Photosynthesis

1
Photosynthesis

• Chapter 6

2
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

3
Linked Processes

• Photosynthesis
• Energy-storing pathway
• Releases oxygen
• Requires carbon dioxide

• Aerobic Respiration
• Energy-releasing pathway
• Requires oxygen
• Releases carbon dioxide

4
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

5
On land, plants such as oak trees and cacti are
the predominant producers
6
In aquatic environments, algae and photosynthetic
bacteria are the main food producers
7
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

8
Location and Structure of Chloroplasts
9
Photosynthesis Equation
LIGHT ENERGY
12H2O 6CO2
6O2 C2H12O6 6H2O
10
Photosynthesis is a redox process, as is cellular
respiration
Reduction
Oxidation

• 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

11
Two Stages of Photosynthesis
12
Visible Light

• Humans perceive different wavelengths as
  different colors
• Violet (380 nm) to red (750 nm)
• Longer wavelengths, lower energy
• Shorter wavelengths, higher energy

13
Pigments

• Light-absorbing molecules
• Absorb some wavelengths and transmit others
• Color you see are the wavelengths NOT absorbed

chlorophyll a
chlorophyll b
Wavelength (nanometers)
14
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

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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

16
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

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Photosystem FunctionPigments

• When excited by light energy, these pigments
  transfer energy to adjacent pigment molecules
• Each transfer involves energy loss

18
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

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Light Dependent Reactions
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Making Sugar from CO2 The Calvin Cycle

• The CalvinBenson cycle makes sugar from CO2.
• Overall reactants
• Carbon dioxide
• ATP
• NADPH
• Overall products
• Glucose
• ADP
• NADP

21
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.

22
Calvin Cycle
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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

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Photosynthesis Summary
25
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

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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

27
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