Introduction to the Light
Reactions of Photosynthesis
"Life is woven out of air by light"
Introduction
• Life on Earth is solar powered.
PHOTOSYNTHESIS
• Plants and other autotrophs are the producers
of the biosphere.
• On a global scale, photosynthesis is the most
important process to the welfare of life on
Earth.
Where does photosynthesis occur?
• Green parts of plants
• Leaves are the specialized plant organs
where most photosynthesis occurs
• A typical leaf parenchyma cell has 30-40
chloroplasts, each about 2-4 microns by 4-7
microns long.
Parts of the Chloroplast
• Thylakoids
– Grana
– Chlorophyll
• a - 2-3x, 430nm (violetblue), 662nm (orangered)
• b - 453nm (blue),
642nm (orange)
– Accessory pigments
(i.e.carotenoids,
xanthophylls)
• Stroma
Photosynthesis
Two steps:
1. Light reactions (=light dependent
reactions)
2. Light independent reactions (= Calvin
Cycle, Calvin-Benson Cycle, Dark
Reactions)
PHOTOSYNTHESIS
6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 + 6H2O
Major Components of Light Reactions
6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 +
6H2O
Light
– Where does it come from?
– How does it get into the leaf?
– How does it get into the mesophyll
cell?
– Why is it needed?
• When light meets matter, it may be reflected,
transmitted, or absorbed.
• The light reactions work with those
wavelengths of light that are absorbed.
• In the thylakoids are several pigments that
differ in their absorption spectrum.
•
Pigments are grouped into
two light collecting
complexes called
Photosynthetic Units (PSI,
PSII).
2 parts to each Photosystem
1. Antenna molecules (many)
2. Reaction center molecules (2
chlorophyll a molecules)
Major Components of Light Reactions
6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 +
6H2O
Water
– Where does it come from?
– How does it get into the leaf?
– How does it get into the mesophyll
cell?
– Why is it needed?
Overview of Light Reactions
Part I: Photosystem II
• Light strikes chlorophyll
• Reaction center molecules excited to a higher
energy level
• Excited electrons captured by electron
acceptor
• Hydrogen from water replaces the ‘hole” left
by excited electrons; oxygen released
• Electron acceptor passes excited electrons to
another acceptor…down an electron transport
chain, ATP formed
Overview of Light Reactions Part
2: Photosystem I
• Light strikes photosystem pigments
• Reaction molecules excited to a higher energy
level
• Excited electrons captured by electron
acceptor NADP+ [= nicotinamide adenine
dinucleotide phosphate]--> NADPH formed
• Excited electrons from reaction center
replaced by electrons from PII electron
transport chain; no oxygen is released and no
water is needed
Bottom Line of Light Reactions
• 6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 +
6H2O
• Energy from sun is captured and converted
to chemical form (ATP, NADPH)
• Oxygen is released
Review Questions
• Why is water needed?
Review Questions
• Why is water needed?
• Supplies the hydrogens to replace the electrons
“lost” from the chlorophyll
Review Questions
• What energy molecules are formed by the
end of the light reactions?
Review Questions
• What energy molecules are formed by the
end of the light reactions?
– ATP and NADPH
Review Questions
• What is the source of energy for ATP and
NADPH?
Review Questions
• What is the source of energy for ATP and
NADPH?
–Sunlight
Review Questions
• What wavelengths of light are most
important for the light reactions?
Review Questions
• What wavelengths of light are most
important for the light reactions?
– Violet-blue (400-500nm) and orange-red (600700nm)
Review Questions
• Is oxygen needed for the light reactions?
Review Questions
• Is oxygen needed for the light reactions?
– No, it is a waste product
Is this all?
• NO!
– The light independent reactions follow the light
dependent reactions.
– The energy molecules formed in the light
dependent reactions are used, along with the
carbon from carbon dioxide, to make glucose.
Why do plants grow better
under certain lighting
conditions than others?
What factors can affect
photosynthesis?
Hydroponic Farming
Pre-Lab Tasks
1. Read the information on photosynthesis
and leaf anatomy from your textbook
2. Complete online tutorial on photosynthesis
3. Complete the Planning Form for “Why Do
Plants Grow Better Under Certain Lighting
Conditions than Others” and turn in by the
deadline
Experimental Set Up
Cuvette with buffered spinach
solution with DPIP (blue
indicator dye) added
Flask filled with H2O and
covered with cellophane
(blue, red, or clear)
Desk Lamp with 60w or 100w bulb
Other Materials
Vernier Lab Pro Colorimeter
Measures the amount of light
transmitted through a sample
Cellophane film
Allows only certain wavelengths of
light to be transmitted through the
film
Blue filter = 413nm; 107 lux
Red filter = 647 nm; 161 lux
Light sensor (Vernier Lab Pro)
Measures light intensity for
visible light in lux
More About DPIP
(2,6-dichloropheno-indophonl)
• Replaces NAD
– Blue (oxidized) [i.e. NAD]
– Colorless (reduced) [i.e. NADPH]
• What will the DPIP allow us to measure?
Other Questions
• What does the desk lamp supply to the
experiment?
• What information can the light sensor
provide?
• What is the purpose of covering the flask
with cellophane?
• Why is the flask filled with water?
• What information will the colorimeter
provide?