Transcript Introduction to the Light Reactions of Photosynthesis

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:
– Light reactions (=light dependent
reactions)
– Light independent reactions (= Calvin
Cycle, Calvin-Benson Cycle, Dark
Reactions)
PHOTOSYNTHESIS
Light Reactions
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 harvesting
complexes called
photosynthetic units
(I, II).
• Antenna molecules
– 200 chlorophyll a
– Chlorophyll b
– Accessory pigments
• Reaction center
– 2 chlorophyll a
molecules
• There are two types of photosystems.
• Photosystem I has a reaction center
chlorophyll, the P700 center, that has an
absorption peak at 700nm.
• Photosystem II has a reaction center with a
peak at 680nm.
• These two photosystems work together to use
light energy to generate energy carrier
molecules ATP and NADPH.
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:
Photosystem II
• Light strikes chlorophyll
• Reaction center molecules excited to a higher
energy level
• Excited electrons captured by electron
acceptor
• The electrons in hydrogen from water replace
the ‘hole” left by excited electrons of reactive
chlorophyll; oxygen released
• Electron acceptor passes excited electrons to
another acceptor…down an electron transport
chain, ATP formed by chemiosmosis
Overview of Light Reactions:
Photosystem I
• Light strikes chlorophyll
• 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
Bottom Line of Light Reactions
• Overall Photosynthetic Reaction:
– 6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 + 6H2O
• Light Reactions
– H2O + NADP + ADP + Pi +light energy---chlorophyll---> ATP
+ NADPH+H + O2
• Energy from sun is captured and converted to
chemical form (ATP, NADPH)
• Oxygen is released
• NADPH+H and ATP are used to power the Calvin
Cycle (Light Independent Rxns).
• Occurs in the thylakoid membrane of chloroplasts
Light Independent Reactions
= Calvin Cycle
• Overall Photosynthetic Reactions
– 6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 + 6H2O
• Light Independent Reactions (Calvin Cycle)
– CO2+ATP+NADPH+H---> C6H12O6 + ADP + Pi + NADP
• NADPH+H and ATP are used to power the Calvin
Cycle (Light Independent Rxns) in the stroma of the
chloroplast.
• The stored energy in ATP & NADPH is used to
convert CO2 (from atmosphere) into glucose
• Glucose is then used to make cell structures like
cellulose or is converted in the mitochondria to make
ATP via cellular respiration just like in you!
Review Questions
• Why is water needed?
• Supplies the electrons (from hydrogens) to
replace the electrons “lost” from the
chlorophyll
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?
• Sunlight
Review Questions
• What wavelengths of light are most
important for the light reactions?
• Violet-blue (400-500nm) and orange-red
(600-700nm)
Review Questions
• Is oxygen needed for the light reactions?
• No, it is a waste product