Digestive System

Why is digestion important?

When you eat foods—such as bread, meat, and vegetables—they are not in a form that the body can use as nourishment. Food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy.

Tests for Chemical Components of Food

CHEMICAL	TEST	POSITIVE RESULT
Protein	Add a few drops of copper sulphate solution to the food sample, then a few drops of sodium hydroxide solution and mix well.	Blue-purple
Starch	Add a few drops of iodine solution to the food sample.	Blue-black
Glucose	Add an equal volume of Benedict's solution to the food sample and warm in a very hot water bath for a few minutes.	Orange-red
Lipid (Fats and Oils)	Rub a food sample onto brown paper and allow it to dry.	Translucent spot

3 Processes in Digestion

1. Ingestion - the mechanical taking in of food, chewing and swallowing
2. Digestion - The breakdown of food into smaller particles by physical or chemical means
3. Absorption - the absorption of these smaller food particles from the digestive tract into the blood

2 Types of Food Breakdown

1. Physical or Mechanical Breakdown - e.g. chewing, mixing with bile
2. Chemical Breakdown - e.g. enzymes, hydrochloric acid

Parts and Functions of the Human Digestive System

Mouth

• Ingests food
• Teeth physically break down food by chewing
• Saliva lubricates food
• The enzyme, salivary amylase also called ptyalin, breaks down starch into simple sugar, glucose

The Tooth

A tooth is a hard structure, set in the upper or lower jaw, that is used for chewing food. Teeth also give shape to the face and aid in the process of speaking clearly. The enamel that covers the crown (the part above the gum) in each tooth can be broken down by acids produced by the mouth for digestive purposes. This process is called "decay". To prevent decay, good oral hygiene, consisting of daily brushing and flossing, is necessary. The hardest substance in the human body is one of the four kinds of tissue which make up the tooth. It is enamel and covers the crown (area above the gum line) of the tooth. A bony material called "cementum" covers the root, which fits into the jaw socket and is joined to it with membranes. "Dentin" is found under the enamel and the cementum, and this material forms the largest part of the tooth. At the heart of each tooth is living "pulp," which contains nerves, connective tissues, blood vessels and lymphatics. When a person gets a toothache, the pulp is what hurts.

Pharynx

• At the back of the mouth cavity
• Both food and air pass through here

Oesophagus

• Tube between mouth and stomach
• A flap called the epiglottis closes over the top of the windpipe or trachea when swallowing, so that food does not enter the respiratory tract
• The walls of the digestive tract from the oesophagus to the anus are muscular, and contract rhythmically to move food. The muscular contractions are called peristalsis.

Stomach

• 2 circular muscles called sphincters surround the entry and exit of the stomach to control the flow of food
• Food remains in the stomach for about 3 hours where it physically broken down by the churning muscular contractions of the stomach wall muscles
• Gastric juice contains hydrochloric acid and has a pH of 1 without food, and 3 with food.
• HCl helps to kill bacteria, and works in association with the enzyme, pepsin, to partially break down proteins.
• The stomach lining contains mucus to prevent damage from the acid
• Only alcohol and a few drugs can be absorbed through the stomach wall into the blood

Small Intestine

• Long tube that is about 7 metres long and 2.5 cm in diameter
• 3 parts of the small intestine are duodenum, jejunum and ileum
• Most of digestion occurs in the small intestine
• The enzymes of the small intestine do not function on the acidic stomach contents. First, an alkaline substance from the pancreas is secreted into the duodenum to neutralise the stomach acid.
• A green substance called bile that is made in the liver and stored in the gall bladder is released into the duodenum to break the large fat particles into smaller fat particles
• Enzymes from the pancreas also break down food chemically. Amylase breaks starch into simple sugars. Lipase breaks lipids into fatty acids and glycerol. Peptidases and Trypsin break down proteins into amino acids.
• After the food into broken into smaller particles, it is absorbed through finger-like projections called villion the walls of the small intestine into blood capillaries.

Large Intestine

• Thicker in diameter than the small intestine
• Water is absorbed from the remains of undigested food to make faeces
• Bacteria produce Vitamin B in the large intestine
• Mucus lubricates the faeces
• The appendix is attached to part of the large intestine. It may be part of the body's immune system.
• Diarrhoea is the condition of more liquid faeces when the large intestine is infected.

Rectum

• Storage area for faeces at the end of the large intestine
• There is a sphincter surrounding the anus, the hole through which faeces passes on defaecation.
• Constipation is the condition of dry hard faeces as a result of low fibre in the diet.

