- Sunlight is a form of electromagnetic energy
- Electromagnetic energy travels in waves
- Wavelength: distance between two adjacent waves
Visible light: wavelengths you see as different colors - Shorter wavelengths have more energy than longer wavelengths
- Wavelengths shorter than visible light have enough energy to damage organic molecules
- Pigments: chemical compounds that give a substance color
- Light waves are either absorbed, transmitted, or reflected
- Chloroplast pigments absorb blue-violet and red-orange light
- Absorbed light energy converted into chemical energy
- Green light is transmited (passes through leaf) or reflected which is why leaves look green
- In thylakoid membrane, clusters called photosystems
- Photosystems contain few hundred pigment molecules
- Clusters act as a light-gathering panel
- When pigment molecule absorbs light energy, electrons gain energy
- Excited electron is very unstable and passes energy to neighboring molecule
- Excited electron in receiver than passes energy to next pigment molecule
- Energy jumps from molecule to molecule until it reaches reaction center of photosystem
- Reaction center consists of chlorophyll a molecule next to primary electron acceptor
- Primary electron acceptor traps excited electron from chlorophyll a
- Other molecules use energy to make ATP and NADPH
- First photosystem traps light energy and transfers electrons to electron transport chain
- Splits water molecules that releases H+ ions and oxygen as a waste product
- Electron transport chain connecting two photosystems releases energy to make ATP
- Mechanism similar to ATP production in cellular respiration
- H+ ions pumped across thlakoid membrane
- Light-excited electrons from photosynthesis travel down chain
- Second photosystem uses excited electrons and H+ ions to make NADPH
- Calvin cycle uses ATP and NADPH produced by light reactions
Vocabulary
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
Concept Check 8.2
1) Explain why a leaf appears green.
A leaf appears green because the chlorophyll in it absorbs blue-violet and red-orange light well. However, green light is either transmitted or reflected so all you see is the green.
2) Describe what happens when a molecule of chlorophyll a absorbs light.
When chlorophyll a absorbs light it transfers electrons from molecule to molecule until it reaches the reaction center which creates ATP and NADPH.
3) Besides oxygen, what two molecules are produced by the light reactions?
The two molecules produced by the light reactions are ATP and NADPH.
4) Where in the chloroplast do the light reactions take place?
Light reactions take place inside the thylakoids.
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