How Cells Harvest Chemical Energy

How Cells Harvest Chemical Energy

Chapter 6: Cellular Respiration

Key Concepts:

  • Electrons

    • Carried via NADH

Glycolysis

  • Glucose is converted to Pyruvate

  • Location: Cytosol

  • Produces ATP

Krebs Cycle (Citric Acid Cycle)

  • Location: Mitochondrion

  • Electrons carried via NADH and FADH2

Electron Transport Chain and Oxidative Phosphorylation

  • Produces ATP via ATP synthase

    • Substrate-level phosphorylation

    • Oxidative phosphorylation

Copyright Notice

  • Copyright Pearson Education, Inc., publishing as Benjamin Cummings.

Word Parts Related to Chapter 6

  • aero- air (e.g., aerobic: using oxygen)

  • chemi- chemical (e.g., chemiosmosis: the production of ATP using the energy of hydrogen ion gradients across membranes to phosphorylate ADP)

  • de- without; -hydro- water (e.g., dehydrogenase: an enzyme that removes water when catalyzing a chemical reaction)

  • glyco- sweet; -lysis split (e.g., glycolysis: the multistep chemical breakdown of a molecule of glucose into two molecules of pyruvate)

Questions Discussed:

  • Why is oxygen necessary for life?

  • Why is it essential for newborns?

Overview of Cellular Respiration

Stages of Cellular Respiration

  • Connections Between Metabolic Pathways

  • Aerobic Harvesting of Energy

  • Fermentation: Anaerobic Harvesting of Energy

Overall Reaction of Cellular Respiration

  •  Overall Equation:

    • C<em>6H</em>12O<em>6+6O</em>2<br>ightarrow6CO<em>2+6H</em>2O+Energy(ATP)C<em>6H</em>{12}O<em>6 + 6O</em>2 <br>ightarrow 6CO<em>2 + 6H</em>2O + Energy (ATP)

Two Main Pathways

  1. Aerobic Path (with O2)

    • 2 Steps: net 36 ATP

    • Efficient

    • Glycolysis

    • Aerobic Respiration

    • Citric Acid Cycle and Oxidative Phosphorylation

  2. Anaerobic Path (without O2)

    • 2 Steps: net 2 ATP

    • Inefficient

    • Glycolysis

    • Fermentation: Lactic Acid or Alcoholic

Pathways of Energy Production

Aerobic vs. Anaerobic Processes

  • Both processes break down sugars to produce ATP.

  • Aerobic Respiration yields 36-38 ATPs:

    • Glycolysis: Occurs in the cytoplasm; net yield is 2 ATP.

    • Pyruvate Oxidation and the Krebs Cycle: Occurs in mitochondria; net yield is 2 ATP.

    • Oxidative Phosphorylation: Occurs in mitochondria; net yield is 32 ATP.

Comparison of Fermentation and Aerobic Respiration

  • Fermentation: Produces a net yield of 2 ATP, sufficient to sustain many single-celled species.

Detailed Stages of Cellular Respiration

Stage 1: Glycolysis

  • Definition: A series of reactions occurring in both fermentation and aerobic respiration.

  • Converts one six-carbon molecule into two three-carbon pyruvate molecules.

  • Functions as a metabolic pathway with the product of one event being the reactant for the next.

  • Intermediates: Form during the process.

  • Contains 10 steps: Starts with hexokinase and glucose.

  • Involves substrate-level phosphorylation.

  • Energy Investment Phase: Steps 1-4.

  • Energy Payoff Phase: Steps 5-9.

  • Summary:

    • Location: Cytoplasm

    • Contribution: Provides only 6% of the energy cell can obtain from glucose.

    • Products: 2 Pyruvic Acids (3-C), 2 NADH (electron acceptors), and 4 ATP total (net gain = 2 ATP).

Stage 2: Citric Acid Cycle (also known as Krebs Cycle)

  • Occurs in the mitochondrial matrix.

