FoodWhen we eat we take in a mixture of macromolecules; starch, protein and fats and micromolecules; vitamins, minerals and water.

Macromolecules need digesting before they can be absorbed whereas, micromolecules are small enough to be absorbed across the membrane of the small intestine without being digested. We also eat fibre which we cannot digest.

Digestive SystemThe function of the digestive system is to break down large, insoluble particles of food (macromolecules) into small, soluble food particles, so that they can be absorbed into the blood from the small intestine. Molecules called enzymes speed up the process.

amylase

EnzymesDigestive enzymes break down macromolecules of starch, protein and fats into smaller soluble molecules so they can be absorbed across the walls of the small intestine.

amylasestarch glucose

proteinpepsin

trypsinamino acids

fatslipase fatty acids &

glycerol

EnzymesEnzymes are specific to the material on which they act, their substrate.

Enzymes are sensitive to temperature.

Enzymes are also sensitive to acidity or alkalinity

Gross Structure of the digestive system

The mouth

Digestion starts in the mouth. Food is mixed with saliva from the salivary glands.

Salivary amylase, found in the mouth, starts the digestion of starch

The OesophagusThe ball of food is swallowed and squeezed down the oesophagus, or gullet into the stomach.

This is done by a process called peristalsis.

The Stomach Food can stay in the stomach for up to 3 hours depending on the protein content of the meal.

Only protein can be digested in the stomach.

The muscular walls mix the food with the gastric juices and begins protein digestion.

This is a slim leaf shaped gland located between the intestines and the stomach, close to the duodenum. It secretes pancreatic juice into the duodenum. This juice is a rich mix of enzymes which break down proteins, fats and carbohydrates.

The pancreas also secretes insulin directly into the bloodstream, where it promotes the uptake of glucose by cells especially cells in the liver and in the skeletal muscles.

The pancreas

The Small Intestine

The small intestine uses enzymes secreted from the pancreas to continue digestion of proteins and starch and the complete digestion of fats.

The nutrients are then absorbed across the walls of the ileum.

Is a 7 metre long, muscular tube, consisting of the;

•Duodenum

•Jejunum

•Ileum

The Small Intestine

The walls of the small intestine have small finger like projections called villi.

These greatly increase the surface area allowing for greater absorption of nutrients into the blood stream.

The intestinal wall also contains glands which secrete enzymes.

The liver Enzymes in the small intestine work best in alkaline conditions. To achieve this, bile is secreted from the liver. Bile is alkaline.

Bile also aids the digestion of fats by emulsifying them.

Would take lipase too long to digest

Bile emulsifies the fats (droplets) and increases their surface area for lipase to act on

The Ileum

The main products of digestion, amino acids and glucose pass into the capillary network which drains into to liver via the hepatic portal vein. The products of fat digestion, fatty acids and glycerol pass into the lacteal and eventually, via the lymphatic system, drain into general circulation.

The products of digestionAmino acids may travel in the bloodstream to areas which are growing, or in need of repair. They are important in making enzymes, some hormones and plasma proteins. Surplus amino acids cannot be stored and are broken down by the liver.

Some of these broken down molecules are used for energy but the nitrogen containing part is converted to urea and filtered by the kidneys to be excreted.

The products of digestionGlucose is either:1.transported to the cells to be used in respiration to release energy2.stored in the liver and muscles as glycogen3.or converted into fat to be stored under the skin or around vital organs.

A macromolecule

The products of digestionGlycerolis used for energy or reconverted into

a form of fat that can be stored. Fatty acids

travel from the lacteals, through the lymphatic

system, into the main veins of the neck. This

allows smaller quantities of potentially harmful

lipids to enter the circulatory system gradually.

The Large IntestineThe large intestine is made up of the colon, the rectum and the anus.

What’s left in the small intestine after digestion and absorption is mainly water, undigested food and bacteria.

The body cannot afford to lose all this water in the faeces so the function of the large intestine is to reabsorb water and form the faeces into stools ready for egesting.

The Large IntestineOnce the water is reabsorbed into the blood the faeces can become semi-solid and eliminated by muscular action of the rectum and relaxation of the anus at a convenient time!

Faeces contains

cellulose (fibre or roughage) from plant cell walls of any fruit and vegetables

dead bacteria, including harmless bacteria living in the large intestine and other bacteria ingested with our food (usually killed off by the acid in our stomach)

cells scraped off the gut lining

some water

All along the gut wall mucus is secreted to reduce friction and ease passage of the chyme and waste as it is moved along by peristalsis