Mammalian digestion
In heterotrophic organisms the digestive system
provides the means by which nutrients are taken in
and broken down. Large insoluble food molecules
are converted into small soluble ones that can be
absorbed and made available to the body cells.
Obtaining nutrients
There are 5 steps involved in heterotrophs
obtaining nutrients:
1. Ingestion: the intake of food
2. Digestion: the breakdown of food into smaller,
soluble chunks that can be easily absorbed. The
end products of digestion are amino acids (from
proteins), simple sugars (from carbohydrates)
and fatty acids (from lipids).
3. Absorption: basic units of food are
absorbed across the digestive tract wall,
into the bloodstream and into cells.
4. Assimilation: the end products of
digestion are built into useful substances
as either biological material or as an
energy source
5. Egestion: the removal of undigested food
as waste.
Describe the role of teeth in increasing the
surface area of complex foods for exposure to
digestive chemicals
The function of teeth is to physically break
down food into smaller pieces. Chewing
food increases the surface area thereby
increasing the rate of reaction with
digestive enzymes (chemical breakdown).
Humans have four types of teeth: incisors, canines,
premolars and molars.
• Incisors: used to grasp, hold and
bite food
•Canines: used for ripping and
tearing food
•Premolars: used for chewing and
cutting flesh
•Molars: used for grinding and
chewing
Diets
Herbivores
Have front teeth (incisors) adapted to tear off
vegetation and back teeth (molars) to chew on
plant material. The molars are specially
designed to help break open the cellulose walls
of plant cells. Cellulose is extremely hard to
break down physically and chemically.
Herbivores cont.
Herbivores need to chew for a long period of
time because:
• To break down the cellulose
• Large quantities of vegetation is needed to
provide sufficient energy for the animal
• To further increase the surface area
Carnivores
They have powerful jaws and well developed
canine teeth designed to hold and kill prey and
tear off meat.
The meat is then chewed by the molars to increase
the surface area before swallowing it.
Explain the relationship between the length and overall complexity of digestive
systems of a vertebrate herbivore and a vertebrate carnivore with respect to:
• the chemical composition of their diet
• the functions of the systems involved
Herbivores
Eat plant material that is high in fibre and starch
and depending on the plant may include sugars,
proteins, oils and other nutrients.
Herbivores have adaptations of their digestive
tract to enable them to digest cellulose.
Herbivores cont.
Large herbivores have microbes living in their
gut who produce an enzyme to break down
cellulose and release energy. This is called
microbial fermentation.
The gut is complex and extremely long providing
space to hold the large quantity of food that
must be eaten. It also gives maximum
opportunity for the microbes to digest cellulose
and the nutrients to be absorbed.
Herbivores cont.
Small herbivores eat plant material with less
cellulose and their diet is predominately nectar.
The gut is usually simple and small because
their diet is easier to digest as it has little or no
fibre and is high in sugar.
Carnivores
Eat animal matter which is more easily digested.
Their diet is high in protein, low in fibre and has
more energy than plant material. This means
that carnivores need to consume lesser
quantities of food than herbivores.
Due to the ease at which food can be digested,
the gut is relatively short. Little undigested
material is egested due to low fibre diet.
Herbivore Adaptations
The microbes living in the gut of animals need to
be protected from being lost with the passage of
food. The microbes are present either in the:
• Stomach (fore-gut fermenters)
• Caecum (hindgut fermenters)
Fore-gut Fermentation
Microbes in the eastern grey kangaroo use
majority of the dietary glucose and protein but
release fatty acids as energy for the host animal.
Some microbes are digested by the host and a
source of amino acids.
Cattle:
• They regurgitate the contents of the first part of their
stomach and re-chew the food to further break it down.
•Ruminant fore-gut fermenters meaning they have four
stomachs: two for microbial fermentation, one for
storage and one which acts as a true stomach.
• Food remains in the two fermentation chambers for a
long period of time so that the microbes have enough
time to break down the cellulose
Hindgut Fermenters
Takes place in the caecum (large intestine)
The main difference is that the microbes cannot
be digested by the host animal as a source of
amino acids. This is because protein digestion
has already occurred before the large intestine
and the amino acids are therefore egested.
This is why rabbits eat their faecal pellets to
digest the nutrient-rich microbes
Carnivore Adaptations
They have a simple stomach and a short
intestine relative to their body size. The caecum
may be absent, of if it is present is greatly
reduced in size.