Enzymes and digestion
Digestion breaks large food molecules into small ones that can pass into the blood. Enzymes do most of that work, and how they behave at different temperatures and pH values is a favourite exam topic.
Part 1 of 3: Learn it
In short
- Cells make tissues, tissues make organs, and organs work together as organ systems.
- Enzymes are proteins that speed up reactions; each one fits only its own substrate.
- Carbohydrases, proteases and lipases break down carbohydrates, proteins and lipids.
Where this is in your specification
Spec points: AQA 4.2.1 and 4.2.2.1 (Combined Trilogy 4.2.1 and 4.2.2.1), Edexcel Topic 1, OCR B1.4
| Board | Topic: Organisation: tissues, organs, digestion and circulation |
|---|---|
| AQA 8461 | 4.2 (8461); 4.2 (8464) |
| Edexcel 1BI0 | Topics 2 and 8 |
| OCR J247 | B2 and B3 |
| Eduqas C400 | 1.3, 2.2, 2.3 |
| Cambridge IGCSE 0610 | 4.1, 5.1, 7.1 to 7.5, 8.1 to 8.4, 9.1 to 9.4, 11.1 |
| National 5 C807 75 | Cell biology: 4 Proteins; Multicellular organisms: 5 Transport systems, 7 Absorption of materials |
Levels of organisation
A tissue is a group of similar cells doing one job, such as muscle tissue. An organ is made of several tissues, such as the stomach. An organ system is a group of organs working together: the digestive system includes the mouth, oesophagus, stomach, liver, pancreas, small intestine and large intestine.
How enzymes work
An enzyme is a biological catalyst. Its active site has a shape that matches one substrate, like a key in a lock. The substrate binds, the reaction happens, and the products are released, leaving the enzyme unchanged and ready to work again.
Warming increases the rate at first because molecules collide more often. Above the optimum temperature (about 37 °C for human enzymes) the shape of the active site is changed for good, so the substrate no longer fits. The enzyme is then denatured. pH far from the optimum distorts the shape in the same way.
The digestive enzymes
| Enzyme | Breaks down | Into | Made in |
|---|---|---|---|
| Carbohydrase, such as amylase | starch | sugars | salivary glands, pancreas, small intestine |
| Protease | proteins | amino acids | stomach, pancreas, small intestine |
| Lipase | lipids (fats and oils) | fatty acids and glycerol | pancreas, small intestine |
Bile
- Bile is made in the liver and stored in the gall bladder, then released into the small intestine.
- It is alkaline, so it neutralises the acid from the stomach, giving the small intestine enzymes a better pH.
- It emulsifies fats into small droplets, which gives lipase a much larger surface area to work on.
What does a protease break proteins into?
Show the answer
Amino acids.
Part 2 of 3: See it worked
Worked examples
Example 1
In an amylase experiment, the starch had all gone after 50 s. Work out the rate of reaction.
- Rate = 1 ÷ time
- Rate = 1 ÷ 50
Answer: 0.02 per second (s⁻¹).
Example 2
Explain why an enzyme from the human gut works much more slowly at 60 °C than at 37 °C.
- At 60 °C the bonds holding the enzyme's shape break
- The shape of the active site changes
- The substrate can no longer bind, so fewer reactions happen
Answer: The enzyme is denatured: its active site has lost the shape that fits the substrate.
Common mistakes
- Saying high temperatures kill enzymes.
- Saying bile is an enzyme or that it digests fat. It emulsifies fat, so lipase can work faster.
- Mixing up where enzymes are made: amylase is not made in the stomach.
- Saying the substrate and active site are the same shape. They are complementary: they fit together.
Give two jobs of bile.
Show the answer
It neutralises stomach acid and emulsifies fats into small droplets.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What does a protease break proteins into?
Amino acids.
Give two jobs of bile.
It neutralises stomach acid and emulsifies fats into small droplets.
What is an organ system?
A group of organs working together to do a particular job, such as digestion.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
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