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Topic 13 - Digestion.docx
Topic_13_-_Digestion.docx
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Topic_13_-_Digestion.docx-Topic 13 – Digestion What
Page 1
Topic 13 – Digestion
What is Digestion?
o
Digestion
: Taking a substance (food) and breaking it up eventually into its
component molecules
o
Animals cannot make their own nutrients from inorganic precursors, so all
animals rely on is ingesting other organisms to provide energy for substrates and
raw materials for synthesizing their own macromolecules
o
Food typically includes macromolecules that can’t be used “as is”
Instead, they must be broken down into smaller components
o
Autotrophs
: Things that can make organic nutrients on their own
All other organisms are heterotrophs
, and they have to eat autotrophs to
get these nutrients
The Four Parts of “Digestion”
o
1) Ingestion
Eating.
Chewing is important because chemical digestion can ONLY
occur at the surface!
o
2) Digestion
The actual breaking down of the food particles
Takes place along the digestive tract, which is divided (both anatomically
and functionally) into distinct regions, and each region carries out specific
phases of digestion
o
3) Absorption
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The nutrients are absorbed across the wall of the digestive tract and then
transferred to the rest of the cells in the body by blood
o
4) Elimination
Any materials that were not digested are excreted
o
As long as digested food is inside the digestive tract, it is useless to the animal
The products of digestion MUST be absorbed across the wall of the
digestive tract and MUST enter the bloodstream to be used
o
Typically, all species have a big tube (digestive tract) with large surface area that
facilitates absorption, and it goes from the mouth (where food enters) to the anus
(where waste is excreted)
The Human Digestive Tract
o
Start at mouth, where we insert food
Salivary glands at the mouth, to let out saliva
o
Then, the food (post-chew) goes into the esophagus
o
Then, from the esophagus, food goes into the stomach
o
Then, from the stomach, goes to small intestine and some branches out to the
pancreas, which then goes to gallbladder, which then goes to liver
o
From small intestine, food moves to large intestine
o
Then from large intestine, goes to rectum
o
Then from rectum, it is excreted by the anus
Page 3
o
o
The stomach has controlling devices (called sphincters
): one at the entrance, one
at the exit
It can be opened or closed, pretty much a valve
Sphincters are all over our body; they’re always a circular muscle wrapped
around a tube somewhere (like a rubber band) that tightens/loosens to
close/open the tube
Nutrients
o
The chemical components of food are called nutrients
o
Nutrients can be categorized as
macronutrients
or
micronutrients
, depending on
how much of them we need to ingest every day
Macronutrients: need them very often in huge quantities; Micronutrients:
don’t need as much of them, but we still do need them (e.g. vitamins,
minerals)
o
The major macronutrients are:
Carbohydrates
: Sugars and polymers made up of sugars
Our main energy source (metabolism of glucose)
Proteins
: Polymers made up of amino acids
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