KS3 Biology › Human digestive system
KS3 Biology › Human digestive system
These are the points the worksheet questions are built around.
Blood is a liquid, and only things that dissolve can travel in it — so before food is any use to the body, it has to be broken down from large, insoluble pieces into small, soluble nutrients that can pass into the blood and be used for energy, growth and repair. This whole process is digestion, and the body does it in two completely different ways at once: one physical, one chemical.
Mechanical digestion is the physical breakdown of food — cutting, tearing, grinding and churning, with no chemistry involved. It starts the moment you bite: teeth slice and grind the food, the tongue rolls it into a ball, and the stomach's muscular walls squeeze and churn it into a thick liquid soup. Nothing has been chemically changed at this point — the food has simply been smashed into smaller pieces. This matters because of surface area: a whole apple only exposes its skin, but chop it into a hundred pieces and there's suddenly a huge amount of surface exposed. Chemicals can only work on food where they can actually touch it, so mechanical digestion sets the job up rather than finishing it — which is also why swallowing food half-chewed slows everything down further along.
Chemical digestion uses chemicals to break the bonds holding large food molecules together, and it happens in three places: the mouth, where saliva (produced at a rate of 1–1.5 litres a day) starts working before you've even swallowed; the stomach, which adds acid and further chemicals; and the small intestine, where most of the chemical work is finished. The chemicals doing this breaking-down work are called enzymes.
Food is mostly made of three things — carbohydrates, proteins and fats — all large, insoluble molecules that need dismantling. Starch, found in potatoes, pasta, rice and bread, is a long chain of hundreds of sugar molecules linked together. Digestion snips that chain apart into individual sugars like glucose, which are small and soluble enough to be absorbed.
Proteins, found in meat, fish, eggs, nuts and beans, work the same way but with different building blocks called amino acids. Digestion breaks a protein down into its individual amino acids, which the body then reuses, joining them back together in different orders to build the proteins it actually needs — muscle, skin, hair, and more.
Fats (or lipids), found in butter, oils, cheese and meat, are the odd one out, since a single fat molecule splits into two different products rather than one: fatty acids and glycerol, both of which get absorbed.
Neither mechanical nor chemical digestion could do this job alone. Mechanical digestion without chemical digestion just produces very small pieces of food that still can't be absorbed. Chemical digestion without mechanical digestion means chemicals working on a huge lump with barely any surface to attack, making everything painfully slow. Together, teeth and stomach create a soup with enormous surface area, and chemicals then swarm across that surface to break the molecules apart — insoluble food goes in, soluble nutrients come out. The whole journey, from the first bite to the end of the digestive system, takes most people somewhere between one and two days.
In short: digestion breaks down insoluble food into soluble nutrients through two processes working together — mechanical digestion (physical breakdown, increasing surface area) and chemical digestion (using enzymes to break molecular bonds). Carbohydrates become sugars, proteins become amino acids, and fats become fatty acids and glycerol — three food groups, three sets of small, soluble nutrients ready for the blood to carry to every cell in the body.
These are the words that appear as key term cards in the video — worth knowing cold before the test.
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The 3 questions from the video, one at a time. See if you caught them.
1. Which of these is an example of mechanical digestion?
Why: Mechanical digestion is physically breaking food into smaller pieces, like the stomach churning it. The other examples are chemicals or enzymes breaking food down, which is chemical digestion.
2. Which of these is NOT broken down by digestion?
Why: Digestion breaks big food molecules like starch, protein and fat into small ones. Water is already a small molecule, so it doesn't need to be broken down.
3. Roughly how much saliva does an adult make in a day?
Why: Your salivary glands make about one and a half litres of saliva every day. It moistens food so you can swallow it, and contains an enzyme that starts breaking down starch.
Score: 0 out of 3 —
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This lesson's quiz questions on one sheet, with the answer key on page 2.
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