Respiration

10.1 Respiration

Respiration happens in every living cell all the time. It is different from breathing (gas exchange). The energy released is used in making ATP, a molecule that cells use as an immediate source of energy (the details are not needed).

Uses of energy in living organisms

• muscle contraction for movement;
• protein synthesis (building proteins from amino acids);
• cell division;
• active transport of molecules and ions across membranes;
• growth;
• passing electrical impulses along neurones;
• maintaining a constant body temperature (in mammals and birds).

Investigating respiration in yeast

Yeast respires glucose. The effect of temperature can be investigated by adding yeast to a glucose solution at different temperatures in a water bath and measuring the rate at which carbon dioxide is made (counting the bubbles, or measuring the gas collected in a given time, or the time for hydrogencarbonate indicator to change colour). The rate rises with temperature up to an optimum of about $35$–$40\,^\circ\text{C}$ (the enzymes work faster), and then falls when the enzymes are denatured.

10.2 Aerobic respiration

$\text{glucose} + \text{oxygen} \to \text{carbon dioxide} + \text{water}$

$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}$

It takes place in the mitochondria (and the cytoplasm), and all the glucose is fully broken down. It is the opposite of photosynthesis, and the equation for aerobic respiration is the reverse of the equation for photosynthesis.

10.3 Anaerobic respiration

Less energy is released because the glucose is only partly broken down.

In humans (muscles):

$\text{glucose} \to \text{lactic acid}$

In yeast (fermentation):

$\text{glucose} \to \text{ethanol} + \text{carbon dioxide}$

Yeast fermentation is used to make bread (carbon dioxide makes the dough rise) and alcoholic drinks (ethanol).

Lactic acid and the oxygen debt

During vigorous exercise, the muscles need energy faster than the blood can bring oxygen to them. The muscles use anaerobic respiration to release extra energy, which makes lactic acid that builds up in the muscles and blood. The lactic acid causes muscle fatigue and cramp.

How the oxygen debt is removed.

• The fast heart rate continues, so that the blood carries the lactic acid from the muscles to the liver.
• Deeper, faster breathing continues, to take in extra oxygen, which is used in the liver to break down the lactic acid (to carbon dioxide and water).

The rates return to normal when the lactic acid has been removed. A fitter person recovers faster.

The muscles made lactic acid by anaerobic respiration, so the body has an oxygen debt. Deep, fast breathing brings in extra oxygen to break down the lactic acid in the liver.