How does the body break down food and absorb the nutrients it needs?
Describe the human digestive system, explain why large food molecules must be digested, and outline how digested food is absorbed in the small intestine
A focused N(A)-Level answer on digestion. The parts of the digestive system, why large molecules must be broken down, the role of enzymes, and absorption in the small intestine.
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What this dot point is asking
SEAB wants you to describe the parts of the human digestive system, to explain why large food molecules must be digested, and to outline how the small intestine absorbs the digested food. The central idea is that digestion breaks large insoluble food molecules into small soluble ones that can pass into the blood.
The answer
The parts of the digestive system
Food passes through a long tube called the gut, helped by several organs:
- mouth: teeth break food up; saliva starts to digest starch,
- stomach: muscles churn the food and acid and enzymes digest protein,
- small intestine: digestion is completed and the small soluble molecules are absorbed into the blood,
- large intestine: water is absorbed from the leftover material,
- liver and pancreas: produce juices and enzymes that help digestion.
Why food must be digested
Many foods contain large molecules, such as starch, proteins and fats. These molecules are too big and insoluble to pass through the gut wall into the blood. Digestion breaks them down into small, soluble molecules that can be absorbed:
- starch is broken down into glucose,
- proteins are broken down into amino acids,
- fats are broken down into fatty acids and glycerol.
The role of enzymes
Enzymes are biological catalysts that speed up the breakdown of food. Different enzymes work on different foods: one type breaks down starch, another breaks down proteins, and another breaks down fats. They work fast at body temperature.
Absorption in the small intestine
Once food is digested into small soluble molecules, it is absorbed in the small intestine. The small intestine is well adapted for this:
- it is very long, giving a large surface area,
- its lining is folded into many tiny finger-like villi that greatly increase the surface area,
- the villi have very thin walls and a rich blood supply, so absorbed molecules pass quickly into the blood.
Examples in context
Example 1. Why a marathon runner takes glucose, not starch, mid-race. Glucose is already a small, soluble molecule, so it can be absorbed into the blood quickly for fast energy. Starch would first need to be digested, which takes time, so it is less useful during a race.
Example 2. How the surface area idea appears elsewhere. The villi increase surface area for absorption in the gut, just as the lungs use many tiny air sacs to increase surface area for gas exchange. A large surface area for exchange is a pattern seen across many body systems.
Try this
- Cue. Name the small molecule that proteins are digested into. Amino acids.
- Cue. Explain why starch must be digested. It is too large and insoluble to be absorbed through the gut wall.
- Cue. State one adaptation of the small intestine for absorption. A large surface area from its length and many villi (with thin walls and a good blood supply).
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marks(a) Explain why large food molecules such as starch must be digested before the body can use them. (b) Name the simple sugar that starch is broken down into.Show worked answer →
(a) Large molecules such as starch are too big to be absorbed through the wall of the small intestine into the blood. Digestion breaks them into small, soluble molecules that can pass through and be absorbed.
(b) Starch is broken down into glucose.
What markers reward: large molecules being too big to be absorbed, digestion making small soluble molecules that can pass into the blood, and glucose as the product.
Original3 marks(a) Name the organ where most absorption of digested food takes place. (b) Describe one way this organ is adapted for absorption.Show worked answer →
(a) The small intestine is where most digested food is absorbed.
(b) Any one of: it is very long, giving a large surface area; its lining is folded into many tiny villi that increase the surface area; the villi have thin walls and a good blood supply for fast absorption.
What markers reward: the small intestine named, and a correct adaptation that increases surface area or speeds absorption.
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