The Digestive System

 Enzymes are biological catalysts.  They are natural substances, which speed up the breakdown of food substances and other materials.  They work by combining with the substrate forming the enzymes – substrate complex.  The enzymes – substrate complex breaks down and forms the product and releases the enzyme.  The enzyme can then be used again.  Enzymes are very specific and will only work on one substrate.  Enzymes are very important in the breakdown of food in our digestive system.  Enzymes work at specific pH’s. Most at pH 7 but pepsin works only at pH 2 & 3.

 The alimentary canal is a long tube going from the mouth to the anus.  Along the tube are glands where substances called enzymes are produced.  The intake of food has five steps: Ingestion Digestion Absorption Assimilation Egestion

 Ingestion is the in take of food.  Digestion is the physical or chemical breakdown of food, so that it can be absorbed into the blood stream.  Absorption is the release of digested food into the blood stream.  Assimilation is the use of digested food in the organisms cells.  Egestion is the removal of indigestible waste materials from the body.

 Digestion in the human occurs in distinct areas:  Mouth (Physical and Chemical)  Stomach (Physical and Chemical)  Duodenum (Chemical)  Ileum (Chemical)  Large Intestine (Chemical)

 Food is mechanically broken down by the teeth, increasing the surface area for the enzyme to work.  Saliva, containing the enzyme Salivary Amylase, breaks down Starch into Maltose.  Food is brought down to the stomach, in a structure called a bolus through the oesophagus by peristalsis.

 The stomach contains Hydrochloric Acid giving it a pH of 1 – 2.  Hydrochloric acid loosens food, kills bacteria, triggers stomach enzymes and turns off amylase.  Inactive Pepsinogen is activated by the low pH turning it into Pepsin (Proteinase).  Proteinase breaks down proteins into peptides.  Peptides are short chains of Amino Acids.

 The stomach churns the food for a number of hours until the food is a thick soupy liquid called Chyme.  Sphincter muscles relax and the chyme enters the duodenum.  Physical digestion by the churning process, chemical digestion by the action of enzymes.

 Lots of enzymes are secreted from the wall of the duodenum, the pancreas and the Gall bladder.  Enzymes including lipase, proteinase (trypsin), amylase, maltase, sucrase, lactase, are found there.  Bile is also secreted from Gall Bladder.  Bile emulsifies fat, so that it can be digested.  Bile also neutralises the stomach acids.

 Intestinal juice secreted from the walls of the small intestine contains the same enzymes as the duodenum.  Most food is digested here, as it is 5.5 metres long.  The wall of the intestine contains millions of finger like projections called villi (villus in singular) so the surface area is increased.  Sugars, Amino acids, vitamins, minerals and water are absorbed into the blood stream.  This material travels to the liver via the Hepatic Portal Vein and then onto the heart.  Fatty acids and glycerol are transported by the lymph vessels within the villus itself.

 Here any remaining broken down food is absorbed.  Water is removed from the waste.  Excess substances are moved to other parts of the body for storage.  Excess Glucose is stored when too much is ingested than needed.  Glucose can be stored in the short term as Glycogen. It is stored in the liver.  The long-term storage of glucose fat.

 Because cows, sheep and horses feed on fibrous materials like grass and hay, their stomachs are adapted to suit their needs.  The ruminant has four compartments in their stomach, the abomasum, the rumen, the reticulum and the omasum.  In the largest compartment, the rumen, there are millions of bacteria and protozoans.  These organisms produce enzymes that break down cellulose into starch.  The organism get what they want and the cows gets the energy value.  This kind of relationship is called a symbiotic relationship.

 The organisms can also make all the amino acids needed for the ruminant so there are no essential amino acids.  These bacteria can make all B vitamins as well.  The rumen is nearly 250 litres in volume and is far bigger than the other compartments. Breakdown of cellulose occurs here.  The reticulum is the smallest of the compartments and is used to bring food back to the mouth to be re-chewed (chewing the cud).  The omasum squeezes the semi – digested food and increases the surface area for the bacteria.  In the abomasum all the processes that take place in the stomach of the human occur here.  No amylase is found in the saliva of cows, so chewing the grass only breaks the food down physically.  This increases the surface area for chemical breakdown in the stomach compartments.