Respiration

Respiration is the body’s way of producing energy from the food we eat. It involves the breakdown of glucose in the presence of oxygen into carbon dioxide and water with the release of energy-generating molecules called ATP. We can also generate energy in the absence of oxygen, but this is a much less efficient process.

ATP

Respiration produces molecules called ATP which are used by parts of the body which energy. ATP is generated in organelles called mitochondria during aerobic respiration. The breakdown of ATP results in the release of energy, which can be used for initiating muscle movement or facilitating biological processes like the absorption of glucose into the bloodstream. A typical person will will use up his or her body weight in ATP over the course of each day (approx 70 kg).

Aerobic vs anaerobic respiration

Aerobic respiration is the breakdown of glucose in the presence of oxygen. If you look at the equation below, you can see it is exactly the same reaction as the combustion of hydrocarbons, for example. This is because we’re basically burning glucose inside our body, which acts as a fuel to release energy, in just the same way as burning petrol provides energy for cars. Aerobic respiration occurs in organelles called mitochondria, which are sort of like the ‘engine-rooms’ of our cells.

Anaerobic respiration is the breakdown of glucose in the absence of oxygen. Humans only carry out anaerobic respiration as a last resort when our blood cannot supply enough oxygen, such as during periods of vigorous exercise. It releases a much smaller amount of ATP compared to aerobic respiration so is a much less efficient process. Muscle cells breakdown glucose into lactic acid, which accumulates in our muscles and must be removed quickly by reacting the lactic acid with oxygen. The amount of oxygen required to remove the lactic acid, and to replace the body’s reserves of oxygen, is referred to as the oxygen debt. This reaction happens in the cytoplasm of our cells.

Anaerobic respiration in yeast

Anaerobic respiration also happens in yeast, but the reaction is slightly different to the reaction that occurs in our muscle. The process is called fermentation and glucose is converted into ethanol and carbon dioxide.

This reaction is harnessed in wine-making and brewing industries, which rely on the production of ethanol by yeast. Bread-making also depends on yeast respiring anaerobically as the production of carbon dioxide causes bread to rise. The ethanol which is produced evaporates off as the bread is baked.

During alcohol production, yeast are grown with sugars in large fermenters. The alcohol content only reaches about 15% because the build-up of ethanol is toxic to yeast, killing them at high concentrations.

Respiration generates heat

Even when we spend a lazy day on the sofa, binge-watching a Netflix series, our bodies are constantly respiring. This is handy since respiration generates heat as a by-product (remember that respiration is basically a combustion reaction) which allows us to maintain our body temperature at a comfortable 37 degrees. Scientists have found that mitochondria themselves are a toasty 10 degrees warmer than the rest of our cells due to the heat released when glucose is burned in oxygen.

Investigating the production of carbon dioxide and heat from living organisms

Carbon dioxide: place respiring organisms e.g. woodlice on top of a piece of gauze within the test tube containing hydrogencarbonate indicator at the bottom. The indicator changes colour from red to yellow when carbon dioxide is present.

Heat: place germinating seeds in a flask and measure the heat produced using a thermometer. Boiled seeds should be used as a control which will not respire as boiling denatures the enzymes in the seed.

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