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AP Biology, Chapter 10
Photosynthesis
Summary
Introduction
PHOTOSYNTHESIS IN NATURE
Plants and other autotrophs are the producers of the biosphere
1. Distinguish autotrophic and heterotrophic nutrition.
a. Autotrophs do not derive energy from other organisms
i. Energy from the Sun or inorganic chemicals
ii. Gather matter in the form of simple organic molecules
iii. Includes plants, protists, and bacteria
b. Heterotrophs derive energy from other organisms
i. May be carnivores, herbivores, parasites, commensals, etc.
ii. Gather matter as a by-product of energy
iii. Includes members of all major taxa
2. Distinguish photoautotrophs and chemoautotrophs.
a. Photoautotrophs
i. Use light energy
ii. Plants, protists, bacteria, archaea
b. Chemoautotrophs
i. Use energy in inorganic molecules; H2S, H2, FeS
ii. Bacteria and archaea
Chloroplasts are the sites of photosynthesis in plants
3. Describe the structure of chloroplasts and indicate their locations within plant cells.
a. Structure
i. Double membrane encloses dense fluid stroma
ii. In the stroma are interconnected stacks of flattened membranes
iii. Flattened membranes = thylakoids; stacks = grana
iv. Chlorophyll is bound to the thylakoid membranes
b. Location: around the periphery of the cell
4. Explain how chloroplast structure relates to its function.
a. Dense stacks of thylakoids efficiently absorb light
b. Light energy is converted into chemical energy at the thylakoid
b. Chemical energy used to build carbohydrate in the stroma
5. Write a summary equation for photosynthesis.
a. 6CO2 + 12H2O + energy  C6H12O6 + 6O2 + 6H2O
THE PATHWAYS OF PHOTOSYNTHESIS
Evidence that chloroplasts split water molecules enabled researchers to track atoms
through photosynthesis: science as a process
6. Explain van Niel's hypothesis and describe how it contributed to our current
understanding of photosynthesis.
a. van Niel's hypothesis
i. Prevailing (wrong) idea: O2 released is from CO2
ii. van Niel's observation
.Green sulfur bacteria perform photosynthesis using H 2S
.6CO2 + 6H2S + energy  C6H12O6 + 12S
iii. Hypothesis: O2 from water, not from CO2
b. Experiments
i. 6CO2 + 12H218O + energy  C6H12O6 + 6H2O + 618O2
ii. 6C18O2 + 12H2O + energy  C6H1218O6 + 6H218O + 6O2
c. Explanation
i. In one phase of photosynthesis water is split and O 2 is liberated
ii. Glucose is built up from reduced CO2
7. Explain the role of redox reactions in photosynthesis.
a. Respiration
i. High-energy electrons are removed from sugar
ii. Oxygen is added = oxidation
b. Photosynthesis
i. Light boosts the energy of electrons in water
ii. Those high-energy electrons are added to CO2 = reduction
The light reactions and the Calvin cycle cooperate in converting light energy to the
chemical energy in food: an overview
8. Describe, in general, the two main stages of photosynthesis.
a. Light reactions take place on the thylakoids
i. Splitting of water
. Chlorophyll absorbs light
. Electrons removed from water absorb the energy
. High-energy electrons and H stored on NADP+
(+ 2H  NADPH + H+)
ii. Electron transport
.High-energy electrons lose energy as they pass
.Energy used to pump H+ into the thylakoid
. H+ allowed to diffuse; energy used to make ATP
b. Calvin cycle takes place in the stroma
i. ATP and NADPH from the light reactions are used
ii. CO2 from the atmosphere is fixed
iii. Glucose is built by reduction
The light reactions convert solar energy to the chemical energy of ATP and NADPH: a
closer look
9. Describe the wavelike and particle-like behaviors of light.
a. Wavelike: diffraction, wavelength, frequency, interference
b. Particle-like: discrete units of specific energy
10. Describe the relationship between an action spectrum and an absorption spectrum.
a. Absorbed wavelengths add energy to electrons in pigments
b. Plot of absorption vs. wavelength = absorption spectrum
c. Plot of function vs. wavelength = action spectrum
11. Explain why the absorption spectrum for chlorophyll differs from the action spectrum
for photosynthesis.
