All living organisms need energy to perform cell activities and maintain organization.

ATP is the molecule that provides energy to cells, and ATP is released during respiration.

Respiration is a chemical reaction where food molecules are broken down to release energy.

There are two types of respiration: aerobic respiration and anaerobic respiration.

Aerobic Respiration

Aerobic respiration happens when there is oxygen. 

In the presence of oxygen, aerobic respiration will release a large amount of energy because glucose is completely broken down:

glucose + oxygen → carbon dioxide + water

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Aerobic respiration takes place in mitochondria of cells.

Anaerobic Respiration

Anaerobic respiration does not require oxygen and releases a relatively small amount of energy because glucose is broken down incompletely.

In mammals:

glucose → lactic acid

During vigorous exercise, not enough oxygen can be provided for aerobic respiration, so energy will be provided through anaerobic respiration. As a result of anaerobic respiration, lactic acid will build up in muscles.

In plants and yeasts:

glucose → carbon dioxide + ethanol

Anaerobic respiration is called fermentation in plants and yeasts and is used for brewing alcohol drinks and bread making.

👆 For IGCSE Biology, it is important to know the word and chemical equation for aerobic and anaerobic respiration 👆 

Experiment to Determine the Production of Carbon Dioxide from Respiration

There are two ways to experimentally determine the production of carbon dioxide from respiration:

1. Using hydrogencarbonate indicator

Hydrogencarbonate indicator shows the presence carbon dioxide:

• When there is no carbon dioxide, the indicator will turn red
• When there is carbon dioxide, the indicator will turn yellow

• Connect four flasks and capillary tubes as shown above 👆 
• The first flask contains sodium hydroxide solution, which will absorb any carbon dioxide present in the air.
• The hydrogencarbonate indicator in the second flask will turn red because all the carbon dioxide in the air was absorbed in the first flask and the air no longer contains any carbon dioxide.
• Carbon dioxide will be produced in the third flask where there are respiring organisms.
• The hydrogencarbonate indicator in the fourth flask will turn yellow because of the carbon dioxide produced by respiration.

2. Using limewater

• The experimental setup is similar to the experiment using hydrogencarbonate indicator. 
• The first flask contains sodium hydroxide solution, which will absorb any carbon dioxide present in the air.
• Since there is no carbon dioxide in the air passing the second flask, the limewater will be clear.
• Carbon dioxide will be produced in the third flask where there are respiring organisms.
• Limewater in the last flask will turn milky.

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