6: Nutrition in Humans

Question 1

List and explain all processes of nutrition in humans.

Answer 1

1. Ingestion: Food is taken in through the mouth.
2. Digestion: Food is broken down into soluble and simple molecules.
3. Absorption: The soluble and simple food molecules enter the circulatory system.
4. Assimilation: The absorbed food molecules are taken up by cells for metabolism.
5. Egestion: The undigested and unabsorbed materials are removed from the body as faeces.

Question 2

Where does nutrition in humans take place?

Answer 2

Nutrition in humans take place in the digestive system, which consists of the alimentary canal and its associated digestive glands.

Question 3

What are the organs in the alimentary canal?

Answer 3

The alimentary canal is a long tube starting from the mouth cavity, which leads to the pharynx, the oesophagus, the stomach, the small intestine (duodenum and ileum), and the large intestine (caecum, colon, and rectum). It ends at the anus.

Question 4

List three digestive glands and their functions.

Answer 4

Salivary glands, pancreas, liver
They produce digestive juices to the alimentary canal.

Question 5

What is the name of the process of chewing food into small pieces?

Answer 5

Mastication

Question 6

List out all types of human teeth.

Answer 6

Incisors, canines, premolars, and molars.

Question 7

What are the shape and function of incisors?

Answer 7

Incisors are chisel-shaped with flat sharp edges. They have one root. They are used for biting and cutting food.

Question 8

What are the shape and function of canines?

Answer 8

Canines are pointed and curved. They have one root. They are used for tearing flesh.

Question 9

What are the shape and function of premolars?

Answer 9

Premolars have broad tops with cusps. They have one or two roots. They are used for crushing and grinding food.

Question 10

What are the shape and function of molars?

Answer 10

Molars have a similar shape to premolars. They have larger broad tops and larger cusps. They have two or three roots. They are used for crushing and grinding food.

Question 11

What is dentition?

Answer 11

Dentition refers to the numbers and arrangement of different types of teeth in a mammal. It can be represented by a dental formula.

Question 12

What does the dental formula describe?

Answer 12

The dental formula shows the numbers and types of teeth on each side of the upper and lower jaws.

Question 13

What is the dental formula for adults?

Answer 13

2123/2123

Question 14

How many teeth are there in a complete set of milk teeth, and how can it be represented with the dental formula?

Answer 14

There are 20 teeth in a complete set of milk teeth. There is no premolar and there are only 2 molars on each side. The dental formula is 2102/2102.

Question 15

List and explain three regions that a tooth is divided into.

Answer 15

Crown, neck, and root. The crown is the region visible above the gum. The neck is the region surrounded by the gum, and the toot is the region embedded in the jaw bone.

Question 16

What are three layers that each tooth consists of?

Answer 16

Enamel, dentine, and pulp cavity.

Question 17

What are the features of enamel?

Answer 17

Enamel is the outermost, hardest part of the crown. It is the hardest tissue in our body. It is non-living and is made mainly of calcium salts. It protects the tooth from wearing down as a result of chewing.
Around the roots, the enamel is replaced by cement.

Question 18

What are the features of cement?

Answer 18

Around the roots, the cement replaces the enamel. Fibres from the cement pass into the jawbone, forming the
periodontal membrane. The cement and periodontal membrane fix the tooth to the jawbone.

Question 19

What are the features of dentine?

Answer 19

Dentine is the middle region of the tooth. It is a bone-like substance, but is not as hard as enamel.
There are strands of living cytoplasm running through the dentine.

Question 20

What are the features of the pulp cavity?

Answer 20

The pulp cavity contains living cells, blood vessels, and nerve fibres. The blood vessels supply oxygen and nutrients to the tooth, and remove wastes from it. The nerve fibres can detect temperature and pressure/pain.

Question 21

Give two examples of common dental diseases.

Answer 21

Tooth decay and periodontal disease.

Question 22

How is tooth decay developed?

Answer 22

Tooth decay starts with the formation of plaque, which consists of food debris and bacteria, on tooth surfaces.
Bacteria in the plaque act on the food debris to produce acids. The acids dissolve the enamel and make holes on the tooth.

Question 23

How is periodontal disease developed?

Answer 23

Periodontal disease starts with the formation of plaque, which consists of food debris and bacteria, on tooth surfaces. The plaque accumulates between the teeth and gums. The bacteria in the plaque produce toxins which cause inflammation of the gums. The gums become red, swollen and can bleed easily. If the inflammation is not treated, the toxins start to break down the cement, the periodontal membranes, and the jawbone. The teeth may eventually become loose and fall out.

Question 24

Why is digestion necessary?

