DIGESTION

This is the process by which animals break down complex food substances into simpler
ones which they can use. The food taken in must enter the blood stream so that it can be
used by the body. The intake of food is called ingestion while the removal of indigestible
food from the body is called egestion. Absorption is the process where small molecules
are absorbed into the blood. Assimilation is the incorporation of absorbed food into
various parts of the body. Simple sugars, water, vitamins and minerals are small
molecules that can be absorbed as they are. Digestion can either be mechanical or
chemical. Mechanical digestion is the breakdown of large food particles into smaller
ones by the teeth and the churning movements produced by muscles. Chemical digestion
involves a chemical change of one molecule to another especially with the help of
enzymes. Holozoic nutrition is feeding by taking in pieces of food which are digested
inside the alimentary canal e.g. in mammals. NB. Mammals digest food extracellularly
(takes place outside the cell).
The teeth, lips, and tongue help with the ingestion of food. As it is chewed, it is mixed
with saliva and forms a bolus. Saliva contains water, mucus, and the enzyme amylase.
Amylase begins to digest starch within the food and forms maltose. The food is then
swallowed. Mucus allows it to slide easily through the alimentary canal. Peristalsis, the
rhythmic contractions of the muscles of the alimentary canal, helps to squeeze the food
along the canal. The epiglottis, a cartilage that covers the trachea, prevents food from
going into the lungs. The oesophagus is found behind the trachea and it allows food to be
transferred into the stomach. A sphincter muscle holds food with the oesophagus
temporarily. The walls of the stomach are very muscular and strong. They churn and
mix the food with enzymes and mucus to form chime. The enzymes pepsin and rennin as
well as hydrochloric acid are released into the stomach. These enzymes work best under
acidic conditions. The temperature within the body is 37◦C. The acid helps to kill any
bacteria in food. Pepsin digests proteins into polypeptides. Renin is produced in the
stomach of young mammals. After about 2 hours the sphincter muscle at the stomach’s
bottom releases the chime into the duodenum.
The small intestine is made up of the duodenum, the jejunum, and the ileum. The
pancreas, a cream-coloured gland allows pancreatic juice to flow into duodenum. The
juice contains the enzymes amylase, trypsin, and lipase.
Amylase breaks down starch to maltose.
Trypsin breaks down proteins to polypeptide.
Lipase breaks down fats to fatty acid and glycerol.
These enzymes do not work best in alkaline condition, hence the pancreatic juice which
contains sodium hydrogen carbonate neutralises the acid from the chyme. Bile, a
yellowish green watery liquid, which is made by the liver and stored in the gall bladder,
is released into the duodenum via the bile duct. It does not contain enzymes but it
emulsifies the fats and makes it easier for lipase to digest them. In the small intestines are
tiny projections called villi, which makes enzymes. The enzymes stay close to the lumen
of small intestines. These enzymes complete the digestion of food. Digestion is
completed in the small intestines.
Maltase breaks down maltose to glucose
Sucrase breaks down sucrose to glucose and frustose.
Lactase breaks down lactose to glucose and galactose.
Maltase, sucrase and lactase are carbohydrases. Proteases break down polypeptides to
amino acids. Lipases break down fats to fatty acids and glycerol. The molecules are able
to pass through the walls of the small intestines and into the blood. Absorption takes
place. These absorbed nutrients are taken to the liver by the hepatic portal vein. Some
food are broken down or converted into other substances; some are stored and the
remainder left unchanged. The dissolved food is taken to other parts of the body to
become assimilated as parts of the cells.
The undigested food continues into the colon. Water and salts are absorbed by the colon.
The indigestible food (roughage), bacteria, and some dead cells are mixed to form faeces
and this passes out through the anus.

