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

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.