Harvesting Energy: Glycolysis and Cellular Respiration

Case Study: Mitochondrial DNA and Human Identification

Mitochondria are organelles responsible for powering the cell and contain their own DNA. This mitochondrial DNA can be used in forensic science to identify human remains, as demonstrated in the identification of King Richard III.

• Mitochondrial DNA: Genetic material found in mitochondria, inherited maternally.

• Application: Used in forensic identification due to its abundance and maternal inheritance.

• Example: Identification of historical remains, such as King Richard III, using mitochondrial DNA analysis.

How Do Cells Obtain Energy?

Overview of Cellular Energy

Cells require a constant supply of energy to drive metabolic reactions essential for life. Energy is transferred from energy-storing molecules to energy-carrier molecules, such as ATP, through cellular reactions.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Metabolic Reactions: Chemical processes that maintain life, requiring energy input.

The Role of Photosynthesis

Photosynthesis is the ultimate source of cellular energy for most life on Earth. Photosynthetic organisms capture sunlight and store it as chemical energy in sugars and other organic molecules. Nearly all organisms rely on the breakdown of these molecules to generate ATP.

• Photosynthetic Organisms: Plants, algae, and some bacteria.

• Energy Flow: Sunlight → Chemical energy (glucose) → ATP (via cellular respiration).

Photosynthesis and Glucose Breakdown: Complementary Processes

The chemical equation for photosynthesis is the reverse of the equation for complete glucose breakdown (cellular respiration).

• Photosynthesis:

• Cellular Respiration (Complete Glucose Breakdown):

• Key Point: Photosynthesis stores energy; cellular respiration releases it.

Pathways for Glucose Breakdown

Cells can use glucose as an energy source through two major processes:

• Glycolysis: The initial splitting of glucose, can occur repeatedly during fermentation.

• Cellular Respiration: Begins with glycolysis and continues with further breakdown in the mitochondria.

• Both processes capture energy in the form of ATP.

Glycolysis: The First Step in Glucose Breakdown

Overview of Glycolysis

Glycolysis (from "glyco" meaning sweet and "lysis" meaning split apart) splits a six-carbon glucose molecule into two three-carbon pyruvate molecules. It occurs in the cytosol and does not require oxygen.

• Stages of Glycolysis:

  • Energy Investment Stage: Two ATP molecules are used to phosphorylate glucose, forming fructose bisphosphate.

  • Energy Harvesting Stage: Fructose bisphosphate is split into two glyceraldehyde-3-phosphate (G3P) molecules, which are converted to pyruvate, generating ATP and NADH.

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

Key Steps and Products of Glycolysis

• Phosphorylation: ATP donates phosphate groups to glucose.

• Formation of NADH: High-energy electrons and hydrogen ions are transferred to NAD+, forming NADH.

• ATP Generation: Substrate-level phosphorylation produces ATP directly.

Cellular Respiration: Extracting Energy from Glucose

Overview of Cellular Respiration

Cellular respiration further breaks down the two pyruvate molecules from glycolysis into six carbon dioxide and six water molecules, producing up to 34 ATP. This process occurs in the mitochondria, often called the "powerhouses" of the cell.

• Mitochondrion Structure:

  • Outer membrane: Encloses the organelle.

  • Inner membrane: Encloses the matrix and contains the electron transport chain.

  • Matrix: Central compartment where the Krebs cycle occurs.

  • Intermembrane space: Between the inner and outer membranes.

Stages of Cellular Respiration

1. Formation of Acetyl CoA and the Krebs Cycle

2. Electron Transport Chain (ETC) and Chemiosmosis

Stage 1: Acetyl CoA Formation and the Krebs Cycle

• Pyruvate Transport: Pyruvate is transported from the cytosol into the mitochondrial matrix.

• Acetyl CoA Formation: Pyruvate is split, releasing CO2 and forming an acetyl group, which combines with coenzyme A to form acetyl CoA. NAD+ is reduced to NADH.

• Krebs Cycle (Citric Acid Cycle): Acetyl CoA combines with oxaloacetate (4C) to form citrate (6C). Through a series of reactions, citrate is broken down, releasing two CO2 molecules, regenerating oxaloacetate, and producing ATP, NADH, and FADH2.

• Products per Acetyl CoA:

  • 1 ATP

  • 3 NADH

  • 1 FADH2

• The cycle turns twice per glucose molecule.

Stage 2: Electron Transport Chain and Chemiosmosis

• Electron Transport Chain (ETC): A series of protein complexes in the inner mitochondrial membrane transfer electrons from NADH and FADH2 to oxygen, the final electron acceptor.

• Proton Gradient: Energy from electron transfer pumps H+ ions into the intermembrane space, creating a gradient.

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

• Oxygen's Role: Oxygen accepts electrons and combines with H+ to form water.

• Total ATP Yield: Up to 36 ATP per glucose (including glycolysis, Krebs cycle, and ETC).

Summary Table: ATP Yield from Glucose Breakdown

Alternative Energy Sources

Cellular respiration can extract energy from various food molecules, not just glucose.

• Fats: Broken into fatty acids and glycerol; fatty acids enter as acetyl CoA.

• Proteins: Broken into amino acids, which can enter the Krebs cycle after deamination.

• Carbohydrates: Other sugars can be converted to glucose or intermediates of glycolysis.

Fermentation: Energy Production Without Oxygen

Overview of Fermentation

Fermentation allows glycolysis to continue in the absence of oxygen by regenerating NAD+. It is an anaerobic process used by many organisms and cells under low-oxygen conditions.

• Pyruvate acts as the final electron acceptor, accepting electrons from NADH to regenerate NAD+.

• Types of Fermentation:

  • Lactate Fermentation: Converts pyruvate to lactic acid (lactate).

  • Alcoholic Fermentation: Converts pyruvate to ethanol and CO2.

Lactate Fermentation

• Occurs in muscle cells during intense activity when oxygen is scarce.

• Regenerates NAD+ for glycolysis, allowing rapid ATP production.

• Used by bacteria in the production of yogurt, sour cream, and cheese.

Alcoholic Fermentation

• Occurs in yeast and some plant cells.

• Converts pyruvate to ethanol and CO2, regenerating NAD+.

• Used in the production of alcoholic beverages and bread.

Summary Table: Types of Fermentation

Importance of Fermentation

• Allows ATP production in the absence of oxygen.

• Essential for certain foods and beverages.

• Contributes to the flavor and texture of fermented foods.

Additional info: The above notes expand on the original lecture slides by providing definitions, context, and examples for each process, as well as summarizing key equations and including tables for clarity.