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Where It Starts: Photosynthesis. Introduction. Before photosynthesis evolved, Earth’s atmosphere had little free oxygen Oxygen released during photosynthesis changed the atmosphere Favored evolution of new metabolic pathways, including aerobic respiration. Electromagnetic Spectrum.

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Where It Starts: Photosynthesis

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  • Before photosynthesis evolved, Earth’s atmosphere had little free oxygen
  • Oxygen released during photosynthesis changed the atmosphere
    • Favored evolution of new metabolic pathways, including aerobic respiration
overview of photosynthesis
Overview of Photosynthesis
  • Photosynthesis proceeds in two stages
    • Light-dependent reactions
    • Light-independent reactions

Summary equation:

6H2O + 6CO2 6O2 + C6H12O6









ADP + Pi






Calvin-Benson cycle



phosphorylated glucose

end products (e.g., sucrose, starch, cellulose)

Fig. 6.13, p.104

sites of photosynthesis chloroplasts
Sites of Photosynthesis: Chloroplasts
  • Light-dependent reactions occur at a much-folded thylakoid membrane
    • Forms a single, continuous compartment inside the stroma (chloroplast’s semifluid interior)
  • Light-independent reactions occur in the stroma
products of light dependent reactions
Products of Light-Dependent Reactions
  • Typically, sunlight energy drives the formation of ATP and NADPH
  • Oxygen is released from the chloroplast (and the cell)

electron transfer chain

light energy

electron transfer chain

light energy


Photosystem II

Photosystem I






(diffuses away)


Fig. 6.8b, p.99

atp formation
ATP Formation
  • In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment
    • A hydrogen ion gradient builds up across the thylakoid membrane
  • H+ flows back across the membrane through ATP synthases
    • Results in formation of ATP in the stroma
light independent reactions the sugar factory
Light Independent Reactions:The Sugar Factory
  • Light-independent reactions proceed in the stroma
  • Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP to start the Calvin–Benson cycle
calvin benson cycle
Calvin–Benson Cycle
  • Cyclic pathway makes phosphorylated glucose
    • Uses energy from ATP, carbon and oxygen from CO2, and hydrogen and electrons from NADPH
  • Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose)
  • Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2
adaptations different carbon fixing pathways
Adaptations: Different Carbon-Fixing Pathways
  • Environments differ
    • Plants have different details of sugar production in light-independent reactions
  • On dry days, plants conserve water by closing their stomata
    • O2 from photosynthesis cannot escape
a burning concern
A Burning Concern
  • Photoautotrophs remove CO2 from atmosphere; metabolic activity of organisms puts it back
  • Human activities disrupt the carbon cycle
    • Add more CO2 to the atmosphere than photoautotrophs can remove
  • Imbalance contributes to global warming
overview of carbohydrate breakdown pathways
Overview of Carbohydrate Breakdown Pathways
  • All organisms (including photoautotrophs) convert chemical energy of organic compounds to chemical energy of ATP
  • ATP is a common energy currency that drives metabolic reactions in cells
pathways of carbohydrate breakdown
Pathways of Carbohydrate Breakdown
  • Start with glycolysis in the cytoplasm
    • Convert glucose and other sugars to pyruvate
  • Fermentation pathways
    • End in cytoplasm, do not use oxygen, yield 2 ATP per molecule of glucose
  • Aerobic respiration
    • Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule
overview of aerobic respiration
Overview of Aerobic Respiration
  • Three main stages of aerobic respiration:

1. Glycolysis

2. Krebs cycle

3. Electron transfer chain

Summary equation:

C6H12O6 + 6O2 → 6CO2 + 6 H2O

glycolysis glucose breakdown starts
Glycolysis – Glucose Breakdown Starts
  • Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate
  • Reactions transfer electrons and hydrogen atoms to two NAD+ (reduces to NADH)
  • 4 ATP form by substrate-level phosphorylation
products of glycolysis
Products of Glycolysis
  • Net yield of glycolysis:
    • 2 pyruvate, 2 ATP, and 2 NADH per glucose
  • Pyruvate may:
    • Enter fermentation pathways in cytoplasm
    • Enter mitochondria and be broken down further in aerobic respiration
second stage of aerobic respiration
Second Stage of Aerobic Respiration
  • The second stage of aerobic respiration takes place in the inner compartment of mitochondria
  • It starts with acetyl-CoA formation and proceeds through the Krebs cycle
acetyl coa formation
Acetyl-CoA Formation
  • Two pyruvates from glycolysis are converted to two acetyl-CoA
  • Two CO2 leave the cell
  • Acetyl-CoA enters the Krebs cycle
krebs cycle
Krebs Cycle
  • Each turn of the Krebs cycle, one acetyl-CoA is converted to two molecules of CO2
  • After two cycles
    • Two pyruvates are dismantled
    • Glucose molecule that entered glycolysis is fully broken down
energy products
Energy Products
  • Reactions transfer electrons and hydrogen atoms to NAD+ and FAD
    • Reduced to NADH and FADH2
  • ATP forms by substrate-level phosphorylation
    • Direct transfer of a phosphate group from a reaction intermediate to ADP
third stage aerobic respiration s big energy payoff
Third Stage:Aerobic Respiration’s Big Energy Payoff
  • Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane
  • Energy released by electrons flowing through the transfer chains moves H+ from the inner to the outer compartment
hydrogen ions and phosphorylation
Hydrogen Ions and Phosphorylation
  • H+ ions accumulate in the outer compartment, forming a gradient across the inner membrane
  • H+ ions flow by concentration gradient back to the inner compartment through ATP synthases (transport proteins that drive ATP synthesis)
the aerobic part of aerobic respiration
The Aerobic Part of Aerobic Respiration
  • Oxygen combines with electrons and H+ at the end of the transfer chains, forming water
  • Overall, aerobic respiration yields up to 36 ATP for each glucose molecule
anaerobic pathways
Anaerobic Pathways
  • Lactic acid fermentation
    • End product: Lactic acid (lactate)
  • Alcoholic fermentation
    • End product: Ethyl alcohol (or ethanol)
  • Both pathways have a net yield of 2 ATP per glucose (from glycolysis)
life s unity
Life’s Unity
  • Photosynthesis and aerobic respiration are interconnected on a global scale
  • In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels