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Class: SS2

SCHEME OF WORK FOR THIRD TERM

BIOLOGY

1,2&3. Digestive Systems I: Alimentary canal of: a)Planaria b) Earthworm c) Insects d) Birds e) Humans.

4. Digestive Systems II: a)Feeding Habits b)Feeding Adaptations

5. Transport Systems

6. Respiratory Systems

7. Excretory Systems

LESSON NOTE FOR WEEK ONE, TWO AND THREE

Class: SS2

Subject: Biology

Topic: Digestion

MAIN OBJECTIVES:

By the end of the lesson, Learners should be able to:

1. Define Digestion

2. Describe the types of Digestion

3. Identify the parts of the alimentary canals of certain organisms and their functions.

4. Describe the relationship between the liver, pancreas, stomach and duodenum.

5. Explain the role of the villi in the absorption of food.

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DEFINITION OF DIGESTION:

Digestion is the process of breaking down complex food into simple soluble substances by mechanical

and chemical means. Due to this, Digestion is divided into two types:

1.Mechanical Digestion: this mainly occurs in the mouth by means of the teeth, through a process called

mastication.

2.Chemical Digestion: this is a chemical process that consist of breaking down large molecules to small

molecules. The large molecules are usually not soluble in water while small ones are soluble. The

chemical substance that aid in breakdown of food substances are called enzymes.

ALIMENTARY CANAL: This is the canal or system which deals with Digestion.

Parts of the alimentary canal:

1.The Alimentary canal of Planaria:

Planaria is a free living flatworm, Digestion in planaria is intracellular,the digested food is distributed by

diffusion to all parts body by the gastrovascular cavity. The digestive tract consist of a ventrally placed

mouth, muscular pharynx and intestine. The pharynx leads into the intestine. The intestine gives rise to

extensive lateral diverticula thus forming gastrovascular cavity.

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2.The Alimentary canal of Earthworm:

The alimentary canal of an Earthworm consists of a mouth,buccal cavity,pharnyx,

oesophagus,crop,gizzard,intestine and anus. Digestion here is extracellular.Mouth and buccal cavity:

the mouth leads into the buccal cavity. Food is sucked into the mouth by muscular pharynx. It helps by

arranging the food substance in position for ingestion.Pharynx leads from the buccal cavity to the

oesophagus. Digestion of proteins starts in the pharynx.Oesophagus is long and connects the pharynx

with the crop.Crop is a temporary storage chamber.Gizzard connects the crop with the intestines. Food

is ground into small particles by the churning action of the gizzard. Grinding is aided by small stones

present in the gizzard. Intestines is the largest part of the alimentary canal and terminates as an anus.

Digestive enzymes are secreted to digest various types of food. Food is distributed round the body by

blood. The indigested materials are removed from the body through the anus as cast.

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3. The Alimentary canal of insects:

The alimentary canal of insects consist basically of three parts. The foregut, midgut and hindgut. The

grasshopper feeds on leaves by pouring saliva on the leaves, glands in the saliva softens the leaves. The

chewed food is stored in the crop and broken into smaller bits in the gizzard.

Between the foregut and the midgut are caeca responsible for the secretion of enzymes rich in digestive

juice into the midgut. Digestion and absorption of food occurs in the midgut. Absorption of water takes

place in the hindgut also, body waste is emptied into the hindgut through the malpighian tubule found

between the midgut and the hindgut.

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4. Alimentary Canal of Birds:

The alimentary canal in birds consist of the pharynx, oesophagus, crop, gizzard, intestine, anus and

cloaca. Birds swallow their food whole and store it in the crop. Then it is passed to the gizzard where it is

broken into smaller pieces by the action of enzymes, coupled with small stones found in the gizzard.

Chemical digestion begins in the gizzard in birds. Digestion is completed in the small intestine,

absorption also take place in the small intestine. Reabsorption of water takes place in the rectum. Solid

waste alongside waste water from the kidney leaves the body through the cloaca.

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5. The Alimentary canal and digestion of food in humans:

The alimentary canal of humans is made up of the mouth, pharynx, oesophagus (gullet), stomach, small

intestine, large intestine, rectum and anus. Liver and pancreas are glands associated with the alimentary

system. The mouth is the first part of the alimentary canal. Mechanical digestion takes place here.

Chemical digestion also begins in the mouth through the action of saliva containing the enzyme ptyalin.

Food mixed with saliva is rolled into bolus and swallowed. The pharynx allows the passage of food from

the mouth into the oesophagus. The oesophagus is a tube linking the pharynx to the stomach. Food is

pushed slowly through the oesophagus by a process called peristalsis. The stomach is the major food

storage organ. Its muscles contracts forcefully and relaxes. This action churns up food turning it into a

paste called chyme. In the stomach, hydrochloric acid produced in the stomach stops the action of

ptyalin on starch. It also provides an acidic medium for the enzymes in the gastric juice (pepsin and

renin). Pepsin acts on proteins converting it to peptones. Renin coagulates milk proteins so that pepsin

can act on it. HCL also kills bacteria and prevent spoilage of food in the stomach. The small intestine is

divided into three parts namely: the duodenum, the jejenum and the ileum. In the duodenum, partly

digested chyme encounters other fluids. Bile neutralizes the acidic chyme from the stomach, breaks fat

to tiny droplets. The pancreas secrets pancreatic juice containing amylase, trypsin and lipase. Amylase

converts starch to maltose, trypsin splits proteins to peptones and amino acids to polypeptides. Lipases

convert fats and oils to fatty acid and glycerol. Digestion is completed in the jejenum and ileum where

maltase converts maltose to glucose. Lactase converts lactose to galactose, sucrase converts sucrose to

fructose, erepsin converts polypeptides or peptones to amino acids and lipase converts fats and oils to

glycerol. In The large intestine, disgestion and absorption does not take place. However, water is

absorbed here. Undigested food (faeces) are stored in the large intestine before egested.

The Relationship Between the duodenum, stomach, liver, gall bladder and pancreas.

The pancreas secretes a juice containing three important digestive enzymes namely Amylase

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(amylopsin), trypsin and lipase. The pancreas also contains islets of langerhans which secretes two

hormones namely: insulin and glucagon. Bile is secreted by the liver cells and stored in the gall bladder.

It is passed into the duodenum by the bile duct. Bile performs an important function in the duodenum

that helps the pancreatic juice function well.

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The Role of the villi in absorption of food.

Absorption of food takes place in the small intestine. The mammalian small intestine is modified to

numerous finger-like projections called villi on its inner surface. The villi increases the absorptive area of

the intestine. The molecules of the digested food (glucose, amino acids) pass into the epithelial cells,

through the walls of the capillaries in the villus into the blood stream. They are then carried away in the

capillaries which join up to form veins. The veins unite to form one large vein called the hepatic portal

vein. The hepatic portal vein carries all the blood from the intestine to the liver which may store or alter

any of the products of digestion. A large portion of the fatty acids and glycerol may combine to form fats

again in the intestinal epithelium. Fatty acids and glycerol are absorbed through the lacteal before

joining the blood stream. Excess amino acids are deaminated into urea and residue is also as glycogen.

Excess fats are stored under the skin.

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