GCSE/iGCSE Biology - Year 1 Week 10
- Sallyann Clark
- 3 days ago
- 8 min read
The Long Journey of a Sandwich
What Really Happens to Food Once You Swallow It?
If you want to use the templates to help you record your work, download them now and print them out. They may not fit perfectly with every week, and you may need extra pages to record some answers, especially when you need to draw a diagram or graph.
The Big Question This Week
"What actually happens to a piece of bread between the moment you swallow it and the moment your body can use it for energy?"
Write this in your learning journal as a title before you begin. Don't look anything up yet — just think. If you prepared this question at the end of Week 9, go back and read what you wrote.
Before You Begin — Predict!
Write down whether you think these are true or false, and why:
"Digestion is mostly about physically grinding and squashing food into smaller pieces."
"Nutrients are absorbed into the bloodstream mainly in the stomach, since that's where digestion happens."
Hold onto your predictions. We will return to them at the end of the week.
Story of the Week
A slice of bread doesn't look like much of a puzzle. But biologically, it's a locked box. The starch in that bread is a huge, tangled molecule your cells cannot use directly, it has to be broken down into small enough pieces to cross into your bloodstream and reach your cells. That's the entire job of your digestive system: take enormous, complex molecules from food such as starches, proteins, fats, and break them down into small, simple molecules small enough to absorb.
Your digestive system does this in two different ways at once. There's mechanical digestion — the physical breaking-up of food by chewing, and the churning and squeezing of muscles along the digestive tract, which increases surface area but doesn't actually change any chemical bonds. And there's chemical digestion, enzymes, like the ones you met back in Week 7, chopping large molecules apart into their smaller building blocks. Both are essential, and they happen together, starting the moment food enters your mouth and continuing all the way through a tube roughly nine metres long, coiled up inside your abdomen.
This week we follow that journey from mouth to small intestine, meet the specific enzymes responsible for breaking down carbohydrates, proteins, and fats, and test some real food samples in the kind of experiment biologists have used for over a century to find out exactly what's inside our meals.
Video Session One
While you watch, look out for:
The four main stages of digestion: ingestion, digestion, absorption, and elimination
What happens chemically and mechanically in the mouth, stomach, and small intestine
What villi are, where they're found, and why their shape matters so much for absorption
Draw a simple diagram of the digestive system in your journal (a mouth, tube, stomach, small intestine, and large intestine is enough) and label each stage of digestion at the correct point along it.
Video Session Two
(Note: this comes from the original 2012 Crash Course Biology series with Hank Green, rather than the newer 2023 Dr. Sammy Ramsey series, because the 2023 series covers digestion only briefly as part of a broader episode on waste. This older episode goes into far more useful GCSE-level detail.)
While you watch, look out for:
Why the phrase "it's all about surface area" comes up again and again in this episode
The specific roles of the liver, gallbladder, and pancreas as accessory organs of digestion
How enzymes from different parts of the digestive tract work on different food groups
⚠️ Common Misconception Alert
Many students assume most nutrient absorption happens in the stomach, since that's where food spends a lot of time being "digested." In fact, the stomach mostly breaks food down chemically and mechanically into a substance called chyme; very little absorption of nutrients actually happens there. The overwhelming majority of nutrient absorption happens further along, in the small intestine, where millions of tiny finger-like villi create an enormous surface area for nutrients to pass into the bloodstream.
Here is a good place to stop if you need to.
Reading Assignment
Booklet Readings 16 and 17 accompany this week, with more detail on enzyme specificity and a story about a very unusual scientific opportunity that came from a hunting accident.
Hands-On Activity — Required Practical: Food Tests
What you need: Small samples of several foods to test (e.g. milk, bread, an egg white, vegetable oil, a potato or apple), Benedict's solution, iodine solution, Biuret solution (or Biuret reagent alternative), ethanol, test tubes, a water bath or beaker of hot water, and adult supervision for the heating steps.
What to do:
Test for starch: Add a few drops of iodine solution directly to a small sample of each food. A colour change from browny-orange to blue-black indicates starch is present.
Test for reducing sugars: Add a sample of food (mashed or dissolved in water if solid) to a test tube with Benedict's solution, and heat in a water bath for a few minutes. A colour change from blue through green, yellow, and orange to brick-red indicates reducing sugar is present, and the specific colour reached gives a rough idea of how much sugar is there.
Test for protein: Add Biuret solution (or the sodium hydroxide plus copper sulfate alternative) to a sample of food. A colour change from blue to purple/lilac indicates protein is present.
Test for lipids (fats): Mix a sample of food with ethanol, shake, then pour the mixture into water. A cloudy white emulsion forming indicates lipid is present.
Record your results in a table (food tested, iodine result, Benedict's result, Biuret result, emulsion test result).
