Chapter 7: Cell Respiration

Learning Objectives

By the end of this chapter, you should be able to:

• Explain the importance of cellular respiration and ATP in living organisms
• Identify the main substrate used for energy production
• Describe the differences between aerobic and anaerobic respiration
• Give examples of aerobic and anaerobic respiration in living organisms
• Relate cellular respiration to everyday biological processes

Overview

Cellular respiration is a vital biochemical process occurring in all living organisms. Organic molecules — primarily glucose — are broken down (oxidised) to release chemical energy, which is then stored in ATP (adenosine triphosphate). ATP is the universal energy currency of the cell, powering everything from muscle contraction to nerve impulse transmission. Respiration can occur with oxygen (aerobic) or without it (anaerobic), with aerobic respiration being far more efficient.

7.1 Introduction to Cellular Respiration

What Is Cellular Respiration?

Cellular respiration is the biochemical process in which organic molecules are oxidised to release chemical energy. This energy is captured and stored as ATP, which cells use to drive life processes.

Why Do Organisms Need Energy?

Living organisms require energy for a wide range of activities:

ATP — The Energy Currency

• ATP (Adenosine Triphosphate) stores energy in its phosphate bonds
• When ATP is hydrolysed to ADP + Pi, energy is released for cellular work
• Cellular respiration continuously regenerates ATP from ADP

$$\text{ADP} + \text{Pi} + \text{Energy} \rightarrow \text{ATP}$$ $$\text{ATP} \rightarrow \text{ADP} + \text{Pi} + \text{Energy (usable)}$$

7.2 Main Substrate for Energy Production

Glucose as the Primary Substrate

Glucose ($C_6H_{12}O_6$) is the principal substrate for cellular respiration in both animals and plants.

Other Substrates

When glucose is scarce, cells can also respire:

• Fats (lipids) — broken down to fatty acids and glycerol; very energy-rich
• Proteins — amino acids used as a last resort (e.g., during starvation)

Key Point: Glucose is always the preferred and most direct substrate. Fats yield more ATP per gram but require more steps to enter the respiratory pathway.

7.3 Types of Cellular Respiration

There are two main types of cellular respiration, distinguished by whether oxygen is required:

Aerobic Respiration

Aerobic respiration is the complete oxidation of glucose in the presence of oxygen, producing a large amount of ATP.

Word equation:

$$\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{Energy (ATP)}$$

Chemical equation:

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$$

Key stages:

Where it occurs:

• Glycolysis takes place in the cytoplasm
• Krebs Cycle and oxidative phosphorylation take place in the mitochondria
• This is why cells with high energy demands (e.g., muscle cells, liver cells) have many mitochondria

Anaerobic Respiration (Fermentation)

Anaerobic respiration occurs when oxygen is absent or limited. Glucose is only partially broken down, producing far less ATP.

Lactic Acid Fermentation (in animals / human muscle cells)

Occurs in human muscle cells during intense exercise when oxygen supply cannot meet demand.

Word equation:

$$\text{Glucose} \rightarrow \text{Lactic Acid} + \text{Energy (ATP)}$$

Effects:

• Lactic acid accumulates → muscle fatigue and soreness
• When oxygen becomes available again, lactic acid is converted back to glucose in the liver (oxygen debt)

Alcoholic Fermentation (in yeast and microorganisms)

Occurs in yeast and some bacteria in the absence of oxygen.

Word equation:

$$\text{Glucose} \rightarrow \text{Ethanol} + \text{Carbon Dioxide} + \text{Energy (ATP)}$$

Applications in daily life:

Comparison Summary

SPM Exam Tip: A very common exam question asks you to compare aerobic and anaerobic respiration. Remember the 4 key differences: oxygen requirement, ATP yield, end products, and site of reaction. Also be able to give a real-life example for each type of anaerobic fermentation (bread/beer for alcoholic; exercise/muscle fatigue for lactic acid).

Practice Questions

1. State three activities in the human body that require energy from ATP.
2. Write the word equation for aerobic respiration.
3. Explain why lactic acid accumulates in muscle cells during intense exercise.
4. A student bakes bread using yeast. Explain the role of anaerobic respiration in making the bread rise.
5. Compare aerobic and anaerobic respiration under the headings: oxygen requirement, ATP yield, and end products.

Summary

• Cellular respiration releases energy stored in glucose and captures it as ATP
• Glucose is the main substrate; obtained from digestion (animals) or photosynthesis (plants)
• Aerobic respiration uses oxygen, completely oxidises glucose, and produces ~36–38 ATP, CO₂ and H₂O
• Anaerobic respiration occurs without oxygen; only produces 2 ATP per glucose
  • Lactic acid fermentation: in animal/muscle cells → lactic acid (causes muscle fatigue)
  • Alcoholic fermentation: in yeast → ethanol + CO₂ (used in bread-making, brewing)
• Aerobic respiration occurs in both the cytoplasm (glycolysis) and mitochondria; anaerobic respiration occurs only in the cytoplasm

Related Topics

• Chapter 5: Metabolism and Enzymes
• Chapter 6: Cell Division
• Chapter 8: Respiratory Systems in Humans and Animals