Digestion can be defined as the breaking down of food into smaller and smaller components, until they can be absorbed and assimilated into the body. This is made possible with the help of the teeth in the mouth. This helps to masticate food into smaller bits which are then acted upon by enzymes in the mouth before it’s fully digested into the stomach.
Chewing is the process in which the food eaten is mixed with saliva in the mouth which begins the process of digestion. It produces a bolus which is then swallowed down through the oesophagus and into the stomach. In the stomach, it is mixed with an acid called the Gastric juice until it passes into the duodenum where its mixes with a number of enzymes which are produced by the pancreas.
The saliva also contains a catalytic enzyme called the Amylase which begins to act on the food in the mouth. Digestion is made easier by mastication of the food by the teeth and also by the muscular action of peristalsis and segmentation contractions.
DESCRIPTION, FUNCTIONS, ND MODIFICATION OF VARIOUS PARTS OF THE ALIMENTARY SYSTEMS
MOUTH: The food is taken in through the mouth. In some animals, e.g. man, the food is masticated by teeth and partly digested in the mouth with the help of enzymes. In some insects, the mouth has biting jaws.
PHARYNX: In some animals, such as planaria and earthworm, the pharynx is muscular and it helps to push the food into the oesophagus.
OESOPHAGUS: By the rhythmic contraction of the muscular walls known as peristalsis, the food is pushed into the stomach. In birds and insects, the posterior end of the oesophagus may be modified to form crop which is a temporary storage organ while in ruminants, e.g. cow, the first three chambers are for temporary storage.
STOMACH: It is muscular and the food is churned by its peristaltic movements. In birds, it is modified into a grinding or milling organ called the Gizzard. Digestion of food by enzymes takes place in the stomach of some animals, e.g. mammals.
INTESTINE: It is divided into the Small and Large intestines. The small intestine is for digestion and absorption of digested food. In mammal, it is modified to form finger-like projections called Villi which increase the surface area for absorption. Large intestine absorbs water in man. At the junction of large and small intestine is the Caecum which houses some bacteria that digest the cellulose contents of the food mainly in herbivores.
RECTUM: Stores faeces before being removed through the anus.
ADAPTIVE FEATURES OF ALIMENTARY SYSTEM OF AN EARTHWORM AND THEIR FUNCTIONS
The alimentary canal of an earthworm is a long straight tube and is divided into the following parts:
The Mouth for ingestion of food
The Pharynx which has a muscular thick wall and glands from which it secretes mucus which lubricates food particles and a protein digesting enzyme.
The Oesophagus which is a narrow tubular wall and has a pair of oesophageal glands that secretes calcium carbonate which removes excess calcium absorbed by the worm’s food.
The Crop which has a thin wall for storing food.
The Gizzard which has a thick muscular wall for crushing food materials with the help of small stones.
The long straight Intestine in which extracellular digestion of food by enzymes secreted from the glandular cells of the intestinal wall occurs.
The digested food mainly vegetable matter diffuses through the intestinal wall into the surrounding blood capillaries. The blood then transports the food to all parts of the body.
FEEDING HABITS IN SOME ANIMALS
1. ABSORBING MECHANISM: In parasitic worms, tapeworms absorbs food through the body wall.
2. CHEWING MECHANISM: In either grasshopper or cockroach, each has an upper lip or labrum which prevents falling of food. Two strong mandibles for chewing, the maxillae which is used for biting and the labium or lower lip which prevents food wastage in the mouth.
3. SUCKING MECHANISM: This is found in insects like mosquito or housefly.
4. GRINDING MECHANISM: This is found in advanced animals like the mammals.
5. TRAPPING AND ABSORBING MECHANISM: This is found in insectivorous plants, e.g. bladder wort.
MODIFICATION IN ORGANISMS TO REFLECT FEEDING HABITS.
1. FILTER FEEDING: A filter feeder takes in a stream of water and filters from it small organisms that maybe present. Examples are the barnacles, clam and mosquito larva.
