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Summary Digestive System Notes
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Notes on the digestive system for GCSE biology
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DigestionDigestion: is the breaking down of complex food molecules into simple molecules by enzymes and
digestive juices that can be absorbed by the bloodstream
Four key stages
1) Ingestion: taking in food and breaking down into smaller pieces by teeth and tongue
2) Digestion: breaking down chemically or mechanically in stomach, small intestine by enzymes
and digestive juices
3) Absorption: through small intestine and passing not the bloodstream
4) Excretion: collection and removal of waste product
Villi in the small intestine
1) Thin walls of small intestine glucose has to pass through meaning diffusion is easier
2) Branched villi throughout the intestine wall increase the SA for diffusion
3) Walls have rich supply of blood capillaries which reduces the distance between the walls of
villi and blood capillaries for diffusion
Fick’s Law of Diffusion ∞ SA X concentration gradient / thickness of membrane
+ thickness = less diffusion - thickness = more diffusion
By the time food reaches the small intestine, it is a liquid. It transfers digested food to blood which
carries it to the body. Used for respiration.
Alveoli in the lungs
1) Thin walls of alveoli so oxygen can pass through meaning diffusion is easier
2) Branched bronchioles(alveoli) increase SA
3) Walls have rich supply of blood capillaries which reduce the distance between the walls of
the villi and blood capillaries for diffusion.
Large Intestine
Undigested food in the small intestine gets transferred to the large intestine.
1) Absorbs all the extra water from the undigested food
2) Then passes the undigested solid waste to the rectum
3) The rectum stores it until its capacity it met and is released by the anus
Diarrhoea
Large intestine is infected, the water is unable to be absorbed back into the body causing the liquid
to be carried to the rectum along with the undigested food. The rectum is unable to stop this waste
and is released as soon as it reaches the rectum.
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