Concept 8.2 Summary

Wave lengths (the distance between two adjacent waves) that are shorter than those of visible light (400nm~700nm, seen as different colours by the eyes) can damage organic molecules such as proteins and nucleic acids.

*Being exposed to UV, which has shorter wavelengths than those of visible light, can cause sunburns and lead to skin cancer.

A substance's color is due to chemical compounds called pigments. When light shines on a material that contains pigments, three things can happen to the different wavelengths: absorbed, transmitted, or reflected.

*Leaves look green-because they don't absorb green light.
-they only absorb blue-violet and red-orange light.

Photosystem
-a cluste containing chlorophyll and other pigment molecules, including chlorophyll a, chlorphyll b, and carotenoids.
-steps: 1. One of the pigment's electrons gains energy when a pigment molecule absobs light energy --> the electron is raised from a low-energy "ground state" to a high-energy "excited state"
2. The excited electron falls back to the ground state.
3. It then transfers the energy to a neighbouring molecule.
4. It excites an electron in the receiving molecule.
5. The electron falls back to the grand state.
6. The energy is transferred in this way, until it reaches the reaction center of the photosystem.
*The reaction center consists of a chlorophyll a molecule and a primary electron acceptor.
7. The primary electron acceptor traps the excited electron from chlorophyll a.
8. Other teams of molecules built into the thylakoid membrane can now use that trapped energy to make ATP and NADPH.

The Steps of the Light Reactions:
1. The first photosystem="water-splitting photosystem"
-traps light energy, and tranfers the light-excited electrons to an electron transport chain.
-releases oxygen (waste product), releases H+.

2. The electron transport cain="connection" between two photosystems
-releases energy, which the chloroplast uses to make ATP.

*It's really similar to the one in cellular respiration.
-similarity: an electron chain pumps H+ across a membrane-inner mitochondrial
-thylakoid membrane
-main difference: -in respiration, food provides electrons for the electron transport chain.
-in photosynthesis, light-excited electrons from chlorophyll travel down the chain.

3. The second photosystem="NADPH-producing photosystem"
-produces NADPH by transferring excited electrons and H+ to NADP+.

Concept Check 8.2:
1. Explain why a leaf appears green.
Because the pigments in the leaf's chloroplasts transmit or reflect the green light, not absorbing it.

2. Describe what happens when a molecule of chlorophyll a absorbs light.
The excited electron then transfers the energy to the chlorophyll a molecule, which then transfers its excited electron to the primary electron acceptor. Then, the light reaction goes on.

3. Besides oxygen, what two molecules are produced by the light reactions?
ATP and NADPH.

4. Where in the chloroplast do the light reactions take place?
In the membrane of thylakoids.

Vocabulary
wavelength-the distance between two adjacent waves
electromagnetic spectrum-the range of types of electromagnetic energy from the very short wavelengths of gamma rays to the very long wavelengths of radio waves
pigment-a chemical compound which a substance's colour is due to
paper chromatography-a laboratory technique in which you could observe the different pigments in a green leaf
photosystem-a cluster which within the thylakoid membrane, chlorophyll and other molecules are arranged in