Download Digestion in Humans

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.
• Carbohydrates
o 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
• Proteins
o 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
• Fats
o 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 glucose and amino
acids, are small, soluble
molecules. They can pass
through the cell membrane.
Nutrients like
starch, proteins and
fats, are large,
complex molecules.
They cannot 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
Product X
Product Y
Enzyme
molecule
Food
molecule
Enzyme-food
complex
Enzyme free
to take part in
next reaction.
Two products
leave the
enzyme.
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
glands
salivary gland
mouth cavity
oesophagus
liver
gall bladder
stomach
pancreas
colon
small intestine
rectum
anus
large
intestine
The mouth
food
windpipe
oesophagus
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; and
o 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
liver
gall bladder
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:
lipase
• Fat
Fatty acids and glycerol
Pancreas
• The digestion of food in the small intestine are
as follows:
o Digestion of starch:
amylase in
• Starch
pancreatic juice
Maltose
maltase
• Maltose
Glucose
o Digestion of small protein molecules
protease
• Protein molecules
amino acids
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
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.
Miscellaneous
• http://kidshealth.org/kid/body/digest_noSW.html
• http://www.ahealthyme.com/topic/digestivesyste
m
• http://hopkinsgi.org/multimedia/database/intro_250_Swallow.s
wf
• http://yucky.discovery.com/noflash/body/pg0000
08.html
• http://hes.ucf.k12.pa.us/gclaypo/digestive_syste
m.html#What%20is%20Digestion