Digestion
Learning objectives
WALT
• To be able to relate the generalised structure of the
gut wall in the oesophagus, stomach, duodenum and
ileum to the functions of these organs.
• To be able to describe mechanisms for the
absorption of food by the ileum, including the roles
of diffusion, facilitated diffusion and active transport.
WILF
• To be able to explain how the structure of the
digestive tract is adapted for absorption
Digestion
• Humans, like all animals, use holozoic nutrition, that
is obtaining nourishment as animals do by ingesting
complex organic matter
• Holozoic nutrition consists of these stages:
– ingestion- taking large pieces of food into the
body
– digestion- breaking down the food by mechanical
and chemical means
– absorption- taking up the soluble digestion
products into the body's cells
– assimilation- using the absorbed materials
– egestion- eliminating the undigested material
How is the human gut adapted for these
functions?
• a long tube, the alimentary canal or digestive
tract (or simply gut) which extends from the
mouth to the anus.
• a number of associated glands.
• The lining wall of the alimentary canal
appears different in different parts of
the gut, reflecting their different roles,
but always has these four basic layers:
The mucosa,
• This secretes digestive juices and absorbs digested food.
• It is often folded to increase its surface area.
• On the inside, next to the lumen (the space inside the
gut) is a thin layer of cells called the epithelium.
• Mucosa cells are constantly worn away by friction with
food moving through the gut, so are constantly being
replaced.
The submucosa,
• This contains blood vessels, lymph vessels
and nerves to control the muscles.
• It may also contain secretory glands.
The muscle layer
• This is made of smooth muscle, under involuntary
control.
• It can be subdivided into circular muscle (which
squeezes the gut when it contracts) and longitudinal
muscle (which shortens the gut when it contracts).
• The combination of these two muscles allows a variety of
different movements.
The serosa
• This is a tough layer of connective
tissue that holds the gut together, and
attaches it to the abdomen
Parts of the Alimentary Canal
tongue
epiglottis
salivary gland
oesophagus
liver
gall bladder
bile duct
duodenum
jejunum
ileum
stomach
pancreas
transverse colon
ascending colon
descending colon
caecum
rectum
anus
• The internal surface area is
increased enormously by three
levels of folding:
• large folds of the mucosa,
• Villi - large structures composed of
many cells that can clearly be seen
with a light microscope
• Microvilli - small sub-cellular
structures formed by the folding of
the plasma membrane of individual
cells.
– Can only be seen clearly with an
electron microscope, and appear
as a fuzzy brush border under
the light microscope.
• Circular and longitudinal muscles bi-directional peristalsis. This also
improves absorption.
Questions
1) (i) Describe how each of the following helps the
oesophagus to carry out its function.
Stratified epithelium
allows for cells damaged by passage of food / resists
damage as food passes;
Muscle layers
move food along / peristalsis;
(2 marks)
(ii) Stratified epithelium is described as a tissue.
Explain what is meant by a tissue.
• cells similar (in structure);
• with same / particular / specific / single /
• one function or job or process;
(2 marks)
2) (a) Name the part labelled A.
• Mucosa / gastric gland (accept: gastric pit)
(1 mark)
(b) Give two ways in which the structure of the wall of the stomach differs
from the wall of the oesophagus
• 3 layers of muscle v 2 layers / oblique layer;
• (Gastric) glands present; (accept 'glands that secrete enzymes’ / equiv)
• Specific cells present, e.g. oxyntic cells that secrete HCl;
• Columnar epithelial cells v stratified;
• All smooth muscle v some striated.
• (differences must be structural, not functional - e.g. enzymes)
.
(2 marks)
(c) Give one way in which the stomach wall is adapted
(i) to churn food;
Muscle layers arranged in all directions / oblique, or 3rd, layer
(1 mark)
(ii) to make food poisoning by bacteria less likely
(Hydrochloric) acid / low pH / (endo)peptidase (kills bacteria)
(1 mark)
(iii) to prevent enzymes produced in the stomach wall digesting the
surface of the stomach.
Mucus secretion / enzymes produced in inactive state
(1 mark)