Aerobic respiration

1. Aerobic respiration is respiration that uses oxygen, and it is the pathway a cell uses whenever enough oxygen is reaching it.
2. The substrate is glucose, and because oxygen is available the glucose is broken down completely.
3. Complete breakdown means every carbon atom in the glucose ends up in carbon dioxide and every hydrogen atom ends up in water, so none of the chemical energy is left locked in a leftover molecule.
4. The word equation is glucose + oxygen $\to$ carbon dioxide + water.
5. The balanced symbol equation is $C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O$.
6. Check the balancing by counting atoms: there are $6$ carbon, $12$ hydrogen and $18$ oxygen atoms on each side of the arrow.
7. The first steps take place in the cytoplasm, and the reactions that need oxygen are completed inside the mitochondria.
8. A cell with a heavy demand for ATP, such as a muscle fibre or a liver cell, is therefore packed with mitochondria.
9. Because the glucose is used up completely, aerobic respiration gives a large yield of ATP, roughly thirty molecules of ATP from each molecule of glucose.
10. Oxygen has to be delivered continuously by the blood, so the maximum rate of aerobic respiration is set by how fast the heart and lungs can supply it.

Anaerobic respiration in animal cells

1. Anaerobic respiration is respiration without oxygen, and an animal cell switches to it when oxygen is being used faster than the blood can deliver it.
2. The clearest example is a muscle during vigorous exercise, when the heart and lungs cannot keep up with the demand of the working fibres.
3. The substrate is still glucose, but without oxygen the glucose is broken down only incompletely.
4. The word equation is glucose $\to$ lactic acid.
5. All of these reactions happen in the cytoplasm, because the stages that use the mitochondria cannot run without oxygen.
6. The yield of ATP is small, only about two molecules of ATP per molecule of glucose, which is roughly fifteen times less than the aerobic pathway gives.
7. The yield is small because lactic acid still holds a great deal of unreleased chemical energy in its bonds.
8. The advantage of the pathway is speed, because it supplies ATP quickly enough to keep a muscle contracting during a short burst of hard effort.
9. The disadvantage is that lactic acid builds up in the muscle and the blood, causing muscle fatigue and pain, so the pathway can only be used for a short time.
10. The lactic acid is later broken down once oxygen is available again, which is why heavy breathing continues after the exercise has stopped.

Anaerobic respiration in plant and fungal cells

1. In plant cells and in fungi such as yeast, anaerobic respiration follows a different route and is called fermentation.
2. The conditions are the same, which is an absence of oxygen, and the substrate is again glucose broken down incompletely.
3. The word equation is glucose $\to$ ethanol + carbon dioxide.
4. These reactions also take place entirely in the cytoplasm, and the yield is the same small amount of about two molecules of ATP per glucose.
5. The energy that is not transferred stays locked in the bonds of the ethanol, which is why ethanol burns so readily.
6. Plant roots ferment when the soil becomes waterlogged, because water fills the air spaces and no oxygen can reach them.
7. Yeast ferments whenever it is sealed away from air, which is what makes it useful for making bread and for brewing.

Comparing the pathways

1. Conditions
   1. Aerobic respiration needs a supply of oxygen, while both forms of anaerobic respiration run only when oxygen is absent or in short supply.
2. Substrate
   1. All three pathways start with glucose, so the substrate is not what makes them different.
3. Extent of breakdown
   1. Aerobic respiration breaks the glucose down completely, whereas anaerobic respiration leaves an energy-rich product behind.
4. Products
   1. Aerobic respiration gives carbon dioxide and water, an animal cell respiring anaerobically gives lactic acid, and a plant or yeast cell gives ethanol and carbon dioxide.
5. Site in the cell
   1. Aerobic respiration begins in the cytoplasm and is completed in the mitochondria, while anaerobic respiration is confined to the cytoplasm.
6. Relative yield of ATP
   1. Aerobic respiration gives roughly thirty ATP per glucose and anaerobic respiration only about two, so the aerobic pathway is around fifteen times more productive.
7. A cell will always use the aerobic pathway when it can, because using glucose anaerobically wastes most of the energy the molecule holds.