34.1 Digestive Systems

Animals get their nutrition from other organisms.

Depending on the diet of the animals, they can be classified into three groups: plant eaters, meat eaters, and omnivores.
The cells can't immediately access the food's macromolecules.
There are a number of processes that modify food within the animal body in order to make it more accessible for cellular function.
The complexity of form and function has led to the evolution of the animal's digestion systems.

Large amounts of plant material can be handled by these animals.

Fruit-eaters, seed eaters, nectivores, and leaf eaters can be classified as herbivores.

The mule deer and monarch caterpillar eat mostly plant material.

Members of the cat family, such as lions and cheetahs, are examples of obligate carnivores, who rely entirely on animal flesh to get their nutrition.

Facultative carnivores also eat non- animal food.
Dogs would be considered facultative carnivores if there was a clear line between them.

The (b) ladybug is a predator that eats small insects.

omnivore means to eat everything.

A bear and a crayfish are omnivores who eat both plant and animal based food.

Different types of digestion systems have been evolved by animals.

This type of digestion is used by the platyhelminthes, Ctenophora, and Cnidaria.

The material enters the mouth through a hollow tube.
The cells within the cavity break down the food.
The food particles are covered by the cells.

An alimentary canal is an example of an animal with an alimentary worm.

Once the food is in the mouth, it passes through the stomach and into the gizzard, where it is churned and eaten.
The food passes through the gizzard and into the anus, where it is absorbed, and the waste is eliminated through the anus.

The vetbrates have evolved more complex systems to deal with their diet.

Some animals have a single stomach, while others have multi-chambered stomachs.
Birds are able to digest unmasticated food.

The process of digestion begins with the mouth.

The teeth break down food into smaller particles when masticating.
The saliva contains enzymes that breakdown food.
The mouth and stomach are connected by a tube.
The food is pushed towards the stomach by the muscles of the esophagus.
The stomach is very acidic, with a pH between 1.5 and 2.5.
The gastric juices act on the food particles and continue the process of digestion.
Further breakdown of food takes place in the small intestine, where the bile from the liver, the small intestine, and the pancreas are produced.
The cells of the small intestines are lined with epithelial cells.
The waste material travels on to the large intestine where water is absorbed and the drier waste material is compressed into feces, which is stored until it is excrete through the rectum.

The small intestine and cecum are enlarged in the rabbit to allow more time to digest plant material.

The surface area of the enlarged organ is larger.
The first time a rabbit eats, it collects in the cecum and then it excretes soft feces called cecotrophes.
The rabbit is able to digest cecotrophes.

Birds face special challenges when it comes to getting nutrition.

Birds have evolved a variety of beak types that reflect the variety in their diet, from seeds and insects to fruits and nuts.
Birds have high metabolisms in order to efficiently process food and keep their body weight low.
Food can be formed from the undigested material.

Most of the chemical digestion and absorption takes place in the gut.

The crop is a pouch that the bird has.

The first of two stomachs is called the proventriculus and it contains juices that breakdown food.
The food enters the second stomach, called the gizzard, from the proventriculus.
Birds store stones in their gizzards to aid the grinding process.
Birds don't have separate openings to excrete.
uric acid is produced in the large intestine and combined with waste from the digestion process.
The waste goes through an opening called the cloaca.

Birds have a very efficient system for digestion.

Recent fossil evidence shows that the evolutionary divergence of birds from other land animals was due to the simplification of the digestive system.
Birds don't have teeth to chew their food.
In place of lips, they have sharp beaks.
The smaller tongue of the birds, lack of jaws, and horny beak can be traced back to their dinosaur ancestors.
The inclusion of seeds in the bird diet seems to coincide with the emergence of these changes.
The two-compartment stomach allows for delegation of tasks and the beaks of seed-eating birds are shaped for grabbing seeds.
Since birds need to remain light in order to fly, their metabolism is very high and they need to eat a lot.
The digestion of plant matter takes a long time in the ruminants.

They have evolved systems that help them digest large amounts of food.

The ruminants have no upper incisor teeth.
They chew their food with their lower teeth, tongue and lips.

The rumen, reticulum, omasum, and abomasum are contained in the stomach.

The chambers contain many organisms that ferment food.
The stomach is called the "true" stomach because it is the same size as the monogastric stomach chamber.
The four-compartment gastric chamber provides larger space and the necessary support to digest plant material in ruminants.
The stomach chamber has large amounts of gas that must be eliminated.
The small intestine plays an important role in absorption and elimination of waste, as it does in other animals.

Animals with four stomachs include goats and cows.

The first two stomachs, the rumen and the reticulum, contain prokaryotes and protists.
The ruminant chews the cud from the reticulum, then swallows it into a third stomach, the omasum, which removes water.
The cud goes into the fourth stomach where it is eaten by the ruminant.

Some animals are pseudo-ruminants.

