Unit 14: Human Digestive System

• provide energy to do work • maintain a healthy body • grow new cells and tissues • repair damaged tissues • Important nutrients: carbohydrates, proteins and fats • If one part of the digestive system is not functioning, all the other parts would be affected & food cannot be completely digested!

• Digestion is the breaking down of large insoluble food molecules into smaller soluble molecules that can be absorbed by the body. • There are 2 types of digestion processes: Physical and Chemical

Physical * It is a mechanical/physical breakdown of large pieces of food into smaller pieces * No new products are being formed To increase the surface area for enzymes to work on • Examples of physical digestion: • Chewing action of food in the mouth • Peristalsis in the oesophagus • Churning action in the stomach • Emulsification in the small intestine

• It is a process whereby large and complex substances are broken down into smaller and simpler substances by the action of enzymes • New products are formed To allow food substances to enter the bloodstream through the intestinal wall via diffusion (or absorption??)

• Enzymes are complex proteins that speed up the rate of chemical reactions • Digestive enzymes in particular speed up the process of digestion by breaking up large food molecules into smaller food molecules • Enzymes are highly specific. Each enzyme can only work on one type of food substance! • Enzymes are not used up after the digestion. They remain chemically unchanged. There are 4 types of enzymes: amylase, maltase, lipase and protease!

Starch Iodine test Brown colour of iodine solution turns blue-black Proteins Biuret test (made up of sodium hydroxide and copper(II) sulfate) Blue solution of Biuret reagent turns violet Fat Emulsification test (containing ethanol and water) Milky solution is observed after shaken Glucose Benedict’s Test Brick-red precipitation is observed (High amount of reducing sugar present)

Initial Product Enzyme Final Product Place of Ingestion Carbohydrates Amylase (Carbohydrase) Maltose Mouth Oesophagus (continued from mouth; no new enzymes mixed) Maltose Small Intestine Maltase (Carbohydrase) Glucose Small Intestine • Amylase breaks down food into maltose, Maltase breaks down maltose into glucose • The ratio will always remain the same! • Carbohydrase = both Amylase and Maltase enzymes

Initial Product Enzyme Final Product Place of Ingestion Initial Product Proteins Protease Amino Acids Stomach Small Intestine

Initial Product Enzyme Final Product Place of Ingestion Fats Lipase Glycerol and fatty acids Small Intestine Ratio of Glycerol to Fatty acids will always be 1:3 !!!

• Ingestion - The intake of food in the mouth • Digestion - The physical as well as chemical break down of large insoluble food molecules to smaller soluble food molecules - Begins in the mouth, continues in oesophagus (but no new enzymes added), stomach and ends in small intestine • Absorption - The entry of soluble food molecules into the blood and then cells - Occurs in the small and large intestine • Egestion - The removal of undigested food from the body stored in the rectum temporarily then through the anus

• The intake of food in the mouth

• The physical as well as chemical break down of large insoluble food molecules to smaller soluble food molecules • Begins in the mouth, continues in oesophagus (but no new enzymes added), stomach and ends in small intestine

• The entry of soluble food molecules into the blood and then cells • Occurs in the small and large intestine

• The removal of undigested food from the body stored in the rectum temporarily then through the anus

• The whole alimentary canal is made up of muscles • Peristalsis is a series of wave-like muscle contractions and relaxation that helps to squeeze food along the alimentary canal.

• Ingestion: • The intake of food through the mouth is called ingestion • Physical digestion: • Chewing by the teeth cuts and grinds food into smaller pieces. o This increases the surface area of the food which makes digestion by enzymes faster • Chemical digestion: • digested in the mouth • Starch molecules are broken down by enzyme salivary amylase (a type of carbohydrase) into maltose molecules

• Physical digestion: • Peristalsis helps move the food down from the mouth to the stomach within about 10 seconds • Chemical digestion: • No enzymes are secreted here. But the digestion of starch by salivary amylase continues here as the food moves down the oesophagus!

• Stomach * Physical digestion: - The churning action (peristalsis) by the muscular walls of the stomach causes food to break up further into smaller pieces. It also helps food mix with gastric juice. - Gastric juice contains water, hydrochloric acid and proteases. - Functions of hydrochloric acid (also function of stomach): • kills bacteria in the food • provides an acidic environment for proteases to work effectively * Chemical digestion: - Complex proteins are broken down into simpler proteins (*polypeptides) by enzyme proteases in the gastric juice • Our stomach contains hydrochloric acid, which is highly acidic and is capable of dissolving an iron nail. • The stomach is adapted to withstand the corrosive effect of the acid as: 1. Stomach cells secrete a thick layer of mucus. • Mucus acts as a barrier between the acid and the stomach walls 2. Hydrochloric acid is only produced when the stomach contains food. When the stomach is empty, hydrochloric acid is not secreted. 3. In any case, the stomach lining is replaced every three days by the constant production of new cells in the inner lining.

• Small Intestine * 2 main processes: emulsification and absorption * Physical digestion: - Peristalsis continues to occur here to move food along the alimentary canal - Bile carries out the process of emulsification, whereby large fat globules are broken down into smaller fat droplets. o This increases the surface area of fats so that they can be digested by lipases faster. - Bile is not an enzyme. It only physically breaks down fat. Emulsification is a form of physical digestion. • Chemical digestion: • Remaining starch molecules are broken down by amylase (a type of carbohydrase) into maltose • Maltose molecules are broken down by maltase (a type of carbohydrase) into glucose molecules • Simple proteins are broken down by protease into amino acids • Fats are broken down by lipase into glycerol and fatty acids • Absorption: • Absorption of food is the movement of food molecules through the walls of the small intestine into the bloodstream. • Food stays in the small intestine for about 5 hours. • The end products of complete digestion of food are glucose molecules, amino acids and Glycerol and fatty acids. • These digested small and soluble food molecules diffuse through the walls of the small intestine and into the blood vessels that surround it. Blood carries these food molecules to the body cells for life processes (e.g. respiration).

The small intestine is very long (~6 m) This increases the time for more complete absorption of food The small intestine has numerous folds in the inner walls, consisting of structures such as villi and microvilli This increases the surface area to volume ratio for faster absorption of food substances The small intestine is supplied with many blood capillaries The blood capillaries help to transport away absorbed nutrients quickly, maintaining a steep concentration gradient for continued absorption to occur (by diffusion) The villi epithelium is very thin, only one cell layer thick This allows for easier absorption by diffusion of food molecules across the thin villi epithelium into the bloodstream

• food in the small intestine is mixed with: 1. Bile produced by the liver and secreted from the gall bladder 2. Pancreatic juice from the pancreas 3. Intestinal juice that it produces itself (small intestine) • The gall bladder, liver and pancreas are not part of the alimentary canal as food does not pass through them!

stores bile and releases bile into small intestine only when food is present

produces pancreatic juice which contains enzymes that can help with digestion in the small intestine

produces bile