AQA AS Biology Unit 1: The Digestive System

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  • Created on: 22-08-13 16:17
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The Digestive System
Digestion breaks down large molecules into smaller molecules.
The digestive system provides an interface with the environment because food substances
enter the body through it.
Each part of the digestive system has a specific function ­ all the organs have a role in
breaking down food and absorbing nutrients.
The lining wall of the alimentary canal appears different in different parts of the gut,
reflecting their different roles, but always has these four basic layers:

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The Mouth
The mouth is the opening to the buccal or oral cavity and is surrounded by muscular lips
which close the mouth to retain food in the cavity.
The buccal cavity is separated from the nasal chamber by the bony hard palate and
cartilaginous soft palate.…read more

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The Oesophagus
The oesophagus is a tube that carries food from the mouth to the stomach.
It is made up of a thick muscular wall.
It is adapted to transport, rather than for digestion or absorption.
It carries the food down the tube using waves of muscle contractions called peristalsis.
Mucus is secreted from tissues in the walls, to lubricate the food's passage downwards.
The Stomach
The stomach is a muscular sac.
It has lots of folds, allowing the stomach to expand.…read more

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The Small Intestine
The small intestine has two main parts ­ the duodenum and the ileum.
The first 30 cm of the small intestine is the duodenum.
Chyme is moved along the small intestine by peristalsis in the duodenum.
Almost all of the digestion takes place in the duodenum, due to two secretions: pancreatic
juice and bile.…read more

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The Large Intestine
The large intestine absorbs water, salts and minerals.
The large intestine has a folded wall which provides a large surface area for absorption.
Most of the water that is reabsorbed comes from the secretions of many digestive glands.
The food within the large intestine therefore becomes drier and thicker in consistency and
forms faeces.
The Rectum
The rectum is the final section of the intestines.
Faeces are stored here then pass through the sphincter muscles at the anus.…read more

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What is Digestion?
Digestion takes place in 2 stages:
Physical breakdown
Chemical breakdown
Physical Breakdown
If the food is large, it is broken down into small pieces by means of structures such as the
This not only makes it possible to ingest the food but also provides a large surface area for
chemical digestion.
Food is churned up by the muscles in the stomach wall and this also physically breaks it up.…read more

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Chemical Digestion
Chemical digestion breaks down large, insoluble molecules into smaller, soluble ones.
It is carried out by enzymes.
All digestive enzymes function by hydrolysis.
Hydrolysis The splitting up of molecule by adding water to the
chemical bonds that hold them together
Enzymes are specific and so it follows that more than one enzyme are needed to break
down a large molecule.…read more


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