Respiration…

I often refer to respiration as the most important chemical reaction on the planet (arguable!). Encyclopaedia Britannica defines cellular respiration as…

the process by which organisms combine oxygen with foodstuff molecules, diverting the chemical energy in these substances into life-sustaining processes and discarding, as waste products, carbon dioxide and water

The Oxford English Dictionary divides respiration into its commonly understood meaning of breathing and its biochemical meaning as…

the metabolic process by which energy is generated from food molecules, involving a series of oxidation reactions, ultimately consuming oxygen and also producing carbon dioxide

It can mean breathing, but when we use the word respiration in science lessons we are always referring to cellular respiration – the release of energy from food. When it involves oxygen it is known as aerobic respiration; this process can be summarised by the following word equation.

The energy given off heats up the cell a little and the rest is bound up in the molecule ATP so, if you are feeling a little braver, you might like to write the equation like this.

One molecule of glucose requires six molecules of oxygen for complete oxidation. This produces six molecules of carbon dioxide and six molecules of water. The water can be used in the body but the carbon dioxide must be excreted via the lungs. Red blood cells (and the haemoglobin they contain) are responsible for delivering oxygen to cells and they also carry carbon dioxide away.

One molecule of glucose (C₆H₁₂O₆) can upgrade 36 ADP molecules to 36 ATP energy-rich molecules. ATP enables other reactions within the cell such as building proteins from amino acids, making muscles contract and sending signals via nerves. ATP is a little like a rechargeable battery.

This is the first 4½ minutes or so of a Khan Academy video that eventually goes into monstrous detail about cellular respiration. It does a nice job of summarising what is covered above in its opening third..

The glucose needed for respiration is mostly provided by carbohydrate in the diet. Starch is made from many glucose molecules joined together and is used by plants as a way of storing their glucose produced during photosynthesis. Fat can also be broken down into glycerol, which can then be converted into glucose.

In the absence of oxygen anaerobic respiration occurs which still produces energy (but less of it) and some unpleasant by-products. In yeast alcohol is produced but in humans and other animals lactic acid is made in the muscles.

Questions…

1. What is the name of the energy rich molecule produced during respiration?
2. What are the reactants in aerobic respiration?
3. What are the products of respiration?
4. What molecule transports O₂ and CO₂ around the body?
5. What are the three major components of the diet other than carbohydrate?
6. What catalyst (pigment) enables photosynthesis?
7. Name an amino acid.
8. For what does a gene code?