Answer 24

Among the seven types of food substances taken into our body, starch and most proteins and lipids are large, complex molecules. Since the wall of the alimentary canal is differentially permeable, the food molecules must be broken down into simple, small, and soluble molecules so that they can pass through the wall and enter the blood. The process of breaking down food into small molecules is called digestion.

Question 25

Give three examples of substances that a can be absorbed into the blood directly without undergoing digestion.

Answer 25

Water, vitamins, and minerals

Question 26

List 2 types of digestion processes in our body.

Answer 26

Physical digestion and chemical digestion

Question 27

What is physical digestion?

Answer 27

Physical digestion is the breaking down of food into smaller pieces by physical actions. It increases the surface area of food in contact with the digestive juices, but it does not change the chemical structure of the food.
After physical digestion, the resultant food pieces are not yet small enough for absorption. They must be further broken down into smaller molecules by chemical digestion.

Question 28

Give 4 examples of physical digestion.

Answer 28

Mechanical actions of the alimentary canal, including
1. Chewing in the mouth cavity (mastication)
2. Churning in the stomach
3. Peristalsis along the alimentary canal.
4. Emulsification of lipids by bile salts in the small intestine.

Question 29

What is chemical digestion?

Answer 29

Chemical digestion involves chemical reactions in which large, complex food molecules are broken down into small, simpler forms. The reactions are catalysed by digestive enzymes.

Question 30

List all types of digestive enzymes in our digestive system.

Answer 30

There are three main types:
1. carbohydrases, which break down complex carbohydrates into simpler sugars
2. proteases, which break down proteins into polypeptides, peptides and amino acids
3. lipases, which break down lipids into fatty acids and glycerol.

Question 31

What kinds of digestion take place in the mouth cavity?

Answer 31

In the mouth cavity, food is chewed into small pieces and digested physically by teeth. It is also mixed with saliva, a digestive juice produced by the salivary glands, which digests starch chemically and breaks it down to maltose. After chewing, the tongue rolls the food into a soft mass called a bolus. The bolus is then swallowed down the oesophagus through the pharynx.

Question 32

What are the components of saliva?

Answer 32

1. Salivary amylase: It is a kind of carbohydrase. It catalyses the breakdown of starch into maltose. It works best in a slightly alkaline medium.
2. Mucus: It is a sticky material that helps bind food particles together. It moistens and lubricates the food, making the food easier to be swallowed.
3. Water: It dissolves soluble substances in the food.

Question 33

How does the bolus move through the pharynx to the oesophagus?

Answer 33

The pharynx leads to both the oesophagus and the trachea. To ensure that the bolus passes into the oesophagus, and not into the trachea and the nasal cavity, a series of events takes place during swallowing:
1. The tongue rises to push the bolus towards the pharynx.
2. The soft palate moves up to prevent the bolus from entering the nasal cavity.
3. The larynx rises so that the epiglottis covers the opening to the trachea. This prevents the bolus from entering the trachea.
4. The bolus enters the oesophagus.

Question 34

What is peristalsis?

Answer 34

Peristalsis is the wave-like movement of the wall of the alimentary canal. It is produced by the alternate contraction and relaxation of the longitudinal muscles and circular muscles in the wall of the alimentary canal.

Question 35

How is the bolus pushed down the oesophagus to the stomach by peristalsis?

Answer 35

Behind the bolus: circular muscles contract, longitudinal muscles relax, the lumen becomes smaller. The food is squeezed forward.
In front of the bolus: circular muscles relax, longitudinal muscles contract. The food slides through the lumen.

Question 36

What is the importance of peristalsis?

Answer 36

1. It pushes food along the oesophagus and other parts of the alimentary canal.
2. It helps mix the food with digestive juices.
3. It increases the contact between the wall of the alimentary canal and digested food to facilitate absorption.

Question 37

What are the names of the rings of muscles guarding the entrance and exit of the stomach respectively?

Answer 37

Cardiac sphincter: guards the entrance of the stomach
pyloric sphincter: guards the exit of the stomach

Question 38

What is the importance of the cardiac sphincter and the pyloric sphincter?

Answer 38

Contraction of the cardiac sphincter prevents food from flowing back into the oesophagus. Meanwhile, the pyloric sphincter controls the release of food into the duodenum. This allows the food to stay in then stomach for a sufficiently long period of time for digestion.

Question 39

What kinds of digestion takes place in the stomach?

Answer 39

Gastric juice secreted by gastric glands on the stomach wall digest the food chemically. Muscles in the stomach lining contract to churn the food into a creamy liquid called chyme, digesting the food physically. The chyme is then released into the duodenum.

Question 40

What are the components of gastric juice?

