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Photosynthesis - PowerPoint PPT Presentation

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Photosynthesis. Chapter 6. Carbon and Energy Sources. Photoautotrophs Carbon source is carbon dioxide Energy source is sunlight Heterotrophs Get carbon and energy by eating autotrophs or one another. Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide.

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carbon and energy sources
Carbon and Energy Sources
  • Photoautotrophs
    • Carbon source is carbon dioxide
    • Energy source is sunlight
  • Heterotrophs
    • Get carbon and energy by eating autotrophs or one another
linked processes

Energy-storing pathway

Releases oxygen

Requires carbon dioxide

Aerobic Respiration

Energy-releasing pathway

Requires oxygen

Releases carbon dioxide

Linked Processes
autotrophs are the producers of the biosphere
Autotrophs are the producers of the biosphere
  • Photoautotrophs capture sunlight energy and use it to carry out photosynthesis.
  • Plants, algae, some protists, and bacteria are photosynthetic autotrophs
    • They are the ultimate producers of food consumed by virtually all organisms
photosynthesis occurs in chloroplasts
Photosynthesis occurs in chloroplasts
  • In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts
  • A chloroplast contains:
    • Stroma (fluid)
    • grana (stacks of thylakoids)
  • The thylakoids contain chlorophyll
    • Chlorophyll is the green pigment that captures light for photosynthesis
photosynthesis equation
Photosynthesis Equation


12H2O + 6CO2

6O2 + C2H12O6 + 6H2O

photosynthesis is a redox process as is cellular respiration
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
visible light
Visible Light
  • Humans perceive different wavelengths as different colors
  • Violet (380 nm) to red (750 nm)
  • Longer wavelengths, lower energy
  • Shorter wavelengths, higher energy
  • Light-absorbing molecules
  • Absorb some wavelengths and transmit others
  • Color you see are the wavelengths NOT absorbed

chlorophyll a

chlorophyll b

Wavelength (nanometers)

excitation of electrons
Excitation of Electrons
  • Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment
  • Amount of energy needed varies among pigment molecules
pigments in photosynthesis
Pigments in Photosynthesis
  • Bacteria
    • Pigments found in plasma membranes
  • Plants
    • Pigments embedded in thylakoid membrane system
    • Pigments and proteins organized into photosystems
    • Photosystems located next to electron transport systems
  • Pigments include chlorophyll a, chlorophyll b and carotenoids
light dependent reactions
Light-Dependent Reactions
  • Pigments absorb light energy, give up e- which enter electron transport systems
  • Water molecules are split, ATP and NADPH are formed, and oxygen is released
  • Pigments that give up electrons get replacements from photosystem
photosystem function pigments
Photosystem Function:Pigments
  • When excited by light energy, these pigments transfer energy to adjacent pigment molecules
  • Each transfer involves energy loss
photosystem function reaction center
Photosystem Function: Reaction Center
  • Energy is reduced to level that can be captured by molecule of chlorophyll a
  • This molecule (P700 or P680) is the reaction center of a photosystem
  • Reaction center accepts energy and donates electron to acceptor molecule
making sugar from co 2 the calvin cycle
Making Sugar from CO2: The Calvin Cycle
  • The Calvin–Benson cycle makes sugar from CO2.
  • Overall reactants
    • Carbon dioxide
    • ATP
    • NADPH
  • Overall products
    • Glucose
    • ADP
    • NADP+
making sugar from co 2 the calvin cycle1
Making Sugar from CO2: The Calvin Cycle
  • The Calvin cycle has four phases:
    • fixation of CO2
    • Energy consumption and reduction
    • carbohydrate production (release of G3P)
    • regeneration of RuBP.
using the products of photosynthesis
Using the Products of Photosynthesis
  • Phosphorylated glucose is the building block for:
    • sucrose
      • The most easily transported plant carbohydrate
    • starch
      • The most common storage form in plants
photorespiration in c 3 plants
Photorespiration in C3 Plants
  • On hot, dry days stomata close
  • Inside leaf
    • O2 levels rise
    • CO2 levels drop
  • Rubisco attaches RuBP to oxygen instead of carbon dioxide
  • Only one PGA forms instead of two
photorespiration in c 4 plants
Photorespiration in C4 Plants
  • Carbon dioxide is fixed twice
    • In mesophyll cells, carbon dioxide is fixed to form four-carbon compound (oxaloacetate)
    • Carbon dioxide is released and fixed again in Calvin cycle
photorespiration in cam plants
Photorespiration in CAM Plants
  • Carbon is fixed twice (in same cells)
  • Night
    • Carbon dioxide is fixed to form organic acids
  • Day
    • Carbon dioxide is released and fixed in Calvin cycle