The Role of the Liver in Digestion

• Bile - The liver produces bile which is stored temporarily in the gall bladder, and then secreted into the duodenum (first part of the small intestine) for the emulsification of lipids. Bile is also alkaline and aids in the neutralisation of stomach acid in the small intestine.
• Sugar Conversion - After a meal, excess simple sugars in the bloodstream pass to the liver and are converted and stored as glycogen. However, between meals, the glycogen is converted back to simple sugars and released into the bloodstream. In this way, the blood sugar remains constant.

Appendix

Digestion takes place almost continuously in a watery, slushy environment. The large intestine absorbs water from its inner contents and stores the rest until it is convenient to dispose of it. Attached to the first portion of the large intestine is a troublesome pouch called the (veriform) appendix. The appendix has no function in modern humans, however it is believed to have been part of the digestive system in our primitive ancestors.

Additional hormones in the digestive system regulate appetite:

• Ghrelin is produced in the stomach and upper intestine in the absence of food in the digestive system and stimulates appetite.
• Peptide YY is produced in the digestive tract in response to a meal in the system and inhibits appetite.

Both of these hormones work on the brain to help regulate the intake of food for energy. Researchers are studying other hormones that may play a part in inhibiting appetite, including glucagon-like peptide-1 (GPL-1), oxyntomodulin (+ ), and pancreatic polypeptide.

Nerve Regulators

Two types of nerves help control the action of the digestive system.

Extrinsic, or outside, nerves come to the digestive organs from the brain or the spinal cord. They release two chemicals, acetylcholine and adrenaline. Acetylcholine causes the muscle layer of the digestive organs to squeeze with more force and increase the “push” of food and juice through the digestive tract. It also causes the stomach and pancreas to produce more digestive juice. Adrenaline has the opposite effect. It relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs, slowing or stopping digestion.

The intrinsic, or inside, nerves make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves are triggered to act when the walls of the hollow organs are stretched by food. They release many different substances that speed up or delay the movement of food and the production of juices by the digestive organs.

Together, nerves, hormones, the blood, and the organs of the digestive system conduct the complex tasks of digesting and absorbing nutrients from the foods and liquids you consume each day.

Comparison between the Digestive Tracts of
Herbivores and Carnivores

• Cellulose is a complex carbohydrate that is a major component of leaves and grasses. Humans refer to cellulose as fibre, as it stimulates peristalsis and passes out in the faeces. Humans cannot digest cellulose.
• Only a few organisms, such as fungi, protozoans and bacteria, can break down cellulose with anenzyme called cellulase.
• Herbivores have a symbiotic relationship with these organisms that live in the herbivores' digestive tracts. The micro-organisms receive food and shelter and, in return, digest cellulose anaerobically into smaller molecules that the host can absorb.
• Some herbivores have a caecum or hind-gut, where the small and large intestines join and where further breakdown of cellulose can occur. However, the faeces of these animals (e.g. horses, koalas, possums and rabbits) contain much cellulose.
• Other herbivores have a more effective rumen or fore-gut before the stomach, where partly digested plant matter can be regurgitated into the mouth for further chewing and swallowed again. Ruminants mammals include cattle, sheep, kangaroos and wallabies.

PHYLUM CHORDATA

All chordates have a number of structures in common:

A notochord (noto = the back; chord = string) is present in all embryos, and may be present or absent/reduced in adults. This is the structure for which the phylum was named.

A dorsal, hollow, ectodermal nerve cord (compare with Annelida and Arthropoda which have ventral, solid, mesodermal nerve cords) typically forms by an infolding of the ectoderm tissue, which then “pinches off” and becomes surrounded by mesoderm. Spinal bifida is the failure of the nervous system to close.

The pharangeal slits (pharynx = throat) originally functioned in filter feeding: water is taken into the mouth and let out via the pharangeal slits. The slits filter out food particles and keep them in the animal’s body so they can be put into the digestive tract. In fish, these have become modified as gills, and in humans as our ears and eustachian tubes.

A postanal tail (post = behind, after; anal refers to the anus) is present and extends behind the anus in many taxa, thus the anus isn’t at posterior tip of body. In humans, the tail is present during embryonic development, but is subsequently resorbed.

The main taxa within Phylum Chordata include:

Subphylum Cephalochordata (cephalo = head), which doesn’t have a “head.” This subphylum includes Lancets, so-named for their shape. These are in genus Branchiostoma (branchio = gill, fish; stoma = mouth) which was formerly know as genus Amphioxus (amphi = on both sides, double) Lancets look a lot like the generic chordate described above. They have a notochord, but no bones. They are about one inch long, and live in the muddy ocean floor.

