Respiration

Cambridge IGCSE Biology 0610 · Core and Extended

Syllabus points

• Describe respiration and state the uses of the energy released.
• Describe aerobic respiration and state the word and balanced chemical equations.
• Describe anaerobic respiration in muscle and in yeast, with word equations.
• Explain oxygen debt and the fate of lactic acid.
• Investigate the effect of temperature on the rate of respiration in yeast.

What respiration is

Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Two things to be clear about from the start.

Respiration is not breathing. Breathing is the movement of air in and out of the lungs. Respiration happens inside every living cell, in every organism, all the time. Plants respire as well as photosynthesise, and they respire day and night.

Energy is released, not made. The energy was already in the glucose; respiration transfers it into a usable form. An answer saying respiration "produces energy" loses the mark.

The energy released is used for muscle contraction, active transport, building large molecules from small ones, cell division, growth, and in mammals and birds keeping the body temperature constant.

Aerobic respiration

Aerobic respiration is the release of a relatively large amount of energy in cells by the breakdown of food substances in the presence of oxygen.

Word equation:

glucose + oxygen → carbon dioxide + water

Balanced equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

It takes place mainly in the mitochondria, which is why cells with high energy demands, such as muscle cells, sperm cells and root hair cells, contain so many of them.

Notice this equation is the reverse of photosynthesis. That is worth seeing, but do not let it confuse the two: photosynthesis stores energy from light in glucose, and respiration releases it again.

Anaerobic respiration

Anaerobic respiration is the release of a relatively small amount of energy by the breakdown of food substances in the absence of oxygen.

The word relatively matters. Anaerobic respiration releases much less energy per glucose molecule, because the glucose is only partly broken down and the products still contain a great deal of chemical energy.

In muscle:

glucose → lactic acid

In yeast and plants:

glucose → alcohol + carbon dioxide

The yeast reaction is fermentation, and it is the basis of both brewing, where the alcohol is wanted, and bread making, where the carbon dioxide is wanted because it makes the dough rise. The alcohol evaporates during baking.

Oxygen debt

During hard exercise the heart and lungs cannot deliver oxygen fast enough, so muscles respire anaerobically as well as aerobically. This produces lactic acid, which builds up and causes muscle fatigue and pain.

The oxygen debt is the extra oxygen the body needs after exercise to break down that lactic acid.

This is why breathing rate and heart rate stay high after exercise has stopped. Blood carries the lactic acid to the liver, where it is broken down using oxygen, and deep breathing continues until the debt is repaid.

Comparing the two

Investigating respiration

Yeast and temperature. Yeast is mixed with a sugar solution and kept at different temperatures, and the carbon dioxide given off is measured, either by counting bubbles or by collecting the gas. The rate rises with temperature to an optimum and then falls, because respiration is controlled by enzymes, which denature above the optimum. A curve that rises and then falls is the signature of an enzyme-controlled process.

Germinating seeds. Germinating seeds respire rapidly. In a sealed flask with soda lime to absorb the carbon dioxide produced, the volume of gas falls as oxygen is used up, drawing liquid along a capillary tube. Boiled seeds are the control: they are dead, so they do not respire, and the liquid does not move. Including a control like that is often worth a mark on its own.

Germinating seeds also release heat, which is why a vacuum flask of them warms up while a flask of dead seeds does not.