1. DIGESTION AND ABSORPTION
by ASLI ALP

2. Digestive System
A complex system of organs that converts food into energy. The human digestive system is a coiled, muscular tube (6-9 metres long) extended from the mouth to the anus.
Several specialized compartments occur along this length; mouth, pharynx, oesophagus, stomach, small intestine, large intestine and anus. Accessory digestive organs are connect to the main system by a series of ducts; salivary glands, parts of the pancreas, liver and gall bladder.
Each part of the digestive system has a specific function (breaking down food and absorbing its nutrients).

3. THE MOUTH
Mechanical breakdown begins in the mouth by chewing and actions of the tongue, the tongue move food around and the saliva produced coats and lubricates the food for easier chewing and swallowing.
Saliva contains an enzyme called “amylase” helps to chemically break down carbohydrates in the meal. Teeth helps to chemically break down food into smaller pieces by tearing and shredding it.
Mixing food with saliva (water, amylase and mucus) parts digest food so it can be swallowed easily. The food is then pushed into the oesophagus. The amylase hydrolyses starch into maltose.

4. THE OESOPHAGUS
The oesophagus is a tube in the chest that connects the mouth and throat to the stomach.
Muscles in the wall of the oesophagus work in a coordinated manner to push food and liquids down into the stomach. This muscular movement is called “peristalsis”
Peristalsis involves a series of muscular contractions and relaxations which propel food forward until it reaches a ring-like valve called the “sphincter”, at the junction of the oesophagus and the stomach.
Mucus is secreted from grandular tissue in the walls, to lubricate food`s passage downwards.

5. THE STOMACH
The stomach is located at the bottom of the chest, just below the oesophagus.
The stomach`s first job is to accept and store food which is enters in. It can hold up to 4 litres of food and liquid.
There are glands in the lining of the stomach produce stomach acid (or gastric acid) another digestive juice.
Muscles in the wall of the stomach helps to move the food and acid around, making sure that they mix throughly. The entrance and exist of the stomach are controlled by “sphincter muscles”.
Gastric juice is the material that breaks down the food. Gastric juice contain HCl and some enzymes, including pepsin, rennin, traces of lipase. Pepsin is an endopeptidase, it hydrolyses peptide bonds in the middle of polypeptide ,molecules, breaking them into smaller polypeptide chains.

6. THE SMALL INTESTINE
The small intestine also known as the “small bowel”.
It is the longest portion of the digestive tract, it is more than 6 metres long and is located within the middle of the abdomen.
It has two sections, the duodenum and ileum.
Much of the small intestine is coiled and suspended in a thin layer of fat, which give the intestine a lot flexibility and mobility.

7. THE DUEDONUM
The Duodenum is located at the junction of the stomach and the small intestine.
The duodenum is the first part of the small intestine. It is C-shaped and about 25 cm long.
This is the place where the ultimate destruction of food digestion reaches its completion and where the acidity of chyme is nullified

8. Bile is a complex fluid containing water, electrolytes and a battery of organic molecules including bile acids
The gall bladder stores and concentrates bile during the fasting state, then it enters the duodenum through the bile duct. Bile duct breaks down fat particles into fatty acids and glycerol to neutralize stomach acid
Pancreatic juice is produced by pancreas and pours into the small intestine through a tube, a duct. The pancreatic juice contains enzymes trypsin amylase and lipase. Trypsin breaks down the partly digested proteins, amylase changes into starch into simple sugars
Exopeptidases hydrolyse peptide bonds found at the end of polypeptide chains, giving free amino acids
Intestinal juice is produced by the walls of the small intestine. It contains more digestive enzymes, more lipase, maltose, exopeptidases and dipeptidases

9. THE ILEUM
The inner linings of ileum contain very small finger-like bumps called “villi”. The presence of these tiny bumps on the inside of the small intestine means that the surface area is much larger than if the lining were just a flat surface. Absorption is through diffusion, facilitated diffusion and active transport.

10. THE PANCREAS
The pancreas is an oblong-shaped organ in the posteriors part of the abdominal cavity, behind the stomach
The pancreas secretes approximately 2 litres of digestive juices into the duodenum per day
It release pancreatic juice into the duodenum via the pancreatic duct.
Pancreatic juice contains amylase, tyrpsin, the endopeptidase chymotrypsin, and lipase

11. THE LARGE INTESTINE
The large intestine is the part of the digestive system where waste products from the food you eat are collected and processed into the faeces.
The large intestine is about 1.5 m long and consists of the caecum, appendix, colon and rectum
The large intestine performs the following functions;
reabsorbs water and maintains the fluid balance of the body
absorbs certain vitamins
processes undigested material
Stores waste before it is eliminated

12. The colon has a folded wall to provide a large surface area for absorption. The faeces are stored in the rectum and then pass through sphincter muscles, at the anus during defecation

13. Enzymes and Digestive Juices have different functions;
site of production
site of action
function
Amylase
Salivary gland
Mouth
Starch to glucose
Pepsin
Stomach
Protein to polypeptides
Amylse
Pancreas
Duodenum
Starch to maltose
Bile salts
Liver (stored in the gall bladder)
Emulsify fats and neutralise the acidity of chyme
Lipase
Fats to fatty acids and glycerol
Maltase
Duodenal glands
Maltose to glucose
Trypsin
Chymotrypsin
Endopeptidases
Stomach, pancreas
Stomach, duodenum
Long polypeptides to shorter ones
Exopeptidases
Pancreas, duodenal glands
Short polypeptides to amino acids
Dipeptidases
duodenum
Double acids to single ones

14. REFERENCES
A-2 Level Biology. AQA A Revision Guide. Coordination Group Publications Ltd. 2005