In your journal, write two sentences summarising which of your tested foods contained which nutrients, and note any surprises.
You can watch the practical here with Mt Exham.
J+Here is a good place for a break!
The Brilliant Mistake — Stories Science Gets Wrong
This Week's Story: The Stomach That Was Thought to Be a Millstone
For a very long time, scientists debated what digestion actually was. One influential idea, championed by figures including the French physicist René Réaumur in the early 1700s, was tested directly: was the stomach purely a mechanical grinder, crushing food the way a millstone crushes grain? Réaumur devised a clever experiment using birds of prey, feeding them meat inside small perforated metal tubes that the birds could not crush or digest mechanically. When the tubes were later retrieved, the meat inside had partly dissolved, showing that something chemical, not mechanical grinding, was breaking it down.
The Italian scientist Lazzaro Spallanzani built on this in the 1780s with an even bolder version of the same idea. Spallanzani swallowed small linen sponges tied to a string, retrieved them after they had spent time in his own stomach, and squeezed out and analysed the liquid soaked into them. He confirmed that the stomach produced a genuine chemical fluid, gastric juice, capable of dissolving food even outside the body, in a test tube, with no grinding or squashing involved at all.
Even so, exactly how gastric juice worked chemically remained mysterious for decades longer, until an American army surgeon named William Beaumont got an extraordinary, if grim, opportunity in 1822. His patient, a young fur trapper named Alexis St. Martin, survived a gunshot wound to the stomach that left a permanent open passage, a fistula, directly into his stomach, which never fully healed closed. Beaumont realised he could literally watch digestion happening in real time. Over the following years, he ran hundreds of careful observations and experiments, lowering food on a string directly into St. Martin's stomach and timing exactly how long different foods took to dissolve, building the first detailed, evidence-based understanding of how gastric digestion actually works.
Journal Prompt:
Why might "grinding" have felt like an obvious, common-sense explanation for digestion before Réaumur and Spallanzani's experiments?
Spallanzani used his own body as an experimental tool. What are the advantages and the ethical complications of a scientist experimenting on themselves?
William Beaumont's most important data came from a situation he never could have planned or ethically created on purpose. What does this tell us about the role of chance and circumstance in scientific discovery?
Scientist Portrait
William Beaumont (1785–1853)
Add William Beaumont to your Scientist journal. Draw or print a portrait. Beneath it, write: "An extraordinary, unplanned window into a living stomach let him build the first real evidence-based map of digestion."
Return to Your Predictions
Go back to the two statements from the start of the week:
"Digestion is mostly about physically grinding and squashing food into smaller pieces."
"Nutrients are absorbed into the bloodstream mainly in the stomach, since that's where digestion happens."
Rewrite each one now, using what you've learned this week.
The Big Question — Final Answer
Return to this week's big question:
"What actually happens to a piece of bread between the moment you swallow it and the moment your body can use it for energy?"
Write a paragraph in your learning journal. A strong answer will mention both mechanical and chemical digestion, name at least one relevant enzyme, and explain where absorption mainly happens.
Practice Questions — 10 Question Set
Name the four main stages of digestion, in order.
What is the difference between mechanical and chemical digestion? Give one example of each.
Name the enzyme that breaks down starch, and the substances it breaks it down into.
Which organ produces bile, and what is bile's role in digestion? (Note: bile is not itself an enzyme.)
Explain why villi in the small intestine are well adapted for absorbing nutrients. Give at least two features.
In the food tests practical, what colour change would indicate a positive test for starch?
In the food tests practical, what colour change would indicate a positive test for reducing sugar?
Réaumur's bird-of-prey experiment showed that meat inside a perforated, uncrushable tube still partly dissolved. What did this demonstrate about digestion?
Explain why William Beaumont's patient, Alexis St. Martin, gave him an unusually direct way to study digestion.
A student says, "The stomach is where most nutrients get absorbed into the blood." Explain why this statement is incorrect, and state where most absorption actually happens.
Week 10 Checklist
Before moving to Week 11, tick off each item:
Watched Amoeba Sisters: Digestive System
Completed the digestive system diagram in your journal
Watched Crash Course Biology (classic series): The Digestive System
Completed the food tests practical and recorded your results
Read the Réaumur/Spallanzani/Beaumont story and answered the journal prompts
Added William Beaumont to your Scientist journal
Rewritten your two predictions from the start of the week
Answered the 10 practice questions
Written a final answer to the Big Question
Looking Ahead — Week 11 Preview
Next week we look at how your cells actually release the energy locked inside glucose, a process happening in every single one of your cells, right now, whether or not you're thinking about it. We'll compare what happens when oxygen is available with what happens when it isn't, and look at why your muscles feel the way they do during hard exercise.
Prepare by writing this question in your journal: "If glucose contains energy, how does your body actually get that energy out of it and into a form it can use?"
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