SENIOR THREE

1.1 Tissue Respiration

Tissue respiration is the breakdown of food substances to release energy. It occurs with the help of enzymes. The major food respired (the respiratory substrate) is a carbohydrate (glucose). All other food compounds are converted into a carbohydrate before they are respired.

The energy released is stored as ATP (Adenosine triphosphate). ATP is a highly energy-rich compound formed by a chemical bond between ADP (Adenosine diphosphate) and an inorganic phosphate group (Pi).

ADP + Pi → ATP

When the energy stored in ATP is required by the body, ATP is broken down (by the enzyme ATPase) into ADP and Pi, releasing energy for body activities:

ATP --ATPase--> ADP + Pi + Energy

Uses of the energy released

Maintaining blood circulationBringing about breathing movementsProducing soundTransmission of nerve impulses from one part of the body to anotherSynthesis of blood proteinsMaintaining a constant blood temperatureCell division (mitosis or meiosis) leading to growthActive transport of materials into or out of the cellSecretion of materials such as hormones and enzymes

Stages of respiration

Respiration occurs as a series of reactions divided into two stages:

Glycolysis - the breakdown of a six-carbon compound (glucose) into two small three-carbon compounds. This occurs in the cell cytoplasm.Krebs cycle - the further breakdown of the three-carbon compounds, releasing more energy than glycolysis. This occurs in the mitochondria.

There are two types of respiration: (1) Aerobic respiration and (2) Anaerobic respiration.

1.2 Aerobic Respiration

This is the breakdown of food to release energy in the presence of oxygen. It produces energy, carbon dioxide and water, and is the most efficient process of energy release because food is completely broken down.

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

(Glucose) (oxygen) (carbon dioxide) (water)

The carbon dioxide produced diffuses from the tissues into the blood and is transported to the lungs for expiration through the trachea and nostrils. In plants, the carbon dioxide produced is either lost to the atmosphere through the stomata (leaves) or lenticels (stems), or used in photosynthesis.

Experiment: To show that living organisms use oxygen in aerobic respiration

Materials:

Conical flaskDelivery tubeBeakerSodium hydroxide solutionWaterGerminating seeds

Procedure:

Place germinating seeds in a conical flask together with a test tube containing sodium hydroxide.Connect a delivery tube to the flask with the other end dipped in a beaker of water.Leave the set-up to stand and observe the water level in the delivery tube.

Observation: After some time, water rises in the delivery tube.

Explanation: As the seeds respire they use oxygen and produce CO2, but the CO2 is absorbed by the sodium hydroxide, so it is not returned to the flask - hence a decrease in air volume, seen as rising water.

Conclusion: Oxygen is used up in aerobic respiration.

Experiment: To show that living organisms liberate CO2 during aerobic respiration

Materials:

Soda lime (sodium hydroxide)Lime waterFilter pumpA toad (or small aerobe)Two delivery tubesThree flasks and corks

Procedure:

A toad/rat is used as the aerobe and the apparatus is set up with three flasks in series, left to stand for 40 minutes.Sodium hydroxide absorbs CO2 from the incoming air.Limewater in flask A confirms the absence of CO2 in incoming air.Limewater in flask B/C tests for CO2 in exhaled air.A filter pump ensures air flows in one direction.

Observation: Limewater in the flask nearer the organism turns milky, while that in flask A (before the organism) remains clear.

Conclusion: Living organisms give out carbon dioxide during respiration.

1.3 Anaerobic Respiration

This is the breakdown of food to release energy in the absence of oxygen. The food is not completely broken down; part of it remains as ethanol in plants or lactic acid in animals. Less energy is released than in aerobic respiration because most of it remains locked in the intermediate substances.

Equation for anaerobic respiration in animals

C6H12O6 → 2CH3CH(OH)COOH + CO2 + 150 kJ (energy)

(Glucose) (lactic acid)

Equation for anaerobic respiration in plants / yeast (fermentation)

C6H12O6 --enzyme (zymase)--> C2H5OH + CO2 + Energy (118 kJ)

(Glucose) (ethanol)

Fermentation is anaerobic respiration carried out in solution, as seen in yeast. It is commercially exploited in beer brewing (to produce alcohol) and in bread baking (to raise dough).

Oxygen debt

During vigorous activity, oxygen supply to the muscles may not meet energy demand, so anaerobic respiration occurs and lactic acid accumulates. Breathing rate stays high even after exercise to supply the extra oxygen needed to oxidise this lactic acid to CO2, water and energy....