Respiration Releases Energy Continuously

1. Living cells respire continuously because they need a constant transfer of energy.
2. Respiration is exothermic because chemical energy from glucose is transferred to the surroundings and to processes in the cell.
3. Cells use the released energy for muscle contraction, active transport and building larger molecules from smaller ones.
4. Energy released by respiration also helps mammals and birds maintain a stable body temperature.
5. Respiration is a series of enzyme-controlled reactions, not a single reaction and not the same process as breathing.

Aerobic Respiration Uses Oxygen

1. Aerobic respiration uses oxygen and completely breaks down glucose.
2. The word equation is $glucose+oxygen\to carbon\; dioxide+water$.
3. The balanced symbol equation is ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\to 6C{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}$.
4. Most aerobic respiration occurs in mitochondria, so cells with high energy demands often contain many mitochondria.
5. Complete breakdown of glucose releases much more energy than anaerobic respiration.

Anaerobic Respiration Uses No Oxygen

1. Anaerobic respiration occurs when oxygen supply cannot meet the demand for aerobic respiration.
2. Glucose is broken down incompletely, so less energy is released from each molecule of glucose.
3. In animal muscle cells, the word equation is $glucose\to lactic\; acid$.
4. Lactic acid can accumulate during vigorous exercise and is associated with muscle fatigue.
5. In yeast and plant cells, the word equation is $glucose\to ethanol+carbon\; dioxide$.
6. Anaerobic respiration occurs in the cytoplasm rather than the mitochondria.

The Two Pathways Have Clear Differences

1. Oxygen: aerobic respiration requires oxygen, while anaerobic respiration does not.
2. Breakdown: aerobic respiration breaks glucose down completely, while anaerobic respiration breaks it down incompletely.
3. Energy: aerobic respiration releases much more energy per molecule of glucose.
4. Animal products: aerobic respiration produces carbon dioxide and water, while anaerobic respiration produces lactic acid.
5. Yeast and plant products: anaerobic respiration produces ethanol and carbon dioxide.
6. Location: most aerobic respiration occurs in mitochondria, while anaerobic respiration occurs in the cytoplasm.

Exercise Can Use Both Pathways

1. At the start of exercise, muscles increase their demand for energy before oxygen delivery has fully increased.
2. Aerobic respiration supplies most energy when enough oxygen reaches the muscles.
3. During vigorous activity, anaerobic respiration adds a faster but smaller energy release when oxygen supply is insufficient.
4. Heart rate and breathing rate rise to deliver more oxygen and glucose and to remove carbon dioxide.