DIGESTIVE SYSTEM

DIGESTIVE SYSTEM

The digestive system includes the alimentary tract and all glands and organs associated with digestion and assimilation of food in animals.

Digestion is the breakdown of large molecules of food into simple and absorbable form for use by animals. Ingestion is the taking in of food from outside through the mouth. Egestion is the discharge of undigested and unabsorbed food from the alimentary canal. Absorption is the passage of food through the wall of the alimentary canal. Assimilation is the manufacture of the body’s own materials from the absorbed substances and their use in metabolism.

TYPES OF DIGESTIVE TRACTS

There are two types of digestive tracts:

A.    Incomplete digestive tract: has only one opening- the mouth which serve for ingestion and egestion e.g Hydra, Planaria

B.     Complete digestive tract: has two openings to the outside world- mouth (for ingestion) and anus (for egestion) e.g. earthworm, goat

Complete digestive system has an advantage that incoming food does not mix with outgoing undigested food materials and each part is specialized to perform different functions.

Alimentary Canal of invertebrates

Animals have different alimentary tracts which vary in size, complexity, mode of feeding or the type of diet.

Alimentary canal of Planaria

Planaria is a free-living flatworm that feeds on small aquatic animals- zooplankton. It has a simple digestive tract with one opening- the mouth. The digestive tract consist of a ventrally placed mouth, muscular pharynx and intestine. The pharynx can be protruded when attacking a prey. Digestion is intracellular and undigested food is egested through the mouth. Digested food is distributed through the body by diffusion.

Alimentary canal of Earthworm

The earthworm has a complete digestive tract.

The mouth has a lip-like structure called prostomium which leads into the buccal cavity. The pharynx secrete mucus and protein digesting enzymes into the pharyngeal cavity. The oesophagus secrete CaCO3 [calcium trioxocarbonate(IV)] which removes excess calcium from ingested food. The crop acts as temporary storage chamber. Food is ground into small particles by the churning action of the gizzard. Digestion of food takes place in the intestine. Indigestible materials are removes from the body through the anus as casts.

Alimentary canal of Cockroach

The digestive tract of Cockroach consist of mouth (mandibles) which is modified for cutting and chewing. Oesophagus is short, narrow and tubular and connects the mouth with the crop. The crop acts as temporary storage. Most digestion occur in the crop. The gizzard is muscular and grinding of food takes place here. Digestion and absorption take place in the midgut. The rectum absorbs water and faeces is passed out through the anus.

Alimentary canal of birds

The mouth parts have been modified to beak with no teeth. Food is passed from the mouth through the oesophagus into the crop where it is stored temporarily. The food then passes to the stomach consisting of the true stomach (proventriculus) and muscular gizzard. The proventriculus also known as glandular stomach has glands which secrete digestive enzymes. The food then proceeds into the gizzard (grindular stomach) where grinding takes place. Bile and pancreatic juice is secreted in the first part of the intestine (duodenum) while digestion is completed and absorption takes place in the lower intestine. Undigested food passes into the rectum and anus (cloaca) to be discharged as faeces.

Similarities in the alimentary canal of Birds and Cockroach

1.         Possession of narrow oesophagus;
2.         Possession of crop;
3.         Possession of caecum;
4.         Possession of muscular gizzard;
5.         Possession of mid-gut/intestine;
6.          Possession of rectum.

Differences between the alimentary Canal of Birds and Cockroach

Bird	Cockroach
-mouth is modified into beak for pecking	mouth modified for chewing and biting;
- tongue is present in the mouth	tongue is absent in the mouth;
- duodenum is present	duodenum is absent;
- alimentary canal is long	alimentary canal is relatively short;
- hindgut terminates into cloaca	hindgut terminates into anus;
- malpighian tubules are not attached to the alimentary canal	malpighian tubules are attached to to the alimentary canal
- liver/pancreas present in the alimentary canal	liver/pancreas absent in the alimentary canal;
- salivary gland absent	salivary gland present.

Alimentary canal of Rabbit

The rabbit is an herbivore that feeds mainly on grasses and leaves. It digestive system consist of mouth, oesophagus, stomach, small intestine, large intestine, caecum, rectum and anus.

Food is physically broken-down in the mouth by the teeth. The food then passes into the stomach where gastric juice breaks down the food into simpler substances. Digested food is absorbed into the bloodstream through the walls of the small intestine. The rabbit has a very large caecum which contains a large number of beneficial microorganisms. These microorganisms digest fine food materials and undigested food from the caecum form soft moist pellets which are excreted through the anus.

