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Digestion |
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Digestion |
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Humans,
like all animals, use holozoic nutrition, which 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
(Do
not confuse egestion, which is the elimination of material from a body cavity,
with excretion, which is the elimination of waste material produced from
within the body's cells.)
The
human digestive system is well adapted to all of these functions. It comprises a
long tube, the alimentary canal or digestive tract (or simply gut)
which extends from the mouth to the anus, together with a number of associated
glands. The digestive systems made up of different tissues doing different jobs.
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, which 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, which contains blood vessels, lymph vessels and nerves to
control the muscles. It may also contain secretory glands.
The
muscle layer, which 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, which is a tough layer of connective tissue that holds the
gut together, and attaches it to the abdomen.
Parts
of the Alimentary Canal [back
to top]
The
teeth and tongue physically break up the food into small pieces with a larger
surface area, and form it into a ball or bolus. The salivary glands
secrete saliva, which contains water to dissolve soluble substances,
mucus for lubrication, lysozymes to kill bacteria and amylase to digest starch.
The food bolus is swallowed by an involuntary reflex action through the pharynx
(the back of the mouth). During swallowing the trachea is blocked off by the epiglottis
to stop food entering the lungs.
This
is a simple tube through the thorax, which connects the mouth to the rest of the
gut. No digestion takes place. There is a thin epithelium, no villi, a few
glands secreting mucus, and a thick muscle layer, which propels the food by peristalsis.
This is a wave of circular muscle contraction, which passes down the oesophagus
and is completely involuntary. The oesophagus is a soft tube that can be closed,
unlike the trachea, which is a hard tube, held open by rings of cartilage.
This
is an expandable bag where the food is stored for up to a few hours. There are
three layers of muscle to churn the food into a liquid called chyme. This
is gradually released in to the small intestine by a sphincter, a region
of thick circular muscle that acts as a valve. The mucosa of the stomach wall
has no villi, but numerous gastric pits (104 cm‑2)
leading to gastric glands in the mucosa layer. These secrete gastric
juice, which contains: hydrochloric acid (pH 1) to kill bacteria (the acid
does not help digestion, in fact it hinders it by denaturing most
enzymes); mucus to lubricate the food and to line the epithelium to protect it
from the acid; and the enzymes pepsin and rennin to digest proteins.
This
is about 6.5 m long, and can be divided into three sections:
(a)
The duodenum (30 cm long). Although this is short, almost all the
digestion takes place here, due to two secretions: Pancreatic juice,
secreted by the pancreas through the pancreatic duct. This
contains numerous carbohydrase, protease and lipase enzymes. Bile,
secreted by the liver, stored in the gall bladder, and released
through the bile duct into the duodenum. Bile contains bile salts
to aid lipid digestion, and the alkali sodium hydrogen carbonate to
neutralise the stomach acid. Without this, the pancreatic enzymes would not
work. The bile duct and the pancreatic duct join just before they enter the
duodenum. The mucosa of the duodenum has few villi, since there is no
absorption, but the submucosa contains glands secreting mucus and sodium
hydrogen carbonate.
(b)
The jejunum (2 m long) and
(c)
The ileum (4 m long). These two are similar in humans, and
are the site of final digestion and all absorption. There are numerous glands in
the mucosa and submucosa secreting enzymes, mucus and sodium hydrogen carbonate.
The
internal surface area is increased enormously by three levels of folding: large
folds of the mucosa, villi, and microvilli. Don't confuse these:
villi are large structures composed of many cells that can clearly be seen with
a light microscope, while microvilli are small sub-cellular structures formed by
the folding of the plasma membrane of individual cells. Microvilli can only be
seen clearly with an electron microscope, and appear as a fuzzy brush border
under the light microscope.
Circular
and longitudinal muscles propel the liquid food by peristalsis, and mix the
contents by pendular movements - bi-directional peristalsis. This also
improves absorption.
This
comprises the caecum, appendix, ascending colon, transverse colon, descending
colon and rectum. Food can spend 36 hours in the large intestine, while water is
absorbed to form semi-solid faeces. The mucosa contains villi but no
microvilli, and there are numerous glands secreting mucus. Faeces is made up of
plant fibre (cellulose mainly), cholesterol, bile, mucus, mucosa cells (250g of
cells are lost each day), bacteria and water, and is released by the anal
sphincter. This is a rare example of an involuntary muscle that we can learn
to control (during potty training).
By
far the most abundant carbohydrate in the human diet is starch (in bread,
potatoes, cereal, rice, pasta, biscuits, cake, etc), but there may also be a lot
of sugar (mainly sucrose) and some glycogen (in meat).
Salivary
amylase starts the
digestion of starch. Very little digestion actually takes place, since
amylase is quickly denatured in the stomach, but is does help to clean the
mouth and reduce bacterial infection.
Pancreatic
amylase digests all the
remaining starch in the duodenum. Amylase digests starch molecules from the
ends of the chains in two-glucose units, forming the disaccharide maltose.
Glycogen is also digested here.
