Lesson focus: villusA tiny finger-shaped bump on the lining of the small intestine that helps absorb food. create a large, thin, well-supplied exchange surface that moves different digestion products into blood or lymph.
Explain how small-intestine structure supports absorption and distinguish routes taken by glucose, amino acids and lipids.
Build on earlier learning
The digestive system: organsThe digestive route produces small soluble molecules in the small intestine. This lesson explains how they cross into internal transport systems.
Practical
Modelling absorption with Visking tubing
- Fill Visking tubing with a starch and glucose mixture and stand it in water.
- After 20 minutes test the water for glucose and starch.
- Glucose passes through the tubing; starch molecules are too large.
Record: Test results and how the tubing models the wall of the small intestine.
Diffusion through visking tubing
Visking tubing works like a membrane with tiny holes. Small glucose particles pass through; large starch molecules cannot.
Glucose Starch
Controls
You will be successful when you can…
- Distinguish digestion from absorption.
- Describe villus and microvillus adaptations.
- Explain how blood flow maintains gradients.
- Compare transport of sugars/amino acids with lipid products.
Words that carry the science
- villus
- A finger-like projection of the small-intestine lining that increases surface area.
- microvillus
- A microscopic projection of an epithelial cell membrane that further increases surface area.
- capillary
- A tiny blood vessel with a thin wall close to absorbing cells.
- lacteal
- A lymph vessel inside a villus that receives much absorbed lipid.
- concentration gradient
- A difference in concentration between two regions.
- active transport
- Energy-requiring movement across membranes through transport proteins, potentially against a concentration gradient.
Connect villus adaptations to transport rate
01A folded surface multiplies contact area
The small intestine is long and its lining is folded into villusA tiny finger-shaped bump on the lining of the small intestine that helps absorb food.. Absorbing epithelial cells also carry microvillusA tiny fold on the surface of a cell lining the gut, giving even more surface to absorb food., creating a very large membrane area for transport.
The epithelial barrier is one cell thick, shortening the diffusion path. The surface must remain moist so dissolved molecules can contact cell membranes.
02Blood flow maintains transfer
Each villus has a dense capillaryThe smallest kind of blood vessel, with a wall just one cell thick so substances can pass through. network. Blood carries glucose and amino acids away, helping maintain concentration differences across the epithelium. Some substances also use active transportMoving particles against a concentration gradient, from a low concentration to a high concentration, using energy from respiration. through membrane proteins.
A short path and maintained gradient increase transfer rate, while surface area increases the number of sites available.
03Lipid products take a different initial route
Products of lipid digestion enter epithelial cells and are repackaged into particles that enter the central lactealA small tube inside each villus that takes in digested fats.. Lymph later joins the blood circulation.
This differs from the simplified route for glucose and amino acids, which enter nearby blood capillaries. Water, ions and vitamins follow routes determined by their properties and transport mechanisms.
Explain rapid glucose absorption
Explain how a villus supports rapid movement of glucose into blood.
- Villi and microvilli provide a large membrane surface area.
- The epithelium is one cell thick, creating a short transport distance.
- Glucose crosses using membrane transport processes, including diffusion and active transport depending on conditions.
- A dense capillary supply carries glucose away and helps maintain gradients.
How to make the reasoning visible: Several adaptations act together; “large surface area” alone is not a complete explanation.
Match structures to causal effects
For each feature, state the direct change and the transport consequence.
| Feature | Direct effect | Transport consequence | Main destination |
|---|---|---|---|
| Many villi | Larger tissue area | More absorbing surface | Blood and lymph |
| Microvilli | Larger cell-membrane area | More transport sites | Across epithelial cells |
| One-cell epithelium | Shorter pathway | Faster exchange | Underlying vessels |
| Dense capillaries | Rapid removal of products | Maintains gradients | Blood |
| Central lacteal | Lymph route available | Carries repackaged lipid | Lymph |
Why does blood flow increase continued glucose absorption?
A diagram labels a villus “one cell thick”. Correct the wording.
Check your understanding
Answer from memory, then check against the suggested answer.
What is a villus?
What does a lacteal carry?
Why is a thin epithelium useful?
Coeliac disease can flatten villi. Predict how this may affect nutrition.
Explain why active transport can continue when the concentration gradient for a nutrient is small.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
The small intestine is lined with villi.
Describe two features of a villus that make absorption fast.
(2)Explain how the blood supply and the lacteal help absorption.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., pair each structural feature with its direct effect on transport.