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Where It Starts:Photosynthesis
Chapter 6
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.
6.1 Green Energy
Before photosynthesis evolved, Earth’s atmosphere had little free oxygen
Oxygen released during photosynthesis changed the atmosphere• Favored evolution of new metabolic pathways,
including aerobic respiration
Introduction
Autotroph organism that makes its own food using carbon from inorganic molecules (CO2) and energy
Heterotroph organism that obtains energy and carbon form organic compounds
Photosynthesis metabolic pathway by which most autotrophs capture light energy and use it to make sugars from CO2 and water
Electromagnetic energy• Travels in waves
• Is organized as photons Wavelength
• Distance between the crests of two successive waves of light
• Measured in nanometers (nm): 25 million nm = 1 inch Visible light A small part of a spectrum of
electromagnetic energy radiating from the sun • Between 380 and 750 nm
6.2 Sunlight as an Energy Source
Electromagnetic Spectrum
Photosynthetic Pigments
Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules
Pigment is an organic molecule that absorbs only light of particular wavelengths• Photons not captured are reflected as color
Pigments Reflect Color
Major Photosynthetic Pigments Chlorophyll a
• Main photosynthetic pigment • Absorbs violet and red light (appears green)
Chlorophyll b, carotenoids, phycobilins• Absorb additional wavelengths
Collectively, photosynthetic pigments absorb almost all of wavelengths of visible light
Figure 6.3 in Text
Chlorophyll a
6.3 Exploring the Rainbow
Engelmann’s Experiment
Fig. 6.4a, p.96
alga
Outcome of T. Engelmann’s experiment.a
Prism
Bacteria (oxygen requiring)
Conclusion: Violet and red light are the best for driving photosynthesis
Absorption Spectra
Fig. 6.4b, p.96
Wavelength (nanometers)
b Absorption spectra for chlorophyll a (solid graphline) and chlorophyll b (dashed line). Compare thesegraphs with the clustering of bacteria shown in (a).
Lig
ht
abso
rpti
on
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)
400 500 600 700
80
0
20
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Fig. 6.4c, p.96
Wavelength (nanometers)
Lig
ht
abso
rpti
on
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)
400 500 600 700
80
0
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c Absorption spectra for beta-carotene (solid line)and one of the phycobilins (dashed line).
Key Concepts: THE RAINBOW CATCHERS
A great one-way flow of energy through the world of life starts after chlorophylls and other pigments absorb the energy of visible light from the sun’s rays
In plants, some bacteria, and many protists, that energy ultimately drives the synthesis of glucose and other carbohydrates
6.4 Overview of Photosynthesis
In the Chloroplast!! Photosynthesis proceeds in two stages
• Light-dependent reactions
• Light-independent reactions
Summary equation:
6H2O + 6CO2 6O2 + C6H12O6
Visual Summary of Photosynthesis
Fig. 6.13, p.104
sunlight
Calvin-Benson cycle
Light-DependentReactions
end products (e.g., sucrose, starch, cellulose)
ATP
Light-IndependentReactions
phosphorylated glucose
H2O
H2O O2
NADPHNADP+
CO2
ADP + Pi
Sites of Photosynthesis: Chloroplasts
Light-dependent reactions occur at a much-folded thylakoid membrane • Forms a single, continuous compartment inside the
stroma
• First stage of photosynthesis • light energy + H2O chemical energy (ATP & NADPH)
Light-independent reactions occur in the stroma (chloroplast’s semifluid interior) • Second stage of photosynthesis
•Use ATP & NADPH to assemble sugars from H2O and CO2
Sites of Photosynthesis
Sites of Photosynthesis
Products of Light-Dependent Reactions
Typically, sunlight energy drives the formation of ATP and NADPH
Oxygen is released from the chloroplast (and the cell)
Key Concepts:OVERVIEW OF PHOTOSYNTHESIS
Photosynthesis proceeds through two stages in chloroplasts of plants and many types of protists
First, pigments in a membrane inside the chloroplast capture light energy, which is converted to chemical energy
Second, chemical energy drives synthesis of carbohydrates
6.5 Light-Dependent Reactions
In the thylakoid membrane
Light-harvesting complexes • Absorb light energy and pass it to photosystems
which then release electrons•Photosystem a cluster of pigments and proteins
that converts light energy to chemical energy in photosynthesis
Electrons enter light-dependent reactions
1. Noncyclic Photophosphorylation
Electrons released from photosystem II flow through an electron transfer chain (ETC)• Electron transfer phosphorylation occurs
•Electrons that flow through the ETC set up a hydrogen ion gradient that drives ATP formation
• At end of chain, they enter photosystem I Photon energy causes photosystem I to release
electrons, which end up in NADPH Photosystem II replaces lost electrons by pulling them
from water (photolysis)• Photolysis process by which light energy breaks
down a molecule
Noncyclic Photophosphorylation
electron transfer chain
THYLAKOIDMEMBRANE
Fig. 6.8b, p.99
NADPH
THYLAKOIDCOMPARTMENT
STROMA
Photosystem IPhotosystem II
electron transfer chainlight energy light energy
oxygen(diffuses away)
2. Cyclic Photophosphorylation
Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I
NADPH does not form, oxygen is not released
ATP Formation
In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment• A hydrogen ion gradient builds up across the
thylakoid membrane
H+ flows back across the membrane through ATP synthases • Results in formation of ATP in the stroma
6.6 Energy Flow in Photosynthesis
6.6 Energy Flow in Photosynthesis
Key Concepts: MAKING ATP AND NADPH
In the first stage of photosynthesis, sunlight energy is converted to the chemical bond energy of ATP
The coenzyme NADPH forms in a pathway that also releases oxygen
6.7 Light Independent Reactions:The Sugar Factory
Light-independent reactions proceed in the stroma
Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP to start the Calvin–Benson cycle • Calvin Benson cycle light-independent
reactions of photosynthesis
• Carbon fixation process by which carbon from an inorganic source gets incorporated into an organic molecule.