Subphylum Urochordata (uro = tail) are called the tunicates. Their larvae show typical chordate characteristics, but the adults have lost many of these organs, and what they do still have have become highly-modified.

Subphylum Vertebrata may have come from an Amphioxus-like ancestor, however vertebrates have a definite head and Amphioxus doesn’t. Another theory suggests that vertebrates may have come from a larval form similar to tunicate larvae that were capable of reproduction in the larval stage, that is, they are sexually mature prior to metamorphosis, which is referred to as paedogenesis (paedo = child — same root as in pediatrician; genesis = origin, birth), defined as the precocious attainment of sexual maturity in a larva. Vertebrate characteristics include:

vertebrae, the skeletal units surrounding the nerve cord

a brain, enclosed within a skull

an endoskeleton which will grow along with the animal (unlike arthropods which must molt).

a closed circulatory system with a ventral heart

excretion via kidneys

separate males and females with sexual reproduction in most, with a few cases of parthenogenesis

The Classes in Subphylum Vertebrata include:

Class Agnatha (a- = not, without; gnatho = jaw) which is the lampreys. They do not have jaws, are eel-shaped, prey on fish, and have larval forms which are different from the adults.

Class Chondrichthyes (chondro = cartilage; ichthys = fish) which includes sharks and rays. They have a cartilage skeleton, not bone. They are not buoyant like other fish so they must swim or sink. Like other fish they have a lateral line system which detects differences in water pressure, the equivalent of our hearing.

Class Osteichthyes (osteo = bone) is the bony fish. This is the most numerous of all vertebrate classes. In fish, O2 is exchanged via the gills, which are covered by an operculum which helps to draw water across/through the gills. Their swim bladder is an air sac used to control buoyancy, thus unlike the sharks, bony fish can hold still at any depth and not sink.

Class Amphibia (amphi = on both sides, double; bios = life) is frogs, newts, and salamanders. They were the first land vertebrates. Frogs, especially, go through metamorphosis. Their eggs have no egg shells, so the sperm can swim through the water to the eggs, and the embryos must develop in water.

Class Reptilia (reptili = creeping) is the dinosaurs (dino = terrible ), snakes, turtles, crocodiles, and lizards. Reptiles have scales and are dry to the touch. Their eggs have leathery shells. Reptiles areexothermic (exo = out, outside), that is they maintain their body temperature through external means such as sunning on a rock or seeking shade. Reptiles need less food/energy to live and live longer than a comparable-sized mammal. Some dinosaurs may have been endothermic.

Class Aves (avi = a bird) is the birds. It is thought that birds are descended from dinosaurs, as evidenced, in part, by the scales on their feet. Also, feathers are modified scales: a key characteristic of birds is that they have feathers. Birds’ bones are light weight for flight. Birds are endothermic (endo = within, inner), that is, they control their body temperature from within (they’re “warm-blooded”). Birds’ vision is the best of all vertebrates: soaring hawks can spot small mice scrambling through the grass in a field far below them. Birds have shelled eggs and so must have internal fertilization — the egg muct be fertilized before the hen’s reproductive tract secretes an eggshell. Generally, mating is accompanied by an elaborate courtship ritual. Eggs and often young birds are more exothermic (are not able to control their body temperatures from within) and so must be brooded/incubated by parents.

Class Mammalia (mamma, mammil = teat, nipple) is the mammals. Key characteristics of mammals are the presence of fur/hair and mammary glands, derived from modified sweat glands, which produce milk for the young. Mammals have a diaphragm to aid in respiration. They are endothermic. Most mammals bear live young.

