1. Food
When 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.

2. Digestive System
The 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

3. Enzymes
Digestive 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.
amylase
starch
glucose
protein
pepsin
trypsin
amino acids
fats
lipase
fatty acids & glycerol

4. Enzymes
Enzymes are specific to the material on which they act, their substrate.
Enzymes are sensitive to temperature.
Enzymes are also sensitive to acidity or alkalinity
Enzymes are specific to the material on which they act, their substrate.
Enzymes are sensitive to temperature and work best at normal body temperature. If the body temperature drops too low the enzyme action slows down. Enzymes are denatured (change shape) at higher temperatures.
Enzymes are also sensitive to the acidity or alkalinity of their environment. Some enzymes like the acidic conditions of the stomach, which secretes hydrochloric acid e.g. pepsin. Whereas lipase prefers the alkaline conditions in the small intestine created by the liver secreting bile salts such as sodium hydrogen carbonate.

5. Gross Structure of the digestive system

6. 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
Digestion starts in the mouth. Food is mixed with saliva from the salivary glands, chewed, or masticated, by the action of the tongue and teeth. This is called mechanical digestion and is an important action to help enzyme action.
Salivary amylase, found in the mouth, starts the digestion of starch

7. The Oesophagus
The ball of food is swallowed and squeezed down the oesophagus, or gullet into the stomach.
This is done by a process called peristalsis.

8. 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.
The lining of the stomach is muscular and glandular. The gastric pits secrete hydrochloric acid and pepsin (a protease enzyme). The muscular walls mix the food with the gastric juices and begins protein digestion. The resultant paste like mix is called chyme.

9. The pancreas
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.

10. The Small Intestine
Is a 7 metre long, muscular tube, consisting of the;
Duodenum
Jejunum
Ileum
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.
It uses enzymes secreted from the pancreas to continue digestion of proteins and starch and the complete digestion of fats.
By the time the food leaves the small intestine it has been broken down from a semi solid liquid to a liquid.
The nutrients are then absorbed across the walls of the ileum.

11. 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.

12. 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

13. 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.

14. The products of digestion
Amino 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.
If there is not enough available glucose in the body proteins can be used in cellular respiration. The by product produced is ammonia (which is toxic to the body), which is then converted to urea – and excreted in the urine. What would happen if the body was very dehydrated and very low in glucose at the same time?

15. The products of digestion
Glucose is either:
transported to the cells to be used in respiration to release energy
stored in the liver and muscles as glycogen
or converted into fat to be stored under the skin or around vital organs.
A macromolecule

16. The products of digestion
Glycerolis 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.
When fat is broken down to use in cellular respiration the bi-product is ketone bodies. This is potentially toxic to the body and must be excreted by the body. This is what happens if a diabetic injects insulin and then does not eat any thing. The ketones can be detected as a small of acetone (pear drops) on the breath.

17. The Large Intestine
The 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.

18. The Large Intestine
Once 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