scheme of work for third term biology
TRANSCRIPT
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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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