Digestive Systems

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SUBJECT: BIOLOGY                                                    PERIOD(S): 3

DURATION : 45 MINUTES

TOPIC: DIGESTIVE SYSTEMS

SUB TOPIC:         ALIMENTARY TRACTS/CANALS IN LIVING ORGANISMS.

SUBTOPIC 2: SIMILARITIES AND DIFFERENCES BETWEEN THE ALIMENTARY CANALS OF A GRASSHOPPER AND A BIRD

SUBTOPIC 3: DIGESTIVE SYSTEM IN MAN.

PREVIOUS KNOWELDGE: STUDENTS ARE FAMILIAR WITH THE TOPIC

OBJECTIVES: at the end of the period(s) students, should be able to:

PERIOD 1: Describe with appropriate illustration of different types of alimentary tracts that can be found in living organisms.

PERIOD 2: Explain the similarities and differences between the alimentary canals of bird and grasshopper.

PERIOD 3: Describe the process of digestion in man.

REFERNECES: Essential Biology and Senior Secondary Schools by MC Michaels pg 262 -273.

Modern Biology for Senior Secondary School 1 – 3 pg 285 – 299

CONTENT

Digestions  is the process whereby the food is broken down into the simple soluble and diffusible substances by mechanical and chemical means. The digestive system is made up of the alimentary canal, the organs and glands associated with the digestion and assimilation (absorption of the soluble food substances) digested into the fluids and transported to body cells) of food in animals. The digestive system varies from one animal to another and it depends on whether the food is in solid form or fluid form. Solid food is taken up using teeth, tentacles, beaks, claws (large food pieces) or filter feeding (Small food pieces / tiny microscopic prey) and in fluid form (fluid feeders) which has  structures for sucking fluid food.

Digestion  and absorption of food in most holozoic animals take place in the alimentary canal. Simple unicellular animals don’t possess alimentary canal. A typical  mammalian alimentary tract/canal has the following parts namely mouth, pharynx, oesphagus/gullet, stomach, small intestine (lieum), caecum, appendix, large intestine  (colon), rectum and anus. However, some parts may be absent in certain animals.

P = President       A – Absent

ALIMENTARY TRACTS IN ANIMALS

1. Amoeba – Amoeba proteus feeds on microscopic organisms or decayed foods substances in muddy water at the bottom of pond and ditches. It has no mouth or alimentary canal but engulfs food particles and form a food vacuole (ingestion) Enzymes from the cell cytoplasm are secreted into the food vacuoles, digested and the soluble products diffuse into the cytoplasm.
2. Paramecium: Feeds on microorganisms (e.g. bacteria ) and decayed food particles which are taken into the gullet and digested in food vacuoles. Undigested waste products are passed out through the anal pores.
3. Hydra(Coelenterate) It is a carnivore which feeds on tiny animals by stinging using the stinging cells of the tentacles then the prey is passed into the gut cavity where digestive juices secreted by gland cells of the gut (endoderm) bring about extracellular digestion of the prey. Afterwards amoeba cells in the endoderm engulf the partially digested solid food particles into food vacuoles where digestion is completed intracellullary. Undigested wastes are got rid of through the mouth.
4. Planaria – a free living flatworm that feeds on zooplanktons (microscopic aquatic animals) it has a simple alimentary canal with just 1 opening, the mouth which opens to a short buccal cavity which leads to the pharynx. Food sucked in through the pumping action of the pharynx into the small intestine is digested. The small intestine has 3 branches (one leading to the hand while the other two towards the tail. The branched small intestine allows digested food e.g. diffuse to all the parts of the body. The wastes are Undigested food) are removed from the mouth.
5. Tapeworm is an endoparasite which has no alimentary canal since it feeds on digested food from the host by absorbing digested food via simple diffusion. It can be found in the intestine of some mammals.
6. Earthworm: The alimentary canal of earthworm how 2 openings. The mouth through which food enters and anus through which undigested food leaves. Alimentary canal of earthwromconsists of mouth and ta thin walled cavity, a muscular thick walled pharynx which secrete muscus to lubricate food particles, a narrow tubular oesphagos which connects the pharynx to the crop, thin walled crop for storing food, thick walled muscular gizzard where food is ground against small stones to break it up, and a long straight intestine where digestion and absorption occurs.

Earthworms feed on vegetable matter in the soil in which it swallows as it burrows. The digested food diffuses through the intestinal wall into the blood capillaries and the blood transports it to all parts of the body.

