Humans, like all animals, use holozoic nutrition,
which consists of these stages:
- taking large pieces of food into the
- breaking down the food by mechanical
and chemical means
- taking up the soluble digestion products
into the body's cells
- using the absorbed materials
- eliminating the undigested material
Egestion is elimination of material
from the body caviry
Excretion is elimination of substances
from within body cells
The human digestive system is well adapted
to all of these functions. It comprises a long tube, the alimentary
canal (digestive tract or simply gut) that runs 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 the same basic layers:
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 the friction of food moving through the
gut, so are constantly being replaced.
contains blood vessels, lymph vessels and
nerves to control the muscles. It may also contain secretory
The muscle layer
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.
A tough layer of connective tissue that
holds the gut together, and attaches it to the abdomen.
of the Alimentary Canal
1. Mouth (Buccal cavity)
The teeth, tongue and chewing action break
up the food physically which increases surface area, and they
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.
2. Oesophagus (gullet)
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
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.
4. Small Intestine
This is about 6.5 m long, and can be divided
into three sections:
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.
The jejunum (2 m long) and
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 move the
liquid food by peristalsis.
5. Large Intestine
This comprises the caecum, appendix, colon
and rectum. Food can spend 36 hours in the large intestine (mind
you that's if your pretty constipated!), 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 cellulose, 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
Chemistry of Digestion
1. Digestion of Carbohydrates
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.
in the membrane of the ileum enzymes attached to the 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
- 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.
2. Digestion of Proteins
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
- 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
- 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.
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
- 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.
The lining of mucus between the stomach wall
and the food also protects the cells from the protease enzymes
once they are activated.
- 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.
3. Digestion of Triglycerides
- 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
- 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.
4. Digestion of Nucleic acids
- 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.
5. Other substances
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
Digestion in Fungi
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. (more
on extracellular digestion)