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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Digestive Enzymes
Digestive enzymes are special proteins that help break down the food we eat into smaller molecules that our bodies can absorb and use. Without these enzymes, we wouldn't be able to get nutrients from our food, no matter how much we ate!
Key Definitions:
The food we eat contains large, complex molecules that are too big to pass through the walls of our intestines and into our bloodstream. Digestive enzymes break these large molecules down into smaller ones that can be absorbed. For example, proteins are broken down into amino acids, carbohydrates into simple sugars and fats into fatty acids and glycerol.
Enzymes work using the 'lock and key' model. Each enzyme has a specific shape that only fits certain substrates - just like a key fits only one lock. When the substrate enters the active site of the enzyme, a reaction occurs that breaks the substrate down. The enzyme then releases the smaller products and is ready to work on another substrate molecule.
There are three main types of digestive enzymes, each responsible for breaking down a different type of food molecule:
Substrate: Starch (carbohydrates)
Products: Maltose and eventually glucose
Where produced: Salivary glands and pancreas
Where active: Mouth and small intestine
Substrate: Proteins
Products: Amino acids
Where produced: Stomach (as pepsin), pancreas (as trypsin)
Where active: Stomach and small intestine
Substrate: Lipids (fats and oils)
Products: Fatty acids and glycerol
Where produced: Pancreas
Where active: Small intestine
Let's follow the journey of food through your digestive system to see how different enzymes work at each stage:
When you eat something starchy like bread or potatoes, digestion begins right away in your mouth. Your salivary glands produce salivary amylase, which starts breaking down starch into maltose (a type of sugar). This is why if you hold a piece of bread in your mouth for a while, it starts to taste sweet!
The food doesn't stay in your mouth long enough for complete digestion, but the process has begun. The food, now called a bolus, is swallowed and travels down the oesophagus to the stomach.
In the stomach, gastric juice contains hydrochloric acid (HCl), which creates a very acidic environment (pH 1-2). This acid doesn't digest food itself but activates an enzyme called pepsinogen by converting it to pepsin.
Pepsin is a type of protease that begins breaking down proteins into smaller chains called peptides. The salivary amylase from the mouth stops working in the stomach because it can't function in such acidic conditions.
After a few hours in the stomach, the food has been turned into a liquid called chyme, which then moves into the small intestine.
The small intestine is where most digestion happens. When chyme enters the small intestine, it triggers the release of several digestive enzymes:
Some people don't produce enough of the enzyme lactase, which breaks down lactose (milk sugar). This condition is called lactose intolerance. When these people consume dairy products, the lactose passes undigested into the large intestine, where bacteria ferment it, causing symptoms like bloating, gas and diarrhoea.
Lactose intolerance affects about 65% of the global population, with rates varying by ethnic group. It's particularly common in people of East Asian, West African, Arab, Jewish, Greek and Italian descent. People with lactose intolerance can take lactase supplements before consuming dairy or choose lactose-free dairy products.
Digestive enzymes, like all enzymes, are affected by their environment. Two key factors are:
Enzymes work best at their optimum temperature, which for human digestive enzymes is around body temperature (37°C). At lower temperatures, enzyme activity slows down. At higher temperatures (above about 40°C), enzymes start to denature (change shape), which means their active site no longer fits the substrate. Once denatured, enzymes can't function.
Each enzyme has an optimum pH where it works best. For example:
After digestive enzymes have broken down food into small molecules, these nutrients are absorbed through the wall of the small intestine into the bloodstream:
The small intestine is specially adapted for absorption with millions of tiny finger-like projections called villi, which increase the surface area. Each villus contains blood capillaries and a lacteal.
Your pancreas produces about 1.5 litres of digestive juices every day! This includes all the pancreatic enzymes needed to digest your food. Without this amazing enzyme factory, you wouldn't be able to get nutrients from your meals.
The digestive enzymes in your body are constantly being produced and recycled. In fact, most of the amino acids used to make new digestive enzymes come from old enzymes that have been broken down - it's a very efficient system!
Digestive enzymes are essential for breaking down the food we eat into nutrients our bodies can use. Each enzyme is specialized to break down a specific type of food molecule and works in a specific part of the digestive system where conditions like pH are optimal for its function.
The main digestive enzymes are:
Together, these enzymes ensure that the complex food we eat is converted into simple molecules that can be absorbed into our bloodstream and used by our cells for energy, growth and repair.
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