  • Completes the energy-yielding oxidation of glucose.

  • Begins with the conversion of pyruvate (3C) into Acetyl CoA (2C), releasing CO2 and producing NADH.

  • Process Breakdown:

    • One glucose molecule produces 2 Acetyl CoA molecules, requiring 2 turns of the Krebs cycle.

    • Results in 6 NADH, 2 FADH2, 2 ATP, and 4 CO2.

Stage 3: Oxidative Phosphorylation

  • Electrons carried by NADH and FADH2 donate electrons at high energy states.

  • Location: Inner mitochondrial membrane (cristae).

  • Process overview:

    • Movement of electrons pumps H+ out of the matrix.

    • Creates a concentration gradient.

    • ATP synthase acts as a channel for H+ ions back into the matrix, facilitating ATP creation via chemiosmosis.

    • Oxygen acts as the final electron acceptor, forming water (H2O).

Summary of Aerobic Respiration

  • 3 Primary Steps:

    1. Glycolysis (in cytoplasm) → 2 ATP, 2 NADH.

    2. Krebs Cycle (in mitochondrial matrix) → 2 NADH, 2 CO2.

    3. Electron Transport Chain (inner mitochondrial membrane) → 34 ATP.

Fermentation: Anaerobic Harvesting of Energy

Overview

  • Begins with glycolysis in the cytoplasm.

  • In fermentation, pyruvate is not fully oxidized to CO2.

  • Electrons do not pass through the electron transport chain, thus no additional ATP is produced but NAD+ is regenerated to maintain glycolysis.

  • Net yield remains 2 ATP.

Types of Fermentation

  1. Lactic Acid Fermentation

    • Location: Cytoplasm.

    • Organisms: Bacteria and muscle cells.

    • Products: Regenerates NAD+, produces lactic acid.

    • Uses: In dairy and pickling industries, and energy production for muscle cells during high activity.

  2. Alcoholic Fermentation

    • Location: Cytoplasm.

    • Products: Regenerate NAD+ and produce Ethyl Alcohol and CO2.

    • Organisms: Yeast and some plant cells.

    • Uses: Brewing (wine and beer) and bread-making processes.

Summary of Fermentation

  • Conversion of glucose into organic compounds via glycolysis, yielding a net of ATP.

Connections Between Metabolic Pathways

Metabolic Interconnectedness

  • Complex carbohydrates, fats, and proteins in food can be converted to feed into glycolysis or the Krebs cycle.

  • Not all molecules used as fuel contribute directly to ATP production. They can also serve as biosynthetic precursors for other organic molecules.

  • Cellular respiration pathways are often under regulation via feedback inhibition to manage energy resources effectively.

Recap of Key Concepts in Cellular Respiration

  • Definitions:

    • Redox Reactions: Reactions involving the transfer of electrons, where oxidation is the loss of electrons and reduction is the gain of electrons.

    • Dehydrogenase: An enzyme that plays a key role in oxidation-reduction reactions by removing hydrogen atoms from the substrates.

    • Importance of Chemiosmosis: This process harnesses the energy from H+ ions diffusing back through ATP synthase, leading to ATP production.

Learning Outcomes

  1. Compare processes and locations of cellular respiration and photosynthesis.

  2. Explain the connection between breathing and cellular respiration.

  3. Provide the overall chemical equation for cellular respiration.

  4. Discuss how the human body utilizes its daily supply of ATP.

  5. Describe the energy release from glucose during cellular respiration.

  6. Explain the role of redox reactions in cellular respiration.

  7. Examine the functions of dehydrogenase, NADH, and the electron transport chain.

  8. Compare reactants, products, and energy returns across the three stages of cellular respiration.

  9. Describe the unique function of brown fat in energy metabolism.

  10. Contrast the products and energy yields of alcoholic and lactic acid fermentation.

  11. Distinguish between obligate anaerobes and facultative anaerobes.

  12. Explain how carbohydrates, fats, and proteins serve as fuel for cellular respiration.