Digestion

Topic Notes

Additional Support Materials

i.e. animations, quizzes, pictures,  worksheets

 
Introduction to Digestion

 

    Digestion Concept Map  
(Chinese University of Hong Kong)
 
Parts of the Alimentary Canal
Digestion Tutorial 
(Thomson Learning)


The GI System 
(The virtual autopsy site
- provided by University of Leicester)

Human Digestive System 
(S-cool)

 
Chemistry of Digestion
Digestive system & nutrition ppt 

Animal Nutrition
 
(Biological Science: Scott Freeman)
  
NEW
Absorption in the Small and Large Intestine (ppt)
 
    
 
 Digestion in Fungi (additional information)
 
Extracellular Digestion in Fungi 
(University of Sydney)

 


 

Digestion
[back to top]

Humans, like all animals, use holozoic nutrition, which consists of these stages:

(Do not confuse egestion, which is the elimination of material from a body cavity, with excretion, which is the elimination of waste material produced from within the body's cells.)

 The human digestive system is well adapted to all of these functions. It comprises a long tube, the alimentary canal or digestive tract (or simply gut) which extends from the mouth to the anus, together with a number of associated glands. The digestive systems made up of different tissues doing different jobs. 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:

 

Parts of the Alimentary Canal [back to top]

  1.  Mouth (Buccal cavity) [back to top]

The teeth and tongue physically break up the food into small pieces with a larger surface area, and form it into a ball or bolus. The salivary glands secrete saliva, which contains water to dissolve soluble substances, mucus for lubrication, lysozymes to kill bacteria and amylase to digest starch. The food bolus is swallowed by an involuntary reflex action through the pharynx (the back of the mouth). During swallowing the trachea is blocked off by the epiglottis to stop food entering the lungs.

 

2.  Oesophagus (gullet)  [back to top]

This is a simple tube through the thorax, which connects the mouth to the rest of the gut. No digestion takes place. There is a thin epithelium, no villi, a few glands secreting mucus, and a thick muscle layer, which propels the food by peristalsis. This is a wave of circular muscle contraction, which passes down the oesophagus and is completely involuntary. The oesophagus is a soft tube that can be closed, unlike the trachea, which is a hard tube, held open by rings of cartilage.

 

3.  Stomach [back to top]

This is an expandable bag where the food is stored for up to a few hours. There are three layers of muscle to churn the food into a liquid called chyme. This is gradually released in to the small intestine by a sphincter, a region of thick circular muscle that acts as a valve. The mucosa of the stomach wall has no villi, but numerous gastric pits (104 cm‑2) leading to gastric glands in the mucosa layer. These secrete gastric juice, which contains: hydrochloric acid (pH 1) to kill bacteria (the acid does not help digestion, in fact it hinders it by denaturing most enzymes); mucus to lubricate the food and to line the epithelium to protect it from the acid; and the enzymes pepsin and rennin to digest proteins.

 

4.  Small Intestine   [back to top]

This is about 6.5 m long, and can be divided into three sections:

(a)   The duodenum (30 cm long). Although this is short, almost all the digestion takes place here, due to two secretions: Pancreatic juice, secreted by the pancreas through the pancreatic duct. This contains numerous carbohydrase, protease and lipase enzymes. Bile, secreted by the liver, stored in the gall bladder, and released through the bile duct into the duodenum. Bile contains bile salts to aid lipid digestion, and the alkali sodium hydrogen carbonate to neutralise the stomach acid. Without this, the pancreatic enzymes would not work. The bile duct and the pancreatic duct join just before they enter the duodenum. The mucosa of the duodenum has few villi, since there is no absorption, but the submucosa contains glands secreting mucus and sodium hydrogen carbonate.

(b)   The jejunum (2 m long) and

(c)   The ileum (4 m long). These two are similar in humans, and are the site of final digestion and all absorption. There are numerous glands in the mucosa and submucosa secreting enzymes, mucus and sodium hydrogen carbonate.

 The internal surface area is increased enormously by three levels of folding: large folds of the mucosa, villi, and microvilli. Don't confuse these: villi are large structures composed of many cells that can clearly be seen with a light microscope, while microvilli are small sub-cellular structures formed by the folding of the plasma membrane of individual cells. Microvilli can only be seen clearly with an electron microscope, and appear as a fuzzy brush border under the light microscope.

Circular and longitudinal muscles propel the liquid food by peristalsis, and mix the contents by pendular movements - bi-directional peristalsis. This also improves absorption.

 
5.  Large Intestine
[back to top]

This comprises the caecum, appendix, ascending colon, transverse colon, descending colon and rectum. Food can spend 36 hours in the large intestine, while water is absorbed to form semi-solid faeces. The mucosa contains villi but no microvilli, and there are numerous glands secreting mucus. Faeces is made up of plant fibre (cellulose mainly), cholesterol, bile, mucus, mucosa cells (250g of cells are lost each day), bacteria and water, and is released by the anal sphincter. This is a rare example of an involuntary muscle that we can learn to control (during potty training).

 

Chemistry of Digestion  [back to top]

1.      Digestion of Carbohydrates  [back to top]

By far the most abundant carbohydrate in the human diet is starch (in bread, potatoes, cereal, rice, pasta, biscuits, cake, etc), but there may also be a lot of sugar (mainly sucrose) and some glycogen (in meat).

 

 

2.      Digestion of Proteins  [back to top]

 Protease enzymes are potentially dangerous because they can break down other enzymes (including themselves!) and other proteins in cells. To prevent this they are synthesised in the RER of their secretory cells as inactive forms, called zymogens. These are quite safe inside cells, and the enzymes are only activated in the lumen of the intestine when they are required.

The lining of mucus between the stomach wall and the food also protects the cells from the protease enzymes once they are activated.

 

3.      Digestion of Triglycerides  [back to top]

 

4.      Digestion of Nucleic acids   [back to top]

 

5.      Other substances  [back to top]

Many substances in the diet are composed of small molecules that need little or no digestion. These include sugars, mineral ions, vitamins and water. These are absorbed by different transport mechanisms:

 

 Digestion in Fungi (additional information)  [back to top]

Fungi are not consumers like animals, but are either saprophytes (decomposers), or pathogens. They therefore use saprophytic nutrition, which means they do not ingest their food, but use extracellular digestion. Fungi secrete digestive enzymes (carbohydrases, proteases and lipases) into the material that surrounds them and then absorb the soluble products (sugars, amino acids, etc).

Fungi are usually composed of long thin threads called hyphae. These grow quickly, penetrating dead material such as leaves, as well as growing underground throughout soil. The cotton wool appearance of bread mould growing on decaying bread is typical of a mass of hyphae, called a fungal mycelium. These thin hyphae give fungi a large surface area to volume ratio. They contain many nuclei, since they are formed from the fusion of many cells.

 

 

[back to top]

Last updated 20/06/2004