Photosynthesis

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

Self-feeding
Green Plants
Algae
Bacteria
- sunlight energy ATP
- ATP used to make glucose
Products of photosynthesis sustain most forms of
life
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II. Overall Equation for Photosynthesis
Light energy
Photo-synthesis
Glucose
Oxygen gas
Carbon dioxide
Water
Unnumbered Figure 7.1
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III. Site of Photosynthesis

• Chloroplasts
• found in the cells of the mesophyll
• Contain stroma, a thick fluid
• Contain thylakoids, membranous sacs

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Chloroplast
Thylakoid membranes
Stroma (space)
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IV. 2 Stages of Photosynthesis

• 1. Light reactions convert solar energy to
  chemical energy (ATP)
• water is split into hydrogen and oxygen
• energy is stored in the coenzyme NADPH
• NADP ? NADPH

Light
Chloroplast
NADP?
ADP P
Light reactions
Calvin cycle

Figure 7.4
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IV. 2 Stages of Photosynthesis
Light
Chloroplast
2. Calvin Cycle Uses energy from ATP and
electrons from NADPH to make sugar from carbon
dioxide
NADP?
ADP P
Calvin cycle
Light reactions

Figure 7.4
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V. Light-Dependent Reactions

• Chloroplasts contain several pigments which can
  absorb light energy and convert it to chemical
  energy

Reflected light
Light

• Chlorophyll a
• Chlorophyll b
• Carotenoids

Absorbed light
Chloroplast
Transmitted light
Figure 7.6
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- Each pigment has its own Absorption Spectrum
The specific wavelengths of light that are
absorbed
Absorption spectra for Chlorophyll a and
Chlorophyll b
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• A photosystem is a light-gathering antenna

organized group of chlorophyll and other
molecules located in the thylakoid membrane

• Chlorophyll molecules absorb photons, discrete
  units of light
• Electrons in the pigment gain energy
• The energy is released and used

Chloroplast
Cluster of pigment molecules embedded in membrane
Photon
Primary electron acceptor
Granum (stack of thylakoids)
Electron transfer
Reaction center
Reaction- center chlorophyll a
Antenna pigment molecules
Thylakoid membrane
Transfer of energy
Photosystem
Figure 7.9
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• The light reactions in the thylakoid membrane

To Calvin Cycle
Light
Light
ADP P
NADP?
Stroma
Electron transport chain
Thylakoid membrane
ATP synthase
Inside thylakoid
1/2
Figure 7.12
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VI. Summary of the Light Reactions
A. Light energy is absorbed by a photosystem
(clusters of proteins and pigment molecules)
B. When light hits a molecule of chlorophyll, it
excites (energizes) an electron which is
passed along to a reaction center
C. Electrons and hydrogen atoms are transferred
through electron transport systems,
resulting in ATP and NADPH formation
D. Oxygen gas is released from the splitting of
a water molecule by the light energy.
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ATP Formation
A. Hydrogen ions (H) are pumped from the
stroma into the thylakoid compartment of
the chloroplast
B. A concentration (and electrical) gradient is
established
C. (H) diffuse back across the membrane down
the concentration gradient through
special ATP- producing molecules (ATP
synthases)
This drives ATP production
Chemiosmosis
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VII. Calvin Cycle
Functions like a sugar factory within a
chloroplast
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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• A review of photosynthesis

Light
Chloroplast
NADP?
ADP P
Calvin cycle
Light reactions
Electrons
G3P
Cellular respiration
Cellulosse
Starch
Other organic compounds
Figure 7.15
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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.
Know how 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.
12. Explain carbon fixation. 14. Describe the
general steps of the Calvin-Benson Cycle. What
is produced in the cycle?