Animal Groups	Description (Which group do you like best?)	Details
	Subclass :egg-laying mammals (monotremata) (trema = hole) Playtypuses, echidnas. Generally regarded as the most primitive mammals since they lay eggs (like reptiles) and do not give birth to formed young. These mammals lays eggs like reptiles, but do have fur and milk. However, they have no nipples: their mammary glands just secrete milk onto the fur, from which the babies lick it. They have one posterior opening for their digestive, urinary, and genital tracts, hence the subclass name.	Families: 2 Genera: 3 Species: 3 show species
	Subclass:marsupials or pouche mammals (marsupialia) marsupi = a bag, pouch) Includes kangaroos, wallabies, wombats, opossums, dunnarts, bandicoots, cuscuses. The young are born as very immature embryos and must crawl to their mother’s pouch to continue their development. Typically, once in the pouch (marsupium) a baby would find a nipple there. Because of continental drift, there is a wide variety of marsupials in Australia, yet few elsewhere on Earth.	Families: 19 Genera: 83 Species: 295 show species
	Subclass Placentals (placent = a round, flat cake) contains most of the animals with which we are familiar. In this taxon, young complete embryonic development within the mother’s uterus and are nourished across a placenta. Some of the Orders in Subclass Placentals include:
	Order: edentata Some of the worlds most bizarre animals - anteaters, armadillos, sloths. They have reduced or no teeth.	Families: 4 Genera: 13 Species: 29 show species
	monkeys and apes (primates) Includes lemrus, bushbabies, lorises, pottos, tarsiers, marmosets, tamarins, monkeys, apes, humansOrder Primates (prima = first) includes lemurs, monkeys, apes, and humans. These have opposable thumbs and eyes that face forward.	Species: 180 show species
	pangolins (pholidota) Seven living species of mammal are included in the Pholidota, the pangolins or scaly anteaters.	Species: 7 show species
	insectivores (insectivora) Includes shrews, moles, golden moles, desmans, hedgehogs, moonrats, solenodons, tenrecsOrder Insectivora (vora = to eat, devour) includes moles, shrews, and hedgehogs which eat insects.	Species: 375 show species
	aardvark (tubulidentata) Aardvark	Species: 1 show species
	carnivores (carnivora) carni = flesh; vore = eat, devour)  Includes big and small cats, dogs, foxes, wolves, hyenas, bears, pandas, raccoons,weasels, stoats, badgers, skunks, otters,mongooses, civets. These are carnivorous and have pointed canine teeth and molars.	Species: 230 show species
	seals (pinnipedia) Seals, sea lions, walruses	Species: 33 show species
	colugos (dermopetera) Also called flying lemurs.	Species: 2 show species
	bats (chiroptera) Includes flying foxes (fruit bats), vampires, and all other bats. (chiro = a hand, like chiropractor; ptera = wing, feather) is the bats. Their fingers are webbed to create wings.	Families: 18 Genera: 177 Species: 993 show species
	rodents (rodentia) Includesrats and mice, dormice, gerbils, beavers, squirrels, porcupines, chinchillas, pacas, voles, hamsters,chipmunks. Order Rodentia (roden = gnaw, gnawing) includes squirrels, beavers, rats, porcupines, woodchucks, guinea pigs, and mice. These have chisel-like, constantly-growing incisors	Species: 1700 show species
	rabbits and hares (lagomorpha) includes rabbits, cottontails, jackrabbits, hares, pikas. which have chisel-like incisors and hind legs modified for jumping.	Species: 80 show species
	whales and dolphins (cetacea) ceta = a whale) toothed whales, baleen, (whalebone) whales, dolphins, propoises. Their front legs (equivalent to our arms) are fin-like, and they have no hind legs. They have a thick layer of blubber for insulation.	Species: 78 show species
	even-toed hoofed mammals (artiodactyla) Includes hippos, pigs, peccaries, camels, llamas, giraffes, deer, chevrotains, gazelles, antelopes, cattle, sheep, goats. which have even-toed hooves and are herbivorous.	Species: 220 show species
	odd-toed hoofed mammals (perissodactyla) horses, asses, zebras, rhinos, tapirs. which have odd-toed hooves and are herbivorous.	Species: 17 show species
	hyraxes (hyracoidea) bush hyraxes, tree hyraxes, rock hyraxes, dassies.	Species: 7 show species
	elephants (proboscidea) African and Indian elephantswhich have long, muscular trunks and thick, loose skin. Their upper incisors are elongated as tusks.	Species: 3 show species
	sea cows (sirenia) Dugongs, manatees.  Their front legs (“arms”) are fin-like, and they have no hind legs.	Species: 4 show species
	scandentia tree shrews, tupais, and dendrogales or tree squirrels	Species: 18 show

Nine Phyla of the Animal Kingdom

1. Phylum Porifera - The Sponges:

 a.) Habitat: mainly marine (salt  water)

b.) Sponges have a porous body wall. The pores or holes allow water to pass through this animal. Floating food particles are caught once they are inside the sponge.

c.) Adult sponges are sessile feeders which means these animals are attached to shells or rocks on the ocean floor as they feed.

d.)  Shape: asymmetrical which means no definite shape.

2. Phylum Coelenterata – The Coelenterates: jellyfish, hydras, corals

 a.) Habitat: marine

b.) Body wall: Their body wall is made of 2 cell layers called the ectoderm and endoderm. The ectoderm is the outside layer while the endoderm is found on the inside layer. A jellylike material is found between these 2 layers.

c.) Digestive System: The digestive system is incomplete which means that coelenterates have just one opening to the digestive cavity. This single opening serves as both its mouth and anus.

d.)  Symmetry: radial

e.) Specialized Cells: 1.Most coelenterates have tentacles that contain stinging cells that are used for protection and capturing food. 2. Their bodies contain a nerve network that allows movement of the tentacles and body.

