Download Digestion - WordPress.com

Digestion
IB Core Topic V
Processing Food





Ingestion: taking food into the digestive system
Digestion: breaking down large molecules into
smaller molecules that can be absorbed
Absorption: Transferring nutrients from the
digestive system to the circulatory system for
transporting around the body
Assimilation: The body’s processing of
digested, absorbed substances (e.g.
transamination and deamination of animo acids)
Egestion: Passing indigestible matter out of the
body
The Alimentary Canal
Basic 4-layered structure:

1.
2.
3.
4.
Inner lining – contains goblet cells that secrete
mucus for lubrication
Nerves, blood and lymph vessels
Involuntary muscle layer that mixes food and
causes peristalsis (pushing food along)
Peritoneum – outer, lubricated layer
Alimentary Canal Sections
 Oesophagus


– “food pipe”
Epiglottis stops food going down the trachea
Uses peristalsis to carry food to the stomach
– muscular bag for storing and
digesting food
 Stomach


Cardiac sphincter relaxes to let food in
Holds ~ 1litre of food for ~ 4hrs
The Stomach
 Stomach
lining is heavily folded, containing
gastric glands
 Glands secrete gastric juice





Mucus – lines stomach to prevent self-digestion
Pepsinogen – converted to pepsin (protease) in low
pH conditions and by pepsin (autocatalytic)
Hydrochloric acid – kills micro-organisms, gives
optimum pH for pepsin, loosens fibrous/cellular food
and deactivates salivary amylase
Young mammals have rennin, and enzyme that
converts caesinogen (soluble milk protein) to caesin
(insoluble) so it can be digested by pepsin
Food + gastric juice = CHYME
Duodenum
 1st
section of the small intestine
 Passage of food from stomach controlled
by pyloric sphincter
 Surface is highly folded, containing villi
(finger-like projections)
 Cells on surface of villi have further
projections – microvilli. These form the
brush border (massive S.A.)
The Pancreas
 Contains




the following:
water and bicarbonate ions to neutralise
stomach acids
Pancreatic amylase, to continue the
breakdown of starch to maltose
Lipase, to break down lipids to fatty acids and
glycerol
Trypsinogen, precursor of the protease trypsin
Gall Bladder
Produces bile, which contains:
Bile pigments
Bile salts

1.
2.
alkali, to neutralise stomach acid

Emulsify fats, to increase SA for lipase
action

Assists in absorption of fat-soluble vitamins
(A, D and K)
The acidic chyme is converted to chyle by the
addition of bile and pancreatic juice

Jejunum and Ileum

Length of the small intestines are covered in villi
 The structure of the villi reflect their function
(absorption of nutrients).




Large surface area
Very long (~8m)
Extremely thin lining (1 cell thick)
Rich blood supply – lots of capillaries
Capillaries absorb small substances (e.g. glucose,
water, amino acids, vitamins, minerals) by a
combination of diffusion (with the concentration
gradient) and active transport (against the
concentration gradient
Large intestines





Consists of caecum, appendix, colon, rectum
and anus
Caecum and appendix are thought to be
vestigial in humans: herbivores host cellulasesecreting bacteria here
Large intestines host bacteria that produce
vitamins (K, B2 group), but some can cause
infection (appendicitis)
Colon reabsorbs water – undigested liquid from
small intestines is converted to semi-solid faeces
(colour due to bile pigments)
Faeces are stored in the rectum and pass out of
the anus under the control of the anal sphincter