Biology 4 AS Digestive System

Topic: Digestive System
Biology 4 - AS  

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Digestive System

The main parts of the digestive system are:

  • Oesophagus
  • Stomach
  • Small intestine (duodenum, jejunum, ileum)
  • Large intestine (colon, cecum, rectum)

Other surrounding organs are:

  • Liver/Gall bladder
  • Pancreas

The Oesophagus has an several layers, including muscle to help food move down. The movement of food by muscles is called Peristalsis. Mucus secreting glands help lubricate the passage of food. 

All digestive processes that happen above the stomach are mechanical.
All digestive processes that happen below (& including) the stomach are chemical.

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Stomach Adaptations

  • The stomach can hold 5 litres of fluid.
  •  Extra muscle layers help to churn the stomach contents.
  • Gastric glands secrets 600ml of gastric juice per meal.
  • The stomach's acid is 0.15M Hydrochloric.
  • The enzymes in the stomach are inactive. Protease/peptidase enzymes have substrate: protein.
  • Enzymes are made of proteins. If the enzymes in the stomach were not inactive, they would self-digest
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Stomach Layers

The inner-most layer after the lumen is the Mucosa. It is folded for expansion and consists of the epithelium and the Muscularis Mucosae.
The next layer, Muscularis Mucosae is a thin layer of smooth muscle, separating the Mucosa from the Submucosa.
Submucosa is the third layer. It is loose connective tissue that suports theMucosa. The first three layers are part of the Mucosal Layer.
The fourth layer of the stomach, the Oblique layer, is part of the Muscularis Externa. These layers are responsible for turning the stomach contents intochyme. The fifth layer, the Circular layer of muscularis, which is where thepylorus is surrounded by a thick muscular wall pyloric sphyncter muscle controls the movement of chyme into the duodenum
The next and final layer of the Muscularis Externa is the Longitudinal layer of muscularis
The final layer is called the Serosa. It is over the muscularis externa and has layers of connective tissue. 

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Adaptations of the small intestine

Small intestine

  • Long coiled tube ~5-6m long
  • The first 30cm is the duodenum - most digestion happens here
  • The ileum forms most of the small intestine. Absorption of digestive food happens here.


  • Alkaline secretions
  • Bile salts from the liver (stored in the gall bladder) to emulsify fats
  • Digestive secretions from the pancreas containing pancreatic enzymes.
  • All of the above are secreted into the duodenum.
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Adaptations for absorption

There are many ways the digestive system is adapted for absorption:

  • Millions of finger-like projections (villi) (20-40/mm²)
  • Each extended 0.5-1.5mm into the lumen.
  • The epithelial cells of the villi themselves have micro-villi, forming a brush border surface to further increase the surface area for absorption.
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Chemical Tests

Biurets: Unknown - Proteins
Reagent: Biurets
Positive Result:Turns Purple

Benedict's: Unknown: Reducing Sugars
Reagent: Benedict's solution
Positive Result: Coloured precipitate (Brick red, orange, green)

Ethanol: Unknown - Lipids
Reagent: Ethanol (alcohol)
Positive Result: cloudy/white emultion

Starch test: Unknown - Starch
Reagent: Iodine
Positive Result: turns black/very dark brown

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Thanks, what exam board is this for?

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