Answer 40

1. Pepsin: It is a kind of protease. It catalyses the breakdown of proteins into peptides. It works best in an acidic medium of about pH 2.
2. Hydrochloric acid: It provides an acidic medium for the action of pepsin. It kills most bacteria in food.
3. Mucus: It protects the stomach wall from being digested by the pepsin and being damaged by the hydrochloric acid.

Question 41

What is the cause of gastric ulcer?

Answer 41

A kind of bacteria called Helicobacter pylori

Question 42

What kinds of digestion take place in the small intestine?

Answer 42

The chyme from the stomach contains partly digested food. The digestion of food is completed in the small intestine, which includes the duodenum and ileum. In the small intestine, food is chemically digested by pancreatic juice and juices excreted by specialised cells on the epithelium of the small intestine. It is also physically digested by bile secreted by the liver.

Question 43

What are the features and components of bile?

Answer 43

Bile is a green alkaline fluid produced by the liver. It is temporarily stored in the gall bladder and is released through the bile duct into the duodenum. It contains no digestive enzyme. It contains three substances:
1. Bile salts: They emulsify lipids into small droplets. This facilitates chemical digestion by increasing the surface area of lipids for lipase to act on.
2. Bile pigments: They are formed from the breakdown of haemoglobin from red blood cells. They are waste products and do not take part in digestion.
3. Sodium hydrogencarbonate: It is alkaline. It neutralises the acidic chyme and provides an alkaline medium for the action of enzymes in the small intestine.

Question 44

What are the features and components of pancreatic juice?

Answer 44

Pancreatic juice is produced by the pancreas. It is carried along the pancreatic duct to the duodenum. It contains four substances:
1. Pancreatic amylase: It catalyses the breakdown of the remaining starch (not digested in the mouth cavity) into maltose.
2. Proteases: They catalyse the breakdown of some proteins into peptides, and some peptides into amino acids.
3. Pancreatic lipase: It catalyses the breakdown of the emulsified lipids into fatty acids and glycerol.
4. Sodium hydrogencarbonate: It is alkaline. It neutralises the acidic chyme and provides an alkaline medium for the action of enzymes in the small intestine.

Question 45

What are the features and components of intestinal juice?

Answer 45

Intestinal juice is produced by some glands in the wall of the small intestine. It is slightly alkaline and mainly consists of water, mucus, and sodium hydrogencarbonate. It provides an alkaline medium for the action of enzymes.
The epithelium of the small intestine has specialised cells that have various enzymes on their cell membranes. They include carbohydrases which catalyse the breakdown of disaccharides into monosaccharides, and proteases which catalyse the breakdown of some peptides into amino acids.

Question 46

Give two examples of carbohydrases that catalyse the breakdown of disaccharides into monosaccharides.

Answer 46

Maltase, which catalyses the breakdown of maltose into glucose.
Sucrase, which catalyses the breakdown of sucrose into glucose and fructose.

Question 47

Where does absorption of food molecules mainly take place?

Answer 47

Absorption of food molecules mainly takes place in the ileum (small intestine).

Question 48

In what ways is the small intestine adapted for absorption?

Answer 48

1. Our small intestine is very long; it is about 7m long in adults. This allows sufficient time for complete digestion snd absorption.
2. The inner wall of the small intestine is highly folded and there are many finger-like projections called villi.
3. Peristalsis of the small intestine brings newly digested food molecules into close contact with the villi for absorption. This also helps keep a steep concentration gradient of food molecules across the wall of the small intestine to increase the rate of diffusion of food molecules.

Question 49

How do the structural features of villi help it adapt for absorption?

Answer 49

1. Villi are finger-like projections. This increases the surface area for absorption.
2. The epithelium of the villi is very thin, being only one-cell thick. This provides a short distance for diffusion of food molecules into the blood. Absorption can take place rapidly.
3. The cell membrane of the epethelial cells of the villi has a large number of microvilli/brush borders. This further increases the surface area our absorption.
4. Each villus has a lymph vessel called lacteal in the centre. The lacteal is surrounded by a network of capillaries. These allow the absorbed food molecules to be carried away rapidly. A steep concentration gradient for molecules is kept across the wall of the small intestine to increase the rate of diffusion of food molecules.

Question 50

How is digested food absorbed into the blood?

Answer 50

Water-soluble food molecules like monosaccharides, amino acids, minerals and water-soluble vitamins (eg: vitamin C), can be absorbed into the capillaries by diffusion and active transport.
As the absorption of digested food into the blood increases the water potential of the content in the small intestine, water is drawn into the blood by osmosis. In other words, absorption of digested food in the small intestine is accompanied by absorption of water at the same time. Over 90% of water in food is absorbed in the small intestine.

