Cellular Respiration

Overview and Purpose

Cellular respiration is a fundamental catabolic pathway by which cells harvest chemical energy from organic molecules, primarily glucose. This process systematically degrades energy-rich compounds into simpler waste products, generating ATP, the universal energy currency of the cell.

• Goal: To generate ATP by oxidizing organic molecules.

• Overall Aerobic Equation (using glucose):

• Energy Transfer: Involves redox reactions (oxidation-reduction) to transfer electrons from food molecules to oxygen, synthesizing ATP.

Key Terms and Concepts

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Electron Carriers: Molecules that temporarily store electrons during cellular respiration. The main carrier is NAD+ (nicotinamide adenine dinucleotide).

• NAD+ as Oxidizing Agent: Accepts two electrons and one proton (H+) to become NADH.

• NADH: Stores energy and donates electrons to the electron transport chain, driving ATP synthesis.

Stages of Aerobic Cellular Respiration

Cellular respiration occurs in three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. Each stage is catalyzed by specific enzymes and occurs in distinct cellular locations.

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytosol. It breaks down one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each).

• Location: Cytosol

• Phases:

  • Energy Investment Phase: The cell uses ATP to phosphorylate glucose and its intermediates.

  • Energy Payoff Phase: ATP is produced by substrate-level phosphorylation, and NAD+ is reduced to NADH.

• Net Yield per Glucose: 2 ATP, 2 NADH, 2 pyruvate

• Key Fact: No CO2 is released during glycolysis.

Pyruvate Oxidation (The Link Step)

In the presence of oxygen, pyruvate enters the mitochondrion and is converted to acetyl CoA, releasing CO2 and generating NADH.

• Location: Mitochondrial matrix (eukaryotes)

• Product: Acetyl CoA, which enters the citric acid cycle

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose by oxidizing acetyl CoA to CO2. It generates high-energy electron carriers and a small amount of ATP.

• Location: Mitochondrial matrix (eukaryotes)

• Key Reactions: Acetyl CoA combines with oxaloacetate to form citrate; CO2 is released; NAD+ and FAD are reduced.

• Yield per Glucose (2 turns): 6 NADH, 2 FADH2, 2 ATP (by substrate-level phosphorylation), 4 CO2

• Energy Fate: Most chemical energy is transferred to NADH and FADH2.

Oxidative Phosphorylation

This stage includes the electron transport chain (ETC) and chemiosmosis, producing the majority of ATP during cellular respiration.

• Location: Inner mitochondrial membrane (cristae) in eukaryotes

• ETC Function: Electrons from NADH and FADH2 are passed through a series of increasingly electronegative protein complexes, ultimately reducing oxygen to water.

• Proton Pumping: The ETC uses released energy to pump H+ ions into the intermembrane space, creating a proton-motive force (H+ gradient).

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

• Total ATP Yield: Up to 32 ATP per glucose (varies by cell type and conditions)

Cellular Respiration in the Absence of Oxygen

Fermentation

When oxygen is unavailable, cells can still produce ATP via glycolysis, but NAD+ must be regenerated by alternative pathways called fermentation.

• Alcohol Fermentation: Pyruvate is converted to ethanol, releasing CO2 and regenerating NAD+.

• Lactic Acid Fermentation: Pyruvate is reduced directly by NADH to form lactate, regenerating NAD+ without releasing CO2.

• ATP Yield: Only 2 ATP per glucose (from glycolysis)

Regulation of Cellular Respiration

Cellular respiration is tightly regulated to match the cell's energy needs. The main control point is the enzyme phosphofructokinase in glycolysis.

• Phosphofructokinase: Allosterically inhibited by high ATP and citrate; stimulated by high AMP (from ADP).

Summary Table: Major Stages of Cellular Respiration

Quiz Questions and Answers (with Explanations)

1. Glycolysis occurs in which cellular location? Answer: Cytosol

2. What is the maximum theoretical net yield of ATP produced per glucose molecule during aerobic cellular respiration? Answer: 32 ATP

3. The process where a substance loses electrons is known as: Answer: Oxidation

4. What is the primary function of NAD+ during the initial steps of glucose breakdown? Answer: To act as an oxidizing agent, trapping electrons from the fuel molecule

5. What is the pacemaker enzyme of glycolysis? Answer: Phosphofructokinase

6. In eukaryotic cells, pyruvate must enter the mitochondrion and be converted to which compound before entering the citric acid cycle? Answer: Acetyl CoA

7. How is the small amount of ATP produced during glycolysis and the citric acid cycle generated? Answer: Substrate-level phosphorylation

8. In aerobic respiration, what molecule captures the electrons at the bottom (lower-energy end) of the electron transport chain? Answer: Oxygen (O2)

9. What is the difference between anaerobic respiration and fermentation? Answer: Anaerobic respiration requires an electron transport chain, but fermentation does not.

Additional info:

• Some details, such as the exact number of ATP produced, can vary depending on cell type and conditions.

• Fermentation pathways are essential for regenerating NAD+ in the absence of oxygen, allowing glycolysis to continue.

• Phosphofructokinase is a key regulatory enzyme, integrating signals from energy status (ATP/AMP) and biosynthetic intermediates (citrate).