The Digestive System
Composed of 4 sections:
1.
2.
3.
4.
Mouth
Stomach
Small intestine
Large intestine
Digestion is the breakdown of large molecules into smaller molecules so that they can be
absorbed by the blood.
Alimentary canal is the tube from the mouth to the anus.
mouth
epiglottis
tongue
salivary gland
trachea (wind pipe)
oesophagus (gullet)
right lung
diaphragm
gall bladder
stomach
bile duct
liver
pyloric sphincter
pancreas
colon
duodenum
ileum
caecum
rectum
appendix
Duodenum
Colon
+
+
Ileum
Rectum
=
=
anus
Small intestine
Large intestine
The Mouth
Digestion begins in the mouth. Saliva is released from the salivary gland and mixes the
food, softening it, dissolving it and making it easier to swallow. Saliva also contains the
enzyme amylase, which begins to break down starch into maltose.
The physical action of chewing (mastication) breaks food into smaller pieces. This makes
it easier to swallow, and increases the surface area for enzymes to work on.
The food (bolus) passes into the throat and the epiglottis should close, preventing the food
entering the trachea.
Eustachian tube
base of skull
food bolus
nasal cavity
soft palate
tongue
squeezes food
against palate
epiglottis
salivary glands
glottis
gullet
windpipe
larynx
cartilage
raised
soft palate seals off
nasal cavity
back of tongue and
epiglottis direct food over
opening of windpipe
circular muscle
partly closes
glottis
The bolus travels down the oesophagus to the stomach by a process called peristalsis. This
is how it travels through the rest of the alimentary canal.
muscular wall of gullet
circular muscle contracting
food
Stomach
It is a large muscular bag, which churns and mixes food. It stores the food temporarily and
releases it gradually into the duodenum. It produces gastric juices containing Hydrochloric
Acid and enzymes. Mucus protects the lining of the stomach from damage by the acid.
Small Intestine
(a)
Duodenum
where most digestion takes place
gall
bladder
Liver
pancreas
bile
pancreatic juice
bile
duct
Bile is not an enzyme: it emulsifies fats making them easier to digest by enzymes.
BILE
large fat
droplet
(b)
Ileum
small droplets, so larger
surface area for enzymes
to work on.
final digestion but mainly absorption of small nutrient molecules
Food
Final product to be absorbed
Carbohydrates
Glucose/Fructose
Proteins
Amino Acids
Lipids
Fatty acids + glycerol
Large intestine
(a)
Colon:
removes water from residual indigestible material. Little water lost in
faeces. Minerals and some vitamins are also absorbed by the colon.
(b)
Rectum:
stores waste material until it is egested.
Digestive Enzymes
Where it is
produced
Secretion
Enzyme(s)
What it does
Salivary Gland
Saliva
Salivary amylase
Breaks down Starch
into Maltose
Walls of stomach
(gastric pits)
Gastric juice
(pepsin and
Hydrochloric Acid)
Pepsin (works best in
acidic conditions of
stomach)
Protein broken down
into Polypeptides
Pancreas
Pancreatic juices
Lipase
Pancreatic Amylase
Trypsin
Walls of the Ileum
Intestinal juice
Maltase
Peptidase
Sucrase
Breaks down fats to
Glycerol and Fatty
acids
Breaks down starch
into Maltose
Breaks down Proteins
into Polypeptides
Breaks down Maltose
into Glucose
Breaks down
Polypeptides into
Amino Acids
Breaks down Sucrose
into Fructose and
Glucose
Protease is the general name for enzymes, which digest protein: pepsin, trypsin and
peptidase.
Absorption
This takes place in the ileum through small finger-like projections called villi.
Muscle layers to allow
peristalsis
Blood vessels to remove
digested food molecules
Bolus
Folded inner wall of
ileum. Increases surface
area for absorption
Lumen – hollow part of
gut
On the surface of the inner folded wall are millions of villi, which greatly increase the
surface area for absorption.
Digested food
Molecules
move into
villus by
diffusion
Wall of villus – only one cell thick
Lacteal
Network of capillaries
Low food molecule concentration
Lymph Vessel
-
Amino acids and glucose diffuse into the capillaries and are taken away in the blood.