Question 51

How is digested food absorbed into the lymph?

Answer 51

Fatty acids and glycerol enter the epithelium of the villi by diffusion. In the epithelial cells, they recombine into fine lipid droplets, which then enter the lymph in the lacteals of the lymphatic system. Lipid-soluble vitamins (eg. vitamins A and D) are also absorbed into the lacteals.
The lipids and lipid-soluble vitamins in the lacteals are transported to the main lymph vessels, and eventually to the bloodstream near the neck region. They are then carried to all parts of the body through blood vessels.

Question 52

Is there any absorption taking place in the large intestine?

Answer 52

Yes: Much of the remaining water is absorbed through the wall of the colon into the blood. Some minerals and vitamins are also absorbed, but very slowly.

Question 53

What is assimilation?

Answer 53

Assimilation is the uptake and use of absorbed food molecules by cells foe metabolism.

Question 54

How are water-soluble food molecules transported to other parts of the body?

Answer 54

In the small intestine, water-soluble food molecules such as monosaccharides, amino acids, minerals and water-soluble vitamins, are absorbed into the capillaries of the villi. Blood in the capillaries eventually flows to the hepatic portal vein, which carries blood to the liver. In the liver, some food molecules are stored. Some are processed and then carried through the hepatic vein and the vena cava to the heart.
When the heart pumps blood, the absorbed food molecules are transported to all parts of the body through the aorta.

Question 55

How are lipid-soluble food molecule transported to other parts of the body?

Answer 55

The lipids and lipid-soluble vitamins absorbed are carried by the lymph in lymph vessels. The lymph vessels eventually join the vein near the neck region. The absorbed lipids and lipid-soluble vitamins are then passed into the blood circulation and enter the heart.
When the heart pumps blood, the absorbed food molecules are transported to all parts of the body through the aorta.

Question 56

What are the fates of absorbed glucose?

Answer 56

Glucose is used by body cells for releasing energy by respiration. Excess glucose is converted to glycogen or lipids in the liver. Glycogen is stored in the liver and muscles. Lipids are stored under the skin or around the internal organs.

Question 57

What are the fates of absorbed amino acids?

Answer 57

Amino acids are used by cells to make different types of proteins for growth and repair. They are also the raw materials for making enzymes, antibodies, and some hormones.
Excess amino acids cannot be stored in our body. They are deaminated in the liver.

Question 58

What are the fates of absorbed lipids?

Answer 58

Lipids are used by cells to make cell membranes and some hormones.
Lipids are energy reserves in our body. They act as a source of energy for cellular activities when the body is short of glycogen.
Excess lipids may be deposited around the internal organs as visceral fat for protection, or build up under the skin as subcutaneous fat to reduce heat loss.

Question 59

What are the roles and functions of the liver?

Answer 59

1. Regulation of blood glucose level: After a meal, the blood glucose level rises. The liver converts the excess glucose to glycogen, which is stored in the liver. Between meals, when the blood glucose level falls below a certain level, the liver connects the stored glycogen back to glucose and releases it into the blood.
2. Storage of glycogen, iron, and lipid-soluble vitamins: The liver breaks down old red blood cells and stores the iron released from the process. The iron is used for making new red blood cells.
3. Production of bile and vitamin A: Carotene is converted in vitamin A in the liver.
4. Deamination: Excess amino acids are broken down in the liver through deamination. The amino groups are converted to urea, while the remaining parts are converted to carbohydrates or lipids.
5. Detoxification: Liver cells contain enzymes which help break down certain mild toxic substances (eg. alcohol and drugs) in the blood to harmless substances in detoxification. The harmless substances are then passed out of the body.

Question 60

What are the features of faeces?

Answer 60

Faeces are formed from the undigested and unabsorbed materials in the large intestine. They are semi-solid wastes containing dietary fibre, bacteria, secretions from the alimentary canal, dead cells from the intestinal wall and a small amount of water. They are brown in colour because of the presence of bile pigments. They are temporarily stored in the rectum.

Question 61

How are faeces expelled from the body?

Answer 61

At the anus, there is a ring of muscles called the anal sphincter. When the sphincter relaxes and the muscles of the rectum contract, faeces are pushed out through the anus. This process of expelling faeces is called egestion or defaecation.

Question 62

List three beneficial actions of bacteria in our intestines.

Answer 62

1. They produce vitamins B and K, which are absorbed into the blood through the wall of the colon. (Vitamin K is a lipid-soluble vitamin involved in blood clotting and bone metabolism.)
2. They protect the intestines from infection by disease-causing microorganisms.
3. They stimulate the development and the activity of the tissues in the immune system.