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)
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
gland outside
the gut, e.g.
pancreas
mucosa
muscularis
mucosa
submucosa
(connective tissue)
outer serosa
gland within the
submucosa
(Brunner’s glands)
villi
lumen
circular muscle
longitudinal muscle
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
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
Epithelial cells, bearing microvilli,
project into the lumen of the gut
Intestinal
gland
between the
villi; contains
enzyme
secreting cells
and hormone
releasing cells
T.S. Ileum
Lumen
villi
mucosa
intestinal gland
Microvilli forming
the brush border
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
mucus-secreting, goblet cell in
the epithelial lining of the villus
epithelial cells
brush
border
brush border
goblet cell
epithelial cell
nucleus of
epithelial cell
microvilli of epithelial
cells (brush border)
mitochondrion
Brush border
Interior of
epithelial cells
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)
Mucosal
membrane
Brush border
Each epithelial cell has a mucosal
and a serosal membrane
Interior of
epithelial cells
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
Serosal
membrane
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
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
Membrane-bound protein
carriers are involved in the
transport of glucose molecules
by facilitated diffusion and
active transport
Facilitated diffusion is a carrier-assisted transport mechanism
in which molecules are transferred across membranes
along their concentration gradients
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
Active transport is a carrier-assisted, energy requiring
transport mechanism that utilises energy from ATP molecules
to transfer molecules against their concentration gradient
Various membranebound 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
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
The sodium gradient
is maintained by
sodium/potassium
pumps located within
the serosal membrane
of the epithelial cells
These energyThe operation of these
Na+/K+ pumps results in
requiring pumps
transfer sodium ions a permanently low Na+
out of the cell into the concentration within the
blood in exchange for epithelial cells and the
maintenance of a Na+
potassium ions
concentration gradient
across the mucosal
membrane
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
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
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