Chapter 9: Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells harvest energy from organic molecules, primarily glucose. This energy is stored in the form of adenosine triphosphate (ATP), which powers cellular activities. The process involves oxidation-reduction (redox) reactions, electron carriers, and multiple metabolic pathways.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

• Oxidation is the loss of electrons from a substance, while reduction is the gain of electrons.

• The electron donor is called the reducing agent, and the electron acceptor is the oxidizing agent.

• Redox reactions are fundamental to the transfer of energy in cellular respiration.l

Redox in Glucose Oxidation

• During cellular respiration, glucose is oxidized and oxygen is reduced.

• Organic molecules rich in hydrogen are excellent sources of high-energy electrons.

• Energy is released as electrons are transferred to oxygen, a lower energy state.

Overview of Cellular Respiration Pathways

Major Stages

• Glycolysis: Breaks down glucose into two molecules of pyruvate in the cytosol.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose in the mitochondrial matrix.

• Oxidative Phosphorylation: Accounts for most ATP synthesis via the electron transport chain and chemiosmosis.

Glycolysis

Phases of Glycolysis

• Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.

• Energy Payoff Phase: 4 ATP and 2 NADH are produced, along with 2 pyruvate molecules.

• Net gain: 2 ATP and 2 NADH per glucose molecule.

• Occurs in the cytoplasm and does not require oxygen.

Pyruvate Oxidation

Conversion to Acetyl CoA

In the presence of oxygen, pyruvate is transported into the mitochondrion and converted to acetyl CoA in three steps:

1. Oxidation of pyruvate and release of CO2

2. Reduction of NAD+ to NADH

3. Addition of coenzyme A to the 2-carbon fragment, forming acetyl CoA

The Citric Acid Cycle (Krebs Cycle)

Cycle Overview

• Acetyl CoA enters the cycle, which completes the oxidation of glucose derivatives to CO2.

• Each turn of the cycle produces 1 ATP, 3 NADH, and 1 FADH2.

• NADH and FADH2 carry electrons to the electron transport chain.

Oxidative Phosphorylation and the Electron Transport Chain

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane (cristae).

• Composed of protein complexes (cytochromes) that transfer electrons from NADH and FADH2 to oxygen.

• Energy released is used to pump protons (H+) into the intermembrane space, creating a proton gradient.

Chemiosmosis and ATP Synthase

• Protons flow back into the mitochondrial matrix through ATP synthase, driving the phosphorylation of ADP to ATP.

• This process is called chemiosmosis.

ATP Yield

• Most energy flows from glucose → NADH → ETC → ATP synthase → ATP.

• About 30–32 ATP are produced per glucose molecule.

Fermentation and Anaerobic Respiration

Fermentation

• Occurs when oxygen is not present.

• Glycolysis continues, but pyruvate is converted to other products to regenerate NAD+.

• Two main types: Alcohol fermentation (produces ethanol and CO2) and Lactic acid fermentation (produces lactate).

Anaerobic Respiration

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

Facultative and Obligate Anaerobes

• Obligate anaerobes can only survive without oxygen.

• Facultative anaerobes (e.g., yeast, muscle cells) can switch between fermentation and aerobic respiration.

Metabolic Integration and Alternative Fuels

Other Fuels in Cellular Respiration

• Carbohydrates, fats, and proteins can all be used as fuel.

• Fats yield more than twice as much ATP per gram as carbohydrates.

• Proteins must be deaminated before entering glycolysis or the citric acid cycle.

ATP: Structure, Hydrolysis, and Function

Structure of ATP

• ATP consists of adenine, ribose, and three phosphate groups.

• The bonds between phosphate groups are high-energy bonds.

ATP Hydrolysis

• Hydrolysis of ATP releases energy by breaking the terminal phosphate bond.

• ATP hydrolysis drives endergonic reactions by phosphorylation of other molecules.

ATP Cycle

• ATP is regenerated from ADP and inorganic phosphate using energy from catabolic reactions (cellular respiration).

• This cycle is essential for maintaining cellular energy balance.

Enzymes and Cellular Work

Role of Enzymes

• Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• ATP powers chemical, transport, and mechanical work in the cell.

Summary Table: Stages of Cellular Respiration