IGCSE Biology 0610: Balanced Equations for Aerobic and Anaerobic Respiration

Heavy rain floods the soil around the roots of a maize (corn) plant for several days. Before the flood, oxygen could diffuse freely through air spaces in the soil to reach the root cells, which respired aerobically. Once the soil is waterlogged, the air spaces fill with water and oxygen can no longer reach the root cells, so they are forced to respire anaerobically instead, breaking glucose down into ethanol and carbon dioxide, in the same way that yeast does.

(a) State the balanced chemical equation for the aerobic respiration that was occurring in the root cells before the flood. [1]

(b) State the balanced chemical equation for the anaerobic respiration now occurring in the waterlogged root cells. [1]

(c) State and explain which of these two respiration reactions releases more energy per molecule of glucose broken down. [2]

(d) Show that the equation you gave in (b), C6H12O6 → 2C2H5OH + 2CO2, has the same number of carbon atoms, the same number of hydrogen atoms and the same number of oxygen atoms on each side. [2]

Part (a): The balanced equation for aerobic respiration

Before the flood, the root cells had a supply of oxygen and were respiring aerobically, completely breaking glucose down into carbon dioxide and water:

C6H12O6 + 6O2 → 6CO2 + 6H2O

Part (b): The balanced equation for anaerobic respiration

Once the soil is waterlogged and no oxygen can reach the root cells, they respire anaerobically instead. The stem tells us the root cells break glucose down into ethanol and carbon dioxide, in the same way that yeast does:

C6H12O6 → 2C2H5OH + 2CO2

Part (c): Comparing the energy released

Aerobic respiration releases more energy per glucose molecule than anaerobic respiration. Aerobic respiration uses oxygen to break glucose down completely into carbon dioxide and water, releasing all of the energy locked up in the glucose. Anaerobic respiration, without oxygen, only breaks glucose down partly, into ethanol and carbon dioxide, so much of the energy in the glucose remains locked up in the ethanol produced rather than being released.

Part (d): Checking that the anaerobic equation is balanced

Left-hand side, C6H12O6:

carbon atoms: 6
hydrogen atoms: 12
oxygen atoms: 6

Right-hand side, 2C2H5OH + 2CO2:

Each C2H5OH molecule has 2 carbon atoms, 6 hydrogen atoms (5 attached to carbon plus 1 in the OH group) and 1 oxygen atom. With the coefficient 2, this gives 2 × 2 = 4 carbon atoms, 2 × 6 = 12 hydrogen atoms and 2 × 1 = 2 oxygen atoms.
Each CO2 molecule has 1 carbon atom and 2 oxygen atoms. With the coefficient 2, this gives 2 × 1 = 2 carbon atoms and 2 × 2 = 4 oxygen atoms.
Adding these together: carbon atoms = 4 + 2 = 6; hydrogen atoms = 12 + 0 = 12; oxygen atoms = 2 + 4 = 6.

Both sides have 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms, so the equation is correctly balanced.

Final answers

(a) C6H12O6 + 6O2 → 6CO2 + 6H2O
(b) C6H12O6 → 2C2H5OH + 2CO2
(c) Aerobic respiration releases more energy per glucose molecule, because it breaks glucose down completely using oxygen, while anaerobic respiration only breaks it down partly.
(d) Both sides of C6H12O6 → 2C2H5OH + 2CO2 have 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms, so the equation is balanced.