Ch+7+whole+FP-module 6

Chapter 7: Cellular Respiration

Energy Source for Cells

  • Energy for life primarily comes from the sun.

  • Plants, algae, and some bacteria convert sunlight into chemical energy via photosynthesis (see Chapter 8).

  • Energy flows in one direction (from the sun), while molecules cyclically move through ecosystems.

Respiration in Humans

  • Cellular respiration is an aerobic process; it requires oxygen.

  • Humans and many animals obtain oxygen through lungs (breathed in).

  • Glucose and other fuel sources come from digestion (food consumption).

  • Blood circulates oxygen and fuels to respiring cells while transporting carbon dioxide (CO2) to the lungs for expulsion.

  • Water may either remain in cells for use or be filtered by kidneys for excretion (as urine).

Conversion of Fuel to ATP

  • Glucose is the most common fuel, but other organic molecules can also serve.

  • Chemical energy from fuels is stored as ATP.

  • Cellular respiration is a multi-step process producing a maximum of 38 ATP per glucose (approximately 40% efficiency).

Energy in Electrons

  • The potential energy of electrons in chemical bonds varies based on their position.

  • When electrons are transferred from glucose to oxygen, energy is released.

  • Redox reactions involve the transfer of electrons between molecules.

  • Movement of electrons correlates with the movement of hydrogen atoms in biological redox reactions.

Redox in Respiration

  • Coenzymes, NAD+ and FAD, act as electron shuttles during respiration.

  • As glucose oxidizes, it loses electrons and hydrogens.

  • NAD+ and FAD gain electrons and hydrogens, becoming reduced to NADH and FADH2.

  • NADH and FADH2 transport electrons to the electron transport chain (ETC) and regenerate NAD+ and FAD for further electron capture.

  • Electrons in the ETC generate ATP.

Steps of Cellular Respiration

Step

Location

Reactants

Products

1. Glycolysis

Cytoplasm

Glucose

2 pyruvate, 2 ATP, 2 NADH

2. Preparation Reaction

Mitochondrial Matrix

Pyruvate, Coenzyme A

Acetyl CoA, CO2, 2 NADH (per glucose)

3. Citric Acid Cycle

Mitochondrial Matrix

Acetyl CoA

2 CO2, 3 NADH, FADH2, 2 ATP (per glucose)

4. Electron Transport Chain

Cristae

NADH, FADH2, O2

H2O and up to 34 ATP

Glycolysis Overview

  • Glycolysis: the breakdown of glucose into 2 pyruvates via 9 enzyme-driven steps.

  • Produces 2 NADH (to mitochondria for later steps) and 2 net ATP via substrate-level phosphorylation (phosphates are directly transferred from substrates to ADP).

Glycolysis Details

  • Intermediates between glucose and pyruvate are formed; memorization is not required.

  • Steps 1-4: energy investment phase consumes 2 ATPs.

  • Steps 5-9: energy payoff phase produces 4 ATPs and 2 NADHs.

  • Glycolysis serves as the sole energy source for some species; others utilize it for short-term energy when oxygen is lacking.

Energy-Investment Phase

  • Steps 1-3: Energization of a fuel molecule using ATP.

  • Step 4: A six-carbon intermediate splits into two three-carbon intermediates.

Energy Payoff Phase

  • Step 5: Redox reaction generates NADH.

  • Steps 6-9: Production of ATP and pyruvate.

Preparation Reaction

  • Before entering the Citric Acid Cycle (CAC), pyruvate undergoes modifications:

    • COO- (carboxyl) group is removed and released as CO2.

    • Electrons and H+ ions are removed by NAD+.

    • Coenzyme A is added to form Acetyl CoA.

  • Each glucose generates 2 Acetyl CoA molecules.

Citric Acid Cycle Overview (Krebs Cycle)

  • Occurs in the mitochondrial matrix.

  • Coenzyme A is detached, allowing the Acetyl group to enter the cycle.

  • Each glucose turns the cycle twice, yielding:

    • 4 CO2

    • 2 ATP

    • 6 NADH

    • 2 FADH2

Citric Acid Cycle Details

  • Acetyl CoA ignites the cycle (the "furnace").

  • Redox reactions produce NADH, ATP, FADH2, and carbon dioxide as waste products.

Electron Transport Chain (ETC)

  • Integrated within the inner mitochondrial membrane (cristae provide surface area).

  • ETC proteins, including cytochromes, facilitate electron transfers from NADH and FADH2.

  • Oxygen functions as the final electron acceptor, producing water.

Chemiosmosis

  • Energy from electrons pumping H+ ions across the membrane creates a concentration gradient.

  • H+ ions accumulate in the intermembrane space, generating potential energy.

  • ATP synthase enzyme channels H+ ions back across the membrane, harnessing this potential energy to generate ATP.

Review of Respiration and Energy Production

  • Overview of processes and ATP production:

    • Glycolysis produces 2 NADH and 2 ATP (by substrate-level phosphorylation).

    • Citric Acid Cycle yields 2 ATP (by substrate-level phosphorylation), 6 NADH and 2 FADH2.

    • Oxidative phosphorylation generates approximately 34 ATP.

  • Maximum ATP yield per glucose is about 38.

Fermentation: Anaerobic Respiration

  • Starts with glycolysis (yielding 2 ATP).

  • In the absence of O2, NAD+ regeneration hampers glycolysis continuation.

  • Fermentation utilizes alternate molecules for electron reception to maintain glycolysis.

  • Types of organisms:

    • Obligate anaerobes: require fermentation for energy.

    • Facultative anaerobes: can switch between fermentation and cellular respiration.

Lactic Acid Fermentation

  • Occurs in muscle cells and some bacteria (including yogurt cultures).

  • Pyruvate is converted to lactate, accepting electrons from NADH.

  • Lactate accumulation causes muscle soreness due to altered pH.

  • This serves as a temporary energy solution; oxygen debt requires heavy breathing afterward to recover.

Alcohol Fermentation

  • Exploited by specific yeasts and bacteria.

  • Pyruvate undergoes processing producing ethanol and carbon dioxide.

  • Applications include baking, brewing, and winemaking.