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Harvesting Energy

Overview of ATP Generation

  • The process of generating ATP depends on the cell's preferences and available nutrients.

Types of Respiration

  1. Aerobic Respiration

    • Most efficient process for generating ATP.

    • Requires oxygen.

    • Utilized by most organisms (not all).

    • Also known as cellular respiration.

  2. Anaerobic Respiration

    • Less efficient than aerobic respiration but similar.

    • Does not require oxygen.

    • Primarily used by anaerobic bacteria (oxygen is toxic to them).

  3. Fermentation

    • The most inefficient process for generating ATP, used when other pathways can't be utilized or when ATP is needed quickly.

    • Does not require oxygen.

    • Important in the production of beverages like wine, beer, and cheese as it manipulates fermentation to create desired flavors.

Aerobic Respiration Process

  • Nutrients (typically glucose) are catabolized into water (H₂O), carbon dioxide (CO₂), and energy stored as ATP.

    • The overall reaction can be expressed as:
      C6H{12}O6 + 6O2 +
      ightarrow 6CO2 + 6H2O + ext{Energy}

    • Glucose is oxidized to CO₂, and oxygen is reduced to H₂O.

Stages of Aerobic Respiration

  1. Glycolysis

    • Occurs in the cytosol (in both prokaryotes and eukaryotes).

    • Converts glucose into 2 pyruvate molecules.

    • Overall production: 2 ATP and 2 NADH molecules generated from 1 round of glycolysis.

    • Can occur under both aerobic and anaerobic conditions.

    • Involves a series of 10 enzymatically catalyzed reactions divided into two phases: energy investment and energy payoff.

Glycolysis Phases

  • Energy Investment Phase

    • Invests 2 ATP.

    • Steps include:

    1. A phosphorylation reaction charges glucose with 2 phosphates, forming fructose-1,6-bisphosphate (F1,6BP).

    2. F1,6BP is split into dihydroxyacetone phosphate (DHAP) + glyceraldehyde-3-phosphate (G3P).

    3. DHAP can rearrange into G3P.

    • Total 2 molecules of G3P are formed.

  • Energy Payoff Phase (occurs twice)

    • Each G3P is converted into pyruvate, yielding a total of 2 pyruvate molecules.

    • Produces:

      • 4 ATP (net gain of 2 ATP as 2 ATP were used in the investment phase).

      • 2 NADH.

  1. Pyruvate Oxidation

    • Occurs in the mitochondria.

    • Converts 1 pyruvate into 1 Acetyl-CoA.

    • Produces 1 NADH and 1 CO₂ per pyruvate.

    • Multiply these values by 2 due to 2 pyruvates, giving a total of:

      • 2 Acetyl-CoA.

      • 2 NADH.

      • 2 CO₂.

  2. TCA Cycle (Tri-Carboxylic Acid Cycle)

    • Also known as the Citric Acid Cycle or Krebs Cycle.

    • Located in the mitochondria of eukaryotes.

    • Total production from 2 turns (1 turn per pyruvate):

      • 6 NADH (3 NADH per turn).

      • 2 FADH₂ (1 FADH₂ per turn).

      • 2 ATP.

  3. Oxidative Phosphorylation (Electron Transport Chain)

    • Takes place in the mitochondrial membrane of eukaryotes.

    • Involves the transfer of electrons from NADH and FADH₂ to a series of membrane-bound electron acceptors, ultimately to oxygen (the final electron acceptor).

    • Aims to generate a proton gradient to power ATP synthase, the last complex of the electron transport chain (ETC).

    • Notice:

      • The protons only flow into the intermembrane space when electrons are dropped off, creating a concentration gradient.

      • Protons flow back through ATP synthase due to the gradient, facilitating ATP production.

    • Chemiosmosis

      • Protons (H⁺) are positively charged and do not cross cell membranes easily.

      • ATP synthase serves as a proton channel, allowing transport powered by the created concentration gradient.

      • High (protons) within the inter3smembrane space

      • Low (protons) within the mitochondrial matrix

Totals from Aerobic Respiration

  • The theoretical yield of aerobic respiration ranges from 36 to 38 ATP molecules from 1 glucose molecule.

  • Breakdown:

    • Glycolysis: 2 ATP, 2 NADH, 2 Pyruvate.

    • Pyruvate Oxidation (from 2 pyruvates): 2 Acetyl-CoA, 2 NADH, 2 CO₂.

    • TCA Cycle (2 turns): 6 NADH, 2 FADH₂, 2 ATP.

  • Calculation for ATP yield:

    • 2 + 2 + 2 + (2 imes 3) + (6 imes 3) + (2 imes 2) = 38 ext{ ATP}

    • Actual yield is about 30 ATP per glucose due to unpredictable energy use and heat loss during glucose breakdown.

    • Efficiency of aerobic respiration is approximately 32%.

Anaerobic Respiration

  • Bacteria in oxygen-limited environments perform anaerobic respiration, where oxygen is toxic.

  • Glycolysis still occurs, but other compounds serve as final electron acceptors (e.g., citrate, sulfate, CO₂).

Fermentation

  • No electron transport chain involved.

  • This is the most inefficient process, yielding a net of 2 ATP per glucose (only glycolysis is used).

    • Focuses on regenerating NAD⁺.

    • Types of fermentation:

  1. Alcohol Fermentation

    • Produces ethanol, CO₂, and NAD⁺.

    • Involves the conversion of pyruvate into ethanol and CO₂ to regenerate NAD⁺.

    • Performed by yeast and bacteria in low-oxygen environments, essential in brewing beer, wine, and baking bread.

  2. Lactic Acid Fermentation

    • Produces lactate and NAD⁺.

    • This type occurs in some bacteria, fungi, and animal cells (especially muscles during exertion when oxygen is scarce).

    • Lactic acid buildup leads to muscle fatigue and soreness, and is utilized to produce cheese, yogurt, and sauerkraut.