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Obesity

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The following molecules, which are the end products of digestion, are directly absorbed by the villi:

Bases and phosphates from nucleic acids

Fatty acids and glycerol

Amino acids

Monomeric carbohydrates, such as fructose, glucose, galactose and ribose.

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Use of dialysis tubing to model absorption

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Use of dialysis tubing to model absorption

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Dialysis is the separation of smaller molecules from larger molecules in solution by selective diffusion through a partially permeable (also known as selectively permeable or semipermeable) membrane.

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The following four modes of absorption are used to execute these two steps:

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.

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Digestion By Enzymes | Organic Chemistry | Chemistry | FuseSchool

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Important

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.

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If you go from outside of the tube towards the inside, you will come across the following tissue layers:
Serosa: the outermost layer consisting of connective tissue that is in contact with body cavities.
Longitudinal muscles: responsible for peristalsis.
Circular muscles: responsible for peristalsis.
Submucosa: connective tissue that supports the mucosa and that contains large veins and arteries which give rise to the capillary bed of the mucosa.
Mucosa: innermost layer forming the soft lining of the tube comprised of epithelium, connective tissue and smooth muscle (villi form part of this layer).

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We cannot think well, love well, sleep well, if one has not dined well.

Virginia Woolf

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How your digestive system works - Emma Bryce

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Peristalsis is a wave of contraction and relaxation of the longitudinal and circular muscles of the alimentary canal, by which the contents are forced along the tube.