Photosynthesis

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Photosynthesis How do Organisms Acquire Energy?
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Photosynthesis is the main metabolic pathway by
which energy is acquired

• Autotrophs

Green Plants
Algae
Bacteria

• 2 steps 1) Use of sunlight energy to produce
  ATP
• 2) ATP used to make glucose

• Products of photosynthesis sustain most forms
  of life

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Overall Reaction for Photosynthesis
12H2O 6CO2 6O2 C6H12O6 6H2O
carbon dioxide
water
oxygen
glucose
water

• Electrons are taken from chlorophyll to produce
  
• ATP and glucose

• These electrons are transferred with the help
  of the
• coenzyme NADP (NADPH)

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Site of Photosynthesis
Chloroplast
Stroma (space)
Thylakoid membranes
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2 Stages of Photosynthesis

• I. Light-dependent reactions (Light reactions)
• sunlight energy is absorbed and converted to
  ATP
• water is split into hydrogen and oxygen

• II. Light-independent reactions (Calvin Cycle)
• ATP donates energy for glucose production

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I. Light-Dependent Reactions
Plants use sunlight energy

• Visible light is contained in discrete packets
  (photons)

• Light is absorbed by photosynthetic pigments
• Chlorophylls (a and b)
• Accessory pigments
• (e.g. carotenes)

• Each pigment has its own Absorption Spectrum

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Absorption spectra for Chlorophylls a and b
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Steps in the Light-Dependent Reactions
(Thylakoid membranes)
1. Light energy is absorbed by a photosystem

2. When light hits a molecule of chlorophyll, it
excites (energizes) an electron which
is passed along to a reaction center
3. Electrons and hydrogen atoms are transferred
through electron transport systems,
resulting in ATP and NADPH formation
4. Oxygen gas is released from the splitting of
a water molecule by light energy.
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Electron Transport and ATP Formation by
Chemiosmosis
1. Hydrogen ions (H) are pumped from the
stroma into the thylakoid compartment of
the chloroplast
2. A concentration gradient is established
3. ATP is produced when (H) diffuse back
across the membrane down their
concentration gradient through ATP synthase
molecules
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II. Light-Independent Reactions (stroma)
The Calvin Cycle (Carbon fixation)

• A carbon atom from CO2 is incorporated into
• a stable organic molecule

• A series of reactions eventually leads to the
• formation of a (6-carbon) sugar phosphate
• (glucose phosphate)

• 6 CO2 molecules are needed to produce 1 sugar
• phosphate

• Intermediates are regenerated

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Study Objectives 1. Define autotroph. Give
some examples of autotrophs. 2. Write the
overall reaction for photosynthesis. 3. What
type of energy do autotrophs use for
photosynthesis? 4. Describe how the structure of
a chloroplast is important to the process of
photosynthesis. Include the terms thylakoid
membranes and stroma in your explanation. 5.
Explain what happens in the light-dependent
reactions of photosynthesis. 6. What is the
role of pigments in photosynthesis? What are
some of the pigments used in
photosynthesis? 7. Define absorption spectrum.
Be able to interpret one of these for a
particular pigment. 8. Explain the role of NADP
in photosynthesis. 9. Describe the role of a
photosystem in the light-dependent reactions.
10. Explain how ATP is produced by
phosphorylation across the thylakoid membranes.
11. Explain what happens in the
light-independent reactions of photosynthesis.
Describe the general steps of the Calvin
Cycle. What is produced in the cycle?