CONCEPT 8.2) The Light Reactions Convert Light Energy to Chemical Energy

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KEY TERMS:

wavelength: distance between adjacent waves.
electromagnetic spectrum: range of types of electromagnetic energy from gamma waves to radio waves.
pigment: chemical compound that determines a substance's color paper.
chromatography: laboratory technique used to observe the different pigments in a material
photosystem: cluster of chlorophyll and other molecules in a thylakoid.

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SUMMARY

• Sunlight is a form of electromagnetic energy that travels in waves.
• Visible light: 400 nm (nanometers) violet ~ 700 nm, red.
• Shorter wavelengths energy > Longer wave lengths energy
• Energy wavelengths shorter than the visible light can cause damage on organic molecules (proteins, nucleic acids). //eg. Ultraviolent radiation
• A substance's color is due to chemical compounds: pigments.
• When light shines on a material containing pigments: ----> 1. absorbed 2. transmitted 3. reflected
• Chloroplasts absorb well: blue-violet, red-orange light
• Chloroplasts do NOT absorb well: green light (transmit, reflect) -> why plants are GREEN!!!

- Paper chromatography is used to observe the different pigments in a green leaf:

• Pigments travel at different rates depending on how easily they dissolve, how strongly they are attracted to the paper.
• This laboratory technique can be used to separate and analyze the pigments in a leaf.

Harvesting Light Energy

• Within the thylakoid membranes is the chlorophyll, other molecules all together: photosystems.
• Each photosystem contains a few hundred pigment molecules (+ chlorophyll a, b & carotenoids).
• A pigment molecule absorbs light energy --> One of the pigment's electrons gains energy ("ground state" to "excited state"-unstable) --> excited electron falls back to ground state --> exits an electron in the next pigment molecule --> so on...."jumping" --> arrives at the reaction center --> primary electron acceptor (traps the excited electron from the chlorophyll a molecule) //
• -----> Now energy is able to make ATP and NADPH in the rest of the thylakoid membrane.

Chemical Products of the Light Reactions


-light strikes photosystem and transfer excited electrons to the primary electron acceptor.
-electrons split water and releases oxygen
-excited electron goes through electron transport chain and pumps H+ions across the membrane to thylakoid.
-light excited electrons get ransferred to NADP.


"Water-splitting photosystem"- releases oxygen (waste product), hydrogen ions:

• First photosystem traps light energy --(light-excited electrons)--> electron transport system//
• --> release of energy --> chloroplast use to make ATP

"NADPH-producing photosystem":

• Photosystem transfer excited electrons & hydrogen ions --> NADP+ --> light reactions (light energy --> chemical energy of ATP & NADPH) --> produces NADPH
• Sugar is to be produced in the Calvin Cycle process.

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CONCEPT CHECK 8.2

1. Explain why a leaf appears green.
A leaf appears green because chloroplasts doesn't absorb green light well. Unlike other colors such as blue-violet and red-orange lights that are well absorbed, green light is rather transmitted or reflected because of the particular wave-length that is not able to get absorbed in the chloroplast pigments.


2. Describe what happens when a molecule of chlorophyll a absorbs light.
When chlorophyll a absorbes light, it makes the molecule get excited and makes it transfer the energy to another near molecule.

3. Besides oxygen, what two molecules are produced by the light reactions?
Hydrogen ions and NADPH is produced by the light reactions, besides oxygen.

4. Where in the chloroplast do the light reactions take place?
It takes place in the chloroplast's thylakiod membrane.