Chapter 6 - Digestion

Chapte r 7

Digestion & Nutrition

You will learn... • • • • • • • • •

the concept of a balanced diet the factors that affect energy intake the problems of world food supply the effects of malnutrition the digestive system the structure and function of a villus the function of the hepatic portal vein fats function as storage substances the role of the liver

Contents

Diet fats-sweets group

dairy products group

vegetables group

protein-rich group

fruits group

grain products group

Diet Food Group

Foods in the Group

Grain products

Products made with whole grains or enriched flour.

Fruits and vegetables

Dark green vegetables (broccoli, spinach) and deep yellow vegetables (carrots).

Nutrients Mostly carbohydrates and fibres, Vitamin B, iron and protein. Most carbohydrates and fibre. Various vitamins and minerals

Protein-rich food

Poultry, eggs, fish, red Rich in proteins, Vitamin B and meat, organ meat such as minerals such as magnesium kidneys and liver. Legumes, and zinc. nuts and seeds.

Dairy products

Milk and milk products such Protein, Vitamins A, B and B2. as yoghurt and cheese. Main source of calcium.

Fats-sweets food

Candy, pastries, salad dressing, coffee and cooking oil.

Supplies fats and sugar. Low in nutrient value.

Balanced Diet

Diet

• A balanced diet is a diet which supplies all the essential nutrients in the correct proportions. • A balanced diet must contain enough:  Protein to provide all the essential amino acids for growth and repair for the body  Energy-food to meet the energy requirements of the individual  Minerals and vitamins to maintain good health  Roughage or dietary fibre to stimulate peristalsis  Water to aid the chemical reactions in the body

Diet • Basal metabolic rate and physical activity determine energy needs. • Basal metabolic rate varies with sex, age and state of health of a person.

Malnutrition

Problems of World Food Supplies

In the underdeveloped nations, malnutrition takes many forms: • Starvation – Insufficient quantity of food to supply the daily energy needs of an individual. • Eating disorders – Genetic or environmental in origin, prevent the digestion of food or the absorption of digested food. • Overweight – due to excessive feeding. The excess food is stored as fats. • Obesity – the state in which a person weighs at least 20% more than his ideal weight. • Over-consumption of cholesterol and saturated fats. • Constipation due to a lack of dietary fibre in the diet.

Marasmus

Problems of World Food Supplies

• Marasmus is the wasting of the body from general starvation. • It develops when both energy food and protein are insufficient. • Most victims are children less than 1 year old who have been weaned too early or have been given a poor substitute for mother’s milk.

Kwashiorkor

Problems of World Food Supplies

• Kwashiorkor causes over half of infant deaths in underdeveloped countries. • Children who survive has retarded brain development and their bodies are dwarfed. • Protein deficiency during infancy is irreversible.

Problems of World Food Supplies

Overeating Saturated Fats

• Increases the level of cholesterol in the blood. • Cholesterol deposited on arterial walls harden and eventually lead to heart disease. • Formation of blood clots. • This can strain the cardiovascular system leading to heart attack and high blood pressure.

Problems of World Food Supplies

Overeating Saturated Fats

Constipation

Problems of World Food Supplies

• Bowel movements are difficult and do not occur often enough. • It is caused by a lack of exercise, emotional distress or misuse of laxatives, and/or a diet low in dietary fibres. • Due to constipation, abdominal pressure causes blood vessels in the rectum to enlarge, producing haemorrhoids.

Problems of World Food Supplies

Eating Disorders

Both anorexics and bulimics have a constant fear of being fat and fear rejection • Anorexia nervosa is characterised by constant dieting, rapid weight loss and feeling too fat in spite of the weight loss. • Bulimia is characterised by eating too much food through vomiting or the use of laxatives; tend to gain weight easily and live in constant fear of becoming obese. Bulimics often have mouth and throat problems due to repeating vomiting.

Problems of World Food Supplies

Causes of World Food Supply Shortage

The Population Explosion • The world population today is 6 billion and is on the rise. • Poverty, starvation and depletion of natural resources is on the increase Famine • Industrialised nations have agricultural surplus due to their advances in technology and their high yielding crops. • This results in unequal distribution of food. Floods • Heavy rainfalls during monsoon season, tidal waves and collapsed dams, all cause widespread destruction of the agricultural land and the crops themselves. Drought • Droughts bring about extreme crop damage and the destruction of farm animals.