They eat a lot of plant material.
The plant cell walls contain a sugar molecule.
The animals can't breakdown the cellulose in their stomachs, but they can break it down.
The bicyle must be able to handle large amounts of roughage.
ruminants have a three-chamber stomach.
The cecum is a pouched organ at the beginning of the large intestine that contains many microorganisms that are necessary for the digestion of plant materials.
These animals do not have a rumen, but they do have an omasum.

The bicyle is designed to facilitate the transformation of food into food components that sustain organisms.

The mastication of the teeth breaks the food into smaller particles.

All mammals have teeth.

The salivary glands produce saliva when food is chewed.

Many animals produce saliva in their mouths.
The parotid, submandibular, and the sublingual are the three major salivary glands.

Ignites and lysozymes are found in saliva and can be used to reduce tooth decay.

Lipases can breakdown triglycerides.
The breakdown of fat in the food begins with the lingual lipase.
The tongue helps in swallowing.
The pharynx leads to the lungs and the stomach.
The epiglottis is a flap that covers the glottis in the trachea.
When swallowing, the epiglottis closes and the food is not in the trachea.
Food can be kept out of the trachea.

Food is masticated by teeth and moistened by saliva.

The saliva begins to digest fats and starches.
With the help of the tongue, the result is moved into the stomach.

The food is chewed and swallowed.

It is not possible to reverse the peristalsis wave, which moves food from the mouth to the stomach.
In response to swallowing, the peristaltic movement of the esophagus takes place.

In response to swallowing, the sphinx opens and the food enters the stomach.

The contents of the stomach are not allowed to travel up the esophagus when there is no swallowing action.
Many animals have a true sphinx, but in humans, the sphinx is closed when there is no swallowing action.
When the acidic juices escape into the esophagus, acid reflux occurs.

The stomach has a pH between 1.5 and 2.5.

The acidic environment is needed for the breakdown of food.
When filled with food, the stomach can expand up to 20 times its resting size, even though it is a small organ.
Animals need to eat when food is available.

The human stomach has an acidic environment.

In animals other than ruminants, the stomach is the main site for digestion.

pepsin is found in the stomach chamber.
A positive feedback mechanism that creates more pepsin can be started by breaking peptide bonds and cleaves proteins into smaller polypeptides.
Parietal cells form hydrochloric acid, the primary acidic component of the stomach juices.

The inactive pepsinogen is converted to pepsin by hydrochloric acid.

The action of the pepsin in the food results in the destruction of many organisms.

The stomach acts as a catalyst for chemical digestion.

About every 20 minutes, stomach contents mix with smooth muscles.

There is further digestion in the small intestine.

Within two to six hours after a meal, gastric emptying occurs.
A small amount of chyme is released into the small intestine.
The movement of chyme from the stomach into the small intestine is regulated.

The stomach lining needs to be protected from being eaten by pepsin.

The stomach lining is protected by two points.
The pepsin is made in the inactive form.
The chief cells are protected because pepsinogen does not have the same function as pepsin.
The stomach has a thick mucus lining that protects the underlying tissue.
Ulcers can form in the stomach when the mucus lining is rupturing.
When the mucus lining is torn and fails to reform, there are open wounds in or on an organ.

From the stomach to the small intestine is where chyme goes.

Microvilli are small projections on the apical surface of the villus.
The villi and microvilli have many folds and increase the surface area of the intestine.
The blood is carried into the hepatic portal vein to carryAbsorbed nutrients in the blood are carried into the hepatic portal vein to carryAbsorbed nutrition in the blood are carried into the hepatic portal vein to carryAbsorbed nutrition in the blood are carried into the hepatic portal vein to carryAbsorbed Toxic substances, including drugs, alcohol, and some pathogens, are removed from the rest of the body by the liver.

The folds on the small intestine lining increase the surface area for absorption of vitamins and minerals.

There are many folds in the small intestine.

The duodenum, jejunum, and ileum are the three parts of the small intestine.

The duodenum is separated from the stomach by the pyloric sphincter.
In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that acts as a buffer.
Pancreatic juices have a number of enzymes in them.
The bile salts in bile are used to make lipids and the pancreas makesamylases, which are used to break down food.
The duodenum is where some chemical digestion of food takes place.
The duodenum is where the absorption of the fatty acids takes place.

In this area, most of the carbohydrates and amino acids are absorbed through the intestinal lining.

The jejunum is home to the bulk of chemical digestion.

The food is sent from the ileum to the colon via peristaltic movements.

The large intestine begins at the ileocecal valve.
The ileocecal valve contains the vermiform appendix.
The appendix of humans has no role in immunity.

The large intestine is larger in diameter than the small one.

The cecum, colon and rectum are part of it.
The cecum joins the ileum to the colon and is the receiving pouch.
The colon is home to many types of flora that aid in the digestion.
The colon can be divided into four regions.
The colon is used to store waste material and to extract water and mineral salts from undigested food.
The large intestine of mammals is shorter due to their diet.

The large intestine stores waste material until it is eliminated.

The rectum stores feces until defecation.

During elimination, the feces are propelled.
The inner and outer sphincters of the rectum and anus are voluntary.

The organs of the bicyle are what passes food through.

There are accessory organs that addenzymes to catabolize food.