In this series of ten blogs, various parts of the GCSE Science, IGCSE Biology specifications will be explored through the context of food. As well as assisting students revising for their GCSE and IGCSE exams, the blogs also provide an every-day context for science which all readers should find accessible, interesting and useful.
Traditionally a marathon runner will eat a large meal containing complex carbohydrates the evening before running a race. This will hopefully contain enough energy to keep the runner going for several hours and avoid them ‘hitting the wall’. In this blog, we will look at how the body works to digest a meal and then release the energy when running.
When we eat a large bowl of pasta, several baked potatoes or a huge serving of rice, the enzymes in our digestive system get to work. Amylase in saliva starts to break the complex carbohydrates down into a sugar molecule called maltose as soon as chewing begins. This process continues down into the small intestine, and a second enzyme, maltase, joins in to break maltose down into glucose. Glucose is very soluble and passes through the walls of the small intestine into the blood. Large amounts of glucose in the blood aren’t needed (and can be dangerous) and so the body has a mechanism to store the glucose for future use between meals.
A hormone called insulin is made in the pancreas when glucose levels increase after a meal. Insulin causes glucose to be stored in its insoluble form, glycogen, in the liver. You can store enough glycogen to sustain you for about 2 hours’ exercise, but with training (so-called carbo-loading) this can be increased – useful for that marathon!
When the gun fires to start the marathon, the glucose present in the blood starts to get used in respiration – releasing energy to make the muscles work. Increased heart rate delivers glucose more rapidly to the muscles and faster breathing increases oxygen intake. As glucose levels start to drop in the blood, insulin’s partner hormone called glucagon is released from the pancreas and it gets to work converting glycogen in the liver back into soluble glucose so respiration can continue at the high rate needed for maximum aerobic respiration.
Top class marathon runners will have enough stored glycogen to see them through the whole race, but those who have trained less (and may be wearing fancy dress) will need to take on extra carbohydrates during the race, or run much more slowly to avoid ‘hitting the wall’ – the term used to describe what happens when glycogen stores are severely depleted; from personal experience not pleasant!