Biology (0610) | Anaerobic respiration

⚡️ Study Notes: Anaerobic Respiration (Biology IGCSE 0610 - Respiration) ⚡️

Welcome to the study notes on Anaerobic Respiration! You've already learned about aerobic respiration—the efficient way cells make energy using oxygen. But what happens when oxygen is scarce, like during a sprint or when yeast is locked away from the air? This chapter explores the body’s ingenious backup plan, which is crucial for survival, industry, and that burning sensation in your muscles!

Don't worry if this seems tricky at first; we will break down the process into simple steps for both humans and yeast.

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What is Anaerobic Respiration?

Respiration is the chemical process in cells that breaks down nutrient molecules (like glucose) to release energy for metabolism.

Core Definition: The Backup Plan

Anaerobic respiration is defined as the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.
Anaerobic literally means "without air" (or specifically, without oxygen).

• It is an incomplete breakdown of glucose.
• It happens in the cytoplasm of the cell (unlike aerobic respiration which finishes in the mitochondria).
• It is used when the oxygen supply is too low to meet the energy demands of the cell (e.g., during intense exercise) or in organisms adapted to oxygen-free environments (like deep-sea bacteria or yeast in brewing).

Energy Released: The Trade-off

The biggest difference between the two types of respiration is the output:

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.

In aerobic respiration, the glucose molecule is broken down completely, releasing a large amount of energy (around 38 ATP molecules). In anaerobic respiration, the breakdown is incomplete, releasing only a small fraction of the energy (typically 2 ATP molecules).

Analogy: Imagine charging your phone. Aerobic respiration is like using the fast charger (efficient and full battery). Anaerobic respiration is like using a slow, weak trickle charger—it gives you a little bit of power fast, but it’s not sustainable!

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Anaerobic Respiration in Yeast (Fermentation)

In certain microorganisms like yeast (a type of fungus), anaerobic respiration is known as alcoholic fermentation.

The Process and Products

When yeast is supplied with glucose but kept away from oxygen, it converts the glucose into ethanol (alcohol) and carbon dioxide.

Core Word Equation:

glucose → alcohol + carbon dioxide + energy

Supplement Balanced Chemical Equation:

\(C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2\)

Real-World Applications of Yeast Fermentation

• Bread Making: Yeast is mixed into dough. It respires anaerobically, producing carbon dioxide gas. This gas is trapped in the dough, causing it to rise and giving bread its light, airy texture. (The alcohol evaporates during baking!)
• Biofuels and Brewing: Yeast fermentation is used industrially to produce ethanol (alcohol) for drinks or as a biofuel.

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Anaerobic Respiration in Human Muscles

When you perform vigorous exercise, such as sprinting or weightlifting, your muscle cells use oxygen much faster than your bloodstream can supply it. To keep moving, muscle cells switch to anaerobic respiration as an emergency energy source.

The Process and Product

In human muscle cells, the incomplete breakdown of glucose produces a substance called lactic acid.

Core Word Equation:

glucose → lactic acid + energy

This provides immediate, fast energy, but it cannot be sustained for long.

The Problem with Lactic Acid

Lactic acid quickly builds up in the muscles and the bloodstream. High concentrations of lactic acid lower the pH of the muscle tissue, causing the familiar feeling of muscle fatigue and sometimes pain or a 'stitch'.

Did you know? Lactic acid used to be blamed entirely for muscle soreness (DOMS), but scientists now know it's primarily responsible for immediate fatigue during exercise, while soreness is due to microscopic muscle damage.

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The Oxygen Debt (Extended/Supplement)

Because lactic acid is toxic and acidic, it must be removed. The body needs extra oxygen after the vigorous exercise stops to break down this accumulated lactic acid. This extra oxygen required is known as the oxygen debt.

What is the Oxygen Debt?

The oxygen debt is the volume of oxygen required after strenuous exercise to aerobically respire the lactic acid built up during the anaerobic phase.

Analogy: Think of the oxygen debt as a financial loan. You borrowed energy (through anaerobic respiration) that didn't require oxygen, but now you must "pay back" the oxygen needed to clear the acidic waste (lactic acid).

Removing the Oxygen Debt (Step-by-Step)

You know you are repaying the debt because you continue to breathe heavily and have a fast heart rate for some time after you stop exercising. This continued activity helps in three ways (Supplement 7):

1. Transporting the Acid: The continuation of fast heart rate ensures blood quickly transports the lactic acid away from the muscles and to the liver.
2. Oxygen Supply: The continuation of deeper and faster breathing ensures a high supply of oxygen is constantly taken into the lungs and delivered to the bloodstream.
3. Breakdown: The lactic acid is transported to the liver where it is broken down through aerobic respiration (or converted back into glucose).

The conversion of lactic acid back into useful products or its complete breakdown using oxygen is essential for returning the body’s metabolic state to normal.