1. Photosynthesis
AP Biology Notes

2. Objectives
Distinguish between autotrophic and heterotrophic nutrition.
Describe the leaf structures that function in photosynthesis.
Describe the structure of a chloroplast.
Write the summary equation for photosynthesis.
Define spectrophotometry and electromagnetic spectrum.

3. Objectives Describe the light dependent reactions of photosynthesis.
Describe the light independent reactions (Calvin Cycle) of photosynthesis.
Describe the role of the Chemiosmotic Theory in photosynthesis.
Compare C4 photosynthesis to C3.
Describe adaptations CAM plants have to minimize water loss during photosynthesis.

4. Photosynthesis is the biosphere’s metabolic foundation.
In photosynthesis, the energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds
(glucose).
Fig
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5. WOW!!!
It is estimated that photosynthesis makes about 160 billion tons of carbohydrates per year!
That’s equivalent to about 60 trillion copies of your textbook!
That’s 17 stacks of books reaching from Earth to the sun!!!

6. The leaf is the main photosynthetic structure in most plants.

7. Chloroplasts are the sites of photosynthesis within the leaf.

8. Check for Understanding
In what leaf tissue does most photosynthesis occur ?
the mesophyll
How are thylakoids and grana related ?
grana are stacks of thylakoids
What is stroma?
The liquid inside the chloroplasts

9. Fig. 10.4
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10. Photosynthesis is a redox reaction.
It reverses the direction of electron flow that occurs in cell respiration.
Water is split and electrons transferred with H+ from water to CO2, reducing it to sugar.
Light boosts the potential energy of electrons as they move from water to sugar.
Fig. 10.3
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11. The entire range of electromagnetic radiation is the electromagnetic spectrum.
Fig. 10.5
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12. A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light.
It beams narrow wavelengths of light through a solution containing a pigment and measures the fraction of light transmitted at each wavelength.
An absorption spectrum plots a pigment’s light absorption versus wavelength.
Fig. 10.7
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13. When light meets matter, it may be reflected, transmitted, or absorbed.
Different pigments absorb photons of different wavelengths.
A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.
Fig. 10.6
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14. The light reactions can perform work with those wavelengths of light that are absorbed.
The thylakoid contains several pigments that differ in their absorption spectrum.
Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.
Other pigments with different structures have different absorption spectra.
Fig. 10.8a
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15. Explain how a spectrophotometer works.

16. Answer
It beams narrow wavelengths of light through a solution containing a pigment and measures the fraction of light transmitted (allowed through) at each wavelength.

17. In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems.
A photosystem acts like a light-gathering “antenna complex” consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules.
Fig
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18. Light Dependent Reactions (noncyclic photophosphorylation)
Energy in light and the electrons in water are used to make ATP and NADPH (energy-rich molecules).
This is the “photo” part (no sugar is produced).

19. Check for Understanding
What is the function of a photosystem?
To gather light
What is a photosystem made of?
Plant pigments – chlorophyll a and b, and carotenoids

20. The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH.
Fig
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21. Fig
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22. Light Independent Reactions – The Calvin Cycle
Carbon dioxide from the atmosphere and energy in ATP and NADPH are used to make glucose.
Occurs in the stroma of the chloroplasts.
This is the “synthesis” part of photosynthesis.

23. Fig
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24. Fig
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25. Fig
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26. Calvin Cycle
Phase I – Carbon Fixation – carbon is attached to 5-carbon RuBP which quickly splits into two 3-carbon molecules
Phase II – Reduction – NADPH gives up electrons, phosphate groups from ATP attach to molecule
Phase III – Regeneration of carbon dioxide acceptor (RuBP) – necessary to continue the cycle

27. Check for Understanding
What is the role of ATP and NADPH in the Calvin Cycle?
ATP is an energy source, NADPH is an electron (H+) carrier
What are the end products of the Calvin Cycle?
G3P/sugar (will become glucose)

28. Chemiosmotic Theory
Similar to process that occurs on mitchondrial membranes.
ADP is phosphorylated to make ATP.
H+ ions flow out of the thylakoid membrane, energy generated by the flow is used to make ATP (like a water turbine generates power).

29. Fig
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30. Check for Understanding
What process does the Chemiosmotic Theory explain?
ATP synthesis
In what two organelles in a plant cell does chemiosmosis occur?
Mitochondria and chloroplast

31. C4 Photosynthesis
More efficient carbon fixation – 4-C compound is first produced during the Calvin Cycle rather than a 3-C one
Adaptive advantage for plants in hot, dry climates (less time for stomata to be open therefore less water loss)
Examples: sugarcane, corn

32. CAM Plant (Crassulacean Acid Metabolism)
These plants close stomata during the day.
In vacuoles, they store C (from carbon dioxide) as organic acids that they make at night. Carry out the remaining steps of photosynthesis during the day.
Examples: succulents (like aloe), pineapple, and cacti

33. Occurs in different types of cells Occurs in same cells at different
times of day
Fig
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34. Check for Understanding
What photosynthetic adaptations to arid conditions have some plants evolved?