Photosynthesis

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Photosynthesis

• Capturing the Energy in Light

6CO2 6H2O -gt C6H12O6 6O2
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I. Energy for Life Processes

1. Plants are autotrophs (make their own food)
2. Photosynthetic organisms plants, protists,
   algae, some bacteria.
3. Photosynthesis is a complex series of chemical
   reactions known as a biochemical pathway.

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II. Light Absorption in Chloroplast

• The first reactions in photosynthesis are called
  light reactions
• A chloroplast contains thylakoids, grana or
  granum (stacks of thylakoids), and stroma
  (solution).

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• Light and Pigments
• Light travels through space as waves of energy
• White light from the sun is composed of an array
  of colors called the visible spectrum (ROY G.
  BIV)
• Different colors have different wavelengths (nm)
• Pigments absorb certain colors of light and
  reflect or transmit the colors of others

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• B. Chloroplast Pigments
• 1. Chlorophylls the type of pigment present in
  the thylakoids (chlorophyll a and chlorophyll b).
• 2. Only chlorophyll a is directly involved in
  the light reactions of photosynthesis\
• 3. Chlorophyll b is called an accessory pigment
  (it assist chlorophyll a in capturing light
  energy).
• 4. Carotenoids also function as accessory
  pigments

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• 5. In leaves (photosynthetic parts) chlorophylls
  are more abundant and they hide the colors of the
  other pigments
• 6. Fruits and flowers colors of other pigments
  might show

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III. Electron Transport

• Chlorophylls and carotenoids are grouped in
  clusters referred to as a photosystem
  (photosystem I and photosystem II)
• Steps involved in the light reaction (inside the
  thylakoid membrane)
• Light excites electrons in chlorophyll a
  molecules of photosystem II.
• These electrons move to the primary electron
  acceptor
• The electrons are transferred along a series of
  molecules called the electron transport chain
  (ETC)

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• 4. Light excites electrons in chlorophyll a
  (photosystem I) as the electrons move to the
  primary electron acceptor, they are replaced by
  electrons from photosystem II.
• 5. The electrons from
• photosystem I are
• transferred along
• a second ETC. At
• the end of the chain
• they combine with
• NADP and a proton
• (H) to make NADPH.

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• Restoring Photosystem II
• The replacement electrons (Step 4) are provided
  by the splitting of water molecules.
• 2H2O 4H 4e- O2
• Protons (H) produced are left inside the
  thylakoid, oxygen diffuses out of the chloroplast
  and can leave the plant. For this reason, oxygen
  is called a byproduct of the light reaction.

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IV. Chemiosmosis

• A. This is a process in chloroplasts and
  mitochondria where protons move across a membrane
  (from high to low concentrations) which also
  synthesizes ATP.

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• B. There is a build up of protons inside the
  thylakoid (from the splitting of water)
• C. ADP P ATP (by the enzyme ATP
  synthase)
• D. The energy that drives the above reaction is
  from the movement of the protons inside the
  thylakoid to the stroma.

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• Describe the structure and function of the
  thylakoids of a chloroplast.
• What role do accessory pigments (hog breath
  fresheners) play in photosynthesis.
• What happens to the electrons that are lost by
  photosystem II ? What happens to the electrons
  that are lost by photosystem I.
• Name the substances that are produced when water
  molecules are broken down during the light
  reaction.
• How is ATP made in the light reaction?
• Explain how the light reactions would be affected
  if there were no concentration gradient of
  protons across the thylakoid membrane.