Chapter 13 Human Digestive System

Chapter 13 Human Digestive SystemWhy do we eat?

1. Go

• Provide energy for work, movement and to maintain body functions

2. Grow

• To make new cells and tissues and to repair worn out cells

3. Glow

• To maintain health

3 Main types of nutrient

1. Carbohydrates

• Immediate form of energy to the body

• Compound of carbon, hydrogen and oxygen

• The main carbohydrates are:

- Starch and sugars: glucose, maltose and sucrose

- Food rich in starch: rice, noodles, bread and potatoes

- Food rich in sugars: honey, sweets and fruits

• Another type of carbohydrate, found mainly in plants (vegetables and salads), is cellulose. This cannot be digested by the human body and acts as roughage (fibre). It is passed out from the body.

2. Proteins

• For growth and repair of worn out tissues

• Compounds of carbon, hydrogen, oxygen & nitrogen

• Some contain sulphur

• Food rich in proteins: meat, fish and eggs

SUBJECT: LOWER SECONDARY SCIENCE - BIOLOGY LEVEL: SECONDARY 2

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3. Fats

• Also supply energy to the body(twice as much per gram as carbohydrate)

• Compounds of carbon, hydrogen and oxygen

• Mainly used as a means of storing food

• Usually solids

• A fat that is a liquid at room temperature is known as oil

• Stored under the skin and around various organs

• Food: butter and cheese

What is digestion?• The breakdown of food into small diffusible molecules

Types of digestion

1. Physical(mechanical) digestion

• The physical breakdown of food into smaller pieces without any chemical changes of the food

• Involves biting and chewing

2. Chemical digestion

• The breakdown of large food molecules into smaller molecules

• Involves the actions of enzymes

What are enzymes?

• Proteins which function as biological catalysts

• Speed up biological reactions in the body

• Specific in action

• Remain chemically unchanged at the end of the reaction

• Sensitive to temperature and pH

What are digestive enzymes?

• Speed up the breakdown of large food substances

• Function as molecular scissors that cut large insoluble molecules into smaller soluble molecules

• Specific in action (each enzyme can only digest one particular food substance)



Examples of digestive enzymes

• Carbohydrates (starch) → Maltose → Simple sugar / Glucose

• Protein → Peptones → Amino acids

• Fats → Fats globules → Fatty acids and glycerol

• Liquid milk proteins → Solid milk

Where does digestion takes place?

• In the alimentary canal / digestive system (10-metre long muscular tube thats is partly coiled up inside our body)

• Comprise of 2 parts: the organs and accessory organs attached to the alimentary canal

Structures and functions of the digestive system

1. 1st stop: Mouth

• Both physical and chemical digestions take place in the mouth

• The teeth break down food mechanically(cut and grind) into smaller pieces to increase surface area of the food exposed to the enzymes

• The food is mixed with saliva produced by the salivary glands

• Saliva contains the digestive enzyme, amylase, which breaks down starch in the food into maltose, a type of sugar

• The other types of food molecules are not broken into smaller molecules in the mouth

• The tongue helps to mix the food with the saliva and rolls the partially digested food into a small ball (bolus) which is then swallowed into the oesophagus

• Epiglottis helps to open and close so that food enters the gullet and air enters the windpipe.

(Estimated time: 20 seconds)



2. 2nd stop: Oesophagus (Gullet)

• The oesophagus (gullet) is a long and narrow tube joining the mouth and the stomach

• When food is swallowed, it does not “drop” or “fall” into the stomach through the oesophagus by the action of gravity

• Rather, the oesophagus has strong muscle in its wall

• These muscles contract and relax to produce a wave-like movement that pushes the food into the stomach

• This voluntary wave-like pattern of contractions is called peristalsis

• Amylase continues to break down starch into maltose

(Estimated time: 10 seconds)

3. 3rd stop: Stomach

• The stomach is a muscular bag which is able to expand to take in different amount of food

• The muscles in the wall of the stomach contract and churn the food to mix it with a digestive juice called gastric juice

• Proteases

- Gastric juice is produced by the lining of the stomach and contains hydrochloric acid and proteases

- The proteases in the stomach digest proteins in the food into shorter chains of amino acids.

• Hydrochloric acid

- Protease in the stomach work well in acidic conditions

- The hydrochloric acids provides the acidic condition in the stomach for the proteases to work

- It also kills bacteria which may have been swallowed with the food

• Food is slowly moved by peristalsis to the small intestine

• Food is turned into semi-liquid called chyme

(Estimated time: 2 to 6 hours)



4. 4th stop: Small intestine

• The small intestine is a long and narrow tube joining the stomach and the large intestine

• The small intestine is where most of the chemical digestion and absorption of food takes place

• As the chyme enters the small intestine from the stomach, it is mixed with secretions from the liver, pancreas and the small intestine itself

• The duodenum is the first part of the small intestine

• Bile

- The liver produces a liquid called bile

- It is stored in the gall bladder and released into the small intestine through the bile duct

- The bile increase the surface area of fats (emulsifies) for lipase to act on into fats globules

• Pancreas

- The pancreas produces pancreatic juice and the small intestine produces intestinal juice

- The pancreatic and intestinal juices contain several types of digestive enzymes:

★Amylase from the pancreas breaks down starch into maltose, which is further broken down into glucose by another type of enzyme (maltase produced by the walls of the small intestine)

★Proteases breaks down proteins into short chains of amino acids which are further broken down into amino acids

★Lipase breaks down fats into fatty acids and glycerol

• Glucose, amino acids, fatty acids and glycerol are the main end products of digestion

• These molecules are now small enough to pass through the wall of the small intestine and into the bloodstream

• Villi (finger-like parts) provides a large surface area for digested food to be absorbed quickly

• Some water, vitamins and mineral salts are absorbed too

• These nutrients then diffuse out of the intestinal cells and most enter the bloodstream through the capillaries

• The end product of digestion are used for cellular processes like respiration, growth and tissue repair

(Estimated time: 5 hours)



5. 5th stop: Large intestine

• The undigested food moves from the small intestine into the large intestine by peristalsis

• The undigested food is made up largely of fibres which come from the cellulose that makes up the cell walls of vegetables and fruits

• This provides roughage, which helps in bowel movement

• Some water is present too

• There are many different types of bacteria living in the large intestine

• Most of them are harmless

• Some are beneficial as they are able to make vitamins which are needed by the body

• The cells lining the large intestine absorb these vitamins as well as mineral salts and excess water

(Estimated time: up to 24 hours)

6. 6th stop: Rectum

• The mixture of undigested food and dead bacteria is called faeces and stored temporarily in the rectum

• Most of the time, the muscles stay contracted

• When the ring-like muscles between the rectum and anus relax, faeces is passed out of the body through the anus

• This is known as defaecation or egestion



Chapter 13 Human Digestive System

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