Biology 1 – Lesson 7: Cellular Respiration – Glycolysis and Fermentation

Overview

Cellular respiration is the essential process by which cells convert nutrients into usable energy (ATP). In this lesson, we focus on the first stage of cellular respiration, glycolysis, which partially breaks down glucose into pyruvate, and explore fermentation, an anaerobic process that regenerates electron carriers in the absence of oxygen. Together, these pathways showcase how cells can extract energy under both aerobic and anaerobic conditions.

1. Introduction to Cellular Respiration

1.1 Energy in Organic Molecules

• Chemical Energy Storage

  • Organisms store energy in organic compounds (like glucose).

  • Electrons in chemical bonds release energy when transferred to electron carriers (e.g., NAD+^++, FAD).

• ATP as the Energy Currency

  • Adenosine Triphosphate (ATP) delivers energy to cellular processes.

  • ATP hydrolysis (removal of a phosphate group) releases usable energy.

1.2 Aerobic vs. Anaerobic Respiration

• Aerobic Respiration

  • Requires oxygen.

  • Yields a high amount of ATP by fully oxidizing glucose to CO2_22​ and H2_22​O.

• Anaerobic Respiration

  • Occurs when oxygen is scarce or absent (e.g., some bacteria, muscle cells under intense exercise).

  • Generates less ATP than aerobic processes.

2. Glycolysis

2.1 Location and Overview

• Location: Cytoplasm of both prokaryotic and eukaryotic cells.

• Purpose: Break down one glucose molecule (6 carbons) into two molecules of pyruvate (3 carbons each).

2. Glycolysis

2.1 Location and Overview

• Location: Cytoplasm of both prokaryotic and eukaryotic cells.
• Purpose: Break down one glucose (6C) into two pyruvate (3C each).

[Diagram: Glucose to Pyruvate Pathway in Glycolysis]

2.2 Two Phases of Glycolysis

1. Energy Investment Phase (Steps 1–5)

   • Requires 2 ATP to phosphorylate glucose and rearrange it into fructose-1,6-bisphosphate.

   • Splits the 6-carbon sugar into two 3-carbon molecules.

2. Energy Payoff Phase (Steps 6–10)

   • Each 3-carbon molecule (G3P) is oxidized, producing NADH and generating 4 total ATP (2 ATP net gain).

   • Results in 2 pyruvate molecules per glucose.

2.3 Net Yield of Glycolysis

• ATP: 2 net ATP per glucose.

• NADH: 2 NADH per glucose.

• Pyruvate: 2 pyruvate per glucose (destined for the mitochondria in aerobic conditions or fermentation in anaerobic conditions).

2.4 Regulation of Glycolysis

• Key Regulatory Enzymes:

  • Hexokinase/Glucokinase (Step 1)

  • Phosphofructokinase-1 (PFK-1) (Step 3, a major control point)

  • Pyruvate Kinase (Step 10)

• Allosteric Regulation:

  • ATP and citrate can inhibit PFK-1 when energy is plentiful.

  • AMP can activate PFK-1 when energy is low.

3. Fermentation

3.1 Rationale Behind Fermentation

• Oxygen Limitations

  • When oxygen is unavailable or limited, cells need a way to regenerate NAD+^++ from NADH (produced during glycolysis).

  • Fermentation provides an anaerobic solution to recycle NAD+^++.

3.2 Lactic Acid Fermentation

• Process

  • Pyruvate is converted into lactate (lactic acid).

  • NADH is oxidized to NAD+^++, which is reused in glycolysis.

• Occurrence

  • Muscle Cells: During strenuous exercise, when oxygen supply is insufficient, muscle cells perform lactic acid fermentation.

  • Bacteria: Yogurt-producing bacteria (Lactobacillus species) ferment sugars into lactic acid.

3.3 Alcoholic Fermentation

• Process

  • Pyruvate is first converted to acetaldehyde and then reduced to ethanol.

  • NADH is oxidized to NAD+^++.

• Occurrence

  • Yeasts: Bread-making (CO2_22​ makes dough rise) and beer/wine production (ethanol as a byproduct).

4. Real-Life Applications

4.1 Food and Beverage Industry

• Bread: Yeast fermentation releases CO2_22​ that makes bread dough rise.

• Dairy Products: Lactic acid bacteria ferment lactose in milk, producing cheese and yogurt flavors.

4.2 Human Physiology

• Muscle Fatigue: Lactic acid buildup may contribute to fatigue and soreness during intense activity.

• Medical Diagnostics: Blood lactate levels can indicate tissue hypoxia or circulatory problems.

4.3 Biofuel Production

• Ethanol: Fermentation of corn sugars by yeast produces ethanol as a biofuel additive.

• Biogas: Anaerobic bacteria can ferment organic waste to produce methane-rich biogas.

5. Exercise: Detecting CO2_22​ from Yeast Fermentation

Objective

Observe carbon dioxide (CO2_22​) generation by fermenting yeast.

Materials

• 1 packet of dry yeast

• 1 tablespoon of sugar

• Warm water (~40°C)

• Balloon and a small plastic bottle (or test tube with stopper)

• Measuring spoon

Procedure

1. Prepare Yeast Mixture

   • Dissolve yeast and sugar in ~100 mL of warm water in the plastic bottle.

2. Seal with Balloon

   • Stretch the balloon over the mouth of the bottle, ensuring an airtight fit.

3. Wait and Observe

   • Let the mixture sit at room temperature for 15–30 minutes.

4. Results

   • The balloon should gradually inflate with CO2_22​ produced by yeast fermentation.

Analysis

• Relate balloon inflation to the production of CO2_22​.

• Discuss why warm water is necessary (enzymes in yeast have an optimal temperature range).

6. Additional Learning Components

6.1 Historical Anecdote: Louis Pasteur and Fermentation

Louis Pasteur’s pioneering work in the 19th century revealed that fermentation was a biological (not purely chemical) process. His research laid the groundwork for modern microbiology and food safety protocols.

6.2 Researcher Spotlight: Otto Warburg

Otto Warburg studied cellular respiration and discovered the “Warburg effect”—cancer cells’ preference for glycolysis even in the presence of oxygen. His findings opened new avenues in understanding tumor metabolism.

6.3 Advanced Reading Suggestions

• “Lehninger Principles of Biochemistry” (Nelson & Cox): Detailed coverage of glycolytic steps, regulation, and fermentation.

• Journal Articles in Biochemistry or Nature Metabolism exploring modern techniques in analyzing fermentation pathways and metabolic engineering.

6.4 Notable Breakthrough: Metabolic Engineering of Yeast

Genetic modification of yeast strains allows for the production of more specialized end products, from high-yield bioethanol to pharmaceuticals. This field combines synthetic biology, genetics, and biochemistry.

6.5 Interactive Concept

Online animations can illustrate each biochemical step of glycolysis, showing how substrates are transformed, energy is captured, and NAD+^++ is regenerated in fermentation.

7. Recall Questions

1. Glycolysis Steps: What are the two main phases of glycolysis, and what is the net ATP yield per glucose molecule?

2. Regulation: Which enzyme is the major control point of glycolysis, and why is it important?

3. Fermentation Types: Compare lactic acid fermentation and alcoholic fermentation. What do they have in common, and how do they differ?

4. Anaerobic vs. Aerobic: Why might a cell opt for fermentation instead of proceeding with aerobic respiration? Provide a specific scenario.

5. Practical Relevance: How does fermentation contribute to food and beverage industries, and why is temperature control crucial in these processes?

Use these questions to test your understanding of how cells extract energy from glucose through glycolysis and fermentation, and how these processes are put to practical use in various fields.