1. Chapter 6 Where It Starts – Photosynthesis

2. Electromagnetic Spectrum of Radiant Energy
range of heat escaping from Earth’s surface
shortest wavelengths (highest energy)
range of most radiation reaching Earth’s surface
longest wavelengths (lowest energy)
visible light
gamma rays
ultraviolet radiation
near-infrared radiation
radiation infrared
x-rays
microwaves
radio waves
Figure 6.2 Properties of light.
400 nm
500 nm
600 nm
700 nm

3. Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin
β-carotene
chlorophyll a
Light absorption
Figure 6.4 Animated Discovery that photosynthesis is driven by particular wavelengths of light.
A Light micrograph of cells in a strand of Chladophora. Theodor Engelmann used this green alga in an early experiment to show that certain colors of light are best for photosynthesis.
B Engelmann directed light through a prism so that bands of colors crossed a water droplet on a microscope slide. The water held a strand of Chladophora and oxygen-requiring bacteria. The bacteria clustered around the algal cells that were releasing the most oxygen—the ones that were most actively engaged in photosynthesis. Those cells were under red and violet light. These results constituted one of the first absorption spectra.
C Absorption spectra of chlorophylls a and b , β-carotene, and two phycobilins reveal the efficiency with which these pigments absorb different wavelengths of visible light. Line color indicates the characteristic color of each pigment.
400 nm
500 nm
600 nm
700 nm
Wavelength

4. Take-Home Message: Why do cells use more than one photosynthetic pigment?
A combination of pigments allows a photosynthetic organism to most efficiently capture the particular range of light wavelengths that reaches the habitat in which it evolved

5. two outer membranes of chloroplast
stroma
part of thylakoid membrane system:
Figure 6.5 Animated The chloroplast: site of photosynthesis in the cells
of typical leafy plants. The micrograph shows chloroplast-stuffed cells in a
leaf of Plagiomnium ellipticum, a type of moss.
thylakoid compartment, cutaway view
Figure 6-5b p105

6. Figure 6.7 Summary of the inputs and outputs of photosynthesis.
Figure 6-7 p106

7. The Noncyclic Pathway
Photosystems (type II and type I) contain “special pairs” of chlorophyll a molecules that eject electrons
Electrons lost from photosystem II are replaced by photolysis of water molecules – the process by which light energy breaks down a water molecule into hydrogen and oxygen
Electrons lost from a photosystem enter an electron transfer chain (ETC) in the thylakoid membrane
In the ETC, electron energy is used to build up a H+ gradient across the membrane
H+ flows through ATP synthase, which attaches a phosphate group to ADP
ATP is formed in the stroma by chemiosmosis, or electron transfer phosphorylation
Electrons from the first electron transfer chain (from photosystem II) are accepted by photosystem I
Electrons ejected from photosystem I enter a different electron transfer chain in which the coenzyme NADP+ accepts the electrons and H+, forming NADPH
ATP and NADPH are the energy products of light-dependent reactions in the noncyclic pathway

8. Noncyclic Pathway of Photosynthesis
to second stage of reactions
light energy
electron transfer chain
light energy
ADP + Pi
ATP synthase
photosystem II
photosystem I
thylakoid compartment
stroma

9. Photophosphorylation
The Cyclic Pathway
When NADPH accumulates in the stroma, the noncyclic pathway stalls
A cyclic pathway runs in type I photosystems to make ATP; electrons are cycled back to photosystem I and NADPH does not form
Photophosphorylation
Photophosphorylation is a light-driven reaction that attaches a phosphate group to a molecule
In noncyclic photophosphorylation, electrons move from water to photosystem II, to photosystem I, to NADPH
In cyclic photophosphorylation, electrons cycle within photosystem I
Figure 6.8 Animated Noncyclic pathway of photosynthesis. Electrons that travel through two different electron transfer chains end up in NADPH, which delivers them to sugar-building reactions in the stroma. The cyclic pathway (not shown) uses a third type of electron transfer chain.
1 Light energy ejects electrons from a photosystem II.
2 The photosystem pulls replacement electrons from water molecules, which break apart into oxygen and hydro-gen ions. The oxygen leaves the cell as O2.
3 The electrons enter an electron transfer chain in the thylakoid membrane.
4 Energy lost by the electrons as they move through the electron transfer chain is used to pump hydrogen ions from the stroma into the thylakoid compartment. A hydrogen ion gradient forms across the thylakoid membrane.
5 Light energy ejects electrons from a photosystem I. Replacement elec-trons come from an electron transfer chain in the thylakoid membrane.
6 The electrons move through a second electron transfer chain, then combine with NADP+ and H+, so NADPH forms.
7 Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane.
8 Hydrogen ion flow causes ATP synthases to attach phosphate to ADP, so ATP forms in the stroma.