Alimentary canal in Man

The mouth contains teeth, tongue and salivary glands. The teeth are used to cut, grind or chew (masticate) the food into tiny particles. The tongue rolls the food into bolus, allows mixing of the food with ptyalin and aids swallowing the food. Saliva contains the enzyme ptyalin, serve as lubricant for the mouth and solvent for the food. Food passes through the oesophagus through peristaltic movement into the stomach.

The stomach secretes renin (curdles milk) and pepsin (breaks down proteins into peptones). It also secretes HCl an acid which helps to kill some bacteria in the stomach. Churning movement of the stomach convert the food into semi-liquid called chyme.

Duodenum secrete amylase (convert starch to maltose), lipase (convert protein into peptones) and bile. The food is now in liquid form- chyle. Absorption and digestion takes place in the ileum. Enzymes secreted in the ileum include lipase, erepsin, maltase, sucrase and lactase.

Water is absorbed in the large intestine while undigested food is converted to faeces and egested through the anus.

The liver secretes bile which is temporarily stored in the gall bladder. Bile aid digestion of fats. The pancreas produce the pancreatic juice which contains digestive enzymes and produces the hormone – insulin which regulates the amount of sugar in the blood.

Digestion in Man

Enzyme	Secretion	Source	Site of action	Substrate	Product
Ptyalin	Saliva	Salivary glands	Mouth	Cooked starch	maltose
Pepsin	Gastric juice	Gastric glands	Stomach	Protein	Polypeptides (Peptone)
Renin	Gastric juice	Gastric glands	Stomach	Soluble casein (milk protein)	Insoluble casein
Amylase	Pancreatic juice	Pancreas	Small intestine (Duodenum)	Starch	Maltose
Trypsin	Pancreatic juice	Pancreas	Small intestine (Duodenum)	Protein	Polypeptides (peptones)
Lipase	Pancreatic juice	Pancreas	Small intestine (Duodenum)	Fats	Fatty acids and glycerol
Erepsin	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Polypeptides (peptones)	Amino acids
Maltase	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Maltose	Glucose
Lactase	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Lactose	Glucose and galactose
Sucrose	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Sucrose	Glucose and fructose
Lipase	Intestinal juice	Intestinal glands	Small intestine (Ileum)	Fats	Fatty acids and glycerol

Similarities in Alimentary Canal of Birds and Grasshopper

1. Both have narrow oesophagus
2. Both have crop for temporary storage of food
3. Both have muscular gizzard
4. Both have midgut

Differences in Alimentary Canal of Birds and Grasshopper

	Birds	Grasshopper
1	Mouth is modified into beaks	Mouth is modified into mandible and maxillae for biting and chewing
2	Duodenum is present	Duodenum is absent
3	The alimentary canal is relatively longer	The alimentary canal is relatively shorter
4	Alimentary canal ends in cloaca	Alimentary canal ends in anus
5	Malpighian tubules are absent	Malpighian tubules are present
6	Pancreas is present	Pancreas is absent
7	Tongue is present in the mouth	Tongue is absent in the mouth

FEEDING MECHANISMS IN SOME ANIMALS

A.          Absorbing mechanism: e.g. Tapeworm. They do not have alimentary canal, they possess a scolex or head with a rostellum surrounded by hooks and four suckers for attachment; they possess a flat body for large surface area for absorption; entire body surface adapted for absorption and thick cuticle to resist digestion by digestive enzymes.

B.           Biting and chewing mechanism: e.g. grasshopper, cockroach. They have four different mouth parts:

• -          Labrum (upper lip): this prevents food from falling off the mouth
• -    Mandibles: these are heavy toothed and law-like structure for cutting and chewing food materials
• -     Maxillae: a biting blade which breaks down the food chewed by mandibles into smaller particles
• -          Labium (lower lip): this prevent wastage of food from the mouth

C.     Sucking mechanism: e.g. mosquito, butterfly and housefly. The mosquito has piercing mouth parts called proboscis, which is used for sucking blood of animals. The mouth produce saliva to prevent clotting of the blood.

The butterfly has long coiled proboscis used for sucking nectars of flowers. The proboscis can be coiled or extended when in use.

The housefly has enlarged labella for sucking liquid food. It uses a sponging mechanism to absorb the food.