Insects – insects such as grasshopper and cockroach have alimentary canal which consists of 3 parts namely the foregut, midgut and hindgut. Grasshoppers feeds on leaves and it uses its mouthparts to cut and crush the leaves. Salivary glands secretes digestive enzymes which works on the leaves softened by the saliva. The food is passed through the oesophagus into the crop where it is stored and broken up further by the gizzard. Tat the junction of the foregut and mid gut are foreger like projections (caeca) which  secretes enzymes rich in digestive juices into the midgut. Digestion and absorption food occurs here. The hindgut (consistiting of ileum, colon and rectum absorbs water and mineral salts and solid faecal pellets and egested from the anus. Body waste is also emptied into the hindgut through the opening of a collection of narrow tubules (malphibian tubules) at the junction of the midgut and hindgut.

3. Bird: The Alimentary canal of a bird consists of the mouth, pharynx, oesphagus, crop, poor entriculus, gizzard, intestine, caecum, rectum, and anus (cloaca). Birds pickup food with theirbeak and it moves through the oesphagus to the crop where it is softened and secretions from the walls of the crop are fermented by some bacteria. The food moves to the proventicrulus (Glandular stomach because it has glands which secret digestive enzymes e.g. pepsin and amylase) where digestive enzymes are secreted on it. The food moves to the gizzard (grindular stomach) where small  stones help to grind the food. From the gizzard, the food moves tot eh duodernum and intestine where gastric and pancreatic juice and intestinal secretions bring about further digestion and absorption. Most of the water is reabsorbed in the rectum. Solid waste, body waste from the kidneys, eggs, and sperms leave the body through the cloaca (anus).

The long alimentary canal of birds provides a large surface area for the digestion and absorption of food.

SIMILARITIES AND DIFFERENCES BETWEEN THE ALIMENTARY CANALS OF A BIRD AND THAT OF GRASSHOPPER.

The similarities include the following:

1. Both have narrow oesphagus
2. Crop is present in both organisms
3. Caecum and gizzard is present in both
4. Midgut is present.

Rabbits: rabbits is a herbivore and it uses its lips (prehensile) to grab plants and pass to the teeth for cutting and grinding. Saliva moisten and lubricate the food for passage through the oesphagus into the stomach. The stomach secretion acids (hydrochloric acid) pepsin and mucus (to protect the stomach from acids and enzymes as well as moisturize the food) mix with the chewed food to form a mixture, chime (digesta) which passes into the small intestine, a flow regulated by pyloric sphincter. Most digestions and nutrient absorption occur in the small intestine which is divided into three sections, duodernum, jejunum and ilieum. Duodernum secrets enzymes like proteolytic, amylolytic (starch digesting) and lipolytic to break down the food. Jejunum, the mid section absorbs glucose, amino and fatty acids. The ileium, the last section of the small intestine, the remaining nutrients and vitamins are absorbed.

          The large intestine (colon)receives the food and fibre from the small intestine where part is further broken and used and the other indigested part passes directly into the large intestine. Water is reabsorbed here and the material is  passed as round droppings. Digestible material (soluble fibre and proteins) are further broken down and moved into the caecum. Bacterial and other microorganisms reside in the caecum where they help in fermenting  or digesting the material passed into the caecum by turning indigestible fibre into digestible nutrients which are absorbed directly across the caecum wall while others are excreted. After 8 hours, materials from caecum is formed into small moist pellets called cecotrope which are nutritious. Once passed out, the rabbit will immediately eat them. Cecotrope pass through the entire digestive system to allow the rabbit get additional nutrients from plant material through a process called cecotrophy.

HCl – reduce stomach pH levels and kill any bacterial in food

Pepsin – breaks protein down

Man: Alimentary Canal of man includes the mouth, oesphagus, stomach, duodernum, small intestine, cacecum, appendix, large intestine (colon) rectum and anus.

1. Mouth: Alimentary canal of man starts from the mouth. The teeth cut and grind food into smaller pieces and the tongue rolls it into bolous enabling the mixing of food with saliva. Chemical digestion starts. The saliva contain ptyalin which acts on cooked starch converting it to complex sugar. Saliva is a watering, slightly alkaline substance. The bolus is then swallowed.

2. Oesphagus: the mouth and nasal cavity opens into the muscular pharynx which opens into the tubules, the trachea (windpipe) which leads to the lungs and the oesphagus (gullet) which is at the back through which food passes to the stomach. The epiglottis flaps down to cover the trachea during swallowing to prevent chocking. The oesphageal wall is muscular and it contracts and relax alternatively to push each bolus of food down slowly in a process called peristalsis.
3. Stomach: food enters the stomach when the ring of muscular (cardiac sphincter) at the entrance of the stomach of the relaxes. The food is churned by the forceful contraction and relaxation of the muscular stomach wall. The churning mixes the gastric juice (secreted by gastric gland) with the churned food. Gastric juice comprises of 2 enzymes, pepsin, a protease which digest proteins to polypeptides, rennin which curdles mild (Coagulate milk into thick curds) by acting on caseinogens (soluble milk protein) and convert it into the insoluble casein and hydrochloric acid which creates acidic medium for the two enymes and kill bacterial present in food. The casein is broken down by pepsin.

