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Cellular Respiration Overview

Cellular Energy

  • Energy in Cells
    • Cells constantly expend energy in the form of ATP hydrolysis.
    • ATP (Adenosine Triphosphate) hydrolysis converts ATP into ADP (Adenosine Diphosphate) and a phosphate group (P).
    • Reaction: ATP → ADP + P
    • After hydrolysis, ATP is resynthesized from ADP and P.
    • Reaction: ADP + P → ATP (recycling energy).
    • Cells utilize stored energy from monosaccharides, fatty acids, and amino acids to resynthesize ATP.

Fuels for ATP Synthesis

  • Key Fuels:

    • Proteins: Derived from amino acids.
    • Carbohydrates: Mainly glucose and other sugars, stored in the form of glycogen.
    • Fats: Including glycerol and fatty acids.

  • Stages of Metabolism:

    • Stage 1: Digestion (in GI tract)
    • Stage 2: Anabolism and formation of catabolic intermediates within tissue cells.
    • Stage 3: Oxidative breakdown in mitochondria, leading to oxidative phosphorylation in the electron transport chain.

Cellular Respiration

  • Main Process: Most cells use glucose with oxygen (O2) to synthesize ATP.
    • Overall reaction: C6H12O6 + 6O2 + 38 ADP + 38 P → 6H2O + 6CO2 + 38 ATP.
    • Notable: The number of atoms in reactants equals those in products.
  • Energy Utilization: ATP produced is used for:
    • Muscle contraction,
    • Ciliary beating,
    • Active transport,
    • Synthesis reactions.

Phases of Cellular Respiration

  1. Glycolysis:

    • Occurs in the cytoplasm.
    • Breaks down glucose into 2 molecules of pyruvic acid through 10 steps.
    • Produces a net of 2 ATP and 2 NADH.

  2. Krebs Cycle (Citric Acid Cycle):

    • Takes place in mitochondrial matrix.
    • Processes Acetyl CoA derived from pyruvic acid.
    • Produces NADH, FADH2, CO2, and ATP through a cyclic series of reactions.

  3. Electron Transport Chain (ETC):

    • Located across the inner mitochondrial membrane.
    • Uses electrons from NADH and FADH2 to establish a proton gradient, powering ATP synthesis via oxidative phosphorylation.

Glycolysis Detail

  • Overview:

    • Glycolysis translates to "sugar breaking".
    • Begins with 1 glucose (6 carbons) and ends with 2 pyruvic acid (3 carbons each).

  • Steps:

    • Energy investment of 2 ATP in the initial steps.
    • Splitting of a 6-carbon sugar into two 3-carbon intermediates.
    • NAD+ is reduced to NADH as 3-carbon intermediates lose H atoms.
    • Total gained ATP: 2 (net), 4 produced but 2 used.

  • Final Products:

    • 2 ATP (net), 2 pyruvate, 2 NADH per glucose.

Pyruvic Acid and Respiration Types

  • Aerobic Respiration:

    • If O2 is present, pyruvic acid converts to Acetyl CoA, leading to Krebs cycle.
    • Produces an additional 36 ATP.

  • Anaerobic Fermentation:

    • Occurs in absence of O2, converting pyruvic acid to lactic acid.
    • Result: no additional ATP, causes muscle fatigue and soreness.

Krebs Cycle (Citric Acid Cycle) Detail

  • Process:
    • Involves the conversion of Acetyl CoA (2 C) into citric acid (6 C) through various enzymatic steps, decarboxylating and oxidizing.
    • Key outputs: 3 NADH, 1 FADH2, 2 CO2, and 1 ATP per cycle.
  • Since each glucose yields 2 Acetyl CoA, total from 2 cycles: 6 NADH, 2 FADH2, 4 CO2, 2 ATP.

Electron Transport Chain (ETC)

  • Function:
    • Comprised of integral proteins creating an H+ gradient in the mitochondria.
    • NADH and FADH2 donate electrons; this energy is used to pump H+ ions across the membrane, enhancing the H+ gradient.
  • ATP Synthesis:
    • H+ flows back through the ATP synthase, facilitating ATP synthesis (oxidative phosphorylation).
    • NADH yields ~3 ATP, FADH2 yields ~2 ATP.

Conclusion

  • Overall ATP yield from cellular respiration: Approximately 38 ATP (2 from glycolysis, 2 from Krebs cycle, 34 from ETC via oxidative phosphorylation).
  • Final consumption of electrons leads to the formation of water as O2 accepts electrons.