Cellular Respiration and Fermentation: Chapter 9 Study Notes

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the main energy currency of the cell. This process occurs in both plant and animal cells and is essential for powering most cellular work.

• ATP (Adenosine Triphosphate): The molecule that stores and supplies energy for cellular processes.

• Photosynthesis vs. Cellular Respiration: Photosynthesis stores energy in organic molecules; cellular respiration releases that energy for cellular use.

• Location: Most steps of cellular respiration occur in the mitochondria.

Catabolic Pathways and ATP Production

Catabolic pathways break down complex molecules into simpler ones, releasing stored energy. This energy is captured in the form of ATP.

• Catabolic Pathways: Release stored energy by breaking down complex molecules.

• Electron Transfer: Movement of electrons from food molecules to other molecules is central to energy release.

Redox Reactions: Oxidation and Reduction

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules, which releases energy used to synthesize ATP.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons (less positive charge).

• Electron Donor: Reducing agent; donates electrons.

• Electron Acceptor: Oxidizing agent; accepts electrons.

Example: $\text{Na} + \text{Cl} \rightarrow \text{Na}^+ + \text{Cl}^-$ (Na is oxidized, Cl is reduced)

Cellular Respiration as a Redox Process

During cellular respiration, organic fuel molecules are oxidized and oxygen is reduced. The oxidation of glucose transfers electrons from a higher energy state to a lower energy state with oxygen atoms.

• Overall Reaction: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$

• Oxygen: Final electron acceptor in the electron transport chain due to its high electronegativity.

Electron Carriers: NAD+ and FAD

Electrons from glucose are not transferred directly to oxygen but are first passed to electron carriers such as NAD+ and FAD.

• NAD+ (Nicotinamide Adenine Dinucleotide): A coenzyme that acts as an electron carrier and oxidizing agent.

• NADH: The reduced form of NAD+, stores energy used to synthesize ATP.

• FAD/FADH2: Another electron carrier involved in the citric acid cycle.

Electron Transport Chain (ETC)

The electron transport chain is a series of molecules embedded in the inner mitochondrial membrane (or plasma membrane in prokaryotes) that transfer electrons to oxygen in a controlled manner, releasing energy to form ATP.

• ETC: Series of redox reactions that transfer electrons from NADH and FADH2 to oxygen.

• ATP Synthesis: Energy released is used to regenerate ATP.

Stages of Cellular Respiration

Cellular respiration consists of three main stages:

1. Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate.

2. Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; completes the oxidation of glucose derivatives.

3. Oxidative Phosphorylation: Includes electron transport and chemiosmosis; produces most ATP.

Summary of electron flow: glucose → NADH → ETC → oxygen

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm and consisting of two phases: energy investment and energy payoff.

• Location: Cytoplasm

• Phases: Energy Investment (uses 2 ATP), Energy Payoff (produces 4 ATP and 2 NADH)

• Net Reaction:

  • Glucose → 2 Pyruvate + 2 H2O

  • 4 ATP formed - 2 ATP used → 2 ATP (net)

  • 2 NAD+ + 4 e- + 4 H+ → 2 NADH + 2 H+

• Does not require oxygen and does not release CO2.

Pyruvate Oxidation

After glycolysis, pyruvate is transported into the mitochondria (in eukaryotes) and converted to acetyl coenzyme A (acetyl CoA), linking glycolysis to the citric acid cycle.

• Steps:

  1. Oxidation of pyruvate's carboxyl group, releasing CO2

  2. Reduction of NAD+ to NADH

  3. Formation of acetyl CoA by combining the remaining two-carbon fragment with coenzyme A

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the energy-yielding oxidation of organic molecules. It consists of eight steps, each catalyzed by a specific enzyme.

• First Step: Acetyl group of acetyl CoA combines with oxaloacetate to form citrate.

• Cycle: The next seven steps decompose citrate back to oxaloacetate.

• Products per turn: ATP, NADH, FADH2

• NADH and FADH2: Carry electrons to the electron transport chain.

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation is the process that generates most ATP during cellular respiration. It involves the electron transport chain and chemiosmosis.

• Electron Transport Chain: Electrons from NADH and FADH2 are passed through protein complexes, releasing energy.

• Proton Gradient: Energy is used to pump H+ ions from the mitochondrial matrix to the intermembrane space.

• ATP Synthase: H+ ions flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi.

• Equation: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

• Proton-Motive Force: The gradient of H+ ions across the membrane that powers ATP synthesis.

Substrate-Level Phosphorylation

Some ATP is produced directly in glycolysis and the citric acid cycle by substrate-level phosphorylation, where an enzyme transfers a phosphate group from a substrate to ADP.

• Occurs in: Glycolysis and citric acid cycle

• Direct transfer: Enzyme-mediated transfer of phosphate group

Fermentation and Anaerobic Respiration

Cells can produce ATP without oxygen through fermentation or anaerobic respiration.

• Anaerobic Respiration: Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate ion).

• Fermentation: Extension of glycolysis that oxidizes NADH by transferring electrons to pyruvate or its derivatives.

• Types of Fermentation:

  • Alcohol fermentation

  • Lactic acid fermentation

• Efficiency: Fermentation is less efficient than aerobic respiration.

Regulation of Cellular Respiration

Cellular respiration is regulated by feedback inhibition to prevent wasteful production of ATP.

• Feedback Inhibition: If ATP concentration drops, respiration speeds up; if ATP is abundant, respiration slows down.

Summary Table: Stages of Cellular Respiration

Stage	Location	Main Inputs	Main Outputs	ATP Produced
Glycolysis	Cytoplasm	Glucose, 2 ATP, 2 NAD+	2 Pyruvate, 2 NADH, 2 ATP (net)	2 (net)
Pyruvate Oxidation	Mitochondrial Matrix	2 Pyruvate, 2 NAD+, 2 CoA	2 Acetyl CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial Matrix	2 Acetyl CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	NADH, FADH2, O2	ATP, H2O	~28

Key Equations

• Overall Cellular Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$

• Glycolysis Net Reaction: $\text{Glucose} \rightarrow 2\ \text{Pyruvate} + 2\ \text{H}_2\text{O}$ $4\ \text{ATP formed} - 2\ \text{ATP used} \rightarrow 2\ \text{ATP}$ $2\ \text{NAD}^+ + 4\ e^- + 4\ H^+ \rightarrow 2\ \text{NADH} + 2\ H^+$

Additional info: The above notes expand on the provided slides and handwritten notes, adding definitions, context, and a summary table for clarity and completeness.