3. Phylum Platyhelminthes – The Flatworms: planaria, tapeworms

 a.) Habitat: fresh and salt water; terrestrial(land)

b.) Body Plan: These animals are given their name because of their flattened bodies. Flatworms have 3 distinct tissue layers called the ectoderm, endoderm, and mesoderm or middle layer. Each layer gives rise to the various organs and systems of this animal.

c.) Digestive System: In free-living species of flatworms the digestive system is incomplete which means that the digestive cavit y has only a single opening. The parasitic tapeworm has no need for a digestive system because it absorbs nutrients that are already digested by the host in which it lives.

d.) Symmetry: Flatworms have bilateral symmetry and they have a definite head and tail region.

e.) Specialized Structures: 1. The planaria has a pair of eyespots at its anterior or front end. These eyespots detect light which the planaria avoids so they are less visible to their predators. 2. The tapeworm like other parasitic worms has a thick protective cuticle on the outside of its body. The cuticle protects the worm from being digested by the strong digestive enzymes of its host.

4. Phylum Nematoda – The Roundworms

a.) Habitat: fresh and salt water; terrestrial

b.) Body Plan: The body of a roundworm is long, smooth and unsegmented. Their cylindrical bodies are tapered at both ends and are covered by a protective cuticle. Three tissue layers; ecto, endo and mesoderm.

c.) Digestive System: Roundworms have a complete digestive system which means their digestive tract has 2 openings; a mouth to ingest food and an anus to egest waste.

d.)  Symmetry: Bilateral symmetry with an anterior end and a posterior end.

e.) Interesting Facts: 1. Free-living roundworms are extremely plentiful in soil and are essential in producing quality soil. 2. Pinworms are common parasitic roundworms found in children.

5. Phylum Annelida – The Segmented Worms: earthworm, leech, sandworm

a.) Habitat: marine, freshwater, terrestrial

b.) Body Plan : similar shape as the roundworm but the body is segmented both internally and externally which allows for a quicker response for movement.

c.) Digestive System: Segmented worms have a complete digestive system and this set-up is often referred to as a tube-within-a-tube body plan.

d.)  Symmetry: Bilateral; anterior and posterior ends; dorsal and ventral surfaces.

6. Phylum Arthropoda – The Arthropods: insects, spiders, crustaceans

a.) Habitat: arthropods are found in all environments.

b.) Numbers: Arthropods are the most successful of any animal group. This is mainly due to the success of insects, which has more than a million different species.

c.) Body Plan: Arthropods have a segmented body with paired jointed appendages that provide excellent movement for walking, swimming, flying, grabbing, fighting, digging and biting just to name a few. In most arthropods the body is divided into a head, abdomen and thorax.

d.) Exoskeleton: The outside skeleton of arthropods is made of chitin which protects the soft body of this animal and prevents water loss allowing them to live successfully on land.

e.)  Symmetry: Bilateral

7. Phylum Mollusca – The Mollusks: clams, snails, oysters, octopus

a.) Habitat: marine and fresh water; terrestrial

b.) Body Plan: Mollusks have a soft, unsegmented body and often move with a strong muscular foot on its ventral surface.

c.) The radula: Mollusks are well known for their tongue-like organ called the radula which has many rows of teeth and is used to scrape food from the surface of plants and rocks.

d.) The mantle: The mantle is a fold of skin that surrounds the body organs. The mantle acts like a gland because it is capable of secretion. These secretions harden to help form the shells of mollusks.

e.)  Symmetry: bilateral

8.   Phylum Echinodermata – The Echinoderms: sea stars, sea urchins

a)   Habitat: all are marine living mainly on the ocean floor.

b)  Body: Echinoderms have an internal, limy skeleton and a spiny outside surface or skin. These structures give both support and protection.

c)   Water-vascular System: Echinoderms like sea stars and sea urchins are well known for their water-vascular system which consist of water-filled tubes that run through their body. By moving water in and out of these tubes echinoderms can move on “jets” of water or use their tubed feet as suction cups.

d)   Digestive System: complete

e)   Symmetry: Radial

9. Phylum Chordata – The Chordates: fish, reptiles, amphibians, birds, mammals

 a.) Habitat: marine, freshwater, or terrestrial

b.) Symmetry: bilateral

 c.) Dorsal nerve cord.

d.) Chordates have a flexible, supporting rod or notochord on their dorsal side. In the invertebrates the notochord remains stiff and flexible. In the vertebrates, cartilage or bone replaces the notochord to form a supporting backbone.