The mosquito larva uses two mouth brushes to create a current of water. Some colloidal particles are filtered or abstracted from water and ingested.
The barnacle and clam use cilia to create a current of water.
2. FLUID FEEDERS: Animals that feed on liquid are called the fluid feeders. Examples are wallowers which rest within their food, e.g. tapeworms and the suckers e.g. aphids, butterflies and mosquito.
Tapeworms absorb the food directly through their body. Suckers make use of modified mouth parts like the rostrum in aphids or the proboscis in the mosquitoes or butterflies.
3. FEEDING ADAPTATIONS IN INSECTS
A. Female mosquito that feed on blood have their mouth parts with exception of the labium modified into piercing stylets and sucking tubes.
B. The housefly has its mouth parts modified into proboscis formed from the labium.
C. The butterfly has its maxillae formed into a long proboscis for reaching down the nectaries.
4. SAPROPHYTIC AND PARASITIC FEEDING
(I) SAPROPHYTIC FEEDING: Saprophytes such as Mucor, Mushrooms and Rhizopus lack pigment called Chlorophyll and, therefore, do not produce their own food like green plants. They feed on dead organic matter. They have hyphae which secretes enzymes onto the organic matter which digest it externally before being absorbed.
(II) PARASITIC FEEDING: Parasitic organisms like the tapeworm and roundworm. The tapeworm and roundworm are endoparasites found in man and so they depend on other organisms for their own food.
A tapeworm has hooks and suckers at the head for attaching itself to the gut of the host in order to avoid being displaced. The body has thick cuticle to resist digestive enzymes and also flat surface which ensures large surface area for absorption. The host’s digested food is absorbed straight into the entire body surface.
5. FEEDING IN PROTOZOA
Amoeba feeds on microorganism or diatoms or particles or organic matter. It engulfs the food particles by forming a cup-shape when the two ends of the pseudopodia touch. They fuse and enclose the food into the body with little amount of water and this forms a food vacuole.
Digestive enzymes stored within the lysosomes hydrolyse the food inside the food vacuole into smaller molecules that can be absorbed through the vacuole membrane into the cytoplasm.
The cilia in the gullet create a swirling current of water which drives bacteria and other small organisms into the mouth-pore or cytosome. The food with a drop of water forms the food vacuole. The vacuole moves along a definite path in the cytoplasm while the food is being digested and absorbed. The undigested food is removed through the Anal Pore.
FEEDING IN MAMMALS
Teeth and Dentition in Mammals
ENAMEL: It’s the very hard part of the teeth and contains 96% mineral salts which is mainly Calcium Phosphate.
DENTINE: It’s the hard substance that surrounds a Pulp cavity.
PULP CAVITY: It contains small blood vessels, nerve fibre and pain sensory receptors.
CEMENT: It’s the bone-like layer which fixes the tooth in its socket.
PERIODONTAL MEMBRANE: This fixes the tooth to the jaw bones.
TYPES OF TEETH
INCISORS: This type of teeth is flat and chisel shaped, for cutting and biting of food.
CANINES: This type of teeth is pointed and its used for tearing.
PREMOLARS AND MOLARS: This type of teeth is flat with cusps and its used for grinding and chewing.
Dentition is the number and arrangement of teeth in an animal.
The dental formula shows the number and types of teeth an animal has in one half of each jaw.
Using dental formulae, dentition in animals can be represented as
(A). MAN: I 2/2 C 1/1 PM 2/2 M 3/3 = 32
(B). DOG: I 3/3 C 1/1 PM 4/4 M 2/3 =42
(C). COW: I 0/3 C 0/1 PM 3/3 M 3/3 =32
I stands for Incisors, C stands for Canine, PM stands for Pre Molars and M stands for Molars. The first set of teeth that appears in mammals are called Milk Teeth while the second set of teeth that appears is called the Permanent teeth.