2. Absorption is the uptake of digested
Absorption of Digested Food
Absorption is the uptake of digested
food molecules from the alimentary canal (gastrointestinal tract) into the blood or lymph
The principal site for absorption of nutrients is the ileum of the small intestine where the epithelial cells lining this gut region are ideally suited to this role
Glucose and amino acids are transported across the epithelial cells to the blood capillaries within the villi, and reconstituted fats are absorbed into the lymphcapillaries (lacteals)

3. Absorption of Digested Food
Absorption involves the transport processes of simple diffusion, facilitated diffusion, active transport and osmosis (for water absorption)
The gross and histological structure of the ileum adapt this region for its absorptive and secretory roles

4. gland outside the gut, e.g. pancreas gland within the submucosa
(Brunner’s glands)
mucosa
villi
muscularis
mucosa
lumen
submucosa
(connective tissue)
circular muscle
outer serosa
longitudinal muscle

5. The duodenum is the site where
pancreatic juice (containing many enzymes) is secreted into the gut and where Brunner’s glands secrete an alkaline mucus to help neutralise and protect its lining from the acid chyme arriving from the stomach
The ileum is the principal site for the absorption of nutrients; it is very long (about 4 metres in length) and the numerous villi with their epithelial linings increase the surface area for absorption

6. The ileum displays adaptations for both absorption and secretion:
Adaptations of the Ileum
The ileum displays adaptations for both absorption and secretion:
The ileum is very long and absorption can occur along its length
The mucosa is highly folded and the numerous ‘finger-like’ projections, the villi, vastly increase the surface area of the epithelium for both digestion and secretion
The epithelial cells of the villi bear microvilli at their luminal surface that project into the lumen of the gut; these microvilli form the brush border and further increase the surface area available for both absorption and secretion
The villi are well supplied with a network of blood capillaries into which glucose and amino acids are transferred and then transported to the liver along the hepatic portal vein
A single, permeable lacteal within each villus transports reconstituted fats away from the intestine

7. Epithelial cells, bearing microvilli, project into the lumen of the gut
Intestinal gland between the villi; contains enzyme secreting cells and hormone releasing cells

8. T.S. Ileum
Lumen
villi
mucosa
intestinal gland

9. Microvilli forming the brush border Numerous mitochondria Nucleus
Microvilli further increase the
surface area of the intestine for efficient absorption of
food molecules
Numerous
mitochondria
Numerous mitochondria provide energy, in the form of ATP, for the active transport of various molecules and ions
Nucleus

10. mucus-secreting, goblet cell in the epithelial lining of the villus
epithelial cells
brush
border

11. brush border
goblet cell
epithelial cell
nucleus of
epithelial cell

12. microvilli of epithelial cells (brush border)
mitochondrion

14. Brush border
Digested food molecules are transported across the epithelial cells of the villi into the blood capillaries (monosaccharides and amino acids) and the lacteals (fatty acids and monoglycerides)
Interior of
epithelial cells

15. Each epithelial cell has a mucosal and a serosal membrane
Brush border
Each epithelial cell has a mucosal and a serosal membrane
The mucosal membrane of the microvilli projects into the lumen of the gut with the serosal membrane being closely apposed to the blood capillaries within the villus
Interior of
epithelial cells
Serosal
membrane

16. sodium-linked active transport
Glucose is transported across the epithelial cells of the villi from the lumen of the gut to the blood capillaries by facilitated diffusion and
sodium-linked active transport
Membrane-bound protein carriers are involved in the transport of glucose molecules by facilitated diffusion and active transport
Facilitated diffusion is dependent upon the existence of a glucose concentration gradient across the epithelial cell membranes
Active transport allows for the transfer of glucose against its concentration gradient

17. Facilitated diffusion is a carrier-assisted transport mechanism
in which molecules are transferred across membranes
along their concentration gradients

18. Glucose is transported into the epithelial cells of the villi by facilitated diffusion as long as a concentration gradient exists between the gut lumen and the interior of the cells

19. Active transport is a carrier-assisted, energy requiring
transport mechanism that utilises energy from ATP molecules
to transfer molecules against their concentration gradient

20. Various membrane-bound carrier proteins in the epithelial cells
of the villi are involved in the sodium-linked, active transport of glucose from the gut lumen to the blood capillaries
Specific carriers located within the mucosal membrane possess binding sites for both glucose and sodium ions
This co-transport of glucose and sodium ions into the cell is dependent upon a sodium ion concentration gradient

21. Specific protein carriers in the mucosal membranes of intestinal epithelial cells possess binding sites for both glucose molecules and sodium ions
The transport of these molecules into the epithelial cells from the gut lumen is dependent upon the sodium ion concentration gradient; so long as this gradient exists, glucose can be transported against its own gradient

22. The sodium gradient is maintained by sodium/potassium pumps located within the serosal membrane of the epithelial cells
These energy-requiring pumps transfer sodium ions out of the cell into the blood in exchange for potassium ions
The operation of these Na+/K+ pumps results in a permanently low Na+ concentration within the epithelial cells and the maintenance of a Na+ concentration gradient across the mucosal membrane

23. Once inside the epithelial cells, glucose molecules are transferred across the serosal membranes into the blood capillaries by facilitated diffusion
In this system, the transport of glucose across the mucosal membranes is coupled to the active transport of sodium ions across the serosal membranes

24. Absorption of Amino Acids
The mechanism for absorption of amino acids is similar to that for glucose absorption, i.e. sodium-linked active transport
The difference between the two processes is such that there are many different co-transport carriers in the mucosal membrane
There are many different amino acids that differ in structure and shape
Individual carriers for the co-transport of sodium ions and each amino acid are located within the mucosal membrane

25. Fatty acids, monoglycerides and glycerol enter the
micelles release
fatty acids,
monoglycerides
and glycerol
Fatty acids, monoglycerides
and glycerol enter the
epithelial cells by simple diffusion where
they are resynthesised
into triglyceride