Background Information

DIGESTION OF FOOD

Heterotrophs are organisms that cannot make their Hfood from simple inorganic substance, rather they feed on already available organic food substances. The complex nature of the organic materials most of these heterotrophs take requires that before the food can be absorbed and assimilated into their bodies, they need not only be ingested but they must be digested (a catabolic process).

Digestion in an organism (Heterotrophs) depends on its complexity and size. The simple unicellular organisms for instance, carry out intracellular digestion within a food vacuole inside the cell while the multicellular organisms usually carry out extra cellular digestion with the gut.

FEEDING MECHANISMS

Organisms have special structures with which they ingest food substances, digest them before they are absorbed into the organism’s body. These structures which are called feeding mechanisms also vary according to the organism’s size, complexity and type of food it takes in.

Feeding mechanisms, modification and adaptations for some organisms are discussed in the table that follows under: Feeding Adaptation and Modication in Amoeba, paramecium, Hydra, Filter feeders, Fluid feeders, Feeding adaptations and modications in insects, saprophyte and parasitic feeding modifications.

Feeding in insect

nsects carry out extra cellular digestion e.g. Buttery, Mosquito Cockroach Grasshopper etc.

Different insects have different mechanisms of feeding process and their various mouth parts are adapted for their kinds of foods eg. Piercing and sucking for mosquito, cutting, crushing and grinding in Cockroaches and grasshoppers, sucking for buttery and housey etc.

Digestion, which is the breaking down of complex and large molecule food substances into simpler and smaller ones takes place in mammals in a long tube like structure called Alimentary canal or Gut. Alimentary canal begins at the mouth and ends at the anus. It also consists of the Gullet or oesophagus, stomach, small and large intestines. Different parts of the alimentary canal are modied into specialized organs and structures, which perform specic functions.

Digestion is of two types: Mechanical digestion and Chemical digestion.

In mechanical digestion, food is physically broken down into smaller particles by chewing in the mouth by the teeth and muscular churning by the muscular walls of the alimentary canal. This helps to prepare the food for quick chemical digestion by exposing large amount of food to action of enzymes through increase in surface area of the food particles.

Under chemical digestion, there is a chemical breakdown of complex molecules of food into simpler ones by the action of enzymes.

Various parts of the alimentary canal and their roles in food digestion

The mouth: Mechanical digestion starts at the mouth by the action of the teeth on the food. Several secretions are added to the food.

1. Mucus secretion from the cells lining the mouth moistens the food
2. Salivary secretions from the salivary gland, which not only moistens the food but starts off chemical digestion by converting starch to maltose. The tongue also helps to put the food in proper position

The Oesophagus: This carries food from the mouth to the stomach by a process which involves rhythmic muscular action called peristalsis and the lubricated oesophagus lining with mucous which eases food passage.

The Stomach: Food is received into the stomach from the esophagus by the relaxation of the rst sphincter muscle. Three kinds of cells make secretions at the stomach, one secretes mucous, the other enzymes while the third one secretes waterand hydrochloric acid. These three secretions together form gastric juice. The Hcl (hydrochloric acid) helps to break up connection tissue and cell membrane in food, it also kills many harmful bacteria. Pepsin breaks down protein into polypeptide. There is also rennin which cuddles the milk. The chemical digestion in the stomach is aided by the churning action of the second sphincter which moves food out of the stomach.The small intestine: The small intestine is of three parts: The duodenum which is the rst part, the Jejunum which is the second part and the third part called the ileum.

Secretions from the pancreas and gallbladder enter the small intestine at the duodenum. These secretions are pancreatic juice and bile (bile is produced by the liver but stored at the gall bladder) pancreatic juice contains digestive enzymes. Amylopsin which breaks down starch to maltose, lipase which breaks down fats and oil to fatty acids and glyceroland trypsin which breaks down proteins to polypeptides.Bile provides an alkaline medium for enzymes of the pancreatic juice to act; it adds water to the chyme (a thick creamy uid the food forms into in the stomach). It also breaks down fats into tiny droplets (emulsication).

At the illium digestion of carbohydrate, protein fats and oil is completed. Food here is more watery and it is called chyle

Digestion here is by the action of enzymes from the intestinal juice maltose is broken down to glucose by maltase. Erepsinbreaks down polypeptides to amino acids while lipase changes fats and oil to fatty acids and glycerol.

Absorption of digested food

Absorption of glucose, fatty acids and glycerol, amino acids, vitamins and mineral salts take place in the small intestine.The small intestine is well adapted to carry out the function by the presence of the following features

• Folds and furrows in the walls of the small intestine which serves to increase its surface area
• The presence of small ngerlike projections called villi which further gives it increased surface area.
• A rich supply of blood vessels and lymphatic vessels containing blood and lymph

All absorbed food are brought to the liver via the lymphatic portal vein before they are assimilated into the body. the liver can convert, break down, store or leave unchanged the egested through the anus after more water absorption has taken place through the anus after more water absorption has taken place through the anus.

