Virginia Woolf 

Movement of food along the small intestine occurs by peristalsis. However, as well as helping the food to move along the gut, the contraction of circular and longitudinal muscles of the small intestine also mixes the food with enzymes.

Enzymes (from pancreas and small intestine) digest most macromolecules in food into monomers in the small intestine. These monomers are then absorbed into the blood or lymph.

The great variety of enzymes secreted by the human body allows the digestion of starch, glycogen, lipids and nucleic acids into their respective monomers. However, cellulose remains undigested, as the enzyme cellulase (which breaks down cellulose) is not produced by humans.

1. Simple diffusion: Occurs when molecules are small and are hydrophobic (so they can pass through the phospholipid bilayers). This occurs mostly with the products of lipid digestion.

2. Facilitated diffusion: Fructose, glucose and other hydrophilic monomers are moved by protein channels. Be aware, this still requires a concentration gradient.

3. Active transport: Is needed when the concentrations are lower in the lumen of the small intestine. Thus, the movement needs to occur against a concentration gradient. Glucose, amino acids and some mineral ions are transported in this way, which requires ATP. The cells of the epithelium have many mitochondria that can synthesise ATP for this process.

4. Pinocytosis: Draws in small droplets of liquid surrounded by a small section of the phospholipid membrane, as shown in Figure 2. This is most likely to occur with fat droplets in the lumen of the small intestine.