Concept 8.2 The light reactions convert light energy to chemical energy

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
  

Light:

• Sunlight form of electromagnetic energy
  
• Electromagnetic energy travel in waves
  
• Forms of electromagnetic energy different wavelength
  
• From short gamma rays to long radio waves, electromagnetic spectrum
  
• Visible light make short part of spectrum, 400 to 700 nm
  
• Short wavelegnths more energy
  
• Waves longer visible light damage C-molecules, protein, nucleic acid, etc
  
• Thus, sunburn and skin cancer
  

Pigment:

• Substance color determined by pigment, chemical compound
  
• Light shines pigment, absorbed, transmitted, or reflected
  
• Chloroplast convert some trapped energy into chemical energy
  
• Pigment in chloroplast well absorb blue-violet, red-orange
  
• Green mostly bounces back
  
• Thus, leaves look green
  

Identify chloroplast pigments:

• Use paper chromatography
  
• Stain filter paper with leaf
  
• Seal paper in cylinder with solvents, in vented lab hood
  
• Solvents move up paper strip, pigments dissolve and move up
  
• Different pigments travel different speed, dissolve and cohesive to paper
  
• Chlorophyll alpha mostly blue-violet, red, reflect green major in light reactions
  
• Chloroplast include helper pigments
  
• chlorophyll beta, mostly blue, orange, reflect yellow-green
  
• carotenoids, mostly blue-green, reflect yellow-orange
  

Absorbing light energy:

• Photosystems in thylakoid membrane, each contain few hundred pigment molecules
  
• Photosystem act like light-gathering panel
  
• When pigment absorb light energy, e- gain energy, excited
  
• Excited state unstable
  
• Almost immediately, e- back to ground state by transfer energy to near molecule
  
• Thus energy jumps from molecule to molecule, arrive at reaction center of photosystem
  

Reaction center:

• Consist chlorophyll alpha molecule next to primary e- acceptor
  
• Primary e- acceptor is molecule traps excited e- from chlorophyll alpha
  
• Other molecules in thylakoid membrane can use trapped energy make ATP and NADPH
  

Chemical products of light reactions:

• One photosystem split water, other make NADPH
  
• O2 released as H2O split recover e- sent to e- transport chain
  
• p+ also released
  
• e- transport pump p+ into thylakoid
  
• e- make NADP+ NADPH
  
• p+ pump through ATP synthase, across thylakoid membrane, make ATP
  
• Cellular respiration food provide e-, photosynthesis light-excited e- from chlorophyll
  
• NADP+ plus e- and p+ make NADPH
  

Concept Check 8.2

1. Explain why a leaf appears green.

Chlorophyll alpha do not absorb green light

2. Describe what happens when a molecule of chlorophyll a absorbs light.

e- excited, transfers e- to next molecule to primary e- acceptor.

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?

Inside and in the membrane of the thylakoid.