Fatty acids and glycerol diffuse into the lacteal and are taken away into the lymph
vessel – they are added to the blood again later.
Advantages
-
Thin membrane means a short diffusion pathway for materials. Therefore absorbs
food molecules quickly.
Vast network of blood capillaries take food material quickly.
Substances taken away quickly so that it maintains a steep concentration gradient.
Advantages of the Small Intestine
-
It is long so that there is time for digestion to take place.
It is folded which makes a greater surface area.
It has a good blood supply so the digested food can be absorbed quickly by the
blood.
It has thin walls so that diffusion can take place – short diffusion pathway.
It has lots of villi, which give more surface area.
It has various enzymes, which break down various foods.
Assimilation
Amino acids
Glucose etc.
Blood capillaries
Fatty acids
Glycerol
Lacteals/ lymphatic system (returned to blood later)
All blood from the intestines is taken to the liver by the hepatic portal vein.
The blood needs to have a constant concentration of various substances (e.g. glucose). The
liver regulates blood from the alimentary canal ensuring these substances remain constant.
Examples in the liver:
1.
2.
Excess glucose is stored as glycogen (using insulin). Glycogen can then be
turned back into glucose (using glucagon).
Further glucose is stored as fat. Fat can be turned back into glucose.
Enzymes
Metabolism
all chemical reactions in an organism
Anabolic reactions
building up smaller molecules into larger molecules (e.g. amino
acids to proteins)
Breaking down large molecules into smaller molecules (e.g.
proteins into amino acids)
Catabolic reaction
Some enzymes work inside cells intracellular enzymes.
Others (like digestive enzymes) work outside cells: extra cellular enzymes.
Enzymes are biological catalysts. They speed up the rate of chemical reactions and control
these reactions within living organisms.
The chemical an enzyme works on is called the substrate.
The chemical(s) produced at the end is the product.
e.g.
Starch
substrate
Amylase
Maltose
product
Properties of enzymes
Enzymes are proteins
this is one reason why we need to eat proteins – to make
enzymes.
Temperature sensitive
a rise in temperature will increase enzyme activity (kinetic
theory). However, if the temperature is too high (normally
above 45ºC) will destroy enzymes – this is permanent –
denaturation.
pH sensitive
all enzymes work within a narrow pH. The pH they work best
in is called their optimum pH.
Enzymes are specific
they only work on one substrate or one group of substrates. E.g.
amylase only breaks down starch – nothing else.
They are not consumed
Enzymes are not consumed during the reaction and so can be
re-used over and over. However, eventually they ‘wear out’
.
Lock and key hypothesis
Enzyme molecule
Substrate molecule
active site
Substrate binds to active site
forming an enzyme-substrate
complex.
The enzyme breaks down the
substrate into the product.
Enzyme is free to break
another substrate molecule
down. It is re-usable.
product
Proteins (enzymes) are altered by changes in pH and in temperature.
Optimum pH is 2.5
Optimum pH is 7
Rate of
reaction
Pepsin
1
2
Trypsin
3
4
5
6
7
8
pH
Enzymes work within narrow pH limits. If they are placed in a pH from their optimum,
they rapidly become less active. Most enzymes work best in a neutral pH. A change in pH
changes the shape of the active site and it no longer fits the substrate.
C
B
D
A
0
5
10
15
20
25
30
35
40
45
50
Temperature (ºC)
Optimum temperature for enzymes vary, but many are around 40ºC.
A
Very slow rate of reaction. Enzymes have little kinetic energy and so they for
enzyme substrate complexes very slowly.
B
Rapid increase in rate of reaction. As temperature increases, the enzyme/substrate
molecules have more kinetic energy so form complex molecules more quickly.
C
Optimum temperature – enzymes and substrates work at fastest rate.
D
Enzymes begin to be denatured. This happens relatively quickly – over about 10ºC.
Bonds in the protein structure of enzyme are broken. Active site loses its shape and
is unable to fit the substrate. Denaturation is permenant.