Digestion in Human Beings

26.1 Why We Need Food?

• We need food to:o provide us with energy for our daily activities

like walking, and to maintain our body temperature;

o maintain a healthy body;o grow new cells and tissues; and o repair worn out tissues.

26.2 What is in the Food We Eat?

• You need the essential nutrients in the food you eat.

• The types of nutrients can be found in the food label on a food package.

• The food label lists the nutrient content of the food.

Main types of nutrients

• There are three main types of nutrients: carbohydrates, proteins and fats.

• Carbohydrateso The main carbohydrates in food are starch and

sugars. Starch can be found in rice, bread, noodles and potatoes.

o Sucrose, glucose and maltose are also forms of sugars. They can be found in fruits like bananas and apples

o Cellulose is another type of carbohydrate, which can be found in plants. Cellulose make up part of the fibre (roughage) that cannot be digested by the body but instead passed out from the body.

Main types of nutrients

• Proteinso Proteins are very large molecules that are

made up of several small molecules called amino acids.

o Proteins are needed to:• build new cells for body growth and for the repair of

worn-out tissues.• make more complex proteins such as enzymes that

carry essential functions in the body.

o Proteins can be found in meat, fish and eggs.

Main types of nutrients

• Fatso Fats are large insoluble molecules that are

made up of glycerol and fatty acids.o Fats provide us with twice as much energy as

carbohydrates.o Fats can be found in food like butter and

cheese.o Fats are stored under our skin to insulate our

body against too much heat loss.

26.3 Why Must Food be Digested?

• We can only use the nutrients in the food we eat when they pass through the gut walls into the blood vessels.

• Here, the nutrients can be carried through the bloodstream to all parts of the body.

• The gut walls and blood vessels are made up of cells. • The cell membrane has small openings (or pores) that

allow small molecules to enter, but not large molecules. • Nutrients like glucose and amino acids are small

molecules. They can pass through the cell membranes easily and enter the bloodstream.

26.3 Why Must Food be Digested?

• But most major nutrients in food are large molecules which cannot pass through the cell membranes.

• They must be broken down into molecules that are small enough to pass through the cell membranes.

• This process is called digestion.• Digestion is the breaking down of large, complex

food molecules into small, simpler molecules. • Our body is able to carry out digestion by producing

complex proteins called enzymes. • The enzymes involved in digestion are called

digestive enzymes.

Nutrients like starch, proteins and fats, are large, complex molecules. They cannot pass through the cell membrane.

Nutrients like glucose and amino acids, are small, soluble molecules. They can pass through the cell membrane.

26.4 What are Enzymes?

• Enzymes are complex proteins that speed up the rate of chemical reactions. Enzymes remain unchanged at the end of chemical reactions.

• Enzymes act like chemical ‘scissors’.

• They break down large molecules into small molecules to speed up the process of digestion.

“Lock and key” hypothesis

active sites

Enzyme molecule

Food molecule

Enzyme-food complex

Enzyme free to take part in next reaction.

Two products leave the enzyme.

Product X

Product Y

Types of digestive enzymes

• A particular enzyme can only bring about one type of chemical reaction.

• For example, enzymes that break down proteins cannot break down starch or fats.

Class of enzyme

Acts on Digested products(s)

Amylase Starch Maltose (a complex sugar)

Maltase Maltose Glucose (simple sugar)

Protease Proteins Amino acids

Lipase Fats Fatty acids and glycerol

26.5 The Human Digestive System

• Food is digested in our body through the digestive system.

• The digestive system is made up a long tube called the gut (or alimentary canal).

• The gut is about nine metres long. The wall of the gut is muscular, which allows food to move along its length.

• Food enters the body through the mouth, and undigested food leaves the body through the anus.

26.5 The Human Digestive System

• Glands connected to the gut are organs which produce special juices containing enzymes.

• These glands are the salivary glands, liver and pancreas.

26.5 The Human Digestive System

salivary glandmouth cavity

oesophagus

stomach

salivary glands

pancreas

colon

rectumanus

large intestine

liver

gall bladder

small intestine

The mouth

food

oesophaguswindpipe

The mouth

• Food is chewed in the mouth with the teeth.

