2
O CO
2
LIGHT
NADP +
ADP
LIGHT
REACTOR
ATP
NADPH
CALVIN
CYCLE
STROMA
(Low H + concentration)
O
2
Photosystem II
[CH
2
O] (sugar)
Cytochrome complex
Light
2 H +
Photosystem I
Fd
NADP + reductase
3
NADP + + 2H +
NADPH + H +
Pq
Pc
2
H
2
O
THYLAKOID SPACE
(High H + concentration)
1
1 ⁄
2
O
2
+2 H +
2 H +
To
Calvin cycle
STROMA
(Low H + concentration)
Thylakoid membrane
ATP synthase
ADP
P
H +
ATP
Figure 10.17

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Carbon enters the cycle in the form of CO
2 leaves in the form of sugar (glucose) and
The cycle spends ATP as an energy source and consumes NADPH as a reducing agent for adding high energy e- to make sugar
For the net synthesis of this sugar, the cycle must take place 2 times

Light
H
2
O CO
2
NADP +
ADP
LIGHT
REACTIONS
ATP
NADPH
CALVIN
CYCLE
O
2
[CH
2
O] (sugar)
3 P
Ribulose bisphosphate
(RuBP)
P
Input
3
CO
2
(Entering one at a time)
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived intermediate
P
6 P
3-Phosphoglycerate
6 ATP
6 ADP
CALVIN
CYCLE

Light
H
2
O
CO
2
NADP +
ADP
LIGHT
REACTIONS
ATP
NADPH
CALVIN
CYCLE
O
2
[CH
2
O] (sugar)
3 P
Ribulose bisphosphate
(RuBP)
P
Input
3
CO
2
(Entering one at a time)
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived intermediate
P
6 P
3-Phosphoglycerate
6 ATP
6 ADP
CALVIN
CYCLE
6 P P
1,3-Bisphosphoglycerate
6 NADPH
6
6 NADP +
P i
6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction
1
G3P
(a sugar)
Output
P
Glucose and other organic compounds

Light
H
2
O
CO
2
NADP +
ADP
LIGHT
REACTIONS
ATP
NADPH
CALVIN
CYCLE
O
2
[CH
2
O] (sugar)
3 ATP
3 ADP
3 P
Ribulose bisphosphate
(RuBP)
P
Phase 3:
Regeneration of the CO
(RuBP)
2 acceptor
5
G3P
P
Input
3
CO
2
(Entering one at a time)
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived intermediate
P
6 P
3-Phosphoglycerate
6 ATP
6 ADP
CALVIN
CYCLE
6 P P
1,3-Bisphosphoglycerate
6 NADPH
6
6 NADP +
P i
6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction
1
G3P
(a sugar)
Output
P
Glucose and other organic compounds

1. Carbon Fixation: 3 CO
2 molecules bind to 3 5-
Carbon sugars, ribulose bisphosphate (RuBP) using enzyme called RuBP carboxylase (rubisco)
• Produces 6 molecules of a 3-carbon sugar,
3-phosphoglycerate

1. Carbon Fixation
2. Reduction: 6 ATP molecules transfer phosphate group to each molecule of 3-phos. to make 1,3diphosphoglycerate
• 6 molecules of NADPH reduce each molecule of 1,3-diphosph. to make glyceraldehyde 3phosphate (G3P)
3. One of the G3P exits the cycle to be used by the plant; the other 5 molecules are used to regenerate the CO
2 acceptor, RuBP: 3 molecules of ATP are used to convert 5 molecules of G3P into RuBP

• 3 more CO
2 molecules enter the cycle, following the same chemical pathway to release another G3P from the cycle.
• 2 G3P molecules can be used to make glucose

Interdependent

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Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis
The closing of stomata reduces access to CO
2 and causes O
2 to build up
These conditions favor a seemingly wasteful process called photorespiration

•
•
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In most plants (C
3 plants), initial fixation of
CO
2
, via rubisco, forms a three-carbon compound
In photorespiration, rubisco adds O
2
Calvin cycle instead of CO
2 to the
Photorespiration consumes O
2 fuel and releases CO
2 and organic without producing ATP or sugar

•

Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O
CO
2
2 and more
In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the
Calvin cycle

Corn
Crab Grass
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C
4 plants minimize the cost of photorespiration by incorporating CO
2 into four-carbon compounds in mesophyll cells
These four-carbon compounds are exported to bundlesheath cells, where they release CO
2 that is then used in the
Calvin cycle

4

PEP carboxylase-high affinity to CO
2 affinity for O
2 and no
, thus no photorespiration possible

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CAM plants open their stomata at night, incorporating CO
2
 into organic acids
Organic acids stored in vacuoles of mesophyll cells until morning, when stomata close
Stomata close during the day, and CO
2 is released from organic acids and used in the
Calvin cycle http://ecology.botany.ufl.edu/ecologyf02/graphics/saguaro.GIF

Sugarcane Pineapple
Mesophyll cell Organic acid
Bundlesheath cell
C
4
CO
2
CALVIN
CYCLE
CO
2
Sugar
Spatial separation of steps
CAM
CO
2 incorporated into four-carbon organic acids
(carbon fixation)
Organic acid
CO
2
CO
2
Organic acids release CO
2
Calvin cycle to
CALVIN
CYCLE
Night
Day
Sugar
Temporal separation of steps

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Are similar in that CO cycle
2 is first incorporated into organic intermediates before it enters the Calvin
Differ in that the initial steps of carbon fixation in
C4 plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times
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Regardless of whether the plant uses C3, C4, or
CAM pathway, all plants use the Calvin Cycle to produce sugar from CO
2