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All the information for topic 6 IB Biology SL
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All the information for topic 6 IB Biology SL
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Human physiology6.1 structure of the digestive system
Ultimately, all that you ingest (eat) needs
Essential idea:
to end up as a usable molecule in your
The structure of the wall of the small intestine
cells. The digestive system has a great
allows it to move, digest and absorb food
deal to accomplish when it breaks down
Living organisms control their composition by a
all these molecules to provide the raw materials your cells must use to build new molecules and
complex web of chemical reactions
perform respiration.
Your body needs to digest this food so that these essential molecules are
separated and absorbed in an efficient and appropriate way. Essential
molecules that are absorbed are then sent to body cells through the
bloodstream to be used for cellular activities: it is this use that
is called assimilation. At the end of the process, your body
needs to egest (get rid of or expel) the waste material.
The processes in digestion can be classified into chemical and mechanical. Each of the parts
of the digestive system have different functions. The process of digestion starts in the mouth:
1. Mouth: chewing (mechanical digestion). Saliva
moistens food to make a bolus for swallowing.
Salivary amylase (enzymes present in saliva)
begins chemical digestion of starch.
2. Oesophagus: (also called the gullet) a wave of
muscle contractions (peristalsis) pushes the bolus
into the stomach.
3. Stomach: muscular contractions continue
mechanical digestion to mix food with acid. Acid
kills bacteria. Pepsin begins digestion of proteins.
Now the bolus is called chyme.
4. Duodenum (small intestine): First section of the small intestine. In the small intestine, the
acid passing from the stomach needs to be neutralized so that enzymes in the small
, intestine can function properly. Bile from the liver and gall bladder neutralizes acid and
emulsifies fats. Pancreatic amylase and lipase digest carbohydrates and fats. Trypsin
digests polypeptides to amino acids.
5. Ileum (small intestine): Lower half of small intestine absorbs nutrients into the blood via
the villi. By the time your food reaches the end of the small intestine, digestion has been
completed and the digested products have been absorbed from the last section, the ileum.
The small intestine is where most of the absorption Peristalsis is a wave of contraction
takes place. and relaxation of the longitudinal
6. Large intestine: water, along with vitamins (mainly K and circular muscles (that work
and B) made by bacteria that normally live in the large antagonistically: when one
intestine, are reclaimed and returned to the blood. contracts, the other relaxes and
The anaerobic bacteria in the large intestine can also vv.) of the alimentary canal, forcing
the contents along the tube.
ferment undigested polysaccharides to produce
energy. In the last parts of the large intestine, faeces
is formed and stored in the rectum prior to egestion.
7. Egestion: Faeces (containing undigested food, dead
cells, and other waste) is forced out of the anus.
Several other organs aid in the digestive process (these organs are the accessory organs):
1. The pancreas secretes amylase, lipase and protease (that aid in the digestion of starch,
lipids and proteins, respectively) into the small intestine. Bile is not an enzyme. Bile
2. The liver secretes bile to emulsify lipids (breaking them breaks large lipid globules
down into smaller sections so they have a higher S:A ratio into smaller lipid globules (this
is what emulsify means). Bile
and enzymatic activity can be more effective). It also
does not facilitate hydrolysis
creates an alkaline environment with the same purpose.
reactions as enzymes do.
3. The gall bladder stores bile.
The mouth
• Ingestion
• Mechanical digestion
o Teeth: mastication (chewing)
o Tongue: mixing and swallowing
• Chemical digestion
o Saliva: moisturizes, starch digestion, lubrificant…
,The oesophagus
• Peristalsis: rhythmical contraction and relaxation of
longitudinal smooth muscle, responsible for the unidirectional
movement of food through the alimentary canal.
The stomach
• Mechanical digestion: churning
• Chemical digestion:
o Pepsine
o HCI
§ Kills bacteria
§ Role in chemical digestion
• Converts pepsinogen (inactive) into pepsin (active)
• Protein denaturation à facilitates pepsin action
• Chyme storage
• Absorption; alcohol, sugars…
Exam tip
You need to know the names of the tissue
layers of the small intestine from the
The small intestine
outside to the inside: serosa, longitudinal
The small intestine plays important roles in
and circular muscles, submucosa, mucosa
digestion and absorption.
and epithelium. On a test, you may be given
• The small intestine is a muscular tube of
a micrograph and be asked to identify the
about 6–7 m in humans that lies between
layers in that micrograph. You also need to
the stomach and the large intestine.
know how to identify the structures that
• It can be divided into three parts, namely
make up an individual villus.
the duodenum, jejunum, and ileum.
• The inner surface of the small intestine is covered with specialized structures called
villi (singular: villus), which increase its surface area by 30–60 fold: this increase in
surface area is essential for absorption.
, If you go from outside of the tube towards the inside, you will come across the following tissue
layers:
• Serosa: the outermost layer consisting of
connective tissue that is in contact with body
cavities.
• Longitudinal muscles: responsible for peristalsis.
• Circular muscles: responsible for peristalsis.
• Submucosa: connective tissue that
supports the mucosa and that contains
large veins and arteries which give rise to
the capillary bed of the mucosa.
• Mucosa: innermost layer forming the soft
lining of the tube comprised of epithelium
(which lines the lumen of the digestive
track), connective tissue and smooth
muscle (villi form part of this layer).
The small intestine completes the digestion of food molecules.
1. Chyme enters the duodenum (first section).
2. Bile from the gallbladder and liver in emptied into the duodenum, neutralizing the acid and
emulsifying fats.
3. Pancreatic enzymes are released (amylase, trypsin).
4. Enzymes are further released into the jejunum.
The ileum is the last stage of the small intestine. Here, the absorption of digested food
molecules takes place. Villi (finger-like projections) increase the surface area for absorption
and have a rich blood supply. A wave of muscle contractions (peristalsis) keeps the mixture of
digested and undigested food moving through the intestine.
The contraction of circular and longitudinal muscle
of the small intestine mixes the food with enzymes
and moves it along the gut.
1. Contraction of longitudinal muscle expand the lumen in front of the food giving it space
to move into.
2. The contraction of circular muscles behind the food propels it forward.
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