Download Reading about Digestion

YR7 Mod 5 lesson 5
Rev 14-05-13
Reading about Digestion
CLICK CLUNK EVRY CHUNK
Digestion happens in the digestive system, which begins at the mouth and
ends at the anus.
After we swallow, our food passes through these organs in turn:
oesophagus or gullet
stomach
small intestine
large intestine.
Stages of digestion
Food is digested in the mouth, stomach and small intestine.
Digested food is absorbed into the bloodstream in the small intestine.
Excess water is absorbed back into the body in the large intestine.
Any undigested food passes out of the anus as faeces (pronounced
"fee-seez") when we go to the toilet.
There are seven different nutrients:
carbohydrates
proteins
fats
minerals
vitamins
fibre
water
Nutrient
Use in the body
Carbohydrate To provide energy
Protein
Fat
Minerals
Vitamins
Fibre
Water
For growth and repair
To provide energy. Also to store
energy in the body and insulate it
against the cold.
Needed in small amounts to
maintain health
Needed in small amounts to
maintain health
To provide roughage to help to keep
the food moving through the gut
Needed for cells and body fluids
Good sources
Cereals, bread, pasta,
rice and potatoes
Fish, meat, eggs, beans,
pulses and dairy products
Butter, oil and nuts
Salt, milk (for calcium)
and liver (for iron)
Dairy foods, fruit,
vegetables
Vegetables, bran
Fruit juice, milk, water
YR7 Mod 5 lesson 5
Rev 14-05-13
Enzymes
Take care - enzymes are not living things. They are just special proteins that
can break large molecules into small molecules. Different types of enzymes
can break down different nutrients:
carbohydrase or amylase enzymes break down starch into sugar
protease enzymes break down proteins into amino acids
lipase enzymes break down fats into fatty acids and glycerol.
Liver and pancreas
The liver and the pancreas play important part in digestion. The liver produces
bile, which helps the digestion of fat. The pancreas produces chemicals called
digestive enzymes.
Carbohydrates
Carbohydrates are digested in the mouth, stomach and small intestine.
Carbohydrase enzymes break down starch into sugars.
The saliva in your mouth contains amylase, which is another starch digesting
enzyme. If you chew a piece of bread for long enough, the starch it contains is
digested to sugar, and it begins to taste sweet.
Proteins
Proteins are digested in the stomach and small intestine. Protease enzymes
break down proteins into amino acids. Digestion of proteins in the stomach is
helped by stomach acid, which is strong hydrochloric acid. This also kills
harmful micro-organisms that may be in the food.
YR7 Mod 5 lesson 5
Rev 14-05-13
Fats
Lipase enzymes break down fat into fatty acids and glycerol. Digestion of fat
in the small intestine is helped by bile, made in the liver. Bile breaks the fat
into small droplets that are easier for the lipase enzymes to work on.
Things that are not digested
Minerals, vitamins and water are already small enough to be absorbed by
the body without being broken down, so they are not digested. Digestive
enzymes cannot break down fibre, which is why it cannot be absorbed by the
body.
These are the processes that happen in the digestive system:
ingestion (eating) → digestion (breaking down) → absorption → egestion
We've already looked at how foods are broken down by enzymes in digestion.
Now we will look at how the products of digestion are absorbed into the body.
YR7 Mod 5 lesson 5
Rev 14-05-13
Absorption
Digested food molecules are absorbed in the small intestine. This means
that they pass through the wall of the small intestine and into our
bloodstream. Once in the bloodstream, the digested food molecules are
carried around the body to where they are needed.
Only small, soluble substances can pass across the wall of the small intestine.
Large insoluble substances cannot pass through. Study the slideshow to
check your understanding of this.
YR7 Mod 5 lesson 5
Rev 14-05-13
The inside wall of the small intestine needs to be thin, with a really big
surface area. This allows absorption to happen quickly and efficiently. If the
small intestine had a thick wall and a small surface area, a lot of digested food
might pass out of the body before it had a chance to be absorbed.
To get a big surface area, the inside wall of the small intestine is lined with tiny
villi (one of them is called a villus). These stick out and give a big surface
area. They also contain blood capillaries to carry away the absorbed food
molecules.
Egestion
Excess water is absorbed back into the body in the large intestine. What is left
then is undigested food. This is stored in the rectum, the lower part of the
large intestine, until we are ready to go to the toilet. It then comes out of the
rectum through the anus as faeces. This process is called egestion.