Cellular Respiration and ATP Production Notes

Introduction to Biological Energy and Metabolism

• Energy and Exercise Connection: The way the human body makes and uses energy is comparable to sports or physical exercise. While it is a complicated process, it provides tremendous biological payoffs.

• The Power of ATP: The process of energy production starts with a molecule called ATP (Adenosine Triphosphate), which is essential for providing power to animal cells.

• Cellular Respiration Defined: This is the process of deriving energy from food, specifically glucose. Most of the food consumed by humans eventually ends up as glucose.

• Molecular Formula of Glucose: The chemical formula for one molecule of glucose is $C_6H_{12}O_6$.

• The General Chemical Reaction: To convert glucose into usable energy, oxygen is required.

  • Reactants: One molecule of glucose ($C_6H_{12}O_6$) and six molecules of oxygen ($6O_2$).

  • Products: Through cellular respiration, these are converted into six molecules of carbon dioxide ($6CO_2$), six molecules of water ($6H_2O$), and energy.

ATP: The Currency of Biological Energy

• Energy Storage: Bodies cannot use raw energy immediately for tasks like running a marathon; it must first be converted into a specific stored form called Adenosine Triphosphate (ATP).

• Economic Analogy: ATP is frequently referred to as the "currency" of biological energy.

  • The Dollar Comparison: Just as an American dollar is required to do business in the United States, ATP is required for cellular business. You cannot use "Chinese yen" or "Indian rupees" (representing other energy forms) at a "Best Buy" (the cell) until they are converted into the local currency.

• Usage of ATP: Cells require ATP to grow, move, create electrical impulses in the nerves and brain, and transport materials across cell membranes.

• Molecular Structure of ATP:

  • Adenine: A nitrogenous base.

  • Ribose: A sugar.

  • Phosphate Groups: Three phosphate groups attached in a row.

• The "Three Kids on a Bus" Metaphor: The three phosphate groups are highly unstable and "uncomfortable" sitting together, similar to three children on a bus seat who dislike each other.

• Energy Release Mechanism: Because the phosphate groups are unstable, ATP shoots one group off, releasing energy and resulting in a molecule with only two phosphate groups called Adenosine Diphosphate (ADP).

• Hydrolysis: This is the process of using water to break down a compound.

  • Etymology: "Hydro" from water and "lysis" from the Greek word for separate.

  • In the conversion from ATP to ADP, a hydroxide ($OH^{-}$) group from a nearby water molecule ($H_2O$) takes the place of the ejected third phosphate group.

The Three Stages of Cellular Respiration

• Timing: While textbooks often describe these stages sequentially, they are actually occurring simultaneously within the cell.

• Total Energy Yield: In a best-case scenario, one molecule of glucose yields heat and approximately $38$ molecules of ATP. In reality, the number is usually closer to $29$ or $30$ ATP.

Stage 1: Glycolysis

• Definition: The breaking down of glucose (the suffix "-ose" denotes a sugar).

• Location: Occurs in the cytoplasm (the fluid medium of the cell where organelles reside).

• Mechanism: Glycolysis breaks the 6-carbon ring of glucose into two 3-carbon molecules called pyruvic acids or pyruvate molecules.

• Anaerobic Nature: Glycolysis does not require oxygen, making it an anaerobic process.

• Investment and Profit:

  • Investment: The process requires an initial investment of $2$ ATP.

  • Total Production: Glycolysis generates $4$ ATP.

  • Net Profit: $2$ ATP.

• Additional Products: The process results in $2$ pyruvates and $2$ molecules of NADH.

• NADH: These are energy-rich molecules created from the combination of a B vitamin called $NAD^{+}$ (Nicotinamide Adenine Dinucleotide), energized electrons, and hydrogen. These act as storehouses of energy to be tapped later.

Stage 2: The Krebs Cycle (Citric Acid Cycle)

• Discovery: Discovered in 1937 by Hans Krebs, an ear, nose, and throat surgeon who fled Nazi Germany for Cambridge. He won the Nobel Prize for Medicine in 1953 for this discovery.

• Location: Occurs across the inner membrane of the mitochondria (the power centers of the cell).

• Aerobic Nature: This is an aerobic process, meaning it requires oxygen.

• The Process:

  1. Pyruvate Oxidation: Pyruvates are combined with oxygen. One carbon from the 3-carbon chain bonds with oxygen and leaves the cell as $CO_2$.

  2. Acetyl Coenzyme A: The remaining 2-carbon compound is called acetyl CoA.

  3. NADH Formation: Another $NAD^{+}$ picks up a hydrogen to become NADH.

  4. Citric Acid Formation: Enzymes join acetyl CoA with a 4-carbon molecule called oxaloacetic acid to form the 6-carbon molecule citric acid (found in orange juice).

  5. The Extraction of Energy: Citric acid is oxidized through several steps, cutting carbons off to return to oxaloacetic acid, making it a cycle.

• Byproducts: The cleaved carbons are released as $CO_2$. Whenever humans exhale, they are breathing out the products of the Krebs cycle.

• Energy Carriers: Aside from a small amount of ATP, energy is stored in NADs and FADs.

  • Niacin and Riboflavin: $NAD^{+}$ and FAD are derivatives of these B vitamins.

  • Battery Analogy: They act like batteries that pick up hydrogen and energized electrons to become charged. The additions turn them into NADH and $FADH_2$.

• Yield per Glucose: Each pyruvate yields $3$ NADH and $1$ $FADH_2$ per cycle. Since there are two pyruvates per glucose, the total is $6$ NADH and $2$ $FADH_2$, plus $2$ ATP molecules.

Stage 3: The Electron Transport Chain (ETC)

• Energy Efficiency: This stage is the "real moneymaker," netting approximately $34$ ATP.

• Mechanism:

  1. Electrons from NADH and $FADH_2$ provide energy for channel proteins to pump hydrogen protons across the mitochondrial inner membrane into the outer compartment.

  2. Protons naturally seek a balance on either side of the membrane and want to flow back in.

  3. ATP Synthase: The protons flow back through this special protein.

  4. Spinning Mechanism: The energy from the proton flow drives a rotation that squeezes ADP and phosphates together to form ATP.

• Conversion Summary:

  • The $10$ molecules of NADH (from previous stages) produce roughly $3$ ATP each ($30$ total).

  • The $2$ molecules of $FADH_2$ produce $2$ ATP each ($4$ total).

Anaerobic Respiration and Fermentation

• Pathways: If oxygen is not present, pyruvates enter a process called fermentation rather than the Krebs cycle.

• Goal: Fermentation frees up $NAD^{+}$ so that glycolysis can continue to produce at least some energy.

• Byproducts in Microorganisms: Yeast produces ethyl alcohol during fermentation.

• Byproducts in Humans: Human muscles produce lactic acid. This occurs when muscles use up available oxygen and must switch to anaerobic respiration. The buildup of lactic acid is responsible for the soreness felt after an intense workout.

Final ATP Tally

• Glycolysis: $2$ ATP.

• Krebs Cycle: $2$ ATP.

• Electron Transport Chain: $34$ ATP.

• Total: Approximately $38$ ATP per single molecule of glucose.

Questions & Discussion

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