1. Chapter 6 Where It Starts – Photosynthesis (Sections 6.1 - 6.4)

2. 6.1 Green Energy
Autotrophs make their own food using energy from the environment and inorganic carbon sources such as CO2
In photosynthesis, plants and other autotrophs capture light energy and use it to build sugars from water and CO2
Heterotrophs get energy and carbon from molecules that other organisms have already assembled

3. Key Terms photosynthesis
Metabolic pathway by which most autotrophs capture light energy and use it to make sugars from CO2 and water
autotroph
Organism that makes its own food using carbon from inorganic molecules, and energy from the environment
heterotroph
Organism that obtains energy and carbon from organic compounds assembled by other organisms

4. Biofuels
There is an increasing demand for biofuels (oils, gases, or alcohols made from organic matter that is not fossilized)
Much of the material currently used for biofuel production consists of food crops – typically expensive, damaging to the environment, and competes with our food supply
Making biofuels from other types of plants requires additional steps to break down abundant cellulose

5. Biofuels
Researchers work to find cost-effective ways to break down the cellulose in fast-growing weeds such as switchgrass

6. BBC Video: Are Biofuels as Green as They Claim?6

7. 6.2 Sunlight as an Energy Source
Visible light drives photosynthesis, which begins when photons are absorbed by photosynthetic pigment molecules
Photosynthesis converts light energy to chemical energy

8. Properties of Light
Light energy travels in waves, and is organized as photons
Visible light is a very small part of the spectrum of electromagnetic energy radiating from the sun
We see light of particular wavelengths as different colors
wavelength
Distance between crests of two successive waves of light

9. Properties of Light

10. Properties of Light range of most radiation reaching Earth’s surface
range of heat escaping from Earth’s surface
shortest wavelengths (highest energy)
longest wavelengths (lowest energy)
visible light
gamma rays
x-rays
ultraviolet radiation
near-infrared radiation
infrared radiation
microwaves
radio waves
400
500
600
700
A Wavelengths of visible light (in nanometers)
Figure 6.2 Properties of light. A Electromagnetic spectrum of radiant energy, which undulates across space as waves that we measure in nanometers. Visible light makes up a very small part of the spectrum. Raindrops or a prism can separate its different wavelengths, which we see as different colors. B Light is organized as packets of energy called photons. The shorter a photon’s wavelength, the greater its energy.
B Higher energy
Lower energy
Fig. 6.2, p. 94

11. ANIMATION: Wavelengths of light11

12. Pigments: The Rainbow Catchers
Pigments are molecules that absorb light of particular wavelengths; photons that are not captured by a pigment are reflected as its characteristic color
Chlorophyll a, the main photosynthetic pigment, absorbs violet and red light, so it appears green
Accessory pigments absorb additional wavelengths

13. Key Terms chlorophyll a pigment
An organic molecule that selectively absorbs light of certain wavelengths
Main photosynthetic pigment in plants

14. Some Accessory Pigments
Accessory pigments color familiar roots, fruits, and flowers:
Beta-carotene in carrots (orange)
Zeaxanthin in corn (yellow)
Lycopene in tomatoes (red)
Anthocyanin in violets (blue)

15. Photosynthetic Pigments
Figure 6.3 Examples of photosynthetic pigments. A Collectively, photosynthetic pigments are capable of absorbing almost all visible light wavelengths. B Structures of two photosynthetic pigments. The light-trapping ring structure of chlorophyll is almost identical to a heme group. Heme groups are part of hemoglobin, which is a red pigment

16. Pigment Function
Absorbing a photon excites electrons in the pigment and boosts them to a higher energy level
Photosynthetic cells can capture energy emitted from an electrons as they return to a lower energy level
When the energy is passed to a special pair of chlorophylls, the reactions of photosynthesis begin

17. Pigment Structure
The light-trapping part of a pigment is an array of atoms in which single bonds alternate with double bonds
Such arrays easily absorb photons, so pigment molecules function a bit like antennas

18. 6.3 Exploring the Rainbow
In 1882, botanist Theodor Engelmann identified the colors of light (red and violet) that drive photosynthesis in a photosynthetic alga (Chladophora)
His results constituted an absorption spectrum, which is a graph that shows how efficiently the different wavelengths of light are absorbed by a substance

19. Engelmann’s Experiment
Engelmann directed light through a prism so that bands of colors crossed a water droplet on a microscope slide

20. Engelmann’s Experiment
bacteria
alga
400
500
600
700
Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light. Theodor Engelmann used the green alga Chladophora A in an early photosynthesis experiment B. His 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.
Wavelength (nanometers)
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.
Fig. 6.4b, p. 96

21. Absorption Spectra

22. Wavelength (nanometers)
phycoerythrobilin
100
chlorophyll b
phycocyanobilin
chlorophyll a
β-carotene 80 60
Light absorption (%) 40 20
Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light. Theodor Engelmann used the green alga Chladophora A in an early photosynthesis experiment B. His 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.
Wavelength (nanometers)
C Absorption spectra of a few photosynthetic pigments. Line color is the characteristic color of each pigment.
Fig. 6.4c, p. 96

23. Key Concepts The Rainbow Catchers
The flow of energy through the biosphere starts when chlorophylls and other photosynthetic pigments absorb the energy of visible light
In plants, some bacteria, and many protists, that energy ultimately drives the synthesis of glucose and other carbohydrates

24. 6.4 Overview of Photosynthesis
Photosynthesis occurs in chloroplasts
Plant chloroplasts have two outer membranes, and are filled with a semifluid matrix called stroma
Stroma contains an inner, folded thylakoid membrane which forms stacks of disks (thylakoids) connected by channels

25. Key Terms
chloroplast
Organelle specialized for photosynthesis in plants and some protists
stroma
Semifluid matrix between the thylakoid membrane and the two outer membranes of a chloroplast
thylakoid membrane
A chloroplast’s highly folded inner membrane system; forms a continuous compartment in the stroma

26. The Chloroplast

27. The Chloroplast two outer membranes of chloroplast stroma
part of thylakoid membrane system:
Figure 6.5 The chloroplast: site of photosynthesis in the cells of typical leafy plants. The micrograph shows chloroplast-stuffed cells of a moss, Plagiomnium affine.
thylakoid compartment, cutaway view
Fig. 6.5b, p. 97

28. ANIMATION: Photosynthesis overview
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29. Photosynthesis An overall formula for photosynthesis:
6CO2 (carbon dioxide) + 6H2O (water) → light energy
→ C6H12O6 (glucose) + 6O2 (oxygen)
Photosynthesis is a series of reactions that occur in two stages: light-dependent reactions and light-independent reactions

30. Key Terms light-dependent reactions
First stage of photosynthesis (“photo-”)
Occur at the thylakoid membrane
Convert light energy to chemical energy (ATP, NADPH)
light-independent reactions
Second stage of photosynthesis (“-synthesis”)
Occur in the stroma
Use ATP and NADPH to assemble sugars from water and CO2

31. Two Stages of Photosynthesis

32. Key Concepts What Is Photosynthesis?
Photosynthesis has two stages in the chloroplasts of plants and many types of protists
In the first stage, sunlight energy is converted to chemical energy
Molecules that form in the first stage of photosynthesis power the formation of sugars in the second stage

33. ANIMATION: Photosynthesis - an overview
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