D.    Grinding mechanism: this is common among mammals. The grinding is aided by hard, strong teeth made of enamel and dentine. The teeth can withstand biting, chewing, grinding or cracking of food. There are different sets of teeth for special purposes- incissors for cutting bits of food, canine for tearing food, pre-molars and molars with wide surfaces for grinding.

E.     Trapping and absorbing mechanism: this is common in carnivorous (insectivorous) plants e.g. bladderwort, sundew. The plants have structures for trapping small insects. Sundew has long hairs which carry digestive glands which digest insects and digested food is absorbed into the plant.

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TISSUE AND SUPPORTING SYSTEM

TISSUE AND SUPPORTING SYSTEM

The skeleton is the hard, bony framework of the body which confers protection, support and shape to organs and soft tissues of the body.

Skeletal Materials in Animals (Forms of Skeleton)

1.     Fluid (hydrostatic) skeleton in earthworm and anemones: fluid pressure is used to provide support.
2.     Bones and cartilages in animals: bone consists of osteocytes (bone cells), protein fibres (collagen) and minerals (mainly calcium phosphate and calcium carbonate). The bone has a hard outer layer (shaft) and a spongy hollow cavity filled with bone marrow. It is found mainly in vertebrates.
3.     Cartilage consists of living cells (chondroblast), carbohydrates and protein fibres. It has a great tensile strength and provides cushioning effect on bones moving against each other. There are three types of cartilage: Hyaline cartilage (found in trachea and bronchi, protruding part of the nose and surface of moveable joints); Fibro-cartilage (found in discs between bones of the vertebral column) and; Elastic cartilage (found in the external ear)
4.      Cuticle (Chitin) in arthropods: chitin is a non-living protein substance with a thin waterproof layer of wax. Organisms with this skeletal material grow by moulting or ecdysis.

Types of Skeleton

1.          Exoskeleton
2.           Endoskeleton
3.          Hydrostatic skeleton

The Mammalian Skeleton

The mammalian skeleton is made up of the axial and appendicular skeleton.

Axial Skeleton

The axial skeleton consists of:

• -          The skull
• -          Vertebral column (backbone)
• -          Ribs

The Skull

The skull is composed of the cranium (brain box) and facial bones.

The cranium consist of several flat bones joined together by saw-like edges called sutures.

The facial bones supports the nose, the eyes and the muscle of the cheeks

Jawbones consist of the upper jaw known as maxilla and the lower jaw known as mandible.

The Vertebral Column

The vertebral column is also known as the backbone or spinal column. The vertebral column is the central supporting structure of the skeleton.

Common Features of the Vertebra

1.         They possess a fairly central hole called neural canal for passage of spinal cord.
2.             They possess a neural spine which projects upwards dorsally for attachment of muscles.
3.        They have two (2) transverse process which projects from the sides for attachment of muscles.
4.        They have centrum which is a solid piece of bone below the neural canal.
5.        They possess facets at the front and back which is a point of contact with another bone.

      Each vertebra is separated from the other by an intervertebral disc.

Bones of the Vertebral Column

	Vertebra type	Man	Rabbit	Rat	Location	Function
1	Cervical	7	7	7	Neck region	Supports the neck
2	Thoracic	12	12-13	13	Thorax (chest)	Articulate with the ribs
3	Lumbar	5	6-7	6	Upper Abdomen	Provide attachment for abdominal muscles
4	Sacral	5	3-4	4	Lower abdomen (Hip)	Form the sacrum
5	Caudal	4	16	27-30	Tail region	Support the tail
	Total	33	44-47	57-60

1.      Cervical Vertebra: they are found in the neck region. Consist of atlas, axis and typical cervical vertebrae.

A.  Atlas Vertebrae: helps with the nodding of the head.

                                         i.   It articulates with the skull

                                       ii.   It has a large neural canal

                                     iii.   It has a flat and broad transverse process

                                     iv.   The neural spine is short/absent

                                       v.   It has no centrum

                                     vi.   It has vertebrarterial canal

B.  Axis Vertebrae: allows twisting of the head.

                                         i.   It has a pin-like transverse process

                                       ii.   It has broad flat centrum

                                     iii.   It has a large and flattened neural spine

                                     iv.   It articulates with atlas through odontoid process

                                       v.   It has vertebrarterial canal

          2.      Thoracic Vertebra

                                         i.   They possess long neural spines

                                       ii.   The transverse process are well-developed

                                     iii.   They have facets for rib articulation

Functions

a). They form thoracic cage with ribs and sternum to protect the heart and lungs

b). They assist in breathing

          3.      Lumbar Vertebrae

                                            i.   They have flattened transverse process

                                          ii.   They have short and flat neural spine

                                        iii.   They have large and thick centrum

Functions

a). They bear the weight of the body

b). They maintain the right and proper gait of the body

        4.      Sacral Vertebrae: they are found in the pelvis. They fuse to form sacrum. They have greatly reduced centrum and they provide support, rigidity and strength.

