1. 8.2 The Light Reactions Convert Light Energy to Chemical Energy
Katrina Garibotto

2. Objectives Explain how light interacts with pigments.
Describe how photosystems help harvest light energy.
Identify the chemical products of the light reactions

3. Vocabulary wavelength: distance between two waves
electromagnetic spectrum: range of types of electromagnetic energy (short to long wavelengths)
pigment: a chemical that creates color
paper chromatography: laboratory technique used to observe the different pigments in a material
photosystem: cluster of chlorophyll and other molecules in a thylakoid

4. Main Idea Chloroplasts are like chemical factories inside plant cells.
The energy to run these factories comes from the sun.

5. Light Energy and Pigments
Sunlight is a form of electromagnetic energy.
It travels in waves.
The distance between two waves is called wavelength.

6. Light Energy and Pigments
The range of types of electromagnetic energy (short to long wavelengths) is the electromagnetic spectrum.
Our eyes can only see some of this spectrum.
Shorter wavelengths have more energy (move faster) than those with longer wavelengths.

7. Pigments and Color Color is due to chemicals called pigments.
When light shines on something, the light can be…
absorbed
transmitted
Reflected
Pigments in chloroplasts absorb blue-violet and red-orange light.
Some of this absorbed light is converted into energy.
Chloroplast pigments do not absorb green light.
Some passes through (transmitted).
Some bounces back (reflected).
Leave appear green because they reflect green light.

8. Pigments and Color

9. Identifying Chloroplast Pigments
Paper chromatography is a lab technique.
shows the pigments in a green leaf

10. Harvesting Light Energy
When a pigment absorbs light, it gets excited. This is unstable.

11. Harvesting Light Energy
Inside the thylakoid membrane (skin), chlorophyll and other molecules are in clusters, photosystems.
Photosystems are like solar panels. They gather light.

12. Harvesting Light Energy
Pigment molecule absorbs sunlight
It gains energy (gets excited).
Almost immediately it calms down, transferring energy (electron) to its neighbor.
This neighbor does the same thing and it continues in a chain.
Eventually the energy reaches the center of the photosystem.

13. Harvesting Light Energy
The reaction center has a molecule of chlorophyll a next to a “primary electron acceptor” molecule.
The primary electron acceptor traps the excited electron.
This trapped energy can now be used by other molecules in the thylakoid to make ATP and NADPH.

14. Harvesting Light Energy

15. Chemical Products of Light Reactions
Two photosystems in light reactions
First photosystem
Traps light energy
Transfers electrons
Splits water into hydrogen and oxygen
Releases oxygen as waste
Two photosystems connected by electron transport chain
Chain releases energy used to make ATP
Second photosystem
Produces NADPH