a. Action spectrum peaks are broader
b. Resembles totaled absorption for chlorophylls a and b and carotenoids
c. Other pigments transfer energy to chlorophyll a
12. List the wavelengths of light that are most effective for photosynthesis.
a. Action spectrum has peaks at 420 and 680 nm
b. Chlorophyll a in photosystem I has peak absorbance at 700 nm
c. Chlorophyll a in photosystem II has peak absorbance at 680 nm
13. Explain what happens when chlorophyll or accessory pigments absorbs photons.
a. Absorbed photon energy is transferred to electrons
b. In photosystem II the electrons in chlorophyll a
c. In photosystem I the electrons from the end of electron transport
14. List the components of a photosystem and explain their functions.
a. A few hundred pigment to funnel absorbed light energy
b. A specific chlorophyll a molecule sits at the reaction center
c. Adjacent to it is a primary electron receptor to immediately capture excited
electrons
15. Trace electron flow through photosystems I and II.
a. Photosystem II first absorbs energy, transfers excited electron to primary
electron acceptor; replaced by electron from water
i.  Plastoquinone
ii.  cytochrome complex
iii.  plastocyanin
b.  Photosystem I
i. Absorbs more light energy  Primary electron acceptor
ii.  ferredoxin
iii.  NADP reductase
iv.  NADP
16. Compare cyclic and noncyclic electron flow and explain the relationship between
these components of the light reactions.
a. Noncyclic
i. As electrons flow H+ is transported into the thylakoid
ii. As it flows back out through a channel linked to ATP synthase
iii. Flow of protons is used to make ATP
iv. Eventually excited electrons are captured by NADP
b. Cyclic
i. Photosystem I feeds electrons back into the cytochrome complex
ii. Makes more ATP but no NADP
c. Realtionship
i. Noncyclic doesn't make the right ratio of ATP/NADP for use in Calcin
cycle
ii. Cyclic makes up the needed ATP
17. Describe important differences in chemiosmosis between oxidative phosphorylation in
mitochondria and photophosphorylation in chloroplasts.
a. Similarities
i. Protons pumped during electron transport
ii. Electron carriers are similar
iii. Proton gradient is called proton motive force
iv. ATP synthase allows protons to flow and uses the energy to make
ATP
b. Differences
i. Mitochondria get electrons from food; chloroplasts by absorbing light
ii. Spatial
.Mitochondria pump H+ into innermembrane space
.Chloroplasts pump H+ into thylakoids
The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar: a closer look
18. Summarize the carbon-fixing reactions of the Calvin cycle and describe changes that
occur in the carbon skeletons of intermediates.
a. Reactions
i. Phase 1: Carbon Fixation
.3 Ribulose bisphosphate + 3CO2  6 3-Phosphoglycerate
.Initial fixation catalyzed by ribulose bisphosphate carboxylase
.Abbreviated as rubisco
ii. Phase 2: Reduction
.6 3-Phosphoglycerate + 6ATP  6 Bisphosphoglycerate +
6ADP
.6 Bisphosphoglycerate + 6NADPH  6 Glyceraldehyde 3phosphate + 6NADP + 6Pi
.One molecule of glyceraldehyde 3-phosphate is removed for
glucose synthesis
iii. Phase 3: Regeneration of Ribulose bisphosphate
.5 Glyceraldehyde 3-phosphate + 3ATP  6 ribulose
bisphosphate
b. Carbon skeletons
i. Phase 1: 3 5-carbon + 3 1-carbon  6 3-carbon
ii. Phase 2: 6 3-carbon  5 3-carbon + 1 3-carbon for glucose synthesis
iii. Phase 3: 5 3-carbon  3 5-carbon
19. Describe the role of ATP and NADPH in the Calvin cycle.
a. ATP adds energy to the carbon skeletons for use in rearrangement
b. NADPH adds high-energy electrons
Alternative mechanisms of carbon fixation have evolved in hot, arid climates
20. What happens to rubisco when, the O2 concentration is much higher than CO 2.
a. Adds O2 to the Calvin cycle instead of CO 2
21. Describe the major consequences of photorespiration.
a. Carbon skeleton breaks apart, CO2 released = photorespiration
b. No glucose, ATP, or NADPH are made
c. Worse in hot, arid conditions
22. Describe two important photosynthetic adaptations that minimize photorespiration.
a. Above description is of C3 plants
i. Carbon fixation yields a 3-carbon compound, 3-Phosphoglycerate
ii. Rice, wheat, and soybeans
b. Adaptations for hot, arid climates
i. C4 plants including sugarcane and corn
.Carbon fixation yields a 4-carbon compound in mesphyll
.CO2 + phosphoenolpyruvate  oxaloacetate
.Oxaloacetate pumped into bundle sheath cells
.Gives off CO2, keeps CO2 high
ii. CAM plants including succulents, cactus, pineapple
.Close stomata during daytime; limiting CO 2
.Fix CO2 during the night and store it in organic acids
.Release CO2 during the day for making glucose
.Called crassulacean acid metabolism
Photosynthesis is the biosphere's metabolic foundation: a review
23. Describe the fate of photosynthetic products.
a. Glucose
i. Broken down in mitochondria for energy
ii. Supplies carbon for anabolism
iii. Exported to the rest of the plant as sucrose
iv. Used to make cellulose
v. Feeds heterotrophs
b. O2
i. Released into the atmosphere
ii. Used in the plant's mitochondria