        5.      Caudal Vertebrae: they are reduced to small centrum bones. They are fused to form coccyx. They have no neural canal, neural spine and transverse process. The function is to stabilize pelvic girdle in man.

The Ribs

There are 12 pairs of rib bones. They articulate dorsally with the transverse process of thoracic vertebrae and ventrally with the sternum. The last pair is unattached ventrally and is known as floating ribs. The ribs protect the heart and the lungs.

Appendicular Skeleton

This consist of 2 pairs of limbs and 2 pairs of limb girdles.

Limbs: the limbs are divided into two

      1)      Forelimbs: these consist of:

a.    Humerus (upper arm)

b.    Radius and ulna (forearm)

c.    Carpal (wrist bones)

d.    Metacarpal (hand bones)

e.    Phalanges (fingers)

      2)      Hind limbs: these consist of:

a.    Femur (thigh)

b.    Tibia and fibula (lower leg)

c.    Tarsal (ankle bone)

d.    Metatarsal (foot bone)

e.    Phalanges (toes)

The limb girdles consist of pectoral and pelvic girdle

     1)      Pectoral girdle: this consist of scapula (shoulder blade), clavicle (collar bone) and coracoid. The scapular is a flat bone with two processes- coracoid and acromion, with a deep notch called glenoid cavity.

      2)      The pelvic girdle consists of the ilium (hip bone), ischium and pubis.

JOINTS

A joint is a place where two or more bones meet. Bones are joined together by ligaments. The articulating surface of bones at movable joints is covered by smooth articular cartilage.

Types of Joints

1.         Ball and socket joint: found in the shoulder and hip joint
2.         Hinge joint: found in the elbow and knee joints
3.         Pivot joint: found between the axis and atlas
4.         Gliding or sliding joint: found at the wrist and ankle

Mechanism of Joint Movement

Muscles are attached to bones by tendons and they control movement at joints. Muscles work by contraction and relaxation. Two muscles are needed to move a joint and their action is antagonistic.

Structure of Joints

A typical joint consist of

1.        Ligament: holds two bones together in a joint
2.        Tendons: connect muscles to bones
3.       Articular cartilage: provide cushioning effect to bones, protecting the articular surfaces of the bone from wear and tear
4.         Synovial fluid: lubricates the joints
5.         Synovial membrane: secretes the synovial fluid

Functions of the Skeleton

1.         Support
2.         Protection
3.         Locomotion (movement)
4.         Shape
5.         Production of blood cells
6.         Assisting in breathing
7.         Storage of mineral salts

Supporting Tissues in Plants

A herbaceous stem is kept erect by a combination of turgor of its living cells, inelastic cuticle and supporting tissues.

Types of Supporting Tissues in Plants

1. Parenchyma tissue: these may be spherical or elongated cells with large vacuoles filled with cell sap. Parenchyma tissues are found at the cortex of stem, phloem, root, leaf, storage tissues and xylem. Parenchyma tissue supports herbaceous plants by turgor, storage of starch and conducts water and gases across the plant.
2. Sclerenchyma tissue: is mostly dead cells with cell walls filled with cellulose or lignin. Sclerenchyma tissue could be:
• Fibres: are elongated cells with tapering ends
• Sclereids: are not so elongated cells
• Sclerenchyma helps in providing rigidity, strengthens and supports the plants
3. Collenchyma tissue: The collenchyma gives the plant strength and provides resilience (flexibility) and support for young stems and leaves, and mechanical support to the plant
4. Xylem tissue/wood: strengthens the plant and conducts water and solutes.

Differences between the functions of the vascular tissues of plants and mammalian skeleton

SKELETON	VASCULAR TISSUES
-aids breathing	does not aid breathing;
-does not conduct water/food	conducts water/food;
-aids movement/locomotion in mammals	does not aid movement/locomotion;
-protects some vital/delicate parts/organs;	does not protect vital/delicate parts/organs;
-composed of bones/cartilage	composed of dense protoplasm and lignified walls.

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