digestion – the chemical and mechanical breakdown of food

Digestion Digestion takes place within a tube called the digestive tract which begins with the mouth and ends with the anus.

Functions of the Digestive Tract To ingest food To transport the food from one end to the other. To digest the food into smaller parts (nutrients that can cross the plasma membrane). To absorb the necessary nutrients into the bloodstream To expel the waste products from the body

Nutrition from Digestion The nutrients that our body must absorb from our food are: Carbohydrates – to store energy Lipids – for insulation, protection, energy storage and membrane structure. Proteins – to make and repair structures, as a form of energy, as hormones, antibodies, etc.

Vitamins – for our metabolism Minerals – to act as building blocks Water – for hydration, transportation, etc.

Digestion involves two main processes that occur at the same time: Mechanical Digestion – physically breaking food down into smaller pieces. Chemical Digestion – enzymes breaking down macromolecules into small molecules that are easily absorbed.  

Mechanical Digestion Mechanical digestion can be broken down into four parts: 1.Mastication Starts in the mouth as the teeth grind, tear and break down food. The tongue manipulates and mixes ingested materials during chewing and helps form the bolus (a lump of chewed food). It also performs important functions in swallowing.

2. Peristalsis a series of wave-like muscle contractions that moves food through the digestive tract. begins in the esophagus when a bolus of food is swallowed and strong wave-like motions of the smooth muscle carry it to the stomach. In the stomach it is churned into a liquid mixture called chyme.

continues in the small intestine where it mixes and shifts the chyme back and forth, allowing nutrients to be absorbed into the bloodstream. concludes in the large intestine where water from the undigested food material is absorbed into the bloodstream. Finally, the remaining waste products are excreted from the body through the rectum and anus.

Peristalsis http://www.youtube.com/watch?v=RsTwabX4ggI

3. Emulsification of Fats: The small intestines receives bile from the liver, which emulsifies fat. Large congealed droplets of fat are separated into smaller, more manageable droplets that can be easily digested. This process is similar to oil and vinegar separating in salad dressing.

4. Defecation Physical removal of solid wastes (feces) from the anus.

Chemical Digestion Chemical Digestion can be broken down into 3 locations: 1) Mouth: Begins in the mouth with the salivary glands which release the enzyme salivary amylase. Salivary amylase begins the process of digesting starch. It breaks starch down into the molecule maltose which will then be digested in the small intestine

2) Stomach: The stomach lining contains gastric glands which secrete gastric juice (a mixture of pepsinogen, hydrochloric acid and mucous). The hydrochloric acid causes the stomach to have a high acidity which kills most of the bacteria present in food. It also activates the enzyme pepsin. Pepsin begins digesting protein in the stomach by breaking it down into peptides.

3) Small Intestine: The small intestine is also involved in digesting proteins. The enzyme trypsin, which is produced by the pancreas, breaks proteins down into peptides.

The enzyme peptidase, produced by the small intestine, breaks the peptides down into amino acids that can then be absorbed by the small intestine. Ducts from the liver and pancreas join to form one duct that enters the duodenum.

The pancreas secretes lipase and trypsin into the small intestine. The small intestine receives bile from the liver which emulsifies the fat in food (turns it into droplets that disperse in water) Once the fat has been emulsified it can be digested by the enzyme lipase.

The starch that was broken down in the mouth to maltose is further broken down into glucose molecules by the enzyme maltase. These glucose molecules can then be absorbed by the small intestine. The enzyme lactase digests lactose which is the sugar found in milk. Many people do not produce lactase and therefore are considered lactose intolerant.