H3.2 Ultrastructure of the epithelial cells of the small intestine.
H3.3 Absorption of digested food in the ileum.
H3.1 Structure of the ileum
The villi increase the surface area of the ileum for the absorption. The villi are finger like projections into the lumen of the gut. The villi increase the available surface area by ten times.
The crypts contain the secretory cells of intestinal secretion.
Mucosa is a mucus secreting membrane.
The circular and longitudinal muscles combine to create the contractions known as peristalsis that maintains the movement of chyme along the alimentary canal.
The serosa is a tough outer membrane composed of collagen.
H3.2 Ultrastructure of the epithelial gut cells
The image to the far left is that of the ileum wall clearly showing the villi.
The nearer image shows the apex of a villus.
The nuclei of each epithelial cell is visible with a central lacteal staining darker (running top right to bottom left).
The microvilli border stains darker purple but is only seen as a darker line around the villus
Epithelial cell: the key features of the epithelial villus cells are highlighted in colour.
Microvilli border increases the surface area for absorption of digested foods. This increases the surface area many hundreds of times.
Tight Junctions in which adjacent cells closely join there plasma membranes to prevent leakage between the cells.
There is a large prominent nucleus in a medial to basal position.
There are many mitochondria reflecting the active transport of some absorbed molecules and the synthesis of lipoproteins along with other molecules.
Pinocytotic vesicles are very small vesicles formed by endocytosis. The purpose of these is to increase the surface area of membrane for other processes such as active transport or facilitated diffusion.
LS Longitudinal section of epithelial microvilli.
Shows the microvilli in longitudinal section.
Note the dark content of the cells.
TS Transverse section of epithelial microvilli
Shows the microvilli in cross section again in the centre the content is the ends of the '9+2' actin protein fibres.
This provides the contractile mechanism for metachronal rhythm
H3.3 Absorption in the small intestine
Absorption of fatty acids . (see Lipid digestions)
(a) Bile salts form a micelle around fatty acids. The phospholipid structure of the salts allows it to fuse with the cell membrane and the fatty acid molecules to pass into the epithelial cells of small intestine villus.
b) The fatty acids and glycerol recombine in the endoplasmic reticulum to form lipid.
c) Protein is added to the lipid to form lipoprotein. This is how lipid is transported around the body.
d) The lipoprotein (called chylomicrons) is formed into small vesicles
e) Exocytosis of the vesicles releases the lipoprotein from the cell
f) The lipoprotein is taken up in the lacteal vessel a branch of the lymphatic system.
g) The lacteals, lymphatic system and the lipoproteins eventually enter the general circulation.
Facilitated diffusion: possible water soluble minerals and vitamins
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Larger molecules move passively through the membrane via channel proteins
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These proteins have large globular structures and complex 3d-shapes
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The shapes provide a channel through the middle of the protein, the 'pore'
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The channel 'shields' the diffusing molecule from the non-charged regions of the membrane.
Active Transport : Glucose absorption and Amino acids absorption into the epithelial cells.
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Active mean that the membrane protein 'pump' requires energy to function
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The source of energy is ATP from respiration
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This moves the molecules from low to high concentration against the concentration gradient
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The energy causes a shape change in the protein that allows it to move the molecule to the other side of the membrane.
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Note that these membrane pumps are often closely associated with membrane immobilised enzymes.(H2.6)
Endocytosis: probably pinocytosis
The formation of tiny vesicles by endocytosis is normally referred to as pinocytosis.
This increases the surface area for the processes of active transport and diffusion.
This should not be confused with fatty acids absorption which in which the micelle fuses with the cell membrane. The absorption of actual lipids would occur passively across the cell membrane.
Many substances in the diet are composed of small molecules that need little or no digestion. These include sugars, mineral ions, vitamins and water. These are absorbed by different transport mechanisms:
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Cholesterol and the fat-soluble vitamins (A, D, E, K) are absorbed into the epithelial cells of the ileum by lipid diffusion
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Mineral ions and water-soluble vitamins are absorbed by passive transport in the ileum
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Dietary monosaccharides are absorbed by active transport in the ileum
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Water is absorbed by osmosis in the ileum and colon.
H3.4 Egestion
Cellulose is an undigested insoluble polysaccharide which forms the fibre in the diet.
Bile Pigments colour the egested faeces and are eliminated. The composition of these pigments often reflect liver function and are sometimes used as a diagnostic for that organs function.
The gut retains its own bacterial populations with their own ecology. There is evidence to believe that normal digestion requires the presence of these populations and that factors that affect these populations (antibiotics) can lead to digestive disorders.
Intestinal cells are constantly removed from the lining of the gut. Gut epithelium has a structure similar to that of skin, with which it shares a common embryological origin. Such cells are egested. An interesting aside is that the collection of animal spoor (faeces) has aided the genetic study of populations like lions. This is because the animals DNA can be extracted from the epithelial gut cells within the faeces.