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Topic H.3 – Absorption of Digested Food
H.3.1 - Draw and label a diagram showing a transverse section of the ileum as
seen under a light microscope.
 The small intestine is divided into three sections; the duodenum, jejunum and the
ileum. The duodenum is the first section where secretions from the pancreas
and liver are added to the chyme which has entered the small intestine from the
stomach. The jejunum and the ileum are much longer sections where digestion is
completed and nutrients are absorbed.
 The longitudinal and circular smooth muscle of the intestines move in rhythmic
contractions known as peristalsis which move food through the small intestine.
 The intestinal mucosa is the innermost lining of the small intestine that shapes
itself into villi; invaginations of the mucosa that greatly increase the surface area
of the small intestine for absorption of nutrients. Digested food molecules come
into direct contact with the mucosa which is responsible for the absorption of the
nutrients. Each villus contains a network of capillaries and a lacteal.
H.3.2 - Explain the structural features of an epithelial cell of a villus as seen in
electron micrographs, including microvilli, mitochondria, pinocytotic vesicles and
tight junctions.
 Epithelial cells line the outside of the villus and it is through these cells that the
digested nutrients are absorbed into either the capillaries or the lacteal. The
epithelial cells contain a few different structural features which makes them
efficient for absorption.
 The microvilli cover the entire surface of the villus. The microvilli extend into the
lumen of the small intestine and significantly increase the surface area of the
villus for absorption. This means nutrients can be absorbed faster (increased rate
of diffusion and facilitated diffusion).
 The epithelial cells will also contain high numbers of mitochondria so that food
nutrients which cannot enter by simple diffusion can be brought into the cells via
active transport. Active transport uses ATP produced by the mitochondria to
move particles inside the cells. Molecules such as amino acids, glucose, and
mineral ions can be absorbed into the cell using active transport.
 Pinocytotic vesicles are also used in the epithelial cells as another form of active
transport to move molecules from the lumen of the small intestine to the interior
of the cells. The pinocytotic vesicles contain droplets of fluid from the small
intestine. Once inside the cell the particles can move out of the vesicle and into
the cytoplasm of the epithelial cell.
 Due to the fact that most cells in the body are surrounded by fluid, molecules can
sometimes slip between cells. It is important however that this does not occur in
the lining of the small intestine. Food molecules must be completely digested
before they can be absorbed and the plasma membrane of the epithelial cells
ensures that absorption is controlled and that only fully digested molecules are
moving into the capillaries and the lacteal. To ensure that this is what occurs the
epithelial cells are packed close together and are sealed to one another by
structures called tight junctions. The adjacent membranes share some
membrane proteins and this help to keep them tightly packed together.
H.3.3 - Explain the mechanisms used by the ileum to absorb and transport food,
including facilitated diffusion, active transport and endocytosis.
 Both active and passive transport are used to move substances from the lumen of
the small intestine inside the epithelial cells. All molecules that move into the
cells have been digested and are now small enough to pass inside of the cells.
 Some molecules such as those that are soluble in lipids pass easily inside the
epithelial cells via simple diffusion.
 Facilitated diffusion is used to transport molecules whose polarity makes it
difficult for them to pass through the hydrophobic center of the plasma
membrane. The channel proteins can help moves these molecules inside due to
their non-polar outer perimeter and their polar interior channel.
 Active transport using membrane pumps can be used to absorb molecules for
which a concentration gradient does not exist. The membrane proteins of the
plasma membrane in the microvilli are used to pump molecules inside using ATP
from the mitochondria.
 Endocytosis or more specifically pinocytosis can also be used to bring in small
droplets of fluid which can later be released into the cytoplasm.
H.3.4 - List the materials that are not absorbed and are egested.
 Materials not digested in the small intestine and are therefore not absorbed but
egested instead include the following:
 Cellulose – found in plant cell walls
 Lignin – also found in plant cell walls
 Bile pigments – give color to feces
 Bacteria – found in large intestine
 Intestinal cells – break off as food moves through lumen