Cellular Respiration: Processes and Pathways

Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), and release waste products. It is essential for the survival of most organisms, providing the energy required for cellular activities.

• Definition: Cellular respiration is the process of breaking down glucose and other molecules to produce ATP, the cell's energy currency.

• Purpose: To generate ATP for cellular work.

• General Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Example: Muscle cells use ATP for contraction; plants and animals rely on cellular respiration for energy.

Overview of Cellular Respiration

Main Steps of Cellular Respiration

Cellular respiration consists of several interconnected metabolic pathways, each with specific inputs, outputs, and locations within the cell.

• Four Major Steps:

  1. Glycolysis (cytoplasm)

  2. Pyruvate Oxidation (mitochondrial matrix)

  3. Krebs Cycle (Citric Acid Cycle) (mitochondrial matrix)

  4. Electron Transport Chain & Oxidative Phosphorylation (inner mitochondrial membrane)

• Key Questions for Each Step:

  • What goes in?

  • What comes out?

  • What is the point (what is used for next steps, what is waste)?

  • What happens to the energy released?

  • Where does each step occur?

Glycolysis

Process and Products

Glycolysis is the first step of cellular respiration, occurring in the cytoplasm. It breaks down one molecule of glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.

• Location: Cytoplasm

• Inputs: Glucose, 2 NAD+, 2 ADP + 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP

• ATP Production: By substrate-level phosphorylation

• Occurs in: All cells (aerobic and anaerobic)

• Example: Anaerobic bacteria and muscle cells during intense exercise use glycolysis for ATP production.

Pyruvate Oxidation

Conversion to Acetyl-CoA

Pyruvate oxidation occurs in the mitochondrial matrix, where each pyruvate is converted into acetyl-CoA, producing NADH and releasing CO2 as a waste product.

• Location: Mitochondrial matrix

• Inputs: 2 Pyruvate, 2 NAD+, 2 Coenzyme A

• Outputs: 2 Acetyl-CoA, 2 NADH, 2 CO2

• Purpose: Prepares acetyl-CoA for entry into the Krebs cycle

Krebs Cycle (Citric Acid Cycle)

Oxidation of Acetyl-CoA

The Krebs cycle completes the oxidation of acetyl-CoA, generating NADH, FADH2, ATP, and releasing CO2 as a waste product.

• Location: Mitochondrial matrix

• Inputs: 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP + 2 Pi

• Outputs: 4 CO2, 6 NADH, 2 FADH2, 2 ATP

• Purpose: Supplies electrons (NADH, FADH2) for the electron transport chain

Electron Transport Chain & Oxidative Phosphorylation

ATP Generation and Electron Flow

The electron transport chain (ETC) is located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through protein complexes, creating a proton gradient that drives ATP synthesis via ATP synthase.

• Location: Inner mitochondrial membrane

• Inputs: NADH, FADH2, O2

• Outputs: ATP (up to 25 per glucose), H2O

• ATP Production: By oxidative phosphorylation

• Oxygen: Final electron acceptor

• Efficiency: Cellular respiration captures about 34% of glucose's energy as ATP; the rest is lost as heat.

Electron Carriers: NADH and FADH2

Role in Cellular Respiration

NAD+ and FAD are coenzymes that act as electron carriers, accepting electrons during glycolysis, pyruvate oxidation, and the Krebs cycle, and donating them to the electron transport chain.

• NAD+: Reduced to NADH

• FAD: Reduced to FADH2

• Function: Transport electrons to the ETC for ATP production

ATP Production Methods

Substrate-Level vs. Oxidative Phosphorylation

ATP is produced in two main ways during cellular respiration:

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP from a substrate (occurs in glycolysis and Krebs cycle).

• Oxidative Phosphorylation: ATP synthesis powered by the movement of protons across the mitochondrial membrane via ATP synthase (occurs in the electron transport chain).

Energy Yield of Cellular Respiration

ATP Scorecard

The total ATP yield from one molecule of glucose during aerobic respiration is up to 29-32 ATP, depending on cell type and conditions.

Process	ATP Produced
Glycolysis	2
Krebs Cycle	2
Electron Transport Chain & Oxidative Phosphorylation	~25-28
Total	~29-32

Anaerobic Cellular Metabolism

Fermentation and Anaerobic Respiration

Some organisms and cells can generate ATP without oxygen through fermentation or anaerobic respiration. These processes are less efficient than aerobic respiration.

• Fermentation: Regenerates NAD+ but does not produce additional ATP beyond glycolysis.

• Anaerobic Respiration: Uses an alternate electron acceptor instead of oxygen (e.g., nitrate, sulfate).

• Strict Anaerobes: Can only survive in the absence of oxygen; oxygen is toxic.

• Facultative Anaerobes: Can use either fermentation or aerobic respiration depending on oxygen availability.

Types of Fermentation

Lactic Acid and Alcohol Fermentation

Fermentation allows cells to produce ATP in the absence of oxygen by converting pyruvate into other molecules.

Type	Organisms	Products	Applications
Lactic Acid Fermentation	Muscle cells, bacteria	Lactate	Cheese, yogurt, muscle activity
Alcohol Fermentation	Yeast, some bacteria	Ethanol, CO2	Bread, beer, wine

Alternative Energy Sources

Metabolism of Fats and Proteins

Cells can also use fats and proteins as sources of energy for ATP synthesis. These molecules enter cellular respiration at different points, such as glycolysis or the Krebs cycle.

• Fats: Broken down into fatty acids and glycerol; fatty acids enter Krebs cycle as acetyl-CoA.

• Proteins: Broken down into amino acids; after deamination, carbon skeletons enter glycolysis or Krebs cycle.

• Purpose: Provides building blocks for biosynthesis and alternative energy sources.

Review Questions

• What is the equation for cellular respiration?

• What is the overall purpose of cellular respiration?

• What are the four steps of cellular respiration? What goes in and what comes out of each?

• Which part of cellular respiration makes the most ATP?

• Which part(s) use oxygen? Which part(s) produce CO2?

• How can organisms make ATP without oxygen?

• Explain the difference between anaerobic respiration and fermentation.

• How is ATP produced from molecules other than glucose?

Additional info: The orangutan image is used as a visual aid to introduce the topic of cellular respiration, emphasizing its relevance to all living organisms, including animals.