ScienceBreathing and digestion30 min★★★ difficulty

Digestion, enzymes and gut bacteria

Food molecules are too big to enter your blood. Digestion is the process of cutting them down, mostly using enzymes, and finishing with help from trillions of bacteria.

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

• Mechanical then chemical
• Conditions matter
• Absorption in the small intestine
• Bacteria doing useful work

This lesson at a glance

30 minutes
19 parts to scroll through
4 quick checks
Marked quiz at the end
Gentle pace: short sittings with pauses

Words to know

tissuesorgansdigestivesystemenzymes

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%

Watch & discover

Part 1 of 19

Watch first

Digestion, enzymes and gut bacteria

An English teaching video on this topic from Amoeba Sisters. Watch it, then scroll down for the lesson.

Do it for real

The practicals below use what the film shows, with a full method you can follow at home.

How fast does exercise change your breathing?Does chewing start digestion before you swallow?

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 19

Before we start

What you already know

You should know food travels from the mouth through the stomach and intestines.

Recap: Breathing, gas exchange and what damages it

Key words

Part 3 of 19

Key words

Words you'll need today

Enzyme: A biological catalyst that speeds up a reaction without being used up.
Substrate: The molecule an enzyme acts on.
Villi: Finger-like folds of the small intestine that increase absorption surface.
Bile: A liquid from the liver that emulsifies fats into droplets.
Microbiome: The community of bacteria living in the gut.

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

The journey and the enzymes

A short piece of teaching, then a check to make sure it has landed.

Explore the idea

Part 5 of 19

Learn

Mechanical then chemical

Teeth and the churning stomach break food up physically, which increases surface area. The chemical work is done by enzymes: amylase on starch, proteases on protein, lipases on fat. Each enzyme has an active site that fits one shape of substrate, which is why one enzyme cannot do another's job.

Explore the idea

Part 6 of 19

Learn

Conditions matter

The stomach is about pH 2, which suits protease and kills many microbes. The small intestine is slightly alkaline, so bile from the liver both neutralises stomach acid and emulsifies fat into small droplets, increasing surface area for lipase. Change the pH too far and enzymes denature: the active site changes shape and stops working.

Quick check

Part 7 of 19

Quick check

Amylase digests...

Quick check

Part 8 of 19

Quick check

Bile helps digestion by...

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 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.

Stop here for now

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Absorption and the microbiome

A short piece of teaching, then a check to make sure it has landed.

Explore the idea

Part 11 of 19

Learn

Absorption in the small intestine

Small soluble molecules - glucose, amino acids, fatty acids - are absorbed through the wall of the small intestine, which is about six metres long and lined with villi and microvilli. That gives a surface area of roughly a tennis court, with capillaries right behind it. The large intestine then absorbs water.

Explore the idea

Part 12 of 19

Learn

Bacteria doing useful work

The gut holds trillions of bacteria. They break down fibre we cannot digest, make some vitamins including K and several B vitamins, and occupy space that harmful microbes would otherwise fill. Antibiotics can disturb this community, which is one reason they can cause stomach upsets.

Quick check

Part 13 of 19

Quick check

Villi increase the rate of absorption because they...

Quick check

Part 14 of 19

Quick check

Gut bacteria are important because they...

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

Enzymes are not living and are not used up by the reaction.
A denatured enzyme is not 'killed' - its active site has changed shape.
Bile is not an enzyme; it emulsifies fat and neutralises acid.
Most absorption is in the small intestine, not the stomach.

Challenge round

Part 16 of 19

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 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

The chemical work is done by ____: amylase on starch, proteases on protein, lipases on fat.

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Digestion, enzymes and gut bacteria

6 questions, marked with the reasoning shown. No timer.

1. An enzyme is a...
2. Stomach pH is about...
3. Denaturing an enzyme changes its...
4. Most water is absorbed in the...
5. Proteases digest...
6. Why must food molecules be broken down?

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

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Games

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Points this lesson

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

Name the variable the club changed the second time, and one variable they kept the same.
Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
Write a conclusion for the club's report, saying how far the evidence supports it.

Method and results
Writing
Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.
1 page + results table
Conclusion
Writing
Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.
6–8 sentences
Labelled diagram
Diagram
Draw the apparatus or the process in pencil, label it fully, and photograph it.
1 labelled diagram
Reading and notes
Reading
Read the lesson text again and write the key ideas in your own words.
Half a page of notes

Write your answer, notes or working here

Saved as you type.

Photos of paper work, diagrams or working outHow many minutes did the written work take?

Part 1 of 19 · Watch & discover

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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