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.

Induced fit theory

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:

IngestionDigestion

AbsorptionAssimilation

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.