1. Where It Starts – Photosynthesis
Chapter 6
Where It Starts – Photosynthesis

2. How Did Energy-Releasing Pathways

3. 6.1 How Do Photosynthesizers Absorb Light?
Most autotrophs photosynthesize
Light is organized in packets of energy called photons which move in waves
Visible light: Small range of the light spectrum
Insert label and caption for any figures here.

4. Properties of Light (cont’d.)
visible light
gamma rays
x-rays
ultraviolet radiation
near-infrared radiation
infrared radiation
microwaves
radio waves
shortest wavelengths (highest energy)
longest wavelengths (lowest energy)
Figure 6.1 – Properties of light.
400 nm
500 nm
600 nm
700 nm A B

5. 6.2 Why Do Cells Use More Than One Photosynthetic Pigment?
In 1882, Theodor Engelmann tested the hypothesis that the color of light affects the rate of photosynthesis
Used motile, oxygen-requiring bacteria to identify where photosynthesis was taking place
Directed a spectrum of light across individual strands of green algae

6. Why Do Cells Use More Than One Photosynthetic Pigment? (cont’d.)
In Engelmann’s experiment, oxygen- requiring bacteria gathered where blue and red light fell across the algal cells
Conclusion:
Blue and red light are best light for driving photosynthesis in these algal cells

7. Why Do Cells Use More Than One Photosynthetic Pigment? (cont’d.)
phycoerythrobilin
phycocyanobilin
chlorophyll b
chlorophyll a
bacteria
β-carotene
algae
Figure {Animated} Discovery that photosynthesis is driven best by particular wavelengths of visible light.
400nm
500nm
600nm
700nm
400nm
500nm
600nm
700nm
Wavelength B C

8. Capturing a Rainbow
Photosynthesis use pigments to capture light of specific wavelengths
Chlorophyll α: most common photosynthetic pigment
Accessory pigments harvest additional light wavelengths

9. Capturing a Rainbow (cont’d.)
Figure Examples of photosynthetic pigments. Left, photosynthetic pigments can collectively absorb almost all visible light wavelengths.
Right, the light-catching part of a pigment (shown in color) is the region in which single bonds alternate with double bonds. These and many other
pigments (including heme, Section 5.5) are derived from evolutionary remodeling of the same compound. Animals convert dietary beta-carotene into
a similar pigment (retinal) that is the basis of vision.

10. What Happens During Photosynthesis? (cont’d.)
Photosynthesis is often summarized as:
CO2 + water sugars + O2
light energy

11. 6.3 What Happens During Photosynthesis?
Photosynthesis converts light energy into the energy of chemical bonds
Bond energy can then power the reactions of life.

12. What Happens During Photosynthesis? (cont’d.)
In eukaryotes, photosynthesis takes place in chloroplasts
Light-dependent reactions on thylakoid membrane
Converts light energy to ATP and NADPH
Light-independent reactions in the stroma
ATP and NADPH drive synthesis of sugars from water and CO2

13. What Happens During Photosynthesis? (cont’d.)
Light-dependent and light-independent reactions

14. 6.4 How Do The Light-Dependent Reactions Work?
Thylakoid membranes contain millions of photosystems
Photosystem contain hundreds of chlorophylls and other molecules
Water is the electron donor; electrons are energized by light absorbed by chlorophyll

15. How Do The Light-Dependent Reactions Work? (cont’d.)
Figure A view of some components of the thylakoid membrane as seen from the stroma. Molecules of electron transfer chains and ATP synthases are also present, but not shown for clarity.
photosystem
light-harvesting complex

16. The Noncyclic Pathway (cont’d.)
Light energy energizes electrons in photosystem II
E.T.C. creates ion gradient; ATP synthases phosphorylate ADP; ATP is formed in the stroma
Light energy energizes electrons in photosystem I
Electrons move through a second electron transfer chain; NADPH forms

17. The Noncyclic Pathway (cont’d.)1
light energy 5
light energy 8
electron transfer chain
electron transfer chain 4
photosystem II 3
photosystem I 6
ATP synthase
thylakoid compartment 2 7
H2O
Figure {Animated} Light-dependent reactions, noncyclic pathway. ATP and oxygen gas are produced in this pathway. Electrons that travel through two different electron transfer chains end up in NADPH.
stroma O2

18. 6.5 How Do The Light-Independent Reactions Work?
The Calvin–Benson cycle:
Build sugars in the stroma of chloroplasts
Driving force is ATP and NADPH that formed in the light-dependent reactions
Uses carbon atoms from CO2 to make sugars (Carbon fixation)

19. Adaptations to Climate
Stomata are tiny gateways for gases
Open stomata: CO2 diffuse into photosynthetic tissues; O2 to diffuse out
Closed stomata to conserve water on dry days
Limit the availability of CO2 for the light- independent reactions; sugar synthesis slows

20. Adaptations to Climate (cont’d.)
C3 plants use only the Calvin–Benson cycle to fix carbon
When CO2 concentration declines, uses oxygen as a substrate in photorespiration
When stomata are closed during the day, C3 plants lose carbon instead of fixing it

21. Adaptations to Climate (cont’d.)
glycolate
RuBP
Calvin–
Benson
Cycle
PGA
Figure 6.9A Anatomical and biochemical specializations minimize photorespiration in C4 plants.
sugar

22. Adaptations to Climate (cont’d.)
C4 plants close stomata on dry days, but their sugar production does not decline
Minimize photorespiration by storing carbon in its cells
Examples: corn, switchgrass, and bamboo

23. Adaptations to Climate (cont’d.)
CAM plants are like C4 plants that conserve water by fixing carbon
Open stomata at night when low temperatures minimize water loss
Examples: Pineapples, Jade Plants

24. Adaptations to Climate (cont’d.)
Figure Crabgrass “weeds” overgrowing a lawn. Crabgrasses, which are C4 plants, thrive in hot, dry summers, when they easily outcompete Kentucky bluegrass and other fine-leaved C3 grasses commonly planted in residential lawns.

25. Adaptations to Climate (cont’d.)
mesophyll cell
bundle-sheath cell
Figure 6.9 Anatomical and biochemical specializations minimize photorespiration in C4 plants. Micrographs show leaf cross sections.