Human Respiration » Anaerobic Respiration Equations

What you'll learn this session

Study time: 30 minutes

The definition and process of anaerobic respiration
The chemical equation for anaerobic respiration in humans and other animals
The chemical equation for anaerobic respiration in plants and yeast
The differences between aerobic and anaerobic respiration
Real-world applications and examples of anaerobic respiration
The importance of anaerobic respiration in sports and exercise

Introduction to Anaerobic Respiration

Respiration is how living organisms release energy from food molecules, particularly glucose. While most of us are familiar with aerobic respiration (which uses oxygen), there's another vital process called anaerobic respiration that occurs when oxygen is in short supply.

Key Definitions:

Anaerobic Respiration: The release of energy from glucose or other organic substances without using oxygen.
Lactic Acid: A compound produced during anaerobic respiration in animals and humans.
Ethanol: An alcohol produced during anaerobic respiration in plants and yeast (also called fermentation).
Oxygen Debt: The amount of extra oxygen needed to break down lactic acid after intense exercise.

🔥 When Does Anaerobic Respiration Happen?

Anaerobic respiration kicks in when:

There's not enough oxygen available
Energy is needed quickly (like during intense exercise)
Organisms live in environments without oxygen

🏃 Why It Matters

Anaerobic respiration is crucial because it:

Provides emergency energy when oxygen is limited
Allows muscles to keep working during intense activity
Enables certain industrial processes like bread-making and brewing

Anaerobic Respiration Equations

The chemical equations for anaerobic respiration differ depending on the organism. Let's look at the two main types:

Anaerobic Respiration in Humans and Animals

When we exercise intensely, our muscles may not get enough oxygen. In these conditions, our cells switch to anaerobic respiration:

The Human Anaerobic Equation

Glucose → Lactic Acid + Energy

In chemical terms: C₆H₁₂O₆ → 2C₃H₆O₃ + Energy (2 ATP)

This process produces lactic acid, which builds up in muscles and causes that burning sensation you feel during intense exercise. It also produces much less energy than aerobic respiration - only 2 ATP molecules compared to the 38 ATP from aerobic respiration.

Oxygen Debt

After intense exercise, you continue breathing heavily even when you've stopped. This is because your body needs extra oxygen to break down the accumulated lactic acid. This extra oxygen required is called oxygen debt.

Lactic acid + Oxygen → Carbon dioxide + Water + Energy

Anaerobic Respiration in Plants and Yeast

Plants and yeast cells use a different form of anaerobic respiration, often called fermentation:

The Plant/Yeast Anaerobic Equation

Glucose → Ethanol + Carbon Dioxide + Energy

In chemical terms: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy (2 ATP)

This process is the basis for many food and drink productions. The carbon dioxide makes bread rise and the ethanol is the alcohol in alcoholic drinks.

🍞 Bread Making

Yeast in dough produces CO₂ bubbles through anaerobic respiration, making the dough rise. The ethanol evaporates during baking.

🍺 Brewing

Yeast ferments sugars in barley or grapes, producing ethanol (alcohol) and CO₂. This is how beer, wine and other alcoholic drinks are made.

🧀 Yogurt Production

Bacteria perform anaerobic respiration in milk, producing lactic acid which gives yogurt its tangy taste and thick texture.

Comparing Aerobic and Anaerobic Respiration

Understanding the differences between these two types of respiration is crucial for your IGCSE exams:

🟢 Aerobic Respiration

Uses oxygen
Produces carbon dioxide and water
Releases a lot of energy (38 ATP)
Equation: Glucose + Oxygen → Carbon Dioxide + Water + Energy
Complete breakdown of glucose
Occurs in the mitochondria

🔴 Anaerobic Respiration

No oxygen required
In humans: produces lactic acid
In plants/yeast: produces ethanol and carbon dioxide
Releases much less energy (2 ATP)
Incomplete breakdown of glucose
Occurs in the cytoplasm

Anaerobic Respiration in Sports and Exercise

Anaerobic respiration is particularly important in sports science and understanding how our bodies respond to different types of exercise.

🏃 Sprinting and High-Intensity Exercise

During a 100m sprint or lifting heavy weights, your muscles need energy faster than oxygen can be delivered. Your body switches to anaerobic respiration, which:

Provides quick energy
Builds up lactic acid
Can only be sustained for short periods
Leads to muscle fatigue and the burning sensation

🏃 Endurance Exercise

During a marathon or long-distance cycling, your body primarily uses:

Aerobic respiration
Steady oxygen supply
More efficient energy production
Can be sustained for longer periods

Case Study: Lactic Acid Threshold Training

Elite athletes often train at or just below their "lactic acid threshold" – the point where lactic acid begins to accumulate faster than it can be removed. This type of training helps the body become more efficient at:

Using oxygen during exercise
Clearing lactic acid from muscles
Delaying the switch to anaerobic respiration
Improving overall performance

This is why you'll often see athletes training with devices that measure their blood lactate levels!

Real-World Applications of Anaerobic Respiration

Beyond sports and brewing, anaerobic respiration has several other important applications:

💪 Muscle Recovery

Understanding anaerobic respiration helps sports scientists develop better recovery techniques for athletes after intense training.

🥩 Biogas Production

Bacteria perform anaerobic respiration on organic waste, producing methane gas that can be used as a renewable energy source.

🧅 Sewage Treatment

Anaerobic digestion helps break down sewage and other waste materials in treatment plants.

Key Points to Remember

For your IGCSE exams, make sure you can:

Write and balance the chemical equations for anaerobic respiration in humans/animals and plants/yeast
Explain when and why anaerobic respiration occurs
Compare the energy yield of aerobic and anaerobic respiration
Describe the concept of oxygen debt and how it relates to lactic acid
Give examples of practical applications of anaerobic respiration

Exam Tip!

Questions about anaerobic respiration often appear alongside questions about aerobic respiration. Make sure you can clearly explain the differences between the two processes and when each one occurs. Remember the equations!

🧠 Test Your Knowledge!