In June 1822, on Mackinac Island in what is now Michigan, a young fur-trade worker called Alexis St. Martin was shot by accident at close range. William Beaumont, a US Army surgeon, treated him. St. Martin survived, but the wound healed with an opening that led straight into his stomach.
Over the following years Beaumont used that opening for experiments. He lowered pieces of food on a silk string into the stomach and pulled them out to see how far they had been digested. He also drew off gastric juice and showed that, kept warm in a vial, it could digest food outside the body. His book came out in 1833. Today historians also question the ethics: St. Martin was poor, worked for Beaumont, and had little real choice about taking part.
Beaumont was looking at chemical digestion. The chemicals that do most of the work are enzymes. This lesson covers the digestive system and enzymes for AQA GCSE Biology Topic 2 (Organisation). It's also useful for Combined Science. The food tests and the amylase pH practical have their own decks.
The digestive system as an organ system
Cells make tissues, tissues make organs, and organs work together in organ systems. The digestive system is one example: the mouth, stomach, pancreas, liver, gall bladder, small intestine and large intestine all play a part in digesting food and absorbing it.
Why digest at all? Most food is made of large, insoluble molecules such as starch, proteins and fats. They can't pass into the blood. Digestive enzymes convert them into small, soluble molecules that can be absorbed in the small intestine.
Enzymes
An enzyme is a biological catalyst: a large protein molecule that speeds up a reaction without being used up. Each enzyme catalyses one specific reaction, because of the shape of its active site. In the lock and key model, the substrate fits the active site like a key fits one lock. A refinement, the induced fit model, says the active site changes shape slightly as the substrate binds.
Enzymes control metabolism, the sum of all the reactions in a cell or the body.
The three groups of digestive enzymes
| Enzyme | Breaks down | Into | Made in |
|---|---|---|---|
| Amylase (a carbohydrase) | Starch | Sugars | Salivary glands, pancreas, small intestine |
| Proteases | Proteins | Amino acids | Stomach, pancreas, small intestine |
| Lipases | Lipids (fats) | Glycerol and fatty acids | Pancreas, small intestine |
The products are used to build new carbohydrates, lipids and proteins, and some glucose is used in respiration.
Bile and stomach acid
The stomach makes hydrochloric acid. The acid suits the stomach's protease and kills many microorganisms in food.
Bile is made in the liver, stored in the gall bladder and released into the small intestine. It does two jobs:
- It is alkaline, so it neutralises the acid coming from the stomach.
- It emulsifies fat into small droplets, giving a larger surface area.
Both speed up the breakdown of fat by lipase. Bile is not an enzyme.
Temperature and enzymes
As temperature rises, molecules move faster and collide more often, so the rate increases up to the optimum temperature. Above it, the active site changes shape and the enzyme is denatured, so the rate drops fast. Cold slows an enzyme down but does not denature it. (The effect of pH is in the enzymes practical deck.)
Common exam mistakes
- "Bile digests fat." Bile emulsifies fat; lipase digests it.
- "Amylase breaks down proteins." Match each enzyme to its own substrate.
- "Lipase makes fatty acids and amino acids." Lipids give glycerol and fatty acids; amino acids come from proteins.
- "The enzyme died." Enzymes aren't alive. Say the active site changed shape.
- "Low temperature denatures enzymes." It only slows them.
Frequently asked questions
What enzymes are involved in digestion for GCSE?
Carbohydrases such as amylase, proteases and lipases. They break down starch, proteins and fats into sugars, amino acids, and glycerol and fatty acids.
Is bile an enzyme?
No. Bile neutralises stomach acid and emulsifies fat, which helps lipase work faster, but it doesn't break down fat itself.
Where is amylase produced?
In the salivary glands, the pancreas and the small intestine.
Why do enzymes stop working at high temperatures?
The active site changes shape, so the substrate no longer fits. The enzyme is denatured, and cooling it down doesn't reverse this.
Try it
Say each answer before you flip the card, then fill in the enzyme table from memory. Then put yourself in Beaumont's place: you have a vial of gastric juice that digests meat outside the body. What would you have tested next?
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