PHOTOSYNTHESIS

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PHOTOSYNTHESIS
Class Notes
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PHOTOSYNTHESIS

• Photosynthesis is ability of a plant to turn
  sunlight, air, and water into sugar (energy).
• The overall reaction is
  
  light
  chlorophyll
  
  CO2 H2O -------gt CH2O O2
• Photosynthesis is vital to life for two reasons

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• 1. The oxygen in the air comes from
• photosynthesis. The plants continue to
• replenish the oxygen in the air.
• 2. All of our food comes directly or
• indirectly from photosynthesis.

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Parts of a leaf
Stomata palisade layer spongy layer veins chloropl
ast thylakoids grana stroma
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The Chloroplast -double membrane, stroma,
thylakoid, grana
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Light

• Photosynthesis uses visible light.
• Only visible light with intermediate wavelengths
  has enough energy to cause chemical change
  without destroying biological molecules.
• What are the colors of the visible spectrum?
• Why does a leaf look green?

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• Light is captured by pigments. The main
  photosynthetic pigment is chlorophyll. There is
  chlorophyll a, b and c. Chlorophyll a is the
  major photosynthetic pigment and is found in all
  photosynthetic plants, protist, and cyanobacteria

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Carotenoids

• Carotenoids are accessory pigments found in all
  green plants. They absorb blue and green
  wavelengths and give a plant a yellow or orange
  color. In the fall when chlorophyll breaks down,
  it is the accessory pigments which give colors of
  fall. The red color of some autumn leaves is due
  to the anthocyanin pigments. These are not
  photosynthetic.

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Stages of Photosynthesis
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The Energy-Capturing Reactions

• Light absorption
• Chlorophyll a and b are antennae pigments
• Electron transport
• Electrons lost by chlorophyll a end up at NADP
• Produce oxygen
• Chemiosmotic ATP synthesis
• Powered by H ion reservoir

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• The Dark reactions use more ATP then NADPH.
• Each turn of the cycle uses 9 ATP and only 6
  NADPH.
• How are more ATP acquired than NAPH since one ATP
  is produced in photosystem II and one NADPH is
  produced in photosystem I?
• Cyclic and noncyclic electron flow

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Noncyclic Electron Flow
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Cyclic Electron Flow
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The Dark Reactions

• The dark reactions take place outside the
  thylakoid membrane in the stroma. The Calvin
  Cycle converts CO2 to sugar in a three phase
  process.
• Carbon fixation - The carbon dioxide is
  incorporated into an organic molecule (1C 5C
  6C) called RuBP by the enzyme Rubisco. This 6
  carbon structure is unstable and immediately
  splits into 2, 3 carbon molecules. This is the
  same molecule that glucose is split into.

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• Reduction Each 3 carbon molecule is
  phosphorylated by ATP and NAPH to create G3P. The
  cycle must turn 3 times for a net gain of one
  molecule of G3P.
• Regeneration of CO2 acceptor The rearrangement
  of 5 molecules of G3P into 3 molecules of RuBP
  required 3 more ATP.
• ( 9 ATP and 6 NADPH yields 1 G3P)

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What controls photosynthesis?

• Intensity of light
• Scarcity of water

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Ecological Aspects

• C4 PLANTS - Use more ATP but at high temperatures
  they can photosynthesize faster -allowing plants
  to grow and reproduce faster. These include many
  weeds and important crops such as corn.

• A special variation of C4 plants are the CAM
  plants. These are mostly desert plants. These
  plants keep the stomata closed during the day and
  open at night. They must conserve water!

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Vocabulary

• Glossary of terms
• blade
• stipules
• petiole
• vein
• midrib

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Vocabulary
NADP NADPH photon photosystem photosystem
I photosystem II pigment stroma thylakoid
carotenoids chlorophyll electromagnetic
spectrum granum