Problems of World Food Supplies

Solutions to World Food Supply Shortage •

Green revolution – enhanced food yields can be obtained using new strains of wheat, maize and rice.

•

Culture fish, lobsters, prawns and crabs by farming the sea. Seaweed and algae could be grown and harvested as well.

•

Genetically modified food – Plants and animals have their genes altered to improve quality. It is the potential solution to a growing world population and dwindling food supplies.

•

Improve the distribution of food from countries with surplus to overpopulated third world countries. Surplus wheat from USA has been distributed to famine stricken nations.

Problems of World Food Supplies

Solutions to World Food Supply Shortage •

Developing farming technology has helped farmers increase output.

•

Enriched food, such as bread have nutrients added to them to improve the quality of food and help prevent malnutrition.

•

Using better methods of pest control can reduce the quantity of food lost during storage

•

Increasing the land under cultivation can increase food production.

The Digestive System

mouth teeth

salivary glands epiglottis glottis

trachea oesophagus

stomach

liver

cardiac sphincter

gallbladder

pancreas pyloric sphincter

duodenum ileum ascending colon caecum appendix

descending colon large intestine rectum anus

The Digestive System Structure

Functions

Mouth Teeth

• Teeth grind and break down food, exposing a large surface area for enzyme action

Tongue

• Rolls food into bolus before swallowing

Salivary glands

• Produce saliva • Saliva dilutes and moistens food • Mucus sticks food together • Lubricates bolus for swallowing

Pharynx

• Bolus is pushed into the pharynx voluntarily • Involuntary action pushes bolus out of the pharynx into the oesophagus during swallowing

The Digestive System Structure

Functions

Oesophagus

• Connects pharynx to stomach • Waves of muscular contractiond during peristalsis push food into the stomach

Cardiac sphincter

• Controls the entry of food into the stomach from the oesophagus

Stomach

• Muscular wall churns and breaks up food • Secretes gastric juice which mixes with the food to form chyme • Stores food for 2 to 6 hours

Pyloric sphincter

• Controls the exit of chyme from the stomach into the duodenum

The Digestive System Structure

Functions

Small intestine

• Keeps chyme in motion by churning movements • Glands secrete mucus, water and digestive juices

Duodenum

• Allows pancreatic juice to digest food in it

Jejunum

• Absorption of digestion food occurs

Ileum

• Absorption of digestion food occurs

Pancreas

• Secretes pancreatic juice • Enzymes of the pancreatic juice hasten hydrolysis of food in the small intestine

Liver and gall bladder

• Liver produces dark green bile • Gall bladder stores bile • Bile flows into the duodenum through the bile duct • Bile emulsifies fats, providing a greater surface area for the action of lipase

The Digestive System Structure

Functions

Large intestine Caecum and appendix Colon

Rectum Anus

• Vestigial organs with no particular function • Forms junction of small and large intestine • Secrete mucus • Contains large number of bacteria which produce vitamins, methane and hydrogen sulphide • Absorbs water from chyme • Peristalsis compact undigested food to form faeces by muscular contraction of the wall of the rectum • Mucus lubricates faeces and protects the walls from digestive juices • Stores faeces before defaecation • Regulate emptying of rectum

What is Digestion?

The Digestive System

Digestion is the process by which the body breaks down carbohydrates, fats and proteins into simpler substances that cells can absorb and use.

Mechanical digestion is the process of chewing, mashing and breaking food into smaller particles. This increases surface area for enzyme action.

Chemical digestion is the process of changing food into smaller particles through the actions of enzymes.

Mechanical Digestion

The Digestive System

The passage of food along the alimentary canal is accomplished by rhythmic muscular contraction called peristalsis. ring of muscles contract bolus

oesophagus cardiac sphincter stomach

The bolus is being pushed towards the stomach. The waves of muscular contraction are called peristalsis.

Chemical Digestion

The Digestive System

• Large molecules such as starch has to be digested to form smaller molecules which can be absorbed through the walls of the villi. • Enzymatic breakdown involves hydrolysis, which splits large molecule by the addition of water. • Enzymes which hasten the breakdown of carbohydrates, fats and proteins into soluble and simple products can be classified into three groups: amylases, proteases and lipases. Amylases: act on carbohydrates and convert them into simple sugars

Proteases: act on proteins and convert them into various amino acids

Lipases: act on fats and convert them into glycerol and fatty acids

Digestion in the Mouth

The Digestive System

salivary gland nose nasal passage buccal cavity tongue epiglottis salivary glands

palate salivary duct pharynx glottis

amylase

• Saliva: starch  maltose. • Saliva: bicarbonate ions (HCO3-) to buffer the saliva ⇒ maintain optimum pH 6.5 ~ 7.5 for starch digestion.