Table 4.4. How the small intestine is adapted for Digesting Food
Feature How This Helps Absorption To Take
Place
1. It is very long, about 5cm in an adult This gives plenty of time for digestion to
be completed, and for digested food to be
absorbed as it passes through.
2. It has villi. Each villi is covered with This gives the inner surface of the small
cells which have even smaller projections intestine a very large surface area. The
on them, called microvilli. larger the surface area, the faster food can
be absorbed.
3. Villi contain blood capillaries Digested food passes into the blood, to be
taken to the liver and then round the body.
4. Villi contain lacteals, which are part of Fats are absorbed into the lacteals
the lymphatic system
5. Villi have cell walls only one cell thick The digested food can easily cross the
wallto reach the blood capillaries and
lacteals.

Diagram of Human Digestive System

Feeding/Digestion in other Animals

Different organisms, depending on their diets, would have differing alimentary canals.
Herbivores, e.g. rabbits are organisms which eat mainly/ only plants while carnivores e.g.
cat are organisms that eat only animal materials.

Summary of the Differences Between the Alimentary Canal

Rabbit, A herbivore Cat, A Carnivore
1. Lower jaw loosely articulated with the (lower) Jaw precisely articulated with
upper jaw, allowing side to side and up and upper jaw, allowing only up and down
down movements. movement.
2. Incisors long, chisel-shaped, forward Incisors short, peg-shaped, backwards
pointing, for cropping grass pointing, for holding prey.
3. No canines Canines long and pointed for killing prey.
4. Diastema (toothless gap between incisors No Diastema
and premolars) for manipulating grass
5. Broad, ridged premolars and molars for Sharp-edged premolars and molars for
grinding grass cutting meat.
6. Open roots (with large pulp cavity, not Constricted openings in roots do not allow
restricting blood flow) allow teeth to grow growth of teeth.
continuously.
7. Long intestine gives time for digestion Shorter intestine, because food does not
take so long to digest.
8. Large caecum and appendix, contain Small vestigial caecum (No apparent
bacteria which secretes cellulase which function)
digests cellulose. [The digested food passes
out the alimentary canal as soft-mucus-
coated pellets. The rabbit eats these
immediately so food can pass into small
intestine and be absorbed.]

Heterotrophic Nutrition

1. Amoeba:
It is a unicellular protozoan which lives in ponds and slow moving streams. Its cell
surface membrane is sensitive to chemicals in water. It uses its pseudopodia (false feet) to
help it find and feed on smaller organisms. The Amoeba finds its prey (e.g. alga or
protoctistan), by moving along a chemical gradient (where the chemical is most
concentrated). The pseudopodia surround the prey. The prey is then enclosed in a vacuole
(food vacuole) which also has some water. Digested enzymes are secreted into the
vacuole. The enzymes breakdown the digestible parts of the prey, so that they become
soluble and are absorbed into the cytoplasm. The indigestible parts of the prey are
egested. NB: Amoeba digests food intracellularly (takes place inside a cell).
2. Insects:
The housefly, for example, allows saliva to flow along its proboscis (sucking mouth part)
onto the food. The enzymes in the saliva digest the food and make it into a solution. The
dissolved food is sucked up through the proboscis (using powerful muscles).
Grasshoppers bite off pieces of food using hard jaws made of chitin. The food is then
digested inside the alimentary canal.

3. Fish and Whales:

These feed by filter feeding, that is, they strain out microscopic organisms, e.g. plankton,
from the water which passes over their gills. They swallow the plankton.

4. Fungi:
Fungi, e.g. moulds (mucor-bread mould) and mushrooms, feed saprophytically. They
possess threads of hyphae, which makes up a mycelium. Example, Mucor, the bread
mould, releases enzymes from the tip of its hyphae and digests bread. The digested food
is now soluble and so diffuses into hyphae. Saprophytic nutrition is very important as it
helps to release nutrients from dead organisms.
NB: Saprophyte (fungi and bacteria) is an organism that feeds on dead organic material
by secreting enzymes onto it and absorbing it in liquid form.