Download The digestive system

Coordinated Science
Reading: Digestive System
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Summary
The digestive system is adapted to break down and absorb food. Physical digestion breaks the food down
into smaller pieces and chemical digestion breaks these pieces into smaller molecules. Various enzymes
and other substances, such as hydrochloric acid and bile, are involved in this process.
You need to be able to describe the position and function of these parts of the human digestive system:
 salivary glands – produce saliva that moistens food and contains carbohydrase enzymes
 stomach – produces hydrochloric acid and protease enzymes
 pancreas – produces carbohydrase, protease and lipase enzymes
 liver – produces bile
 gall bladder – stores bile
 small intestine – produces carbohydrase, protease and lipase enzymes, and absorbs digested
food
 large intestine – absorbs water
The digestive system
When you eat a piece of bread, you don't wake up next day to discover it growing out of your arm! The
food we eat has to be broken down into other substances that our bodies can use. This is
called digestion. Without digestion, we could not absorb food into our bodies and use it.
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.
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.
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.
Absorption and egestion
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.
Absorption
Article source: www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/the_living_body/digestion/revision
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Coordinated Science
Reading: Digestive System
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.
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.
Physical vs chemical digestion
Physical digestion
Physical digestion involves breaking food down into smaller pieces without making any chemical
changes. Physical digestion happens in:
 the mouth when food is chewed
 the stomach when food is squeezed and moved around
Physical digestion is important because it lets the food pass through the digestive system more easily,
and it provides a larger surface area for enzymes [enzyme: Proteins which catalyse or speed up chemical
reactions inside our bodies. Enzymes are a vital in chemical digestion of food in the gut.] to work on.
Chemical digestion
Chemical digestion involves breaking large food molecules into smaller molecules using chemical
reactions. These smaller molecules can then be absorbed into the blood.
Enzymes speed up chemical digestion. These are natural catalysts [catalyst: A catalyst changes the rate of
a chemical reaction without being changed by the reaction itself.] made of proteins. Different enzymes
catalyse different digestion reactions:
Enzyme
Reaction catalysed
Carbohydrase (eg amylase) Starch → sugars
Protease
Protein → amino acids
Lipase
Fat → fatty acids + glycerol
Stomach acid provides the correct pH for stomach protease to function properly.
Different enzymes are produced in different parts of the gut:
Enzyme
Where produced
Carbohydrase (eg amylase) Salivary glands, pancreas, small intestine
Protease
Stomach, pancreas, small intestine
Lipase
Pancreas, small intestine
Overall, this means that:
 carbohydrase enzymes catalyse the breakdown of starch [starch: A type of carbohydrate. Plants
can turn the glucose produced in photosynthesis into starch for storage, and turn it back into glucose
when it is needed for respiration.] into sugars in the mouth and small intestine
Article source: www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/the_living_body/digestion/revision
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Coordinated Science
Reading: Digestive System
protease enzymes catalyse the breakdown of proteins into amino acids [amino acid: Complex
molecules, which form the building-blocks of proteins.] in the stomach and small intestine
lipase enzymes catalyse the breakdown of fats and oils into fatty acids andglycerol [glycerol: A
soluble carbohydrate which is coverted into glucose by the liver.] in the small intestine.
Absorption and egestion
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.
Absorption
The products of chemical digestion are absorbed into the body in the small intestine:
 sugars and amino acids pass into the bloodstream by diffusion [diffusion:The movement of
particles (molecules or ions) from an area of higher concentration to an area of lower
concentration.]
 fatty acids and glycerol pass into the lymph [lymph: Fluid carried in the lymphatic system.]
The bloodstream and lymph carry them to the body’s tissues.
Read on if you're taking the higher paper.
Absorption Adaptations
The intestine has several adaptations for the efficient absorption of small molecules from digestion:
 it has a thin lining
 it has a good blood supply
 it is very long and has a large surface area
Tiny finger-like projections called villi provide a large surface area for absorption to take place. They also
have a rich supply of blood vessels to carry away absorbed molecules.
Article source: www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/the_living_body/digestion/revision
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Coordinated Science
Reading: Digestive System
The villi are an adaptation to efficient absorption in the small intestine
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.
Digestion – Higher tier
Starch digestion
Starch is a complex carbohydrate [complex carbohydrate: More than two sugar molecules joined together
into a chain. Starch is a complex carbohydrate.] . Each molecule consists of very many glucose [glucose: A
simple sugar made by the body from food, which is used by cells to make energy in respiration.] molecules
joined together. The digestion of starch is a two-stage process:
1. it is first digested to form maltose (each maltose molecule consists of two glucose molecules
joined together)
2. the maltose is then digested to form glucose
Changing pH
The pH [pH: Scale of acidity/alkalinity. pH below 7 = acidic, pH above 7 = alkaline.] is
at alkaline [alkaline: Having the properties of an alkali, such as a pH above 7. Alkaline substances react
with acids, neutralising them, and forming salts.] levels in the mouth and small intestine, but the pH in the
stomach is at acidic levels. The enzymes there work at different optimum pH levels. For example:
 salivary amylase works best just below pH 7
 stomach protease works best at around pH 1.6
 pancreatic lipase works best at around pH 8
Bile
As food passes from the stomach into the small intestine, the pH must be raised. Bile - an alkaline
substance - is released into the small intestine to achieve this.
Bile is released into the duodenum (part of the small intestine)
Bile also emulsifies (breaks down) fats in the small intestine. This is important, because it provides a
larger surface area in which the lipases can work.
Additional online resources: http://www.neok12.com/Digestive-System.htm ;
http://www.brainpop.com/games/buildabodydigestivesystem/ ;
http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/common_systems/digestionact.shtml
Article source: www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/the_living_body/digestion/revision
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