Aerobic respiration releases energy

Explanation

Aerobic respiration is the release of energy by the breakdown of glucose in the presence of oxygen. It is a series of enzyme-controlled reactions in cells. Much of the process takes place in mitochondria, although the initial breakdown steps occur in the cytoplasm.

The word equation is glucose + oxygen -> carbon dioxide + water. The balanced symbol equation is C6H12O6 + 6O2 -> 6CO2 + 6H2O. Energy is released and transferred for cell activities; it is not a new kind of matter created by the reaction.

The equation balances atoms. One glucose molecule supplies six carbon atoms, so the products include six carbon dioxide molecules. Twelve hydrogen atoms form six water molecules. Eighteen oxygen atoms occur on each side when all reactants and products are counted.

Cells use released energy for activities such as muscle contraction, active transport and building large molecules. These activities continue when a person is resting, so respiration does not stop between breaths or when exercise ends.

Oxygen is a reactant, not an energy source that replaces glucose. Carbon dioxide is a waste product that blood carries to the lungs, where it can diffuse into alveoli and be exhaled.

A respirometer measures how fast living organisms take in oxygen. Germinating seeds or woodlice sit in a sealed tube above sodium hydroxide solution, which absorbs carbon dioxide. A capillary tube with a coloured drop of liquid is attached. As the organisms take in oxygen and the carbon dioxide they release is absorbed, the volume of gas in the tube falls, so the drop moves towards the organisms. The distance moved per minute measures the rate of oxygen uptake. A control tube holds glass beads of the same volume instead of living organisms, showing that the drop moves only because of respiration. Sodium hydroxide is corrosive, so it is kept below a gauze where the organisms cannot touch it.

Small mammals respire faster for their size. A mouse has a much larger surface area to volume ratio than a human, so it loses heat to its surroundings faster for each gram of body mass. To keep its body temperature constant, its cells must respire faster per gram, releasing more heat, so it takes in oxygen faster per gram than a large mammal.