Respiration

Define respiration

Respiration is the process where glucose is broken down to release energy, in the form of ATP, in all living cells

Define aerobic respiration

The breakdown of glucose in the presence of oxygen with the release of 38 ATP molecules. Carbon dioxide and water are released as waste products

Glycolysis

1 glucose molecule is broken down to form 2 molecules of pyruvate and 2 ATP molecules (1 pyruvate: 18 ATP)

Equation for aerobic respiration

Word equation: Glucose + Oxygen → Carbon Dioxide + Water + ATP

Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Define anaerobic respiration

The breakdown of glucose in the absence of oxygen, resulting in the release of lesser energy than aerobic respiration

What happens in anaerobic respiration

1. Glycolysis of one molecule of glucose in the cytosol yields two molecules of pyruvate and two molecules of ATP

2. In the absence or low concentration of oxygen, the pyruvate will not enter the mitochondrion. Instead, pyruvate is converted into lactic acid/ethanol and carbon dioxide, which are removed as metabolic waste products

Alcohol fermentation

Pyruvate converted into ethanol and carbon dioxide

Glucose —>(Glycolysis)—>Pyruvate + 2ATP —>(Yeast) —> Ethanol + Carbon dioxide

Overall Eqn:

• Glucose → Ethanol + Carbon Dioxide + ATP (small amount)

• C₆H₁₂O_{6 ⟶} 2C₂H₅OH + 2CO₂ + ATP (small amount)

Lactic acid fermentation

pyruvate is converted to lactic acid (no carbon dioxide)

Glucose —>(Glycolysis)—>Pyruvate + 2ATP —>(Humans/Bacteria) —> Lactic Acid

Overall Eqn:

• Glucose → Lactic Acid + ATP (small amount)

• C₆H₁₂O₆ → 2C₃H₆O₃ + ATP (small amount)

Uses of ATP in the body of humans

• Muscle contraction

• Protein synthesis

• Cell division

• Active transport

• Growth

• The passage of nerve impulses

• The maintenance of a constant body temperature

Describe the production of lactic acid in muscles during vigorous exercise

• During vigorous muscular contractions, the muscle cells first respire aerobically

• Breathing rate increases to remove carbon dioxide and take in oxygen at a faster rate. Heart rate will also increase so that oxygen can be transported to the muscles at a faster rate. However, there is a limit to the increase in the rate of breathing and heartbeat, therefore, there is insufficient oxygen to meet the demands of the vigorous muscular contractions

• In such cases, muscle cells also respire anaerobically for short durations in order to meet the energy demands of the activity in addition to aerobic respiration that is already taking place

• The extra energy released by anaerobic respiration supplements the energy released by aerobic respiration to allow the muscles to continue contracting

• When anaerobic respiration occurs, there is a buildup of lactic acid in the muscle cells

• Since there is insufficient oxygen to meet the demands of the vigorous muscular contractions, the muscles incur an oxygen debt

• Lactic acid concentrations build up slowly in the muscles and may eventually become high enough to cause fatigue and muscular pains

Describe what happens during the recovery period

• During rest period, breathing rate continues to be high to provide “extra” oxygen to repay oxygen debt

• Lactic acid is removed from the muscles and transported to the liver

• In the liver, the lactic acid is oxidised (using oxygen taken in) to release energy. Energy is used to convert the remaining lactic acid back into glucose

• When all the lactic acid has been converted to glucose, the oxygen debt is repaid

• Glucose transported back to muscles