Bio 11 - -Digestive System

Digestion

All organisms are composed of four complex biological molecules: lipids (or fats), proteins, carbohydrates, and nucleic acids.  All are composed of carbon, hydrogen, oxygen and some have nitrogen.

The chemical and physical breakdown of complex biological molecules into their basic components is the process of digestion.

They are important as a structural element in bones, cartilage, hair, nails, and cell membranes.

They are also important as enzymes, hormones, antibodies, hemoglobin and in membrane  transport.

Why are proteins important?

Amino Acids

What are proteins broken down into?

Fish, eggs, meat, and beans

Food that is high in protein

They are a fundamental component of cell membranes and may be used for energy storage.  

They also protect and insulate internal organs.  Certain ones help nerve cells function to conduct impulses. 

Why are lipids important?

Fatty acids and glycerol

What are fats broken down into?

Butter, oil, and nuts

Foods high in fat

They include simple sugars such as glucose and sucrose and more complex sugars in the form of starch, cellulose and glycogen.

When broken down, they are structural components that can be used as a source of ATP.

Why are carbohydrates important?

Potatoes, cereal, pasta, fruits, and vegetables

Foods high in carbohydrates

Simple sugars such as glucose and fructose

What are carbohydrates broken down into?

The digestive system

In most animals the digestive system is made up of a tube (alimentary canal) that runs the length of the body.

Generally the food moves in one direction and different parts are responsible for doing different jobs in the digestive process.

There are also accessory organs that are important in digestion that connect to the alimentary canal via ducts.

Mouth

opening where food enters body, the teeth chew food to help with physical digestion and salivary glands secrete an enzyme that helps to chemically break down the food

Esophagus

muscular tube that connects the mouth with the stomach about 25cm long. Food, now referred to as a bolus, is moved through this structure due to peristalsis.

Stomach

large muscular organ; churns and mixes the food. Produces gastric juices to help with chemical digestion. The pH of this structure ranges from 2-4 depending on its contents.

Where chemical digestion of proteins start.

Cardiac sphincter

Controls the entrance of food to the stomach

Pyloric sphincter

Regulates the release of chyme (soupy mixture of food as it leaves the stomach) from the stomach. 

Composition of gastric juice

Water - Making solution fluid-like. Some enzymes require water in order to function.

Mucous - It eases the passage of food through the gastrointestinal tract and protects the lining of the stomach from being attacked by acid. 

Hydrochloric Acid - This potent acid is secreted by the parietal cells of the stomach. It kills bacteria and other potential pathogens in food and converts the enzyme pepsinogen into pepsin.

Pepsinogen - Pepsinogen is secreted by chief cells in the stomach. Once it's activated by low pH it is converted in to pepsin which begins the chemical breakdown of proteins.

Gastric Lipase - This is an enzyme made by chief cells in the stomach that aids in breaking up short-chain and medium-chain fats.

Small intestine

Long twisted tube which averages about 4-6m (15-20ft) long. This structure is divided into three sections.

Duodenum

First 25-38cm of the small intestine. Receives secretions from the liver and the pancreas. Walls of this section also secrete digestive enzymes to complete chemical digestion. 

Jejunum

2.5m long. Second part of the small intestine. Absorption of nutrients

Ileum

2-4m long. Third part of the small intestine. Further absorption of vitamins and minerals 

Liver

produces bile which is used to emulsify fats.

Pancreas

Secretion of enzymes.

Gallbladder

Storage of bile.

Large intestine

Functions to reabsorb water and salts. Bacteria present produce vitamins which can be absorbed by the colon.   

Waste moves, due to peristalsis, slowly through the various parts of this structure. 

Digestion of proteins

Any enzyme that digests proteins is called a protease

Chemical digestion of proteins begins in the stomach.   

HCl (hydrochloric) acid serves two functions: denatures proteins which changes the shape of the proteins exposing the bonds holding the protein together. Stimulates the secretion of pepsinogen which is then converted to pepsin. 

Pepsin then breaks the proteins into shorter chains called polypeptides. 

The small intestine carries out further digestion with trypsin, which is secreted by the pancreas and small intestine. This breaks the polypeptides down into peptide chains. 

Erepsins secreted by the small intestine break the bonds in the peptide chains so that individual amino acids can be absorbed.

Digestion of lipids

It begins in the stomach with gastric lipase, however; much more is accomplished in the small intestine with the secretion of lipase from the pancreas. 

The physical break up of fat into small droplets which can be distributed in the water of the small intestine is referred to as emulsification.

Bile which is produced by the liver, stored in the gall bladder and pumped into the small intestine when lipids are present is responsible for emulsification. 

Once emulsified, they may be digested into their subunits (glycerol and fatty acids) by digestive enzymes called lipases. They are produced in the stomach and the pancreas (released into the duodenum).

Digestion of carbohydrates

The digestion begins by converting polysaccharides (long chains of simple sugars) and disaccharides (two sugars linked together) into monosaccharides (simple sugar units) that can be absorbed by body cells

It begins in the mouth and is completed in the small intestine (they are not digested in the stomach)

Amylase is the primary enzyme responsible for digesting starch 

It can be found in the saliva as well as in the small intestine secreted by the pancreas.