• Rubisco carbon fixing enzyme
Calvin–Benson Cycle
Cyclic pathway makes phosphorylated glucose• Uses energy from ATP, carbon and oxygen from
CO2, and hydrogen and electrons from NADPH
Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose)
Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2
Light-Independent Reactions
Fig. 6.10, p.101
6CO2
12 PGA 12 ATP
12 ADP + 12 Pi
12 NADPH
12 NADP+
12 PGAL
phosphorylated glucose
1 Pi
10 PGAL
4 Pi
ATP
6 ADP
6 RuBP
Calvin-Bensoncycle6
f It takes six turns of theCalvin–Benson cycle (sixcarbon atoms) to produceone glucose molecule andregenerate six RuBP.
e Ten of the PGAL get phosphate groups from ATP. In terms of energy, this primes them for an uphillrun—for the endergonic synthesis reactions thatregenerate RuBP.
d The phosphorylatedglucose enters reactions that form carbohydrateproducts—mainly sucrose, starch, and cellulose.
a CO2 in air spaces inside aleaf diffuses into a photosyntheticcell. Six times, rubisco attaches acarbon atom from CO2 to the RuBPthat is the starting compound forthe Calvin–Benson cycle.
b Each PGA molecule getsa phosphate group from ATP, plus hydrogen and electronsfrom NADPH. The resultingintermediate is called PGAL.
c Two of the twelve PGALmolecules combine to forma molecule of glucose withan attached phosphate group.
6.8 Adaptations: Different Carbon-Fixing Pathways
Environments differ• Plants have different details of sugar production
in light-independent reactions
On dry days, plants conserve water by closing their stomata gaps that open on plant surfaces that allow water vapor and gases to diffuse across the epidermis• O2 from photosynthesis cannot escape
Plant Adaptations to Environment
C3 plants• High O2 level; Rubisco
attaches to O2 instead of CO2 to RuBP; Photorespiration reduces efficiency of sugar production
Plant Adaptations to Environment
C3 plants• Photorespiration
•Reaction in which rubisco attaches oxygen instead of CO2 to ribulose bisphosphate
•Plant loses carbon instead of fixing it.
• Extra energy is need to make sugars on dry days
Plant Adaptations to Environment
C4 plants• Carbon fixation occurs twice, in two different cells to
minimize photorespiration
• First reactions release CO2 near rubisco, limit photorespiration when stomata are closed
Examples: Corn, bamboo
Fig. 6.11b2, p.102
CO2 from inside plant
Calvin-Benson
cycle
RuBP
sugar
PGA
C4cycle
CO2
oxaloacetate
b C4 plants. Oxygen also buildsup in the air spaces inside the leaveswhen stomata close. An additionalpathway in these plants keeps theCO2 concentration high enough toprevent rubisco from using oxygen.
Plant Adaptations to Environment
CAM plants • Type of C4 plant that conserves water by fixing carbon
twice, at different times of the day in the same cell•Day time C4 reactions•Night time Calvin-Benson cycle
• Open stomata and fix carbon at night
Fig. 6.11c2, p.102
Calvin-Benson
cycle
C4cycle
sugar
nightday
CO2 from outside plant
PGA
CO2
oxaloacetate
RuBP
c CAM plants open stomata andfix carbon with a C4 pathway at night.When stomata are closed during theday, organic compounds made duringthe night are converted to CO2 thatenters the Calvin–Benson cycle.
Key Concepts: MAKING SUGARS
Second stage is the “synthesis” part of photosynthesis
Enzymes speed assembly of sugars from carbon and oxygen atoms, both from carbon dioxide
Reactions use ATP and NADPH that form in the first stage of photosynthesis
ATP delivers energy, and NADPH delivers electrons and hydrogens to the reaction sites
Details of the reactions vary among organisms
Animation: C3-C4 comparison
Animation: Calvin-Benson cycle
Animation: Energy changes in photosynthesis
Animation: Noncyclic pathway of electron flow
Animation: Photosynthesis overview
Animation: Sites of photosynthesis
Animation: Wavelengths of light