10. Energy flow in the noncyclic reactions of photosynthesis
light energy
Excited P680
energy
P680
(photosystem II)
light energy
Excited P700
P700
(photosystem I)
Figure 6.9 Energy flow in the light-dependent reactions of photosynthesis. The
P700 in photosystem I absorbs photons of a 700-nanometer wavelength. The P680
of photosystem II absorbs photons at 680 nanometers. Energy inputs boost P700
and P680 to an excited state in which they lose electrons.
A The noncyclic pathway involves a one-way flow of electrons from water, to
photosystem II, to photosystem I, to NADPH. As long as electrons continue
to flow through the two electron transfer chains, H+ continues to be carried
across the thylakoid membrane, and ATP and NADPH keep forming. Light
provides the energy boosts that keep the pathway going.
Energy flow in the noncyclic reactions of photosynthesis
Stepped Art
Figure 6-9a p108

11. P700 (photosystem I) Excited P700
light energy
Excited P700
energy
Figure 6.9 Energy flow in the light-dependent reactions of photosynthesis. The
P700 in photosystem I absorbs photons of a 700-nanometer wavelength. The P680
of photosystem II absorbs photons at 680 nanometers. Energy inputs boost P700
and P680 to an excited state in which they lose electrons.
B In the cyclic pathway,
electrons ejected
from photosystem I are
returned to it. As long
as electrons continue to
pass through its electron
transfer chain, H+ continues
to be carried across
the thylakoid membrane,
and ATP continues to
form. Light provides the
energy boost that keeps
the cycle going.
Energy flow in the cyclic reactions of photosynthesis
Stepped Art
Figure 6-9b p108

12. Take-Home Message: What happens during the light-dependent reactions of photosynthesis?
In light-dependent reactions, chlorophylls and other pigments in thylakoid membrane transfer light energy to photosystems
Photosystems eject electrons that enter electron transfer chains in the membrane; electron flow through ETCs sets up hydrogen ion gradients that drive ATP formation
In the noncyclic pathway, oxygen is released and electrons end up in NADPH
A cyclic pathway involving only photosystem I allows the cell to continue making ATP when the noncyclic pathway is not running; NADPH does not form; O2 is not released

13. Take-Home Message: How does energy flow during the reactions of photosynthesis?
Light provides energy inputs that keep electrons flowing through electron transfer chains
Energy lost by electrons as they flow through the chains sets up a hydrogen ion gradient that drives the synthesis of ATP alone, or ATP and NADPH

14. Light-Independent Reactions
The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis
Calvin-Benson cycle
Enzyme-mediated reactions that build sugars in the stroma of chloroplasts
Carbon fixation
Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule
Builds glucose from CO2
Uses bond energy of molecules formed in light-dependent reactions (ATP, NADPH)

15. The Calvin-Benson Cycle
The enzyme rubisco attaches CO2 to RuBP
Forms two 3-carbon PGA molecules
PGAL is formed
PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH
Two PGAL combine to form a 6-carbon sugar
Rubisco is regenerated

