Nutrient Absorption (SQA National 5 Biology): Revision Note

The small intestine is a tube in the abdomen, located between the stomach and large intestine; in the small intestine:

• enzymes complete digestion

• villi absorb the products of digestion

Absorption is the movement of small, digested food molecules from the digestive system into the blood

• Absorption of small soluble molecules occurs by diffusion and active transport

The small intestine has adaptations that increase its surface area

• Tt is very long

• Its surface is made up of millions of villi

Villi are finger-like projections lining the small intestine; their role is to aid in absorption of the products of digestion

The adaptations of villi include:

• large surface area

  • There are many villi, so the collective surface area is large

  • Microvilli on the surface of each individual villus further increases the surface available for absorption

• short diffusion distance

  • The wall of a villus is only one cell thick

• steep concentration gradient

  • The villi contain a network of blood capillaries that transport glucose and amino acids away from the small intestine; this maintains the concentration gradient between the intestine and the blood

  • A lacteal runs through the centre of each villus to transport fatty acids and glycerol away from the small intestine; this maintains the concentration gradient between the intestine and the lacteal