General Questions

1.The organic chemical substances that bring about chemical digestion is called digestive

2. must take place before a complex food substances like starch can be absorbed into the blood stream.

3.In feeding mechanism, cilia is required in paramecium while pseodopods are needed for

4.In paramecium egestion takes place through the

5.Intracellular digestion occurs in unicellular protists like amoeba while in multicellular animals like earthworm digestion is

6.Peristalsis is a rhythmic muscular action at the alimentary canal while emulsication is a process of

7. Mechanical digestion takes place in the mouth mostly by the action of the teeth but in the stomach by

8. is an adaptation for lter feeding , A proboscis also is an adaptation for

9. State the three parts of the small intestine

10.In a hydra, in the endoderm brings about intracellular digestion after the initial extra cellular digestion on the wall of the gut.

11.Identify the parts labeled (i) (ii) (iii) (iv)

12.Write down two plants that are carnivorous or intectivorous

14. ___________ is an example of a saprophytic micro organism.

15.The number, kinds and arrangement of teeth in the lower and upper jaw bones of a mammal is called

16.a. Is the organism a herbivore or a carnivore?

In g 2.94, the teeth is adapted to (i) And (ii)
(iii) is it open or closed teeth?

17. A bile is produced by the liver and stored at the

18.Before egestion takes place through the anus, absorption of water occurs at the

19.Pepsin acts in an acid medium while Erepsin acts in an

20.Write two examples of a ruminant

The diagram below shows the alimentary canal of a bird. Label the parts according to their numbers

26.What is the function of the gizzard in digestion in birds?

Expression Exercise on Digestive systems and Mechanisms

1.(a) Write five examples of animals that are uid feeder

(b)Write down two adaptive features of carnivorous plants in their mode of feeding

2.(a) How is the food in the stomach of mammals changed to chyme?

(b)Is there a pH change in medium for food digestion when food enters the deodernum? If so what is the change?

(c) Write down the end product of the following food substances in the table below

3.(a)What is the cause of the difference in length between the alimentary canal of herbivores and that of other mammals

(b) State three special adaptation of herbivores that enable them to digest cellulose

4.(a)On the diagram below, label the parts that show lacteal and capillary

b)Within the table below, summarize activities of the stated digestive enzymes during digestion. (Follow the rst example)

Practical Activities

Activity 1

Observing the alimentary of a small mammal eg. Rat, rabbit or guinea pig

Method /procedure

Questions

(i)Write down the lengths of the intestine observed :
Small
Large

(ii)Compare your observed real alimentary canal of a small mammal and the one drawn on paper.

(iii)Write down your observation when you used a hand lens to see through the ileum

(iv)Write down one other function of the small intestine apart from digestion

(v)Write down two adaptative features that enable small intestine to perform such function.

Activity 2

Examming the alimentary canal of a bird (a pigeon or fowl)

Materials required

1. Freshly dissected fowl with its alimentary canal exposed
2. Pen knife;
3. Hand lens

Method/procedure

(i)Carefully examine various parts of the birds alimentary canal, identifying the positions of the following: The crop, esophagus, gizzard, liver, pancreas, small intestine and anus

(ii)Compare your observations with that of the small mammal alimentary canal in activity 1Write down your observations and comparisons

(iii)What happens to food taken in by the bird when it reaches the gizzard

Draw the diagram of the alimentary canal in your practical note book and compare it with the one in question 21-25

a. Identify the diagram specimens above and label the parts indicated with; , ii, iii, iv, v and vi.

b. Compare B and C in relation to their modes of feeding

a. The diagram specimen shows the jaw bone and teeth of a herbivore and a carnivore. State how their dentitions are well adapted for their feedings

b. Identify the diagram specimens in fig 2.99 A - B by name (without reasons) and label the parts indicated with; , ii, iii, iv, and vi..

3. The diagrams below show the mouth parts of insects

a. What are the mouth parts of B adapted for

b. Which of the mouth parts are adapted for cutting and chewing

c. Write the names of three organisms that their mouth are shown in A, B, C in order of

The diagram specimen in gs. 2.992 and 2.993 show the jaw bone and teeth of rabbit and dogs state how their dentition are well adapted for their feeding

Write down the dental formula of each of the organisms under the respective jaw bone shown.

DIGESTIVE SYSTEM SSCE PAST QUESTIONS

JUNE 2000 PART II Qa,b

You are provided with the following specimens: a bird and buttery as specimens G and H respectively

(a) Identify specimens G, and H, without reasons

(b) State one observable feature by which each of specimens G and H obtain their food

(c) State three observable features which each contribute to the survival of specimen G and H