Disaccharidases
in the membrane of the ileum epithelial cells complete the digestion of
disaccharides to monosaccharides. This includes maltose from starch
digestion as well as any sucrose and lactose in the diet. There are three
important disaccharidase enzymes:
The
monsaccharides (glucose, fructose and galactose) are absorbed by
active transport into the epithelial cells of the ileum, whence they diffuse
into the blood capillaries of the villi. Active transport requires energy in
the form of ATP, but it allows very rapid absorption, even against a
concentration gradient. The membrane-bound disaccharidases and the
monosaccharide pumps are often closely associated:
The
carbohydrates that make up plant fibres (cellulose, hemicellulose, lignin,
etc) cannot be digested, so pass through the digestive system as fibre.
Rennin
(in gastric juice) converts the soluble milk protein caesin into its
insoluble calcium salt. This keeps in the stomach longer so that pepsin can
digest it. Rennin is normally only produced by infant mammals. It is used
commercially to make cheese.
Pepsin
(in gastric juice) digests proteins to peptides, 6-12 amino acids long.
Pepsin is an endopeptidase, which means it hydrolyses peptide bonds
in the middle of a polypeptide chain. It is unusual in that it has an
optimum pH of about 2 and stops working at neutral pH.
Pancreatic
endopeptidases continue
to digest proteins and peptides to short peptides in the duodenum. Different
endopeptidase enzymes cut at different places on a peptide chain because
they have different target amino acid sequences, so this is an efficient way
to cut a long chain up into many short fragments, and it provides many free
ends for the next enzymes to work on.
Exopeptidases
in the membrane of the ileum epithelial cells complete the digestion of the
short peptides to individual amino acids. Exopeptidases remove amino acids
one by one from the ends of peptide chains. Carboxypeptidases work
from the C-terminal end, aminopeptidases work from the N-terminal
end, and dipeptidases cut dipeptides in half.
The
amino acids are absorbed by active transport into the epithelial cells of
the ileum, whence they diffuse into the blood capillaries of the villi.
Again, the membrane-bound peptidases and the amino acid transporters are
closely associated.
Protease
enzymes are potentially dangerous because they can break down other enzymes
(including themselves!) and other proteins in cells. To prevent this they are
synthesised in the RER of their secretory cells as inactive forms, called zymogens.
These are quite safe inside cells, and the enzymes are only activated in the
lumen of the intestine when they are required.
Pepsin
is synthesised as inactive pepsinogen, and activated by the acid in
the stomach
Rennin
is synthesised as inactive prorennin, and activated by pepsin in the
stomach
The
pancreatic exopeptidases are activated by specific enzymes in the duodenum
The
membrane-bound peptidase enzymes do not have this problem since they are
fixed, so cannot come into contact with cell proteins.
The
lining of mucus between the stomach wall and the food also protects the cells
from the protease enzymes once they are activated.
Fats
are emulsified by bile salts to form small oil droplets called
micelles, which have a large surface area.
Pancreatic
lipase enzymes digest
triglycerides to fatty acids and glycerol in the duodenum.
Fatty
acids and glycerol are lipid soluble and diffuse across the membrane (by
lipid diffusion) into the epithelial cells of the villi in the ileum.
In
the epithelial cells of the ileum triglycerides are re-synthesised (!) and
combine with proteins to form tiny lipoprotein particles called chylomicrons.
The
chylomicrons diffuse into the lacteal - the lymph vessel inside each
villus. The emulsified fatty droplets give lymph its milky colour, hence
name lacteal.
The
chylomicrons are carried through the lymphatic system to enter the
bloodstream at the vena cava, and are then carried in the blood to all parts
of the body. They are stored as triglycerides in adipose (fat) tissue.
Fats
are not properly broken down until they used for respiration in liver or
muscle cells.
Pancreatic
nuclease enzymes digest nucleic acids (DNA and RNA) to nucleotides in the
duodenum.
Membrane-bound
nucleotidase enzymes in the epithelial cells of the ileum digest the
nucleotides to sugar, base and phosphate, which are absorbed.
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:
Cholesterol
and the fat-soluble vitamins (A, D, E, K) are absorbed into the epithelial
cells of the ileum by lipid diffusion
Mineral
ions and water-soluble vitamins are absorbed by passive transport in the
ileum
Dietary
monosaccharides are absorbed by active transport in the ileum
Water
is absorbed by osmosis in the ileum and colon.
Digestion
in Fungi
(additional information)
Fungi
are not consumers like animals, but are either saprophytes (decomposers),
or pathogens. They therefore use saprophytic nutrition, which means they
do not ingest their food, but use extracellular digestion. Fungi secrete
digestive enzymes (carbohydrases, proteases and lipases) into the material that
surrounds them and then absorb the soluble products (sugars, amino acids, etc).
Fungi
are usually composed of long thin threads called hyphae. These grow
quickly, penetrating dead material such as leaves, as well as growing
underground throughout soil. The cotton wool appearance of bread mould growing
on decaying bread is typical of a mass of hyphae, called a fungal mycelium.
These thin hyphae give fungi a large surface area to volume ratio. They contain
many nuclei, since they are formed from the fusion of many cells.
Last updated 20/06/2004