Cellular Respiration Overview

By the end of this lesson, you will be able to write and explain the overall cellular respiration equation, describe the three stages and their locations, explain how ATP is generated, and compare aerobic and anaerobic respiration.

The Big Idea

Cellular respiration is how cells extract energy from glucose and convert it to ATP — the usable form of energy for cellular work. It's essentially the reverse of photosynthesis in terms of reactants and products, but the mechanism is completely different. Respiration occurs in virtually all living organisms, 24/7, whether or not they photosynthesize.

The Overall Equation

$$\text{C}6\text{H}{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP (energy)}$$

Reactants: glucose, oxygen

Products: carbon dioxide (exhaled), water, ATP

Three Stages of Aerobic Respiration

Stage 1: Glycolysis (cytoplasm)

• Input: glucose (C₆) + 2 ATP + 2 NAD⁺
• Output: 2 pyruvate (C₃) + 4 ATP (net gain: 2 ATP) + 2 NADH
• Doesn't require oxygen (can proceed anaerobically)

Stage 2: Krebs Cycle / Citric Acid Cycle (mitochondrial matrix)

• Input: each pyruvate → acetyl-CoA (2C) + CO₂; acetyl-CoA enters cycle
• Output per glucose: 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂
• Carbon skeletons are completely oxidized here → CO₂ released

Stage 3: Electron Transport Chain + Oxidative Phosphorylation (inner mitochondrial membrane)

• NADH and FADH₂ donate electrons to the ETC
• Electrons flow through protein complexes → H⁺ pumped across membrane → ATP synthase → ATP
• O₂ is the final electron acceptor → forms H₂O
• ~32–34 ATP produced here (by chemiosmosis)

ATP Yield Summary

Stage	Location	Net ATP	NADH	FADH₂
Glycolysis	Cytoplasm	2	2	0
Pyruvate oxidation	Mitochondrial matrix	0	2	0
Krebs cycle	Mitochondrial matrix	2	6	2
ETC + oxidative phosphorylation