Digestiongr9

Digestion in Human Beings

Why We Need Food?
We need food to:
provide us with energy for our daily activities like walking, and to maintain our body temperature;
maintain a healthy body ;
grow new cells and tissues; and
repair worn out tissues.

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 .
Carbohydrates
The main carbohydrates in food are starch and sugars . Starch can be found in rice, bread, noodles and potatoes.
Sucrose , glucose and maltose are also forms of sugars. They can be found in fruits like bananas and apples
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.

Proteins
Proteins are very large molecules that are made up of several small molecules called amino acids .
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.
Proteins can be found in meat, fish and eggs.

Fats
Fats are large insoluble molecules that are made up of glycerol and fatty acids.
Fats provide us with twice as much energy as carbohydrates.
Fats can be found in food like butter and cheese.
Fats are stored under our skin to insulate our body against too much heat loss.

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.

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.

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.

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.

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 .

The Human Digestive System salivary gland mouth cavity oesophagus stomach salivary glands pancreas colon rectum anus large intestine liver gall bladder small intestine

The mouth food oesophagus windpipe

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:
Wets the food, so that it is easier to swallow.
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:
enzymes which digest proteins; and
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:
Intestinal juice from the walls of the intestine.
Pancreatic juice from the pancreas.
Bile from the liver.

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 rectum anus

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.

Homework
Do digestion activity in notes
“ Where does the hamburger go?”

Digestiongr9

by Numandi Marais on Jan 29, 2012

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