Aerobic respiration uses oxygen to break down glucose completely. It takes place in the mitochondria of cells and releases a large amount of energy.

The word equation: Glucose + Oxygen → Carbon dioxide + Water + Energy

The chemical equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$

Aerobic respiration produces carbon dioxide and water as waste products. It occurs in plants, animals, and humans during normal breathing.

Anaerobic respiration occurs when there is no oxygen available. It releases less energy because glucose is not completely broken down.

In Plants and Yeast (Fermentation)

Plants and yeast break down glucose to produce ethanol and carbon dioxide: $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + Energy$

Glucose → Ethanol + Carbon dioxide + Energy

In Animals (Including Humans)

During vigorous exercise, animal cells produce lactic acid when oxygen is scarce: $C_6H_{12}O_6 \rightarrow 2C_3H_6O_3 + Energy$

Glucose → Lactic acid + Energy

The buildup of lactic acid in muscles causes fatigue and creates an oxygen debt—the body must breathe deeply after exercise to repay this debt by converting lactic acid back to carbon dioxide and water.

Experiment 1: Investigating the Significance of Oxygen in Respiration (Activity 6.5)

Materials:

• Two conical flasks, bean seeds, cotton wool, pyrogallic acid, water, test tubes, corks, glass tubes

Procedure:

1. Place wet cotton wool with bean seeds in two conical flasks (A and B)
2. In flask A, suspend a test tube containing pyrogallic acid (which absorbs oxygen)
3. In flask B, suspend a test tube with water (for comparison)
4. Seal both flasks with corks
5. Leave for five days and observe

Expected Results:

• Flask A (no oxygen): Seeds do not germinate or grow poorly
• Flask B (with oxygen): Seeds germinate and grow normally

Conclusion: Oxygen is necessary for effective respiration and growth.

Experiment 2: Investigating Anaerobic Respiration in Yeast (Activity 6.8)

Materials:

• Yeast, glucose solution, test tubes, delivery tube, lime water, oil, warm water bath

Procedure:

1. Mix 2g yeast with 5ml glucose solution in a test tube
2. Add a layer of oil on top to prevent oxygen from entering
3. Connect a delivery tube to another test tube containing lime water
4. Place in a warm place for one hour
5. Observe changes in lime water

Expected Results:

• Lime water turns milky (cloudy) because carbon dioxide is produced
• Bubbles may be seen in the mixture

Conclusion: Yeast carries out anaerobic respiration (fermentation), producing carbon dioxide and ethanol.

Experiment 3: Demonstrating Respiration in Animals

Using a mouse, lime water, and a bell jar setup (as in Activity 6.6), students can show that exhaled air contains carbon dioxide because lime water turns milky when exposed to the mouse's exhaled breath.

In Tanzania, understanding anaerobic respiration is important for brewing traditional drinks like mbege (banana beer) and kangara (palm wine). Local brewers rely on yeast fermentation—the anaerobic respiration of yeast—to produce alcohol. Knowing how temperature affects this process helps brewers control fermentation to get good yields. For example, placing fermenting mbege in a cool, shaded place during hot weather in Dodoma or Arusha prevents the yeast from being killed by high temperatures, ensuring the drink ferments properly.