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Key concept
Villi are tiny finger-like projections lining the small intestine that greatly increase the surface area for absorption. They have thin walls (one cell thick), a rich blood supply to carry absorbed nutrients away, and lacteals for absorbing fatty acids and glycerol.
The small intestine is the main site of nutrient absorption. Its inner surface is covered with millions of villi, each maximising the efficiency of absorption. This is a classic "structure adapted for function" topic: expect to explain how each adaptation increases absorption rate.
Adaptations of Villi for Absorption
Adaptation
How it increases absorption
Many villi (finger-like projections)
Massively increase the surface area of the small intestine wall: more area for absorption to occur.
Microvilli on the surface of each villus cell
Further increase the surface area at the cellular level (the "brush border").
Very thin walls: one cell thick
Short diffusion distance: nutrients pass quickly from the intestine into the blood.
Dense network of blood capillaries
Maintains a steep concentration gradient by constantly carrying absorbed nutrients away.
Lacteal (lymph vessel inside each villus)
Absorbs fatty acids and glycerol into the lymphatic system.
Good blood supply
Brings oxygen for active transport and carries absorbed nutrients to the liver via the hepatic portal vein.
How Nutrients Are Absorbed
Glucose and amino acids are absorbed by a combination of diffusion and active transport:
Initially, concentration is higher in the intestine → diffusion.
As concentration equalises, active transport moves remaining molecules against the gradient using energy from respiration.
Fatty acids and glycerol are absorbed into the lacteal (not directly into the blood). They are reassembled into lipids and transported via the lymphatic system.
Water: absorbed by osmosis in both the small and large intestine.
Minerals and vitamins: absorbed by diffusion and active transport.
After Absorption: Assimilation
Once absorbed, nutrients travel in the blood to cells where they are used:
Glucose → respired for energy, or stored as glycogen in the liver and muscles.
Amino acids → used to build new proteins for growth, repair, enzymes, antibodies.
Fatty acids and glycerol → stored as fat under the skin and around organs, or used for cell membranes.
Excess amino acids cannot be stored: the liver deaminates them (removes the amino group), converting the amino group to urea (excreted by kidneys) and using the rest for energy.
Try a focused question (4 marks)
Explain how two features of a villus are adaptations for efficient absorption of nutrients. (4 marks)
Show the answer and marking guidance
Villi have a very large surface area due to their finger-like shape.
This increases the area over which absorption can occur, so more nutrients are absorbed per unit time.
Villi have a dense network of blood capillaries.
Blood flow maintains a steep concentration gradient by continuously carrying absorbed nutrients away, increasing the rate of diffusion.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying villi "digest" food: villi absorb nutrients; digestion has already occurred before food reaches the villi.
Forgetting microvilli: the "brush border" of microvilli further increases surface area at the cellular level.
Confusing lacteals with capillaries: lacteals absorb fats; capillaries absorb glucose and amino acids.