topic 10

Lecture Notes for Biology 107: Cellular Respiration and Fermentation

Overview of Cellular Respiration

This section introduces the fundamental processes involved in cellular respiration, focusing on how glucose metabolism leads to ATP production, which powers cellular activities.

Introduction to Cellular Respiration

1. Definition: Cellular respiration refers to the metabolic processes that convert glucose's chemical energy into adenosine triphosphate (ATP), providing energy for cellular activities.

2. Overall Reaction:

   • $ext{C}<em>6 ext{H}</em>{12} ext{O}<em>6 + 6 ext{O}</em>2 <br>ightarrow 6 ext{CO}<em>2 + 6 ext{H}</em>2 ext{O} + ext{Energy}$

   • This is an exergonic process carried out by all eukaryotes, including plants and certain bacteria.

3. Stages of Metabolism:

   • Glycolysis

   • Pyruvate Oxidation

   • Citric Acid Cycle

   • Oxidative Phosphorylation

Glycolysis: Step-by-step Breakdown

1. Overview: Breaks down glucose (6 carbons) into two pyruvate molecules (3 carbons each).

2. Location: Cytosol.

3. Phases:

   • Energy-Requiring Phase: Invests ATP to activate glucose.

   • Energy-Releasing Phase: Produces ATP and NADH.

4. Overall Reaction for Glycolysis:

   • $ext{1 glucose} <br>ightarrow 2 ext{pyruvate} + 2 ext{ATP} + 2 ext{NADH}$

5. Important Enzymes:

   • Kinases: Transfer a phosphate group from one molecule to another.

   • Isomerases: Rearrange molecules.

   • Dehydrogenases: Transfer electrons, aiding in oxidation reactions.

6. Substrate-Level Phosphorylation: ATP is created via direct transfer of a phosphate group from a high-energy substrate to ADP.

7. Net Yield from Glycolysis:

   • 4 ATP produced, 2 ATP consumed, net gain of 2 ATP.

Fate of Pyruvate

1. With Oxygen (Aerobic Conditions):

   • Pyruvate enters the mitochondria for further processing in cellular respiration, leading to more ATP production.

2. Without Oxygen (Anaerobic Conditions):

   • Pyruvate undergoes fermentation in the cytosol, producing no additional ATP.

3. Types of Fermentation:

   • Ethanolic Fermentation: Converts pyruvate to ethanol, used in brewing and baking.

   • Lactic Acid Fermentation: Reduces pyruvate to lactic acid, occurring in muscle cells and certain bacteria.

Pyruvate Oxidation and the Citric Acid Cycle

1. Pyruvate Oxidation:

   • Occurs in the mitochondrial matrix, converting pyruvate to acetyl-CoA.

   • Reaction products: 2 Acetyl-CoA molecules, 2 NADH, and 2 CO2.

2. Citric Acid Cycle (Krebs Cycle):

   • Occurs in the mitochondrial matrix; cycle regenerates oxaloacetate.

   • Purpose: To oxidize acetyl-CoA, transfer high-energy electrons, and produce ATP via substrate-level phosphorylation.

   • Net reaction per acetyl-CoA:

     • $1 ext{Acetyl-CoA} <br>ightarrow 2 ext{CO}<em>2 + 3 ext{NADH} + 1 ext{FADH}</em>2 + 1 ext{ATP}$

   • For glucose: 2 acetyl-CoA produce 4 CO2 + 6 NADH + 2 FADH2 + 2 ATP.

Oxidative Phosphorylation

1. Electron Transport Chain (ETC):

   • Location: Inner mitochondrial membrane.

   • Composed of protein complexes where electrons are transferred through redox reactions, releasing energy.

   • Source of electrons: Derived from glycolysis and the citric acid cycle (NADH, FADH2).

   • Electrons are passed along the chain until they reach oxygen, creating water as a byproduct.

2. Chemiosmosis:

   • Protons are pumped into the intermembrane space creating a proton gradient.

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

Summary of Energy Yields

1. Total ATP Yield from One Glucose:

   • Glycolysis: 2 ATP (substrate-level phosphorylation) + 2 NADH

   • Pyruvate Oxidation: 2 NADH (oxidative)

   • Citric Acid Cycle: 2 ATP (substrate-level), 6 NADH, 2 FADH2 (oxidative)

   • Overall ATP yield ~32 ATP (under optimal conditions).

   • Only 34% of energy in glucose is captured as ATP, the rest lost as heat.

Regulation of Cellular Respiration

1. Feedback Inhibition:

   • End products inhibit enzymes earlier in the pathway to prevent resource wastage.

2. Allosteric Regulation:

   • Molecules bind to sites other than the active site, affecting enzyme activities of pathways such as glycolysis and citric acid cycle (e.g., ATP inhibits phosphofructokinase).

Importance of Fermentation

1. Living in Anaerobic Conditions:

   • Fermentation allows ATP production in the absence of oxygen.

2. Net Energy Production: Glycolysis produces only 2 ATP compared to approximately 32 from complete cellular respiration.

Background Reading and Resources

1. Textbook Assigned:

   • 4th Ed. Biology: Exploring the Diversity of Life - Chapter 3, Section 3.6c; Chapter 5, Sections 5.1 - 5.7.

   • 5th Ed. Biology: Exploring the Diversity of Life - Chapter 3, Section 3.6c; Chapter 5, Sections 5.1 - 5.7.

2. Further Reading:

   • Links to supplementary readings on OpenStax pertaining to energy in living systems, glycolysis, and other metabolic pathways can be accessed for additional detail.