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.

Concept 8.1 Photosynthesis uses light energy to make food

Key Terms

• chloroplast: organelle found in some plant cells and certain unicellular organisms where photosynthesis takes place
  
• chlorophyll: pigment that gives a chloroplast its green color; uses light energy to split water molecules during photosynthesis 
  
• stroma: thick fluid contained in the inner membrane of a chloroplast
  
• thylakoid: disk-shaped sac in the stroma of a chloroplast; site of the light reactions of photosynthesis
• light reactions: chemical reactions that convert the sun's energy to chemical energy; take place in the membranes of thylakoids in the chloroplast
  
• Calvin cycle: cycle in plants that makes sugar from carbon dioxide, H+ ions, and high-energy electrons carried by NADPH

Photosynthesis feature:

• Photosynthesis takes place chloroplast
  
• Chlorophast contain chlorophyll, make green
  
• Leaves are major site of photosynthesis
  

Structure of a leaf:

• In a leaf, chloroplast concentrated in mesophyll
  
• Stomata (stoma) on surface of leaf
  
• CO2 enter and O2 exit through stomata
  
• Veins branch in leaf, deliver produce of leaf to other parts
  

Structure of chloroplast:

• Inner and outer membrane
  
• Inner holds thick fluid, stroma
  
• Stroma suspend thylakoid, disk-shaped stacked in grana (granum)
  
• Chemical reactions in thylakoid membrane or stroma
  

Photosynthesis:

• Cellular respiration of falling e-
  
• Photosynthesis rise of e-, by energize sunlight
  
• Chloroplast use excited e- and p+ to produce glucose
  
• Of light reactions and Calvin cycle

Light reactions:

• Convert energy of sunlight to chemical energy, NADPH and ATP
  
• Need molecules in membrane thylakoid
  
• Chlorophyll molecules in membrane capture light energy
  
• Chloroplast use energy remove e- from H2O, waste O and p+
  
• O2 leave through stomata
• Left e- and p+ used make NADPH, e- carrier similar NADH
  
• Chloroplast also use energy make ATP
  

Calvin cycle:

• Make sugar from CO2, p+, e- from NADPH
  
• Enzymes for Calvin cycle outside thylakoid, dissolved in stroma
  
• ATP made by light reaction used make sugar
  
• AKA "light-independent reactions," not directly need light
  

Concept check:

1. Draw and label a simple diagram of a chloroplast that includes the following structures: outer and inner membranes, stroma, thylakoids.

2. What are the reactants for photosynthesis? What are the products?

Reactants CO2 and H2O, products glucose and O2.

3. Name the two main stages of photosynthesis. How are the two stages related?

Light reactions and Calvin cycle, supply chain. Calvin cycle requires energy produced during the light reactions. Both work together to make glucose for cellular respiration.