HUMAN HEALTH AND PHYSIOLOGYDIGESTION

Assessment Statements

Explain why the digestion of large food molecules is essential

Explain the need for enzymes in digestion

State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase.

Draw and label a diagram of the digestive system.

Outline the function of the stomach, small intestine and large intestine.

Distinguish between ‘absorption’ and ‘assimilation’.

Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.

Why is Digestion Necessary?

When we eat, we take in food consisting of large, complex organic molecules, which are not suitable to be used since they are too large.

Large molecules can’t pass through membranes to enter the cells that line the intestine or pass on into the bloodstream

Digestion- the biochemical breakdown of large, insoluble food molecules into small, soluble molecules.

This process is essential because only small molecules can enter cells and be used in the body.

Molecules produced by digestion pass through the wall of the intestine by diffusion, facilitated diffusion or active transport.

They enter the bloodstream and travel to the cells, where they are reassembled into new structures.-

Three main types of food molecule that must be digested are carbohydrates, proteins and lipids.Type of Molecule Form of molecule

In ingested foodProduct of Digestion

Carbohydrates Monosaccharides,disaccharides, polysaccharides

Monosaccharides (glucose)

Proteins Proteins Amino acidsLipids Triglycerides Fatty acids and

glycerolNucleic acids DNA, RNA Nucleotides

Digestion of large molecules occurs very slowly at body temperature.

Enzymes are essential to speed up the rate of digestion so that it is fast enough to process nutrients to supply our needs.

There are many different enzymes in the human digestive system.

Different enzymes are released in different sections of the digestive system and each one is specific for one type of food.

Some enzymes are specific to the different carbohydrates that we eat, and others work one after another to digest foods such as proteins in a series of stages.

All digestive enzymes help to catalyze hydrolysis reactions and work best at 37C. Enzyme Type

Example Source Substrate

Products Optimum pH

Amylase Salivary glands

Salivary glands

Starch Maltose 7

Protease Pepsin Gastric glands in stomach wall

Protein Polypeptides 2

Lipase PancreaticLipase

Pancreas Triglycerides

Fatty acids and glycerol

7

Water molecules are used to break the links between the subunits of the large molecules.

The Digestive System

The digestive system consists of long, muscular tubes, also called the gut or alimentary canal.

Associated with it are a number of glands that secrete enzymes and other digestive juices.

The gut extends from the mouth to the anus and is specialized for the movement, digestion and absorption of food.

Mouth, Oesophagus and Stomach

In the mouth, food is broken down into small pieces by the jaws and teeth, and mixed with saliva containing the enzyme salivary amylase, which begins the digestion of any starch the food contains.

The food is then passed down the oesophagus to the stomach by a sequence of muscle contractions known as peristalsis.

http://www.youtube.com/watch?v=o18UycWRsaA

The stomach is a muscular sac that holds food for up to four hours while digestion proceeds inside it.

As the muscles of the stomach contract, food and enzymes are mixed- this gives maximum contact between food and enzyme molecules, and speeds up the digestive process.

http://www.youtube.com/watch?v=URHBBE3RKEs

Digestion of proteins begin here, catalyzed by the enzyme pepsin, which is secreted in gastric juice produced by millions of gastric glands in the stomach wall.

Gastric juice contains pepsin in an inactive form.

Hydrochloric acid activates the pepsin and maintains a pH of 1.5-2 in the stomach.

This pH is optimum for protein digestion and also kills many of the bacteria present in the food we eat.

Goblet cells in the stomach lining secrete mucus to protect the interior of the stomach from the acid and enzymes, which would otherwise digest it.

Food is transformed in the stomach to a semi-liquid called chyme and is ready to the small intestine.

Roles of the Small Intestine

Little by little, chyme leaves the stomach through a valve into the 5 meter long small intestine.

Digestion is completed in the first section of the small intestine.

Digestive juices are secreted from the liver, gall bladder, pancreas and intestine walls.

Bile is added from the liver and gall bladder, and the pancreas secretes pancreatic juice containing trypsin, lipase, amylase and bicarbonate ions.

The acidity of the chyme is reduced by these ions, allowing the enzymes to work at their optimum pH

http://www.youtube.com/watch?v=umPJNAF4VKo

The inner surface of the small intestine is greatly folded to form thousands of villi.

Each villus contains a network of capillaries and a lacteal (vessels for the lymphatic system).

Villi greatly increases the surface area of the small intestine improving its ability to absorb nutrients.

As small molecules such as glucose, Amino acids, fatty acids and glycerol, come into contact with villus, they are absorbed, either passively or by active transport, into the single layer of epithelial cells.

Amino acids and glucose then enter capillaries and are carried away in the bloodstream. Fatty acids and glycerol are taken into the lacteal and travel in the lymphatic system.

After digested food has been absorbed, it is assimilated into the bodyAnd enters cells to become part of the body’s tissues or reserves. Glucose is transported to the liver, which maintains constant level of blood sugar.

Amino acids form part of the reserve of amino acids used to build new proteins in cells all over the body, and fatty acids and glycerol enter the bloodstream from lymph vessels near the heart to be used as an energy source or to build larger molecules.

Absorption- the process by which small molecules are taken through the cells of the intestine and pass into the bloodstream.

Assimilation- the process by which products of digestion are used or stored by the body.

Role of the Large Intestine

By the time food reaches the end of the small intestine, most useful substances have been removed.

Any remaining undigested material passes into the large intestine, which also contains mucus, dead cells from intestine lining and large number of naturally occurring bacteria.

Bacteria here are mutualistic organisms, gaining nutrients and a suitable habitat, while synthesizing vitamin K for the benefit of the human host.

The main role of the large intestine is reabsorbing water and mineral ions such as NA+ and chloride Cl-

Water in the gut contents comes not only from our diet, but also from the many additional liters that are added into the intestine in digestive juices. What remains of the original food now is now feces and is egested, or eliminated from the body via the anus.

Egestion- the process by which undigested material leaves the body at the end of the gut.

Find 5 diseases or complications of the digestive system. What does it do? What causes it? What is the treatment?

List he reasons why digestion is essential.

Distinguish between absorption and assimilation

State why enzymes are needed in digestion.

List the ways in which the villus is adapted to increase the efficiency of absorption of nutrients.