          Food is retained in the stomach for 3-4 horus and it is a thick, creamy fluid called chime which moves out through the ptyalotic sphincter at the bottom of the stomach into the duodernum.

4. Small Intestine: The small intestine is made up of 3 parts, the duodernum, jejunum and ileum respectively. The duodernum contains the pancreas which secrets the pancreatic juice, a watery alkaline liquid which contains 3 important enzymes namely Amylase, which breaks starch to maltose, trypsin, a protease, which breaks down peptones to polypeptides and lipase which breaks down fats and oils to carboxylic acids and glycerol. These 3 enzymes require alkaline medium to work. Fats and oils digestion is aided by bile, a green alkaline liquid secreted by liver and stored in the gall bladder. The bile contains high percentage of water, and adds water to chime and it also contains sodium salts which neutralize the hydrochloric acid in the chime thus enabling the pancreatic enzymes to begin work and bile causes fat emulsification, that is, reduces surface tension of fats and break it up into tiny droplets. At the end of the duodernum the chime becomes more watery and referred to as chyle.

In the ileum, the intestinal wall secretes intestinal juice which contains enzymes which complete the digestion of food. These enzymes are lipase which converts fats and oils to fatty acid (carboxylic acid) and glycerol, maltase which converts maltose to glucose, erepsin converts polypeptides to amino acids, lactase which converts lactose to glucose and galactose and sucrose which converts sucrose to glucose and fructose. Food digestion of food ends in the small intestine. The end products of digestion are amino acids (proteins) fatty acids and glycerol (fats and oils) and glucose (starch)

The small intestine contains folds and furrows (which increase the surface area) as well as small finger like projectiosn villi(singular villus) which take part in the absorption of the and products of digestions, that is, glucose, amino acids, fatty acids and glycerol, vitamins and minerals. The inner surface layer (epithelium) of each vilus is this which allows diffusion of the end products by either diffusion or active transport. The glucose and amino acids are easily absorbed by blood capillaries through the epithelium while fatty acids and glycerol enter the lacteal (a  blind lymphatic tube) are carried through the lymph vessels which eventually empty their contents into the blood vessels near the heart. The blood then carries the fats and other food materials to various parts of the body where they are needed. Excess fats are stored in fat cells which form adipose tissue under the skin and around organs the absorbed food substances and transported from the small intestine by the hepatic portal vein to the liver where it becomes processed.

5. Large intestine: undigested food passes into the colon (large intestine) and water is absorbed. This concentrates the waste products and makes them semi-solid (faeces) which is passed into the rectum and out through the anus. Movement of faeces is facilitated by dietary fibre (roughage) which gives the gut muscles something to squeeze against. Insufficient roughage causes constipation.

The food we eat during a meal takes about one and a half day to pass through the gut before the undigested remains are egested as faeces.

INSTRUCTIONAL MATERIALS: Practicals on digestive systems.

Teaching methods procedures

Step 1: Teacher revises the last topic and introduces the new topic

Step II: Teacher defines digestion and explains what the alimentary canal comprises of:

Step III: Teacher explains the alimentary canals of Amoeba, paramecium, hydra, earthworm, tapeworm, planeria, insects, birds and rabbits

Step IV: Teacher explains the great detail the process of digestion in man.

Step V: Teacher allows students to ask questions, make the student notes and make corrections where necessary.

CLASSROOM (ACTIVITIES)

Period 1: Students define and discuss the process of digestive in man.

Step V: Teacher allows students to ask questions, marks the students notes and makes corrections where necessary.

CLASSROOM ACTIVITIES

PERIOD 1: Students define and discuss the process of food digestion in living organizations apart from man.

PERIOD 2: Students discuss digestion in man.

EVALUATION

PERIOD 1:

1. Define Digestion
2. Mention the parts that make up the digestive system

PERIOD 2:

1. What is cecotrophy?
2. What are the end products of digestion?

SUMMARY / CONCLUSION: Teacher summarises the topic, makes the students notes and make corrections where necessary.

ASSIGNMENT(S)

PERIOD 1: Explain how the gizzard helps in the grinding of maize in a bird.

PERIOD 2: where is pancreatic juice made? Where is it secreted into?

PERIOD 3: Discuss why an athlete is given glucose after a race instead of a piece of bread.