• Chewing helps to cut and grind the food the smaller pieces.

• This increases the surface area and allows the food to be digested faster.

• As food is chewed, salivary glands in the mouth secrete saliva.

The mouth

• Saliva serves two purposes:o Wets the food, so that it is easier to swallow.o Digests starch into sugars with the help of an

enzyme called salivary amylase.

• While chewing food, the tongue rolls the food into small balls when are pushed to the back of the mouth and squeezed into the oesophagus. This is known as swallowing.

The oesophagus

Muscles contract to push the food ball down.

Muscles relax, and the tube widens for food to move.

The oesophagus

• The oesophagus is a long muscular tube leading to the stomach.

• By contracting and relaxing, the muscles help to push the food down to the stomach.

• This is how food moves along the rest of the gut too.

• No digestion occurs in the oesophagus. • However, the digestion of starch by amylase

may continue as the food moves to the stomach.

The stomach

stomach

The stomach

• The stomach is a muscular bag that lies in the upper part of the abdomen.

• Its muscles contract and relax, causing food to break up into even smaller pieces.

• This movement also mixes the food well with gastric juice for better digestion.

• Gastric juice is secreted by glands in the stomach walls, into the stomach cavity. It contains:o proteases which digest proteins; ando hydrochloric acid, which helps proteases to work.

• Hydrochloric acid kills any bacteria in the food.• Food stays in the stomach for a few hours before

passing into the small intestine, bit by bit.

The small intestine, liver and pancreas

gall bladder

liver

small intestine

pancreas

The small intestine

• The small intestine is a long muscular tube, which is about 6 m long.

• The liver and the pancreas are connected to the small intestine.

• Food is mixed with 3 fluids in the small intestine to aid digestion:o Intestinal juice from the walls of the intestine. It

contains the enzymes maltase, proteases and lipases.

o Pancreatic juice from the pancreas.o Bile from the liver.

Pancreas

• The pancreas produces alkaline pancreatic juice.

• The juice contains the enzymes amylase, protease and lipase.

• The digestion of food in the small intestine are as follows:o Digestion of fats:

• Fat Fatty acids and glycerol lipase

Pancreas

• The digestion of food in the small intestine are as follows:o Digestion of starch:

• Starch Maltose

• Maltose Glucoseo Digestion of small protein molecules

• Protein molecules amino acids

amylase in pancreatic juice

maltase

protease

Liver

• The liver produces a yellowish-green fluid called bile.• Bile is stored in the gall bladder.• The gall bladder has a duct (a small tube) that carries

bile into the small intestine.• Bile does not contain digestive enzymes, but helps to

break up fast into smaller oil droplets in a process known as emulsification.

• This increases the surface area of the oil and allows the fats to be digested quickly by the lipases in the pancreatic and intestinal juices.

Emulsification

Bile

Large oil drop

Small oil droplets

Absorption in the small intestine

• The small intestine allows only small molecules like sugar and amino acids to pass through its wall and into the bloodstream.

• Large molecules like starch and proteins cannot pass through the walls of the small intestine.

• Digestion ends in the small intestine. • The final products of digestion are glucose,

amino acids, fatty acids and glycerol.

Absorption in the small intestine

• After digestion, the smaller digested food molecules can pass through the wall of the small intestine and into the bloodstream.

• Undigested matter is mostly made up of fibre. • Together with water and mineral salts, the

undigested food passes into the large intestine.• Fibre can be found in vegetables. It is important

for the gut muscles to move the food along the gut.

The large intestine

large intestine

rectumanus

The large intestine

• The large intestine is about 1.5 m long. • Its function is to absorb water and mineral

salts.• This takes place in the colon.• What is left now is a nearly solid waste

called faeces. • This is temporarily stored in the rectum,

before being expelled through the anus is a process called egestion.

Miscellaneous

• http://kidshealth.org/kid/body/digest_noSW.html• http://www.ahealthyme.com/topic/digestivesyste

m• http://hopkins-gi.org/multimedia/database/intro_2

50_Swallow.swf• http://yucky.discovery.com/noflash/body/pg0000

08.html• http://hes.ucf.k12.pa.us/gclaypo/digestive_syste

m.html#What%20is%20Digestion