How Cells Acquire Energy

1
How Cells Acquire Energy

• Chapter 6

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Life in the Sun

• Light is central to the life of a plant
• Photosynthesis is the most important chemical
  process on Earth
• It provides food for virtually all organisms
• Plant cells convert light into chemical signals
  that affect a plants life cycle

3
PhotosynthesisAn Overview

• Chloroplasts are made up of
• Stroma -liquid
• Thylakoids- disks containing chlorophyll
• Two stages of Photosynthesis
• Light-dependent reactions - need sunlight
• Light-independent reactions- do not need light

4
Experiment 1
Notlabeled
Experiment 2
Labeled
Figure 7.3B
Reactants
Products
Figure 7.3C
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AN OVERVIEW OF PHOTOSYNTHESIS

• Photosynthesis is the process by which
  autotrophic organisms use light energy to make
  sugar and oxygen gas from carbon dioxide and
  water

Carbondioxide
Water
Glucose
Oxygengas
PHOTOSYNTHESIS
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7.4 Photosynthesis is a redox process, as is
cellular respiration

• Water molecules are split apart and electrons and
  H ions are removed, leaving O2 gas
• These electrons and H ions are transferred to
  CO2, producing sugar

Reduction
Oxidation
Figure 7.4A
Oxidation
Reduction
Figure 7.4B
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Energy and Materials for the Reactions

• Light Dependent
• Sunlight energy is absorbed and converted to ATP
  energy
• Water molecules are split - releasing oxygen
• NADP (coenzyme and electron carrier) picks up
  hydrogen and electrons to form NADPH

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Energy and Materials for the Reactions

• Light-Independent
• ATP donates energy to the cycle
• NADP donates hydrogen
• CO2 donates Carbon and Oxygen
• Glucose (C6H12O6) made from CO2 and Hydrogens
• Different sites in chloroplast
• Stroma
• Granum (in stroma) - stacks of thylakoids

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Absorption Spectra

• 2 kinds of chlorophyll absorb wavelengths of
  visible light

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The Chemical Basis of Color

• Pigments
• Chlorophylls a b-green
• Carotenoids-orange
• Phycobilins - yellow

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What Happens to the Absorbed Energy?

• Light energy is passed from pigment to pigment
  until it reaches the reactioh center

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Light-Dependent Reactions

• In Thylakoid membrane (granum)
• Pigments absorb photon energy
• Transfer of electrons and hydrogen through
  electron transport system
• ATP and NADPH formation
• Replacement of electrons to pigments

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Cyclic and Noncyclic Electron Flow

• Cyclic Pathway
• P700
• Type I photosystem
• Electron transfer creates ATP only
• Noncyclic Pathway
• P680 Electrons not cycled
• Type II and I photosystems
• Electrons from water molecules passed along
• Photolysis- breaking water
• ATP and NADPH made

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ATP Formation in Chloroplasts
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ATP Formation in Chloroplasts
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ATP Formation in Chloroplasts

• Oxygen released from photolysis of water
• H concentration and electric gradients form
  across the thylakoid membrane
• Flow of ions from thylakoid compartment into the
  stroma drives ATP formation

12H20 6CO2 ---gt 602 C6H12O6 6H2O
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• An overview of photosynthesis

H2O
CO2
Chloroplast
Light
NADP
ADP
P
LIGHTREACTIONS(in grana)
CALVINCYCLE(in stroma)
ATP
Electrons
NADPH
O2
Sugar
Figure 7.5
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Light-Independent Reactions

• Synthesis of sugar
• Calvin-Benson Cycle

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Calvin-Benson Cycle

• Carbon Fixation
• RuBP (ribulose biphosphate) attaches to carbon
  dioxide
• RuBP carboxylase ( RUBisCO)-enzyme that puts
  together RuBP and carbon dioxide
• PGA (Phosphoglyceraldehyde) - The 3 carbon sugar
  produced
• Stroma
• Diffusion of CO2 and O2 through pores called
  stomates

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Fixing Carbon

• C3 Plants -normal photosynthesis
• Kentucky blue grass, crops
• C4 Plants- hide the carbon dioxide in a 4 carbon
  sugar deeper in the leaf so when they need carbon
  dioxide they have it
• Crabgrass

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Fixing CarbonThe C3 Pathway
Evergreen trees and shrubs and nonwoody plants of
temperate zones
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Fixing CarbonThe C4 Pathway
Grasses and other tropical plants - Corn,
crabgrass, and sugarcane
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Fixing Carbon CAM Plants
Hot dry climates - cacti, pineapple, orchids,
and Succulents Stomates only open at night
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Autotrophs, Humans, and The Biosphere
Satellite images Red-orange signifies chlorophyll
concentrations
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In Conclusion

• Plants and other autotrophs use carbon dioxide as
  their source of carbon and sunlight as the energy
  source
• Photosynthesis is the main process by which
  carbon and energy enter the web of life
• Photosynthesis has two stages the
  light-dependent and the light-independent
  reactions

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In Conclusion

• Light-dependent reactions proceed in the
  thylakoids
• Light-independent reactions occur in the stroma
• Chlorophyll a is the main photosynthetic pigment
• Accessory pigments absorb wavelengths that
  chlorophyll a cannot

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In Conclusion

• In chloroplasts, photosynthetic pigments are part
  of photosystems
• Thylakoid membranes have photosystems I II
• The light-independent reactions in the stroma
  deal with the Calvin-Benson cycle

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In Conclusion

• Summary of photosynthesis

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In Conclusion

• Photorespiration predominates in C3 plants
• C4 plants fix carbon twice and CAM plants fix
  carbon at night
• developed by M. Roig