Digestion, enzymes and the gut
Food molecules are far too large to enter your blood. Digestion is the controlled breakdown that makes them small enough to be absorbed and used.
Part of your national curriculum
- Biology: structure and function: Understand the tissues and organs of the human digestive system, including the role of enzymes and the importance of bacteria in the gut
Lesson overview
What you'll learn in this lesson
Understand the tissues and organs of the human digestive system, including the role of enzymes and the importance of bacteria in the gut
Key learning points
- • The route through the system
- • Enzymes as biological catalysts
- • Bile and surface area
- • Absorption and gut bacteria
This lesson at a glance
- 30 minutes
- 27 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 27
Visual introductionPicture this
Digestion, enzymes and the gut
Food molecules are far too large to enter your blood. Digestion is the controlled breakdown that makes them small enough to be absorbed and used.
In a nutshell
Understand the tissues and organs of the human digestive system, including the role of enzymes and the importance of bacteria in the gut
What you already know
Part 2 of 27
Before we start
What you already know
You should already know the main organs of the digestive system in order, and that food supplies the body with energy and materials.
Key words
Part 3 of 27
Key words
Words you'll need today
- Enzyme
- A protein that speeds up a reaction without being used up.
- Bile
- A liquid made in the liver that breaks fat into droplets.
- Absorption
- Digested food passing from the gut into the blood.
- Villi
- Finger-like folds in the small intestine that increase surface area.
- Catalyst
- Something that makes a reaction faster without changing itself.
Reset break
Part 4 of 27
Pause
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Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 5 of 27
Learning cycle 1 of 2
The journey and the chemistry
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 6 of 27
Learn
The route through the system
Food passes from the mouth down the oesophagus by waves of muscle contraction called peristalsis, into the stomach, then the small intestine where most digestion and absorption occur, then the large intestine where water is reabsorbed. The liver and pancreas are organs that release substances into the system rather than carrying food.
Explore the idea
Part 7 of 27
Learn
Enzymes as biological catalysts
Amylase breaks starch into sugars, protease breaks protein into amino acids and lipase breaks fats into fatty acids and glycerol. Each enzyme has an active site shaped to fit one type of substrate, which is why they are specific. High temperature or the wrong pH changes the active site shape, denaturing the enzyme so it can no longer work.
Reset break
Part 8 of 27
Pause
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Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 9 of 27
Quick check
Part 10 of 27
Learning cycle
Part 11 of 27
Learning cycle 2 of 2
Surface area and absorption
A short piece of teaching, then a check to make sure it has landed.
Reset break
Part 12 of 27
Pause
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Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 13 of 27
Learn
Bile and surface area
Bile is made in the liver, stored in the gall bladder and released into the small intestine. It neutralises stomach acid, giving lipase the alkaline conditions it needs, and emulsifies fat into tiny droplets. Bile is not an enzyme: it increases surface area so lipase can work faster.
Explore the idea
Part 14 of 27
Learn
Absorption and gut bacteria
The small intestine wall is folded into villi with microvilli, giving a huge surface area with a thin wall and a rich blood supply. Helpful gut bacteria break down substances we cannot digest, produce some vitamins and compete with harmful microbes, which is why the balance of gut bacteria affects health.
Quick check
Part 15 of 27
Reset break
Part 16 of 27
Pause
That's sitting 4 of 7 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 17 of 27
Explore the idea
Part 18 of 27
Worked example
Worked answer: explain why bile is needed for the digestion of fats (3 marks)
Bile neutralises the acid arriving from the stomach, creating the slightly alkaline conditions in which lipase works best (1). It emulsifies large fat droplets into many small ones (1), which greatly increases the surface area available for lipase to act on, so fat is digested faster (1). Stating that 'bile digests fat' is wrong: bile contains no enzymes.
Common mix-ups
Part 19 of 27
Common mix-ups
Things people often get wrong
- People often think bile digests fat chemically. Bile only breaks fat into small droplets; enzymes do the chemical work.
- People often think food is absorbed in the stomach. Almost all absorption happens in the small intestine.
Reset break
Part 20 of 27
Pause
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Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 21 of 27
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 22 of 27
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
The liver and pancreas are ____ that release substances into the system rather than carrying food.
Challenge round
Part 23 of 27
Game · Recall cards
What does amylase break down?
Card 1 of 4
Reset break
Part 24 of 27
Pause
That's sitting 6 of 7 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Exit quiz
Part 25 of 27
Exit quiz
Show what you've learned
6 questions, marked together at the end. Nothing is timed.
1. An enzyme is best described as a...
2. Bile works by...
3. Where does most absorption happen?
4. Villi increase the rate of absorption mainly by increasing...
5. The stomach is acidic. One useful effect is that it...
6. The large intestine mainly...
Mastery quiz
Part 26 of 27
Marked quiz
End of lesson quiz: Digestion, enzymes and the gut
4 questions, marked with the reasoning shown. No timer.
1. What does amylase break down?
2. Where does most digestion and absorption occur?
3. What is one benefit of gut bacteria?
4. Which enzyme breaks down protein?
Lesson round-up
Part 27 of 27
Lesson round-up
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Understand the tissues and organs of the human digestive system, including the role of enzymes and the importance of bacteria in the gut