Digestion in the Stomach

The Digestive System

• Gastric juices are secreted into the stomach by gastric glands in the walls of the stomach. oesophagus • Gastric juices contain water, hydrochloric acid and enzymes. cardiac • The hydrochloric acid lowers the sphincter pH of the stomach contents to a stomach pH of 2.0. • Protein digestion begins with action of pyloric proteases – pepsin and rennin. sphincter • Pepsin catalyses the hydrolysis of protein to form polypeptides. • Rennin catalyses the hydrolysis of milk caseinogen to form milk curds (casein).

duodenum

The Digestive System

Digestion in the Small Intestine Organ

Gland

Secretion

Enzyme

Duodenum (pH 8.5)

Pancreas

Pancreatic juice

Intestinal glands

Intestinal juice (succus entericus)

Enzyme Action Substrate

Product

Trypsin

Protein

Polypeptides Peptones

Amylase

Starch

Maltose

Lipase

Emulsified fats

Fatty acids and glycerol

Maltase

Maltose

Glucose

Sucrase

Sucrose

Glucose and fructose

Lactase

Lactose

Glucose and Galactose

Peptidases

Peptones and polypeptides

Amino acids

The Digestive System Bile • Is a thick, yellow-green fluid produced by the liver. • Helps to digest fats in the body. • Is either secreted directly into the duodenum through the bile duct or stored in the gall bladder. • Contains sodium hydrogencarbonate, bile salts and bile pigments.

The Digestive System Absorption of Digested Food About 90% of the digested food is absorbed in the small intestine. liver

gall bladder

hepatic portal vein

pancreas

bile duct

pancreatic duct duodenum

The small intestine and accesssory digestive organs.

The Digestive System Absorption of Digested Food Features of the intestine: • It is the longest part of the alimentary canal, consisting of the duodenum, jejunum, ileum. • The internal surface of its wall is folded to increase the surface area for absorption. • Its surface is covered with villi to increase the absorptive surface. • Each villus is covered with epithelial cells. The plasma membrane of each cell has minute fingerlike projections called microvilli to increase surface area. • The villi move back and forth independently of each other. Motion is fast after a meal and absorption proceeds faster. • Absorption of nutrients across the membrane occurs by diffusion and active transport.

The Digestive System Absorption of Digested Food microvilli

nutrients enter villus villi

blood capillaries

microvilli

lacteal epithelial lining of the villus

lymph to the neck before entering the heart

lymphatic vessel blood vessel

blood from heart blood carries non-fatty nutrients to the liver

Structure of the villus.

The Digestive System Absorption of Digested Food hepatic vein

to heart posterior vena cava

liver

stomach

hepatic portal vein

intestinal vein

gastric vein small intestine

Blood is carried directly from the stomach and the intestine to the liver via the hepatic portal vein. Hepatic veins then convey it to the heart via the posterior vena cava.

The Digestive System Assimilation • Is the uptake and use of digested food products by the cells.

• oxidised  water + CO2 • excess glucose  glycogen • glycogen stored in liver cells and muscle tissues

• a portion of absorbed fats are stored in adipose tissues as fat droplets • some are use in the synthesis of cell membrane and the rest for energy fat droplet nucleus

large fat globule formed

The Digestive System Assimilation • Is the uptake and use of digested food products by the cells.

• for synthesis of proteins for growth and development; enzymes and hormones

• food residues that cannot be digested leave the small intestine  large intestine

• excess amino acids  ammonia + glycogen

• waste material  compacted into faeces • faeces propelled along colon and rectum, then expelled

The Digestive System Functions of the Liver • • • • • • • • • •

Metabolism of glucose Metabolism of amino acids Production of bile Storage of iron Excretion of bile pigments Synthesis of plasma proteins Detoxification Storage of vitamins Reservoir for blood Source of heat energy

Inquiry-based learning • Divide class into 3 groups • Each group to brainstorm questions related to one of the following facts on chemical digestion in – – – –

Mouth Stomach Small intestine Large intestin