A-Level Biology: Respiration - The Link Reaction

Introduction

• Topic: Specification point D (OCR specification for A-level biology).
• Focus: Processes involved in the link reaction.
• Context: Second stage of the aerobic respiration cycle.
• Previous Lesson: Glycolysis.

Link Reaction Overview

• Location: Mitochondrial matrix.
• Frequency: Occurs twice for every one molecule of glucose (due to two molecules of pyruvate produced from glycolysis).

Process

1. Transport: Pyruvate (from glycolysis in the cytoplasm) is actively transported into the mitochondrial matrix.
2. Conversion to Acetate:
   • Pyruvate undergoes decarboxylation (loses one carbon to form CO2).
   • Pyruvate also loses a hydrogen electron to NAD, forming reduced NAD (NADH) = Oxidation/Dehydrogenation.
3. Formation of Acetyl Coenzyme A:
   • Acetate (2-carbon compound) combines with coenzyme A to form Acetyl Coenzyme A.

Role of Coenzymes

• Coenzyme A & NAD: Act as carriers (like postmen).
• Coenzyme A: Transports acetate into the Krebs cycle, then returns to transport more acetate (reused).

Products per Glucose Molecule

• Two Rounds of Link Reaction:
  • 2 molecules of CO2.
  • 2 molecules of reduced NAD (NADH).
  • 2 molecules of Acetyl Coenzyme A.

Summary

• Repetition: Essential to practice and rewrite the process until it is memorized accurately.
• Scientific Terminology: Avoid vague terms (like "it", "they", "amount", or "size") in explanations. Use precise scientific language.

Exam Preparation

• Practice: Rewrite steps and summary to ensure thorough understanding.
• Terminology: Use specific and correct scientific terms to describe processes.
• Good Luck: Reminder to focus and prepare well for exams.