17. Calvin– Benson Cycle other molecules glucose 1 42 4
Calvin– Benson Cycle
Figure 6.10 Animated Light-independent reactions of photosynthesis. The
sketch shows a cross-section of a chloroplast with the light-independent reactions
cycling in the stroma.
The steps shown are a summary of six cycles of the Calvin–Benson reactions.
Black balls signify carbon atoms. Appendix V details the reaction steps.
1 Six CO2 diffuse into a photosynthetic cell, and then into a chloroplast. Rubisco
attaches each to a RuBP molecule. The resulting intermediates split, so twelve
molecules of PGA form.
2 Each PGA molecule gets a phosphate group from ATP, plus hydrogen and
electrons from NADPH. Twelve PGAL form.
3 Two PGAL combine to form one glucose molecule.
4 The remaining ten PGAL receive phosphate groups from ATP. The transfer
primes them for endergonic reactions that regenerate the 6 RuBP.
glucose 3
other molecules
Stepped Art
Figure 6-10 p109

19. Take-Home Message: What happens in light-independent reactions of photosynthesis?
Light-independent reactions of photosynthesis run on the bond energy of ATP and energy of electrons donated by NADPH; both formed in the light- dependent reactions
Collectively called the Calvin–Benson cycle, these carbon-fixing reaction use hydrogen (from NADPH), and carbon and oxygen (from CO2) to build sugars

22. Adaptations: Different Carbon-Fixing Pathways
Environments differ, and so do details of photosynthesis:
C3 plants
C4 plants
CAM plants
Stomata
Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems
Allow CO2 in and O2 out
Close on dry days to minimize water loss

23. C3 Plants palisade mesophyll cell spongy mesophyll cell C3 plants
Plants that use only the Calvin–Benson cycle to fix carbon
Forms 3-carbon PGA in mesophyll cells
Used by most plants, but inefficient in dry weather when stomata are closed
Example: barley
When stomata are closed, CO2 needed for light-independent reactions can’t enter, O2 produced by light-dependent reactions can’t leave
Photorespiration
At high O2 levels, rubisco attaches to oxygen instead of carbon
CO2 is produced rather than fixed
spongy mesophyll cell

24. mesophyll cell CO2 O2 glycolate RuBP Calvin– Benson PGA Cycle ATP
NADPH
Figure 6.11 Animated Light-independent reactions
in C3 plants.
B On dry days, stomata close and oxygen accumulates inside leaves. The excess causes rubisco to attach oxygen instead of carbon to RuBP. This is photorespiration, and it makes sugar production inefficient in C3 plants.
sugars
Figure 6-11b p110 24

25. C4 Plants
C4 plants
Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco
Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar
Examples: Corn, switchgrass, bamboo
C4 plants. Oxygen also builds up inside leaves when stomata close during photosynthesis.
An additional pathway in these plants keeps the CO2 concentration high enough in bundle-sheath cells to prevent photorespiration.

26. C4 Cycle Calvin– Benson Cycle
mesophyll cell
CO2 from inside plant
B C4 plants. Oxygen also builds up inside leaves when stomata close during photosynthesis.
mesophyll cell
oxaloacetate
C4 Cycle
bundle-sheath cell
CO2
An additional pathway in these plants keeps the CO2 concentration high enough in bundle-sheath cells to prevent photorespiration.
RuBP
Calvin– Benson Cycle
Figure 6.12 Animated Light-independent reactions
in C4 plants.
PGA
bundle-sheath cell
sugars
Figure 6-12b p110

27. A CAM Plant: Jade Plant CAM Plants
CAM plants (Crassulacean Acid Metabolism)
Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot
Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for sugar production
Example: succulents, cactuses

28. mesophyll cell CO2 from outside plant oxaloacetate C4 Cycle night day
RuBP
Calvin– Benson Cycle
Figure 6.13 Animated Light-independent reactions in CAM plants.
A A CAM plant: Crassula argentea, or jade plant.
B CAM plants open stomata and fix carbon using a C4 pathway at night. When the plant’s stomata are closed during the day, the organic compounds made during the night are converted to CO2 that enters the Calvin–Benson cycle.
PGA
sugars
Figure 6-13a p111

29. Take-Home Message: How do carbon-fixing reactions vary?
When stomata are closed, oxygen builds up inside leaves of C3 plants; rubisco then can attach oxygen (instead of carbon dioxide) to RuBP; photorespiration reduces the efficiency of sugar production, so it can limit the plant’s growth
Plants adapted to dry conditions limit photorespiration by fixing carbon twice: C4 plants separate the two sets of reactions in space; CAM plants separate them in time